The Quest for Optical Excellence

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...

For the "Optically Correct"

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

to: top of page

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, sometimes 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!

to: top of page

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 (sum 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.

to: top of page

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.

to: top of page

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 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 manages 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 (alomg with scope type).

to: top of page

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...

to: top of page

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!

to: top of page

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.

to: top of page

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. But when showing off the quality of your scopeto 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!

to: top of page

In Conclusion...

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 to 1/5th wave region. Definitely on the "best of breed" edge for aa mass produced achromat...

Focused Star Disparate Double Extrafocal Startest Planet Saturn Planet Jupiter

Overall I 'd say that 78 percent of the light from a star entering Argo ends up just where its supposed to. While through the Pup maybe 80 - 85 percent does so - but for only 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

to: top of page

to: All Things Telescope Index Page

to: Astro.Geekjoy Home Page

Email: Astro.Geekjoy