Startesting And Scope Tuning Backgrounder

AstroTalk @ Astro.Geekjoy.Com's
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Startesting And Scope Tuning Backgrounder

Introduction: For the "Optically Correct"
High Fidelity Optics
Stereoscopic Telescopy
The First Degree: Chromatic Aberration
The Second Degree: Obstructions in the Light Path
The Third Degree: Exclusion of Stray Light
The Fourth Degree: "Stellar" Performance
The Fifth Degree: Ease of Focus
The Sixth Degree: The Extrafocal Star Test
In Conclusion...
Let the Dialogue Begin!

For the "Optically Correct"

Hi All,

The most important thing about your scope is the fact that it gets used regularly and brings you joyful and satisfying views of the cosmos. If this is the case then you have an excellent telescope and have no need to go any further.

However, there are those amongst us who will never be happy until we achieve the "bliss of perfect optics". (Or at least optics that is as perfect as we can afford and collimate.)

There are also those among us who wish to get a deeper understanding into how a telescope works and works well.

Such observers aspire to be "optically correct".

Thats what this thread is about - getting ourselves and our scopes "optically correct".

But be forewarned when it comes to evaluating a scopes optical performance a little bit of knowledge can be a dangerous thing! (I know - I'm dangerous...)

Another warning: "If the engine runs well, don't start poking around under the hood." Keep in mind that if you look too closely at anything you can't help but notice "flaws". Many of which are quite trivial and have no real impact on performance.

Clear and Steady,

jeff

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High Fidelity Optics

There are those of us who remember something of the "bad old days" of recorded music. A time when recording albums had limited frequency response, were monaural, and included a great deal of harmonic distortion and unintended sound (noise).

As I recall, sometime in the early 60's record albums started getting stamped with the following phrase on the packaging: "High Fidelity". This occured about the same time that records also began to include two tracks (stereophonic) and a few adventurous types (the bleeding edge) began investing in modular sound components (turntables, preamps, power amps and speakers).

Later (by the early 1970's), high fidelity sound and equipment had become commonplace. Especially as quality improved in the audio industry and the price of good components fell as a result of surging demand and improved manufacturing techniques and electronics.

Now I see this same phenomenon happening all over again - in optics. And for the same reason, amateurs want "high fidelity" equipment. And they want it reasonably priced.

So what does "high fidelity" mean anyway?

For me it means that the original and its reproduction are so identical to the senses as to be indistinguishable...

And in telescopy, this means that when you observe any study in the heavens, you see it as though you were looking at it with the power of your own eye expanded to the size and degree of magnification that the telescope provides!

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Stereoscopic Telescopy

For many decades folks were very happy with the fact that both of their ears could hear music - even if it meant that the sound they heard was exactly the same (that is lacking stereophonic sound modulation).

BUT eventually the requirement for "High Fidelity" sound demanded that the sonic experience of the listener duplicate that of being in the concert hall. As such, depth of audio perception became an important attribute of the listening experience.

Such a "stereo" experience however, is not to be expected in amateur astronomy. For as we know, to see even the nearest stars shift against the background of space requires that optical signals from two scopes located on opposite sides of the Earth's orbit around the Sun (some 186 million miles) be presented to the eyes simultaneously.

But, even a monaural listening experience allows both ears and more importantly both halves of the human brain to participate.

Of course this is not true in amateur astronomy. We observers are only able to admit light into one eye - and by extension only one half the brain's ability to process optically encoded stimulation is engaged.

In this way we have exposed one potentially serious deficiency related to our observing experience. AND that deficiency is a strike against most astronomical equipment now generally in use.

CONCLUSION ONE: SOMEDAY THE USE OF BINOVIEWERS WILL BE THE NORM AMONGST AMATEURS - NOT AS NOW - THE EXCEPTION.

NOTE: Several months after writing the above I had my first extended opportunity to view the Night Sky through a binoviewer.

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The First Degree: Chromatic Aberration

To this point we've identified two divergences between the audio revolution of the 60's and the current state of optical empowerment at the beginning of the new millenium:

One of the goals of the audio movement was to extend the range of audio reproduction until the full sonic envelop (from 20hz to 20Khz) could be reproduced with a "flat frequency response". This meant that no particular tone on play back should be any "louder" or "softer" than was the intent of the musician, conductor, or audio engineer.

Now the corollary to "frequency response" in a telescope is its ability bring to focus all the colors of the rainbow - from deep red to near ultraviolet purple. In general, telescopes that use reflective surfaces for organizing photons are able to do this extremely well. And again, in general, scopes that use refractory elements tend to do so with great difficulty. Of course, all scopes have at least one component that includes refractory elements (the eyepiece) and are subject to some degree of chromatic aberration.

As a result, certain types of scopes (mirror-based newtonian reflectors for instance) enjoy a great deal of popularity specifically for this reason - such scopes have "high chromatic fidelity" (and are also relatively inexpensive to manufacture).

Other types of scope (the achromatic refractor) can be handicapped chromatically. And from the beginning of the optics revolution (beginning in the 17th century) there were only two ways to offset this problem:

Today, of course, there is a class of refractors which have closed the chromatic fidelity gap by using exotic materials and extremely close toleranced lens shaping, alignment, and positioning. Such scopes are extremely expensive (typically ten times a comparably apertured achromat or newtonian) and are highly prized (for reasons to be discussed later.)

Since this discussion is about "testing" and "tuning" your scope, your first optical test (should you decide to accept) is to make this simple check:

Any sign of color seen either along the limb of the Moon, or in the defocused image, means that the scope is not completely doing the job of "properly organizing photons" in a way which empowers the eye to see things as they really are. Such, less well organized photons can and will be used against you as you view the denizens of the night sky...

NOTE: Most achromatic refractors will show a greenish cast outside focus and a magenta cast inside focus. However, this assumes slight undercorrection in the optics. Some achromats may actually be "overcorrected". This can reverse the color scheme described above. Meanwhile, there is a second type of chromaticism seen in scopes. Whereas the inside /outside focus type is "longitudinal", there can also be a latitudinal spread of colors across the field of view. (Like a spectrum but you tend to see blue one side of the study and orange on the other.) This type chromaticism is often eyepiece engendered but can also be induced by low sky position or other optical elements.

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The Second Degree: Obstructions in the Light Path

Our discussion of chromatic aberrations introduced the idea that improperly organized light has a dual effect on the quality of the observing experience.

First, is that the "discolored" in focus image does not faithfully represent the actual appearance of the study within the field of view. Effectively the image of that study takes on hues and colors which are natively present, BUT are not properly distributed and balanced vis a vis complementary colors from that same study.

The second, and more devastating impact is that photons which should come to precise focus don't. Certain colors on the extreme of the spectrum are actually slightly out of focus. This introduces a loss of clarity and contrast. Effectively, the eye has to overcome photonic distractions and (in concert with the brain) must work harder at interpreting what information it does receive...

As mentioned earlier, there is one advantage to the modern refracting telescope - especially those scopes that employ exotic materials (low dispersion glass, flourite crystal etc.) to suppress chromatic aberration. Such scopes do not have a "blockage" in the optical path. Effectively all the light passing though the elements manage to get to the eye.

The relatively chroma-free newtonian reflector however, does have such a blockage, a blockage that prevents high quality on-axis light from entering the eyepiece. The effect of such a blockage (or obstruction) is similar to that noticed when stereo speakers are separated so widely that the music no longer "synthesizes" in the minds ear. The result is a type of neuroprocessing confusion that detracts from the quality of the experience and makes interpretation difficult.

The effect of a central obstruction also causes a purely physical problem. When light passes around any transitional boundary, it is slightly diffracted. As such, the waveform is disturbed and its ability to re-integrate itself is hampered. Effectively, photons that would normally come to a point (in an airy disk for example) are dispersed into the diffraction rings around that disk. As a result, clarity is lost due to the dispersion of photons into a larger area.

This is the price payed for the use of mirrored surfaces. A slight loss in contrast due to the presence of the secondary mirror and (less significantly) the support vanes within the main tube.

NOTE: There are designs of reflecting scopes (the gregorian for instance) that do not use a secondary mirror. Such designs have their own inherent problems. It remains quite likely, however, that some enterprising optical designer will develop a design to overcome these difficulties and an obstruction-free reflector will emerge that can be mass produced and made generally available to the astronomical community.

ANOTHER NOTE: It is well established amongst the astronomical community that scopes with linear central obstruction percentages less than 20% the aperture of the scope are virtually indistinguishable from unobstructed scopes in terms of contrast and fine detail.

Once again, because this thread is about testing and tuning a telescope try this:

To give a sense of the effect of central obstruction on detail, I used Cor Boerrevoet's aberrator program to create three views of Jupiter. Each assumes a six inch scope with slightly better than "diffraction-limited" optics used through near-perfect skies. One scope is free of an obstruction (an APO refractor). The second possesses a linear central obstruction of 20% total aperture (a Maksutov-Newtonian). The third has a 35% central obstruction (a Maksutov-Cassegrain). None includes a spider vane to support the secondary. The challenge is to determine which scope is which based on the quality of the image...

Example One
Example Two
Example Three

NOTE: Once you've come to a conclusion, right click on each image and inspect it's name property. Linear central obstruction percentage is embedded in the file name (along with scope type).

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The Third Degree: Exclusion of Stray Light

When listening to a Beethoven sonata through a really good sound system, it is very important that unrelated sound (noise really) be kept to a minimum in order to have the best possible chance of capturing subtle moods and nuances in the music.

There is also a corrallary in telescopy. While observing the faint glow of a distant galaxy, subtle structure may be easily lost to "stray light" entering the optic tube. For this reason better scope designs include a series of "light baffles" designed to prevent stray light from entering the field of view. It is for this reason that the interior of all well made scopes is as black as possible. (This black should have as dull a finish as possible to prevent light from bouncing around inside the tube.)

Meanwhile, other enhancements are possible. In a refractor a series of descendingly sized rings can be placed in the tube. Each ring is sized in such a way as to allow the convergent light cone to pass straight through (without vignetting) while at the same time blocking off-axis light from making its way inside due to moon, sky, neighbors well lit windows, and outdoor lighting etc. The more of this type light excluded, the darker the background sky is seen to be. The effective result is an increase in magnitudinal reach along with enhanced image contrast.

Certain scope designs are more difficult to baffle than others. Of all designs, scopes of the Maksutov-Cassegrain (MCT) type are perhaps the most difficult to baffle. Such scopes use the opposite of a converging light cone - the meniscus (at front) actually "splays" the beam outward. Because of this, it is impossible to place a series of "down-sized" concentric down-tube "baffle rings" of the type used in a refractor. However, just because it is difficult to baffle an MCT does not mean it can not be done - and done well. Cassegrain, Maksutov-cassegrains, and Schmidt-Cassegrains can have both ring and tubular baffles added. The optical tube is frequently much larger in diameter than the primary mirror. And baffle rings must have larger diameters as they move away from the meniscus. (Whereas in a Newtonian they can remain all the same size.) Tubular baffles are used exclusively in Cassegrain type scopes. Such baffles may be added around the secondary and surrounding the port in the middle of the primary which admits the light cone into the visual back and focuser.

Generally though, scopes of the newtonian type are least likely to be well baffled. One issue is the fact that the focuser looks toward the far side of the tube. If the tube is open (of the truss type) or is made of bright or glossy material much ambient light may enter the field of view by "leaking" around the smallish secondary mirror and housing.

To test for stray light rejection:

At focus: On a clear, dark night place the highest power magnification your scope is capable of in the focuser (add a barlow if you have one). Direct the scope straight overhead. Inspect to see whether or not the field stop in the eyepiece is distinctively darker than the field of view. If so, then there may be an excess of stray light finding its way to the field of view.

OR on a similar night, view the edge of the Moon. Inspect for "glow" around the edge. (The best scopes show the night sky outside the limb of the moon almost pitch black - the "color" of the eyepiece field stop.)

A-Focal: Again on a dark, steady night, turn the scope on Polaris. Inspect the diffraction rings that should be visible around the star. Notice the difference between the "bright" and the "dark" interference regions. If it is hard to distinguish them from one another, it may be due to "stray light" entering the field. (Or it may be due to problems with the optics.)

Finally you can also check a scope visually for good baffling by simply inspecting the optical path using a cheshire eyepiece. All you should see with the cheshire in place is light from the main mirror, object glass, meniscus, or corrector plate. If you see light surrounding any of these sources through the cheshire (at point of focus) there is insufficient baffling present in the optical tube assembly...

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The Fourth Degree: "Stellar" Performance

Everything talked about so far has dealt with the "macro level" performance of the optical train. We've discussed "overall" issues of "frequency response", and "presence" (contrast) plus the need to eliminate stray light. Such factors relate primarily to the basic design of the scope, and the materials out of which optical components are constructed. These factors are generic to a brand and model and should be considered in advance of purchase by reviewing scope tests and reviews on the Internet and elsewhere.

But now we are at a point where we begin discussing micro level issues. Issues that relate to how well a particular scope compares to others of it's kind and manufacture. This in turn relates to a particular scopes "character", "quality", or "integrity" and thus its unique ability to accurately reproduce celestial (or terrestrial) objects for apprehension by eye (and mind).

It is only at this point that we can address the question of "star testing" a particular scope and its accessories. But first keep in mind that the basic goal of a star test is to provide definitive information related to how well a particular scope is assembled and aligned. A secondary goal is to determine if any particular accessory is in good working order (an eyepiece or diagonal if one is installed) and a third (more painful result) is to determine if any serious problem exists with the main optical components of the scope (primary or secondary mirror of a reflector, or object glass or any petzval of a refractor).

There is one main reason that all telescopes, without exception, use round object glasses or mirrors. The laws of optics limit the ability of the optical train to creating something other than "perfect infinitesmal points or light". All stars are such points, but they can only be seen as tiny disks. If mirrors and lenses were not round, images of stars would take on whatever shape the lens or mirror has. So since most observers prefer "round " to orange hearts, pink moons etc. (the stuff of "Lucky Charms" kids cereal) scopes are round and so should stars being seen through them.

The only in focus test to be made during star testing is this: At moderate magnifications (30x per inch aperture or so), inspect the image of a second or third magnitude star. Be sure the scope is temperature stabilized. And make sure that there is a steadyness to the atmosphere through which you are looking.

As you look at such a star inspect it for "roundness". It should appear a perfect little "ball of light" surrounded by one or more thin, concentric hoops or rings, (The number and perfection of these rings depends greatly on the magnitude of the star, seeing stability, transparency, and a number of scope related factors.) There should be no sign of "oblateness" to the stars shape. Nor should the rings appear oblong or otherwise deformed.

Generally the above test is only valid at the center of the field of view. Eyepieces can have a significant impact on the way in which off-axis stars are seen. Even more significant is the effect of collimation (or the alignment of the various image handling surfaces and lenses in the optical train). Stars that look like arcs, have tails, or throw off a lot of spurious light consistently to a particular direction are NOT a good thing.

But if the scope shows brighter stars as perfect little disks. And shows faint stars as perfect ilittle pinpoints of light - you have a fine scope - regardless of whatever other little ideosyncracies you may discover along the way.

To be sure, seeing stars in this way may only be possible on better nights of seeing. Large aperture scopes are less likely to achieve this "vision of stellar perfection" than smaller ones. Also NOT being able to see these disks or pinpoints is not necessarily a sign that you have an intrinsically poor scope, but may simply be a matter of a poor accessory (barlow, diagonal, or eyepiece) or more likely an improperly aligned optical or mechanical components.

Finally, even though you may see such airy disks or pinpoints of light on a regular basis, your scope could still be hampered in a way that results in less than satisfactory performance while viewing the Moon or planets, globular clusters, or disparate magnitude double stars.

To guage this next level of performance you will need to perform two more types of test:

The Plot Thickens!

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The Fifth Degree: Ease of Focus

A very important key to seeing fine detail on the Moon, planets, resolving compact globular clusters or doubles with large differences in stellar magnitude (disparates) is the ability to achieve "perfect focus".

As we all know, focus is achieved by starting on one side and adjusting the focuser past best image quality and back again. The finer the optical train, the more precise the mechanics of focusing, the stabler the telescope mount, and the better the alignment of the optical train, the easier it is to achieve "perfect" focus.

Finding yourself "fiddling" with focus often during observing is a clear sign that there is some kind of problem. But with what?

Problem 1: Seeing stability is very poor in the region of sky you are viewing through.

Problem 2: There is "slop" in the focuser mechanism.

Problem 3: The mount is unstable and you are unable to determine when the image has best focus due to gyrations.

Problem 4: Too much magnification is in use for the scope, mount, sky, or object of study.

Problem 5: The scope is out of alignment (collimation).

Problem 6: Some component in the optical train is improperly shaped, surfaced, coated, or is contaminated by fluid.

Problem 7: Your observing eye is tired and you need a break.

When achieving proper focal traverse, the goal is to systematically eliminate problem sources. Since we all have a native genius for this type troubleshooting, I will simply describe the "focus traversal startest method" and leave you to figure out what steps must be undertaken to isolate and remedy the problem.

Step One: Pick a star bright enough to easily see, but not so bright that you can not make it out for all the flashing that goes on around it. (You want to see an airy disk if at all possible.)

Step Two: Rack the focuser backward until you see a "round globe of light". (The further extrafocal you have to go to see the globe as round the more likely that there is a problem with the scope!)

Step Three: Slowly move the focuser in and observe whether or not the globe collapses into a perfect "circle" or it starts to flare noticeably in one direction or another.

Step Four: Pass through the point of best focus and carefully watch as the "globe of light" expands again. Note whether the globe reverses direction (this is called coma) or shoots off at a right angle (astigmatism).

If coma is present, it is likely that only one surface is askant in the optical train. If astigmatism is seen, then more than one surface (or lens) is misaligned.

If neither coma or astigmatism is seen, then your scope passes the traversal of focus test - but may still have serious flaws in shaping or alignment that preclude its ability to provide "on the edge" optical performance (or achieve the ultimate level of optical fidelity). Such flaws fall along two main lines:

1. Spherical aberration.

2. Zones or improperly shaped (turned) edges somewhere in the optical train.

To diagnose these two type problems you will need to do an afocal star test by examining the interference lines created when the light from a point source (a star) converges (inside focus) and diverges (outside focus) from the point of best focus.

NOTE: Zonal and spherical shaping errors can also be diagnosed at focus but requires a well trained eye and a night of superb seeing. In such a case the number, intensity, and position of diffraction rings around a bright stars airy disk is examined for various irregularities. But throwing the scope out of focus makes the task much easier visually.

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The Sixth Degree: The Extrafocal Star Test

So far the perfect telescope may be said to embody the following virtues:

OK, this last test is very "esoteric" and perhaps a bit of overkill anyway - but it does have its place and I personally have been scratching my head over the issue since I first heard mention of it. But the mechanics aren't very difficult and that's always a good place to start...

On a night of great atmospheric stability and decent transparency, using moderate to high magnifications (~50-75x per inch aperture) and after allowing the scope sufficient cool down time (to eliminate thermal distortion):

Step One: Select a star that is bright enough to show an airy disk - but not so bright as to "flash" at you. (The star should be roughly 11 magnitudes brighter than the dimmest star visible through your scope - or roughly magnitude 2 in a six inch instrument.)

Step Two: Adjust focus inward (intrafocally) until the star just expands into an outer ring of light (a torus). This torus may encompass a void, or show a central starlike point. Make a mental note of how bright and evenly illuminated the intrafocal torus is.

Step Three: Adjust focus outward (extrafocally) until the torus expands to roughly the same size as intrafocally. Note how bright and evenly illumined this extrafocal torus is.

Step Four: Mentally compare the two afocal tori. If one is easily visible but the other is only seen with great effort, the scope probably has severe spherical aberration. If the two are indistinguishable, spherical aberration is negligible. The degree of disparity between the two outer tori is proportional (more or less) to the degree of undercorrection (bright torus inside focus) or overcorrection (bright torus outside focus) displayed by the optics.

NOTE: It is important to make this check without cranking the focuser too much. As travel increases, the difference between the two tori is substantially reduced. As a rule, crank focus only so far as to show one additional interference (fresnel) ring between the star point (or central void) and the torus. (So when showing off the quality of your scope to others - crank it all you want!)

Step Five: Return inside focus and examine interference rings seen around the star. This time try to make at least four rings total appear by extending the focal travel. Note whether all rings are perfectly concentric and well-defined all the way around.

Step Six: Note whether all rings are equally spaced from one another. If not, some coma, astigmatism, or pinch is present in the optics. These problems can be addressed mechanically through proper tensioning of optical retaining elements and mechanical collimation. You should also try other eyepieces, diagonals etc.

Step Seven: Note whether each ring (inside the torus) is roughly equal to the others in "thickness"? (Does one, for instance, look like an "inner tube" while a neighbor looks like a "piston ring"?)

Step Eight: Follow each ring around its circumference and look for perfect symmetry of luminosity, thickness, and spacing at all points.

Repeat steps five through eight with the star outside focus.

In general, if intra-toroidal interfernce rings are well defined, concentric, and evenly illumined, the optic displays good smoothness and relative absence of zones. If poorly defined, this suggests that the scope tends to "spray light" chaotically around the field. If soft in some spots and sharp in others, there is probably a region of the optic (a zone) that is improperly figured.

That's all there is to it... (yah, right!) These silly rings tell you everything you need to know about your optics.

And of course, if everything is absolutely perfect both sides of focus, your scope was made by a defence contractor who develops imaging systems for spy satellites. That scope was later personally collimated by Harold Suiter, Roland Christian, Cor Berrevoets, or Otto Piechewski!

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In Conclusion...

Well Folks,

I hope the mystery has been taking out of the star test. I also hope that you will still love your scope after doing it. I have to tell you that 150mm MCT Argo is almost, but not quite, perfect. Inside focus I can detect no flaw whatsoever in ring presentation. But outside focus there is a slight loss of clarity and definition. All this says the optics is pretty smooth. But what about spherical aberration?

Frankly its hard to tell any real difference between the intra and extrafocal tori. But the difference is there. So yah, these MK-67's come in somewhere bewteen 1/6th and 1/8th wavelengths corrected. But due to that large plug in the middle of the tube, this means the scopes do not perform quite as well as six inch refractors of 1/4 wave correction...

Focused Star Disparate Double Extrafocal Startest Planet Saturn Planet Jupiter

Like Argo, the 80mm Pup achromat gives a fine view inside focus. But there are complications. Chromaticism makes a good view of diffraction rings difficult. (Lately I've taken to using a medium blue filter.). Outside focus, along with the chromatic aberration, the Pup shows a thinning of diffraction rings as they move toward the star point in the middle. The Pup also fails to hold collimation well - due to a lack of collimation adjustments. In the spherical aberration area, the Pup is in the 1/4 wave region. Definitely on the "best of breed" edge for a mass produced achromat...

Focused Star Disparate Double Extrafocal Startest Planet Saturn Planet Jupiter

Overall I 'd say that slightly less than 80 percent of the light from a star entering Argo ends up just where its supposed to. (i.e. in the stars airy disk.) While through the Pup maybe 85 percent does so - but only for the yellow region of the spectra. All that chromaticism means mybe 75% of the light ends up in the airy disk..

Even so I am quite satisfied with the views through both scopes. Argo is capable of cutting edge observation in every area an amateur may be interested in (except face on galaxies where a dearth of aperture handicaps the scope severely.) Meanwhile, the Pup is excellent for wide field sweeps, grab and go views of the Moon, Saturn's ring system, Jupiters belts, matched doubles down to Dawes limit and moderately disparate double stars.

But in neither Argo's case nor that of the Pup has this happy state of affairs come easy. Both scopes were in my possession for many "moonths" before finally collimated to an ultimately satisfying degree.

Steps taken to collimate the Pup are documented on the web at:

"Sky Training the Pup"

Argo's ordeal is also documented for public perusal:

"Argo's Perilous Journey"

As this thread further develops I hope to add some very specific suggestions to help others get through the fine tuning process. Hopefully others who have expertise in this area will also join in. It is my hope that everyone participating in astrotalk will eventually make the most of themselves and their scopes.

That is after all about the best reason I can think of for coming together in the AstroTalk online community.

Carpe Scopem,

jeff

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Let the Dialogue Begin!

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Jeff, I was thinking about your words about baffles....is it possible that the comparative lack of baffles in our Mak casses has a more detrimental impact on extra-focal images than on intra-focal images? due to more extraneous light seeping through due to the narrower cone of light?

~Otto

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Hi Otto,

I have noticed that the central obstruction in Argo is much better defined inside focus than out. I have also noticed that the innermost and outermost "rings" in the defocused image both sides of focus is "weird" - certainly not a true diffraction ring. (More of a kind of "tunneled light".)

Frankly I too am puzzled why the CO looks so ratty outside focus (as opposed to inside). I imagine that if the primary baffle tube in our scopes is improperly positioned that some degree of "vignetting" would be seen outside focus that would not be seen inside focus. BUT that would show up as a "larger hole in the center" proportional to the cross section and not necessarily as a loss of definition.

No, the loss of CO definition outside focus must have more to do with better organization of the photons inside focus than out. (For instance, in the Pup focus is sharper when a little magenta is seen - indicating a slight inside focus preference.)

Possibly this loss of CO definition is one sign of slight "undercorrection". Hopefully Cor will weigh in on this!

But as to baffling in MK-67's. They are very good. In fact almost 20% of the CO is due strictly to the large secondary baffle surrounding the mirror. So somebody has gone to a lot of trouble (and weve taken a hit) in terms of the secondary obstruction on this model.

Good question,

jeff

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Hi Jeff/Otto,

Did you also notice that the shadow of the secondary is actually smaller inside focus vs. outside focus ?

Cor

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You know, Cor, that never crossed my mind. Of course, the secondary obstruction would appear different extra vs. intrafocal.

Can your aberrator program...or can you...determine how much impact on the extra and intrafocal images there will be because of (1) the smaller diagonal shadow and (2) more stray light entering the extra focal image due to a thinner light cone passing by (any) baffles?

Otto

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Hi Otto,

The Aberrator cannot help us out at this stage but look at this image from Markus Ludes: http://aberrator.astronomy.net/scopetest/html/body_maksutov150_1.html

Here you can clearly see that the % of the obstruction increases from inside to outside focus. And that is for an optical normal/good Maksutov.

greetings

Cor

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Hi Cor,

It is helpful to know that the delta in CO is "healthy" but the mystery remains: Why does it exist? Is there vignetting going on with the baffle or is there some deep dark secret of optics that says the emergent light cone has different properties than the convergent light cone? And doesn't this just boil down to the fact that there is some kind of correction imbalance?

Just when you think you have a handle on things, you start to lose the grip...

jeff

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Hi Otto/Jeff,

Here's a clear example of a not equal showing image of an obstructed telescope. This was simulated for a 35% obstructed telescope at 5 waves of defocus. The left is inside focus and the right outside focus.

As you can see this looks rather different and what is the quality of this scope ....

1/6 wave PV and 1/20 wave RMS. That is a rather decent scope in optical terms. The concoction I used to brew this was -1/5 wave Lower Spherical aberration and +1/10 wave Higher spherical aberration. As you can see aberrations do interact and counteract. In this case (optics) the chain is not as weak as the weakest link, we can add a stronger link into the optical train to correct that weakness and order the photons to line up once more. Optics and light .... beautifull pair.

Cor

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Hi All,

Another tastey product from Cor's Advanced Muffin Research Laboratory!

Who'd have thought that something so divergent could result in "diffraction limited" optical performance?

Effectively Cor makes a very good case for putting more weight on a scopes in focus performance. However, I might observe that a scope showing such a delta may require an adept hand to achieve good focus.

jeff

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The extra-focal image you gave us presents the mushiness I speak of, and the intra-focal has something of the dark middle area I am use to seeing. What I am also noticing is that the intrafocal image when defocused even further presents a beautiful pair of concentric rings fairly far out. At the same time the extra focal image also has a pair of rings but much fainter and mushier.

Thank you again. Otto

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Otto,

The important thing indeed is that the characteristics are really different. The outer-rings are softer and the light is wider spread. Also notice that the inside focus has a few well defined rings around the central dot. This image shows what happens to the diffraction-pattern if you turn from inside focus (left) to outside focus (right). They are slices over the focussing-range and the diffraction patterns. Notice that the focus-point is well defined and that you can actually have good focus over a small range. In spherical aberrated instruments the focus-point is far less clear and it is difficult to find the best spot

Cor

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Thank you, Cor. That diagram shows exactly what I am experiencing. But, y'all, I still don't understand why the system should have dissimilar extra and intrafocal images. I keep hearing of refractors providing identical images.

Is it the nature of a well corrected mak-cass optical system to provide dissimilar afocal images, with a good central focus? If so, what is the optical physics that explains this? Otto

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I for one have a little difficulty with the concept of "spherical aberration", so let me try this concept out...

(Longitudinal) chromatic aberration occurs when light of different colors (wavelengths) comes to focus at different points on the focal plane. Typically red light is refracted less than green, green less than violet etc. So you get this idea of distinctly different "cones of light" converging at different points on the focal plane. Another way of saying this is that a chromatically afflicted scope has say an focal ratio of "F5.97" for violet light, F5.98 for blue, F5.99 for green, F6.00 for yellow. F6.01 for orange, and F6.02 for red.

Now let's see if we can't apply this same concept to "spherical aberration".

Let's say that the very center of lens (or mirror) has a focal length of F6.01. The one inch radius, F6.00, the two inch radius F5.99, and the three inch radius F5.98. So now, as you go through focus each convergent light cone is slightly offset with a tendency for most of the rays to converge inside F6.00.

Now here's my first question: Is such a scope over or undercorrected? (I assume "undercorrected" since the bulk of the area of the mirror/lens comes to focus at the shorter focal lengths.)

Here's my second question: So now at focus such a scope comes to best focus at about F5.99 but inside and outside focus there is a 'smearing" or "softening" of the diffraction pattern to one side or the other. Will the resulting "mushyness" be seen inside focus or outside?

Now as to my third question: Does the above make sense to explain spherical aberration?

If so then everything starts to make sense. Newts have a paraboloid shape in order to cause the rays near the center to more sharply focus (in concert with the outside more spherical parts of the mirror). SCTs add a correction lens at front because newts tend to have very poor "off-axis" focus (coma) because the paraboloid is not particularly good at correcting off axis beams hitting the paraboloid. MCT's enjoy both good off axis and on axis correction because the meniscus ifs better equipped to correct both on axis and off axis beams.

But this final element is missing, why (as Otto asks) is it necessary for an MCT to have a slight preference for inside focal ray convergence than outside? Is it possible that the meniscus should be shaped with a slightly aspherical component but this would add additional complexity, cost and quality control problems to the design?

Finally, I have an idea for a scope design that may resolve a lot of these issues. Why not create a large cassegrain (say an 18 inch scope) with a spherical primary and a slightly corrected 6 inch secondary that bounces the rays to the focuser through a hole in the main mirror? Basically why bother to perfectly shape a huge 18 inch mirror when you only have to shape a six inch???

Thanks Dudes,

jeff

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That was helpful Jeff. Which also means that it is helping me frame my question better.

Suppose we have an F6 system mirror system. Let's say 90% of the light focuses at F6, 5% at F5.9 and 5% at F6.1.

It strikes my attention that the light at 5.9 is just as afocal as the 6.1 but the light at 5.9 is afocal at a much steeper angle then the light afocal at 6.1. Might the steepness of that angle be related to why the inside is clean and neat and the outside is mushy?

And then there is the question of why refractors seem to have identical intra and extra but mak-casses do not.

Otto

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Yo Otto!

Intriguing thought. A little bit of undercorrection might very well be "amplified" in the outfocus image. Whereas a little overcorrection (if I get these terms straight) may have less impact on what's seen inside focus. Thus the tendency to "mushyness" is more an extrafocal property while let's say a tendency to thinner / poorer illuminated ring structure is a byproduct of overcorrection.

If this is the case, then it may be a general rule of optics that undercorrection is better than overcorrection (meaning that the majority of the convergent beams tend to coincide inside rather than outside focus) then it may possibly explain why Maks get such incredible in focus images...

But of course, we need for Cor to get back from "counting geese" before any of this speculation can be substantiated by someone who really knows whats up!

jeff

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Hi Jeff,

I still got blisters on my fingers using the tally-counter on the geese. Al together over 10 000 geese/swanbs divided over the whole range of species

A shorty : I indeed think that inside-focus is more important in terms of quality assesment than outside-focus. But as I have tried to show with the example, things are complex in many ways.

Hope you all find time to record some Leonids also on my behalf, wish them well

Nearly forgot, if you want to use my tally-counter ....

Cor

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Gents,

I sure hope you got to see those Leonids flying in formation on a "crisp fall night". Simply spectacular!

(Hope to write up something and publish within the next few hours.)

And oh yah, looks like Cor confirmed our guess that undercorrection is preferred to overcorrection (and by implication validated the model that "undercorrection" means the bulk of the photons come to focus "inside focus" rather than out and that undercorrection means better defined fresnel patterns intrafocally.)

BTW: Cor, I'm afraid I would have worn out your "geese counter" if I'd had borrowed it last night - there was one flurry where 5 metors streamed within a single second of observation!

jeff

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No jeff, you would have to count at least 40 000 meteors to "fill" my counter, it consists out of 4 separate counter that all have a 4 digit display. When used carefully you could even could until 10^16 .... that is a huge number I think.

Still great to hear the Leonids did a better show than last year and that some of my friend have seen them.

Cor

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Hi Cor,

I decided to leave the meteor counts up to T, who did an admirable job in a report at:

The Daily Observing.

Meanwhile, back on the startesting thread...

Spent a lot of time playing with aberrator. Could not find any intra/extra focal parameters that looked anything like the following:

Given your expertise in this area I thought you might like to take a stab at it.

Of course this is only a "word paint" approximation of what I've seen - so don't pay too much attention to the details. Suffice to say there are two large, ragged bright rings. One wrapped around the central obstruction and the other corresponding to the central primary baffle tube inside diameter. Between these two are 2, 3, or 4 nicely contrasting, relatively thin concentric evenly matched and balanced luminosity "real" difraction rings. The only difference between inside and out, is that the rings are not quite as nicely defined - the CO is poorly-defined at center, the two bright baffle rings are present, but there is more diffusion of the light around the fresnels and more light in the "de-inforcement" (dark) rings between the bright ones.

Note also the in focus star. Magnitude 2.8 (Eta Draconis) on a 9/10 night two perfect concentric rings (1st and 4th harmonics). Vega shows as many as 4 (when steady) and 5 when comatose (due to atmosphere - not collimation).

I believe aberrator assumes 0 magnitude star so you might want to redo your in focus star disk or say something like "only applies to stars that show a pure airy disk" since the greater the aperture the dimmer the star has to be to show an airy. (Most of the time any star brighter than magnitude 2.5 or so shows a flashing spurious image in Argo. And as we know the larger the aperture the dimmer a star must be to avoid "flashing".)

To be honest with you, one reason I never spent much time with the original aberrator (the one that only replicated startesting) was that it never could show me anything that resmbled Argos star test - and I KNOW Argo is in good shape optics-wise... Now that you can show the planets and stuff compensated for aperture, CO etc. it's a lot more fun!

Adios me compadres,

jeff

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Hi Jeff,

I remember long ago, that Cor provided me with a set of parameters that did give me the double concentric ring phenomenon intrafocus.

Yes, I too have noticed the first and fourth ring phenomenon.

Otto

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Hi Otto,

This "CO/baffle-ring" phenomenon is clearly depicted in the Markus Ludes startest photo on Cor's site. (Cor sited this URL earlier in this thread.)

http://aberrator.astronomy.net/scopetest/html/body_maksutov150_1.html

I guess my question comes down to this: If these are true "diffraction rings" (and not the product of the CO & primary tube baffle) then there is no way I would ever call our Maks 90-95% efficient (strehl ratio percentage). However if they are only an artifact of defocusing then go no further - these maks are "dream scopes".

Now the reason I assume these "bounding rings" are artifacts is because you see 'em, I see 'em, and Markus Ludes startest photo shows 'em and our two scopes give superb in focus performance...

So these bounding rings with their excess of photonage (way out of balance with the interstitial rings) effectively "disappear" at focus.

Still scratching my head though,

jeff

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There are three other things I notice on Markus' site of the two 150 mm mak-casses (1) the central obstruction in those photos is not a central obstruction, it is (what did he call it?) it is an artifact of the testing process (maybe the thing that holds the mirror). That is why the central dark area is the same size in all eight images. (2) The top four intra and extra photos are what I see with my scope, the intra pair being darker than the extra pair, which are mushier...but do you notice that Markus did not have that set collimated correctly..I can see it in the far right image.[I think we talked about this once upon a time.] (3) The bottom four photos are weird, the intrafocal are mushy and the extra focal are about as dark and clear as the intrafocal.

~Otto

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Hi Otto,

Sharp eye ! Indeed the central obstruction is a fake, the artificial star is generated by creating a beam out of a rather large telescope. However that telescope does have an obstruction by itself and therefore the obstruction you see is the obstruction on the other telescope. This makes the images slightly different from reality I think. You can actually see that the rings are slightly mis-aligned also in the diffraction pattern.

Cor

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Hi Jeff,

Quote:"Spent a lot of time playing with aberrator. Could not find any intra/extra focal parameters that looked anything like the following: Given your expertise in this area I thought you might like to take a stab at it" ~ jeff.

Everybody has read you are ready to take a stab

Well my dear friend, the startest is most sensitive .... when used nearer to focus.

To produce this image :

I had to defocus 20 waves and only then you see three rings between the obstruction-edge and the aperture-edge. The source on star-testing is Harold (D i c k, try writing Dick in this Yabb-systemm) Suiters book, he states "you should not defocus the image too far. If you do you will find no errors". So in defocussing that much most of the test-value is hardly present. For good evaluation Suiter keeps within the -10 to +10 defocussing waves.

Aberrator was made especially for this but you can (as you see above) trick aberrator to show larger defocussed images. Just type in the number in the defocussion waves section also make sure to enlarge the diameter of the telescope else the image will be too large. The aperture in Aberrator is only functional when we try to convolute planetary images.

And here's the one with that 4th ring more clearly. I only added some additional light to the image to show them more clearly.

greetings Cor

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Hi Cor,

After writing up my comments and drawing what I see through Argo, I revisited aberrator with an eye to assuming that the bright (CO/baffle) rings were the real rings and not those cute little "interstitials".

So, based on your input, I've got to go out now and redo my startest without cranking the focus out so much (very pretty isn't it when you do that!) and make another run at interpretation.

To be clear then, those honest to diety real rings are the bright ones right? Who'd a thought it!

By the way when I crank out just a little those rings are very pretty too! But I need to pay more attention to them to be sure.

Thanks Cor, for exposing my ignorance in this matter. Frankly I think focal traverse testing probably gives a more immediate sense of the quality of the optics than these ring thingy dingies. You either collapse into perfect focus with nice nice round diffract(s) or you don't...

Frankly, it's really a pity that this sort of test doesn't get more air play than the Suiter stuff. If you don't crank the focus out very far it is awefully difficult to see much in those other two rings other than there being concentric no matter what the power used. AND its those real outfocus interstitials that give you the best clues when collimating - not the bright ones!

Again thanks for your expertise,

jeff

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Quote: " Hi Cor, After writing up my comments and drawing what I see through Argo, I revisited aberrator with an eye to assuming that the bright (CO/baffle) rings were the real rings and not those cute little "interstitials". So, based on your input, I've got to go out now and redo my startest without cranking the focus out so much (very pretty isn't it when you do that!) and make another run at interpretation."~jeff

Yep, you got to my friend I am ever so sorry to annoy you this much. But you were asking me to stab you. Remember that I have been eating from the dough of this cookie since last year summer and I still am not able to bake that apple-pie.

Quote: "To be clear then, those honest to diety real rings are the bright ones right? Who'd a thought it! By the way when I crank out just a little those rings are very pretty too! But I need to pay more attention to them to be sure."~jeff

Just keep the focus-travel less than 2 mm on either side. That will be 5 waves of defocus the most.

Quote: "Thanks Cor, for exposing my ignorance in this matter. Frankly I think focal traverse testing probably gives a more immediate sense of the quality of the optics than these ring thingy dingies. You either collapse into perfect focus with nice nice round diffract(s) or you don't... Frankly, it's really a pity that this sort of test doesn't get more air play than the Suiter stuff. If you don't crank the focus out very far it is awefully difficult to see much in those other two rings other than there being concentric no matter what the power used. AND its those real outfocus interstitials that give you the best clues when collimating - not the bright ones!"~jeff

Suiter says: The first procedure to try is called the "snap" test. Using high magnification wiggle the focusser back/forth through best focus. Good scopes will snap into crisp focus quickly. And furthermore : If the optics are perfect the defocussed images are fairly uniform-illuminated disks. They appear the same at similar distances inside and outside of focus.

It is a well known fact that this is ever so true for many instruments as a good test. But for the Maks things are different, even Roland C. stated that it is not necessary to have identical images in a Mak. When producing Maks it seems he had to do additional tweaks to get identical images, he knew many people where going to feast on this if they were not.

Diffraction really waves the rules as you see.

Cor

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Hi Cor, We are fighting an uphill battle my friend. So many words have been tossed around about "startesting" (perhaps even a few in the way Suiter described) and so few about focal traverse (not the same as "snap test" in my estimation since the goal is to watch things from one side of focus to the other and not just "how easy it is TO focusl" ). NOTE: Suiter may be seeing the same thing as focal traverse in his book but "snap to focus" means something else to most amateurs.) Meanwhile, now I am "hot to trot" for a stable night and the opportunity to pay more attention to those "real rings" and try out aberrator against them. Learning more an more all the time and in the best way possible (through experience, conjectiure and dialogue), jeff BTW: How come Markus defocuses 16 waves? If this had not been the case, then it is less likely that I would have put so much emphasis on those "interstitials". (Surely Markus knows what D I C K does. Or does Markus have a contrary opinion on the matter?

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Hi Jeff,

Markus shot those image indeed a bit too far out. Only later we have started to add closer to focus images, that in themselves needed far more magnification.

Here's a good quote from the internet, this is the stuff I was talking about with assymetrical images from Maks by Roland C. The original source is at:

Original article

21st jan 2000 article quote from Roland Christian:

I should start out saying that I have the highest regard for Suiter and his book, there are however enough holes in the book vis a vis refractors and catadioptrics that it cannot be used for all occasions. If you want to see some wildly or perhaps mildly asymetrical printouts, contact TEC Telescopes. They can send you some Zemax results of Mak-Cass designs that may want you to start nibbling.

Now on to my soapbox:

The situation with the star test is more complicated than manufacturers being afraid to be found out now that amateurs have an easy test to evaluate the optics. When the optical system gets more complex than a simple parabolic mirror, then there are inherent aberrations that affect the star test. An example is the Maksutov Cassegrain. This system can be manufactured many different ways, but one popular way is to have all surfaces spherical, which cuts down the need for hand work. Machines exist now that can lay down a 1/20 wave or better spherical surface on a piece of glass without any human intervention. In this pure form, the Mak-Cass has left over 5th order aberrations and, depending on design, these can be less than 1/10 wave on the wavefront. By the way, fast Apo refractors have these same aberrations also.

The RMS value will be better than 1/50 RMS and the Strehl ratio will be exceedingly high. In other words, the optic will deliver a very high contrast image, consistent with the high wavefront rating. When tested on the night sky, the inside and outside diffraction patterns will be quite different. Any beginner will see it so and may conclude that the optic is of poor quality.

As an example, I have recently finished exhaustive tests of different 10" F14 Mak-Cass systems, some with these inherent aberrations left in, some with them meticulously removed. All the optics tested between 1/10 and 1/12 wave with Strehl ratios of 98% or better. The system with the uncorrected or pure Maksutov curves had the central hole of the donut break out 4 times farther on one side of focus than the other. It would be judged by the Suiter star test method as being maybe barely 1/4 wave, if that. The hand aspherized version, which was no better on the interferometer and had similar Strehl ratio, showed a donut breakout approximately at the same point on either side of focus - in other words textbook perfect. At focus, both scopes showed a tight central Airy disc with a faint first diffraction ring. At focus, it was impossible to tell them apart. The ratio of brightness of the Airy disc and first diffraction ring was essentially identical. Both scopes showed the same high contrast on Jupiter and Saturn. Both scopes could split doubles with equal precision.

The dilemma for manufacturers then is, should we do our best to produce smooth high contrast optics, or should we please the star test crowd and do some hand aspherizing to get a more pleasing out-of-focus star image? I can tell you that it is easy to do some rough compensation with quick local polishing at several zones to get more equal inside and outside star patterns, but the result will almost certainly be a loss of contrast. Add to that a nice big central obstruction to get rid of the offending inner zones, and presto! you have a nice "fast food" Mak-Cass that doesn't work any better than a typical SCT.

In our case, we will do our utmost to produce the closest faximile to the star patterns in Suiter's book, but they will never be exactly equal. The overriding concern will be that the optic has a very smooth and accurate wavefront to produce the highest contrast possible in the final image, which I assume will be in focus.

Roland Christen

ASTRO-PHYSICS"

~Cor

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Hi Cor,

Fascinating input. "The star test is dead, long live the star test!"

Quote: "Markus shot those image indeed a bit too far out. Only later we have started to add closer to focus images, that in themselves needed far more magnification." ~Cor

Many folks will turn to standard reference works (like Suiters) and will learn as the Moon reflects the light of the Sun. Others (very few) will examine a phenomena, make conjectures and test their hypothesis against the light of real world experience. In looking at Markus' MCT image I made the assumption that those interstitial rings mattered and not the real bright ones. However, I was aware of an interesting inconsistency - I would often judge an optic to be better than Markus. And this led me to begin exploring this whole star test thing via this thread.

So my next step is to revisit all those star test images on your site and see if that inconsistency is illuminated. HOWEVER, one very important clue I could have gotten from Markus' images would have been that a note was added saying something like "Typically star testing is done at less than or equal 10 waves of defocus. For (whatever) reason Markus has chosen to do so at a much higher extra-focal ratio."

As to Rolands discussion - I feel his pain. Suiter's work does not appear applicable to anything other than paraboloid newtonians. And Astro-Physics (along with Cervalo etc.) are put in the position of looking like they are "rationalizing" poor star test results in refractors and MCTs. Too bad...

Happily, all this gives an EXCELLENT opportunity here at AstroTalk to have some INCREDIBLE FUN figuring out what makes for a TRUELY USEFUL star test methodology.

Kewl huh?

Sounds like dblGUTTeam time all over again! Lets get on it dudes!

Who wants to become part of the StarTestGUTTeam???

jeff

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I understand that the far defocused images (i.e. where we are seeing a double concentric ring like those on Markus' image of the 150mm mak-casses) are not of use in startesting a scope. However, would I be correct in assuming that one can use these highly defocused images for purposes of collimation?

Secondly, perhaps we are being polite, but let me be blunt...are we saying that the type of highly defocused images like we saw for the 150mm mak-casses, in fact cannot be used to determine a scope's wavefront error (e.g. 1/4 wave or 1/8wave)?

~Otto

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Hi Otto,

I agree 100%, the interstitial rings are essential for good collimation. If those little puppies touch one another anywhere its back to the nuts and bolts.

And yes personally my faith in startesting has been shaken. Right now I am looking at :

http://aberrator.astronomy.net/scopetest/html/refractor150_1.html

This refractor has the best inside outside ring structure of any other refractor seen (except for some coma and an artificial CO) and it is rated as 1/4 wave while stuff that shows obvious imbalances in thickness, luminosity, and presence is rated 1/10 to 1/12. Meanwhile that same 150mm gives oustanding views of the planets and is a sharpsplitter!

See for an example of a highly touted startest:

http://aberrator.astronomy.net/scopetest/html/refractor63_1.html

Man o' man is this a morass or what?

jeff

PS: Cor, how ever did you manage to wade through all this!

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Hey Guys,

Check this one out! Looks like an "overcorrected" Mak rated at 1/7th wave and has a far poorer inside-outside balance than factory 1/6th wave rated Argo:

http://aberrator.astronomy.net/scopetest/html/maksutov180_1.html

(I really must be missing something here.)

This is too much fun!

jeff

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Yo,

Finally a point of agreement! This scope shows exactly what I'd think was a "slightly better than diffraction limited" optic.

http://aberrator.astronomy.net/scopetest/html/maksutov_180_2.html

jeff

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Hi Otto/Jeff,

As to the too far out of focus images and collimation, well it is my belief that you could not care a pinch of salt about those. Get them photons to line up in the area closest to focus and forget about the rest. That is also why the high magnification can be driven to absurd levels and why you can use bright stars for this too. So just make the rings concentric very near to focus inside and outside and you are there. Collimation whilst seeing the shadow of the secondary is only useful as a rough guide.

And indeed this cookie is a tough cookie, its more like strolling around in the photon-jungle on a rainy afternoon.

And Jeff, how did I get through this. Well I did eat quiet a fair share of letters to get started. And just building Aberrator from literally scratch with hardly any sources to use was an experience on its own. Later I got rewarding advice from several great team-mates. Some just planted seeds in my brain, others managed to align my neurons so I actually understood more of this whole photon-jungle at every step. But I still need my references at hand to pinpoint accurately how things are/could be. But that is what learning is often about nowadays, knowing where to find knowledge has become nearly as important as having all the knowledge in ones brain.

If you like this you are going to have a nice venture into photon-space. Lots of good reading is around on the net, there are many sources I can recommend for this. Search for Wyant and Optics in any good search-machine and you will get proper info on optometrics and such.

Cor

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OK Dudes,

So after going through all the startest images, lets chime in. What do you think, diffraction limited (80% strehl / 1/4 wave) or not?

http://aberrator.astronomy.net/scopetest/html/cassegrain200_3.html

jeff

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Thank You Cor,

Research is important and it is just possible I may do some scrambling on the theory stuff. But, as you know, what I have the most fun at is "modeling reality in my head - ex nihilo". Learning from others is a valuable skill and most folks can do this well enough at this stage of human evolution. Doing independent study and reading from authoritative sources is probably a higher skill since you must take the initiative and try to pull the pieces together based on what hints you can glean. But, "learning from within" is the ultimate skill and that is my path in life.

Of course, as you know self-learning often leads to mistakes BUT if you remain humble and continue to scratch those tiny anomoly-itches you soon forsake the Earth-centric solar system and put the Sun (more or less) "squarely" in the middle!

Let's keep the dialogue going! Right now, my working theory is that a Mak should show 2 evenly luminous concentric, well defined rings of light on both sides of focus with allowance for a "softening" extrafocally. Ideally the focus should be set to show only these two rings with a gap between them. That "interference gap" should be as dark, circular and well-defined as possible. Because we are not supposed to "rack in or out" the focuser we have to star test using the highest possible magnification.

However, when collimating Cor, there is nothing you can say to convince me that those interstitial rings from too much focuser travel aren't the "cats meow" for getting "purr"fect collimation.

This has all been very en-photon-ing...

jeff

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Gents,

Spent a LOT of time with aberrator this evening. Made a whole bunch of notes about how HSA and LSA interact as scalars and by sign. This what I came up with:

Perfect Optics, De-focus 3 wavelengths:

LSA +.1 wavelengths HSA Zero:

LSA -.1 wavelengths HSA zero:

HSA +.1 wavelengths LSA Zero:

HSA -.1 wavelengths LSA Zero:

LSA .1 wavelengths HSA

LSA -.1 wavelengths HSA -.1:

CO 35%, Perfect Optics, De-focus 3 wavelengths:

LSA +.1 wavelengths HSA Zero:

LSA -.1 wavelengths HSA zero:

HSA +.1 wavelengths LSA Zero:

HSA -.1 wavelengths LSA Zero:

LSA .1 wavelengths HSA .1:

LSA -.1 wavelengths HSA -.1:

Then I had a thinning of the clouds. Vega could be seen unaided. At 210x I got a startest that best resembled this from aberrator:

This is at 3 wavelengths defocus.

At 10 wavelengths you get this:

I'm too embarrased to tell you what values I put in to get the above aberrator depictions. I found that if I pushed the numbers beyond what I used for the above the outside inner ring would not have been as distinct as it was.

Keep in mind that all i need to do to minimize my afocal wavelengths was to not let that little starpoint dissappear from the center.

jeff

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Jeff,

You are using the "old" aberrator I see. Please use the new one it is better at certain features. Also use a larger aperture to display lots of defocus. There is no need to keep the aperture at 150mm since we are looking at the physical properties of startests. So push the diameter to at least 400mm or so and generate those out of focus images.

On the collimation side, it takes time to get grip I understand. Here's some additional proof why collimation is not sensitive far away from focus.

As you can see in the 20 waves defocus you can see hints of miscollimation, at 2 waves things are very clearly misaligned. So the sensitivity increases when you come closer to focus. That is why most of the collimation sources tell you to start further out of focus, in the SCT/MCT the "doughnut" is the starting point. But the finishing touch that remains important should be done nearer to focus.

~Cor

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Quote: "OK Dudes, So after going through all the startest images, lets chime in. What do you think, diffraction limited (80% strehl / 1/4 wave) or not? http://aberrator.astronomy.net/scopetest/html/cassegrain200_3.html" ~jeff

Hi Jeff,

This is a problem zone, as we increase the aperture the physical diameter of the diffraction image decreases and therefore Markus had to go further out of focus to get something onto the CCD. As you can see the definition of the rings becomes problematic due to this. The only way to get by this is decreasing the pixel-size in the CCD and increasing the magnification used. The latter and the former are limited in their freedom.

Cor

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Hi All,

This AstroTalk thread has been extremely valuable in exposing some basic ideas about evaluating the current status of telescopy and clarifying scope testing techniques and interpretation. But in order to keep things well organized I've changed the topic name and limited it's "scope". New threads on startesting and corrective action are in the offing. In addition I hope to start one that exposes some of the capabilities inherent in Cor's Aberrator program.

So the topic "Startesting and Tuning Your Scope" is now locked and is renamed "Startesting And Scope Tuning Backgrounder"

Onward and Upward!

jeff

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