Introduction
Argo's Story
Argo's Issues
Corrective Action
Collimation Blues
Collimation Mechanism Re-design
Next Pass at Collimation
Should I? or Shouldn't I?
In the (Star) Field
Field Flatness Issue Update
On (Orion Ultrascopic) Eyepieces
Telescopic Limiting Threshold Magnitude
Low Power Field Astigmatism
A Statement of Comparative Worth
Intes Replacement Kit and Primary Removal

Like most scopists, I have a "personal" relationship with my scope. Our relationship flows out of the many months we've spent together on mutual quest for meaning and fulfillment. For you see, we are both very small parts of a "much greater universe". And it is through participation in the re-discovery of that universe that Argo and I have been thrown together.

Much of what follows is the story of my technical relationship with Argo. Although he started out as a fine example of his kind, somewhere along the line Argo ended up as a "seconded" scope at the distributor (Orion Binocular and Telescope). So, of course, with my personal penchant for pushing the envelope, I wasn't about to leave Argo "as is" and go about observing the heavens without also getting deeply involved in wringing the highest levels of performance out of him. So before too much in the way of "negative momentum" is established, I'd like to recount some of the pluses associated with Argo's performance as a budding "research grade" scope on his way to relative "perfection".

• Outstanding Views of Saturn's Ring System at Outrageous Magnifications I have personally had clear, sharp views of Saturn at 600X (100X per inch aperture). This is an unheard of figure and is only possible with the finest optics - irrespective of telescope archetecture. Under such magnifications, Saturn's bright Ring B bleeds through into the darker inner Crepe Ring, the Encke Minima becomes apparent, and irregularities in the edge of Saturn's South Equatorial Belt are possible. All this, of course, is only feasible under 8+/10 stability skies.
• Globular Clusters: On a clear dark night, globular clusters as dim as magnitude 8.4 (M56) have revealed something of their stellar components. This through rapid eye movement (REM) scintillation, direct surface grainyness, or averted vision. On nights of exceptionally calm skies, the showpiece globular M13 has revealed hundreds of stars - many discernable across the core. This level of performance is comparable to an average 200mm SCT, or 8" Dob-Newt. Effectively you get 8 inch performance in a compact 6 inch package.
• Nebulae and Dim Star Clusters: In general the optics on the scope are of such a quality as to show a wealth of background sky texture wherever pointed. This means that, on a clear dark night, the scope is sensitive to very small changes in surface brightness. The wealth of detail implied here is staggering. It is, in fact well beyond the ability of the mind to interpret or the tongue to verbalize. From my own experience - on a very good 6.0ZULTM night - I have clearly and amazingly made out all three regions of the extraordinary Flame Nebula (NGC2024) even while the bright star Alnitak remained in the same field of view.
• Galaxies: There are reports on the Internet, of a few, very experienced observers using this model to detect 1 arc-minute apparent-sized galaxies of photographic magnitude 14.0. This should not be surprising given the scopes ability to reveal "the texture" of space wherever it is turned on a clear, dark, night. Through 5.5 ULM skies, and using all the tricks of the visual trade I barely detected 1 arc-minute sized 13.7 magnitude NGC3073 in Ursa Major. Generally it isa commonplace to detect galaxies with average surface brightnesses to magnitude 14.5. (Of course only the bright condensed cores of such galaxies are visible.) 2 arc-minute apparent-sized galaxies of magnitudes 12.0 to 12.5 visual can be confidently seen - even under sub-5.5 ULM skies. Among the "showcase galaxies", the Whirpool - M51 has revealed its spiral arms - but only under 6.0+ ULM conditions.
• Matched Double Stars: Perhaps the scope's finest skill is separating extremely close double stars of similar magnitude. Recently (June '02) had the pleasure of seeing a clear gap between the .76 arc-second Zeta Bootes pair. Of course, such an extreme split is only possible on nights of exceptional stability (and at very high magnifications - 500X plus). Since the pair lies below "Dawes Limit" for the aperture (~.81 arc-seconds) the result shows that the scope's Russian-made optics are capable of "sub-Dawesian" performance.
• Disparate Double Stars: In the area of doubles of disimilar magnitudes, (those with a secondary at least 4 times - 1.5 magnitudes - dimmer than the primary), the Nu UMA pair (magnitudes 3.7&10.1, separation 7.3") lies just this side of "no pairs land" in the 5 to 10 arcsec separation range. Meanwhile between 2.5 to 5 arcsecs, Chi Leonis (mags 4.7/11.0, sep 3.6") takes a similar post. Among disparates that include sub-3.0 magnitude primaries, the summer pair Eta Draconis (2.9&8.2, 4.8") proved difficult but likely surpassable. While the toughest pair at or near the first diffraction ring (Rayleigh's limit), Iota Leonis (4.1/7.3, 1.3") is also likely to be superceded as observation continues. Finally, right at Dawes limit (defined as 120/Dmm), several mildly disparate pairs (delta=1.5 magnitudes) have proven resolvable in the past. These have always required magnification in the range 360 - 540x and exceptional stability. Experience in this observational realm suggests that decisive resolution of disparate doubles is perhaps the "acid test" of any scope's performance. And in this area - due primarily to the large size of the scopes central obstruction - performance is equivalent to that of the finest 4 inch apochromatic refractors.

SEPTEMBER, 19, 2001 NOTE: As you read this document keep in mind that over the last six months I have carefully, and methodically groomed Argo's optical train to achieve the highest levels of collimation possible. This process may never be complete - but is very close.. For example, extensive testing of the difficult Delta Cygni disparate double (magnitudes 2.8/6.4 at 2.4 arc-seconds) revealed that Argo's collimation was a tad off. Less than perfect collimation resulted in subtle forms of "primary flair" and spotty resolution of Delta's dimmish companion. Using this pair as a test case, I implemented a series of "tweaks" to Argo's primary and secondary. This brought the scope to the point where .55 arc-second 3.6 / 4.6 magnitude Beta Delphinus could be resolved as a "teardrop shaped" pair of conjunct airy disks". And, of course, resolution of Delta's companion is now a commonplace - even under 6/10 seeing stability. Other improvements have been seen as well. Can now hold five "brightenings" directly on the floor of Plato on or about the time of the full moon under 7/10 stability conditions. (Several others are possible averted - one has been seen as a "craterlet".) Can now detect the Encke Minima within Saturn's Ring A at 180x under moments of 8/10 stability. (Previously required twice that magnification.) And recently (January 18, 2002), detected numerous brightenings and maria on the dark side of a 5 day Moon. The procedure used to accomplish this "final quanta of collimation" is embodied in an article on MK-67 collimation written by Otto Piechowski here on Astro.Geekjoy.

But let's get on with the historical perspective, shall we...

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There is nothing special about Argo (except perhaps the fact that it is "my" scope and therefore deserves a little extra respect - from me). Argo is actually my second MK-67 OTA. The first was purchased at an Orion Telescope and Binocular Parking Lot Sale in September 2000, That MK-67 was hopelessly comatose on arrival. There was no way to collimate the thing. In fact the telescope tube was not even square with itself (There was a tube length differential of about 1 mm. This caused the meniscus to skew laterally.) It's remarkable the thing ever shipped. (There must have been a "mix-up" in the warehouse. The junk scope got shipped to Orion instead of Siberia where it belonged.)

While at the parking lot sale, I had an inner debate. The dialog centered on picking up a Chinese Synta SkyWatcher 120mm achromatic refractor or the Russian Intes 150mm Argonaut. (Both of these formerly rabid communist countries have moved toward a free-market based economy in the last decade or so.) The refractor appealed to me on the basis of it's unobstructed aperture. A clear 4.7 inch without the "plug" in the middle. Ultimately, I figured that the Russian economy needed a little more help so I "voted" for the Intes.

The final determinator came down to two fundamentals: Light grasp and color correction. The 150 MCT would obviously be superior in light grasp. And since it also possessed fewer refractive elements (a Maksutov-Cassegrain does have a correction lens), it would also prove superior in chromatic abberation. (Recently Synta released a 150mm version of the Skywatcher. I have reason to believe that, even a chromatically incorrect 6 inch achromatic refractor, would prove superior to the 150mm MCT in revealing low contrast planetary detail -- such as belt festoons on Jupiter). Had the 150 Synta been available, I still may have selected the Argonaut because of its great portability. (If you've read my observing reports you will note that I travel -- locally -- to improve my deepsky reach.)

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Argo's Issues

As I write this, it is early March 2001. There's rain and the weather is not expected to clear until Thursday. (Today is Sunday.) Argo lies disassembled on the kitchen table. I am cleaning the optics. While waiting for condensation to dry on optical surfaces I reflect on why I am at this point in my relationship with Argo. You see I am not satisfied with Argo's behaviour. There are problems, some I may be able to address, others not.

Problem One: Too much light scatter in the field of view. On even the darkest/driest of nights, it is very difficult to distinguish "star haze" from nebulosity. I've missed far too many otherwise susceptible deep sky objects because of this. Subtle nebulosity in the presence of even ninth magnitude stars is indistinguishable from the stars own "corona" of light scatter. This is not good. Things may improve with drier weather. But my experience with "the Pup" (an F5 80mm Synta achromatic refractor) suggests the problem may be with Argo.
Problem Two: Low contrast on Jupiter's main belts. Part of this relates to problem one above, but another may be due to the "coverage gap" that exists between the baffles that extend from the secondary to those of the primary. The first hint of this problem surfaced when I noticed that, while waiting for Jupiter to enter the field of view (with the clock drive clutched), a huge splinter of light would precede it. Later, while aligning the scope, I noticed a "ring of light" within the tube while the eye is positioned where the eyepiece ought to be. That gap exists because the adjustable baffle extending from the primary is misadjusted. (It should extend just far enough to close the coverage gap from the perspective of the at focus eyepiece, but not so far as to cause "vigneting" - edge truncation of the light cone which robs the scope of aperture.)
Problem Three: Poor field flatness. Beginning about 15 arc-minutes from the center of the field of view stars are no longer pin-points. At about 25 arc-minutes they display coma (and can not be brought to proper focus whatsoever). This degrades the observing experience to an unfortunate (but not intolerable) degree. A scope possessing this scopes on axis optics should show stars as pinpoints across (at least) a one degree (60 arc-minute) field of view. (In fact the Pup shows pinpoints across a two degree field!) I now suspect the problem lies in the relationship between the primary and secondary mirrors. Since the secondary is convex, the image from the primary must engage the correct part of the convex secondary to maintain a "flat" virtual image at the eyepiece. If the secondary is too close to, or far away from, the primary, focus may still be achieved at eyepiece central but there is a rapid drop off as stars move toward the edge.

Problem Four: Visible mismatch of extra/intra-focal star images. A telescope with truly excellent optics, tested on a night of good to excellent seeing, should show at least four concentrically (not necessarily equally) spaced, virtually identical, inside and outside de-focus rings around a first (or second) magnitude star. Argo falls down somewhat here. It's extra-focal image is slightly "fuzzy/mushy" compared to its incredibly sharp (almost transcendently so) inside focus image.

Problem Five: Focal length is too long (1800mm). Even the longest focal length eyepieces (50mm) can't get down to the idealized 30X needed for extended star-field and nebulosity "sweeping". (Besides I really don't want to spend $500 on a 50mm ep plus the 2 inch star diagonal needed to support it.)

So, at a parking lot sale, I took receipt of a modest but adequately apertured, easily transported, chromatically correct telescope platformed on a CG-4 equivalent equatorial mount for well less than $1000. After performing the not unexpected re-collimation of the optics and enjoying its use for a six-month period, I started to notice a few "flaws". Is this just delayed "buyers remorse"? Am I taking other's glowing reports of the optical excellence of this particular style scope too literally? Or am I just indulging a craving for a new scope? Should I have purchased the 120mm achromat?

Well no, of course not. I could easily live with this scope. There's not a single issue cited above that is truly intolerable. But in their sum, well, as you may have noticed I am not completely satisfied.

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Corrective Action

So the bowels of Argo now lie exposed on the kitchen "operating" table. I hope to address one-half the light scatter problem by thoroughly cleaning all optical surfaces. The big issue here is to avoid scratching anything. Scratches could contribute more to light scatter than cleaning will offset.

In cleaning the optical surfaces the first thing I have to do is practice patience. Before removing the meniscus, I make sure to mark both the tube and the meniscus ring to properly restore the secondary alignment on re-assembly. Once the meniscus is removed I have to wait for any condensation to evaporate. (Cleaning the lens wet leaves streaks on the glass. Let the water evaporate off the surface first then apply three drops of optical cleaning fluid on the dry surface. Use new cotton balls to gently wipe the surface. Repeat until the surface is "squeeky" clean. This makes sure oils are removed -- and oils absorb / refract light.) The cotton balls leave fibers behind. I gently brush them off the surface with a fine optician's brush. The meniscus is clean. I move onto the secondary mirror.

The secondary can be physically separated from it's mount. I choose not to do this since it is now properly collimated. It is cleaned using precisely the same method as the meniscus. But instead of cleaning two surfaces I only have to clean one. The problem with not disassembling the secondary is that the baffle gets in the way of cleaning. I have to avoid causing particulate on the inside surface of the baffle from dropping onto the secondary.

As expected, the secondary mirror is convex. This is important to verify. Why? Because I have a pet theory that primary-secondary-meniscus separation may be a critical factor affecting field flatness. The secondary itself is roughly 42mm in diameter. This is 10 millimeters less than that of the secondary obstruction itself. If the manufacturer had chosen to aluminize the meniscus instead of installing a separate adjustable convex surface, the obstruction ratio of this scope would have been a respectable 28% (rather than 34%). Some believe that image contrast is significantly improved if the obstruction ratio is smaller. However, I believe the manufacturer has done extensive testing to demonstrate that it is more important to have excellent anti-stray light baffling than smaller obstruction ratios. The additional 10mm of obstruction size on this scope exists because of this testing. This reinforces my belief about the need to bridge the coverage gap in the baffling system.

The meniscus and secondary are now dry. But, there are spots all over the surface. The cleaning fluid is not optically correct. I want to see spots on Jupiter -- not my optical surfaces. There is no choice, I reapply a little cleaning fluid and rub repeatedly in a circular motion to remove the fluid. Evaporation alone doesn't cut it. Fortunately, I am able to do this without scratching the surfaces.

I point the spots out to my wife. Unlike myself, Sharon is willing to read the label on the bottle. (I wouldn't go near the thing - it might actually have instructions printed on it telling me how to clean the optics.) The bottle says that it leaves spots and that you must gently rub these away with optical tissue.

The meniscus is now reasonably clean. All oils are removed. Between the use of the optical brush and hurricane bulb I am able to remove the leftover cotton filaments on the surface. So far so good.

I inspect the back of the scope. Cleaning the primary will be much easier if it can be removed from the tube. Despite removing all visible hardware fasteners, the primary mount and cell refuses to budge and I don't want to force it. The mirror will have to be cleaned in situ. A very difficult and possibly ineffectual task. I try anyway. The mirror is very difficult to get at. After about a half hour of skinnying up my hands and grasping the cotton balls with two and three middle finger combinations I succeed. A few "dull" areas are still perceptible. (Mostly near the inside and outside perimeter of the mirror.) In cleaning optics, along with cultivating patience you need to know when to quit.

Before cleaning the mirror I try to rotate the threaded baffle assembly. No way. This thing is "fused" in place. The manufacturer has made another mistake here. It should be possible to both remove the primary mirror assembly and tune the primary baffle length without excessive force. It is now clear that I will not be able to extend the baffle to bridge the gap that causes the "splinter of light" from entering the eyepiece.

I reassemble the optical tube assembly. But before doing so I make a few quick measurements of its physical characteristics. The values I record are not precise, but they are helpful in better understanding how the scope is archetected. Depending on where you position your eye, the meniscus is anywhere from 150 to 152 millimeters in diameter. The inside diameter of the tube is 180mm. There is a 5mm gap between the inside of the tube and the edge of the primary mirror. The primary is 170mm in diameter. The scope's designers did their homework. They understood that the 150mm concave meniscus "splays" the light cone outward. To fully exploit the 150mm of aperture, the primary would have to be significantly larger. Based on my measurement, the ratio is roughly 1.13:1. The point of focus of the spherical mirror is about 34 inches above the level of the back of the tube (the table on which it sits). I estimate that the outside perimeter of the mirror is roughly 2.25 inches above the table top. This means the radius of curvature for the mirror is 790 millimeters. As such, it has a focal ratio of approximately 4.7:1. The concave secondary mirror corrects this to the published 12:1.

The light cone from the spherical mirror is intended to contact the secondary, and be reflected back down the tube into the focuser-mount assembly where it is redirected by the mirrored star diagonal to form part of a virtual image beneath at the eyepiece. The size of this virtual image is based on the focal length. The "shape" of this image in space is primarily determined by the quality and physical relationships between the three optical surfaces. The meniscus shapes the light first. Any parallel beam of light that passes mear it's edge should, ideally, just touch the corresponding outside edge of the primary. It is then reflected back to the outside edge of the secondary and from there come to a point in the center of the field of view (along with all other parallel beams from any on-axis point source like a star.) If the meniscus is repostioned (in relationship to the secondary) the beam will make contact with a different part of the primary. It will also make contact with a different part of the secondary. No big deal. All these parallel beams will come to a point somewhere. That point can simply be found by repositioning the eyepiece to achieve focus.

The big problem is with off-axis parallel beams of light. Such beams pass through an extremely complex path based on a what can be a less than optimally positioned set of optical components. The mean free path of the many parallel beams (from an off-axis point source) are skewed in such a way that natural offsets available to on-axis beams progressively break down. The more off-axis the point source, the more severe the effect. The eyepiece sees a curved virtual image rather than a flat one. The observer experiences this as poorly focused off-axis star images. Any attempt to refocus causes the on axis image to lose focus. At the extremes, star images appear as comas rather than points.

This is my problem. The relationships between all the optical surfaces are messed up. Several days ago I made an effort to correct this problem. I had noticed that most MK-67 owners complained of not having enough "backfocus". Many were adding extension tubes to their scopes to compensate. My problem is one of not having enough front focus. (I could not thread an OIII filter onto my 35mm eyepiece and get it to come to focus with the focuser.) Based on this hint, I figured that I needed to shift the primary mirror forward to cause the focal point to shift backward.

The operation was dutifully performed. My method was to adjust the primary forward while maintaining a distant target centered in the field of view. Sounds like a nice way to adjust without causing problems. Right?

Wrong! Moving the primary forward in this way caused an unoticed shift in the image plane of the convergent light cone. Sure it stayed centered in the eyepiece, but meanwhile, later in the evening while viewing Jupiter I noticed an immoderate shift in its eyepiece position as I passed through focus. Instead of observing, I found myself re-collimating the secondary. Then "pop". One sprung adjustment screw separated and the secondary jumped completely off-axis. I took the scope home and dissassembled the secondary. In so doing I found that many of the threads inside the adjustment backing plate were stripped. I removed the overstrong springs, butted the focus pivot setscrew against the back of the adjustment plate and reassembled the scope.

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Collimation Blues

On Saturday I used my collimation target (an antenna tower several miles away on a distant hillside) to redo everything. It was in this effort that I discovered that the image plane was whacked out. For you see, I had to make sure that all my eyepieces would come to focus - with and without the barlow lens installed. While so doing I noticed about a 15 arc-minute delta in the position of the target with and without the barlow lens. Ultimately, I was able to close this delta to about 3 arc-minutes using an approach based on adjusting the primary alignment exclusively while centering the target in the 25mm eyepiece with barlow installed and adjusting the secondary exclusively without.

Now, keep in mind that I had to remove those over-strong springs in the secondary assembly to be able to re-collimate at all. In so doing I also had to shift the secondary back further than it would otherwise be. In addition, I've shifted the primary forward, closing the gap somewhat to the meniscus. Now the geometries of the image handling surfaces are different from what they were several days ago. The meniscus, now closer to the primary, means that the refractive splay will not go as far out onto the edge of the primary. The primary, now closer to the secondary, will cause those same reflected beams to touch further out on the edge of the convex secondary. The center of the virtual image is now shifted back down the focuser tube.

Now I have two final issues: What will happen to the field flatness? And will the non-sprung version of the secondary adjustment mechanism hold collimation?

I reassemble the scope and place the OTA back on the mount. Align the finderscope on the most distant object possible (clouds hide the targeting tower). Check the view with and without the Barlow through the 25mm eyepiece. The finderscope and main tube hold alignment quite well. The 25mm pretty much shows the same treetop near the center of the field of view with and without the barlow. I am pleased. Despite the manhandling needed to disassemble the tube and clean the optics things held collimation well. The thread stripping springs are gone and the focal point is shifted away from the OTA.

By experience I know that a field flatness check can not be made without a star field. By experience, I also know that it will be necessary to finalize collimation using a bright star referencing the shape of intra and extra-focal images. But everything is now in place to accomplish these two checks.

Once the weather clears...

It's now Monday evening, March 5th. I had this hunch that by moving the primary forward, I not only shifted the focal point backward, but also moved the primary baffling forward. (Something I should have thought of yesterday.) In checking for this, after returning home from work, I found that it was no longer possible to see the "ring of light" from the eyepiece position. This is good news and re-inforces the idea that, somewhere along the line, I may have mispositioned the primary mirror during collimation. (Back when I knew even less than I know now!)

While making the baffle check, I noticed that one of my less expensive eyepieces (25mm Explorer II Kellner) had a serious "ring of light" of it's own. In taking it apart I found that both of it's internal spacer-sleeves had glossy internal finishes. Definite no, no. I used a coarse piece of sandpaper to roughen up the plastic then reassembled. No chance to check in the scope - yet...

On Tuesday the sixth, the sky cleared enough in the early evening to get a quick look at the Moon and Jupiter. Field flatness appeared about the same (while viewing the moon) but I hadn't really shifted the focus back significantly (so I didn't expect much). In viewing Jupiter I was aghast to see that traversing intra to extra-focus caused a "comatic" shift in the sharpness of Jupiter's edge. (The out focus "halo" shifted from one limb of Jupiter to the other even with the planet in the middle of the field of view.) This is a clear sign of serious mis-collimation. Later, it grew dark enough do a quick star test on Sirius. Coma was quite apparent in both the in and out focus images. I made a few adjustments but there was no real improvement. It was clear that I would have to completely rebuild the adjustment mechanism to ensure that I could get precise, repeatable, and stable collimation results.

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This morning (Wednesday) I thought through the entire adjustment mechanism and came up with (what I consider) to be a huge improvement in approach. The problem with the original Intes design is that the adjustment screws actually rotate to reposition the backing plate. Since the backing plate is made of aluminum, the threads in the plate wear down with use. The problem is exacerbated by the strength of the springs Intes installs to maintain tension between the aluminum backing plate and face plate (through which the phillips head adjustment screws pass). Any improvement in the design would require that the screws be static yet still allow the distance between the back of face plate and the front of the adjustment plate to vary in a controlled manner. I came up with the idea of re-installing the tensioning springs (in their usual position as before). But instead of the leadscrew approach used by Intes, I'd install long setscrews (instead of phillips) with wingnuts that could be turned on the outside for adjustment.

Of course, for this scheme to work, I would have to secure the ends of the setscrews into the secondary mirror-supporting adjustment plate. This requires removing the secondary mirror from the plate and gluing the ends of the setscrews in place. Fortunately I didn't really need to separate the secondary manually. For you see, last night I accomplished this by installing over-long 3mm allen head screws into the mount and having there heads "pop" the secondary off the mount during an attempt to collimate the scope. So today, all I need do was secure the setscrews, reglue the secondary mirror and re-assemble everything. Easy huh?

Unfortunately, neither of the two local hardware stores had 3mm setscrews long enough to accommodate this approach. One did have 4mm allen head screws and wingnuts. So I spent an hour removing the allen heads and re-tapping the backing plate to 4mm. With this done, I verified that the mechanics of this scheme would work. (Admirably.) With the "setscrews" and secondary mirror glued into the adjustment plate, the scope was re-assembled and readied for collimation.

By 1:00 this afternoon I had re-collimated the scope. This was done using the "look down the tube and adjust the primary and secondary mirrors until all the reflective circles line up concentrically" method. Complicating the process was the need to make sure that all the eyepieces would come to focus. (The 35mm Ultrascopic is not parfocal with the others.) I made several attempts to ensure that the barlow could also come to focus (in both the 2X and 3X configuration) but decided to bail and forget about the barlow. Why? I want to test my idea that field flatness will improve with the primary and secondary shifted closer to one another. If I were to set everything up so I could use the 3X barlow (by maintaining the more widely spaced primary secondary relationship) I'd be back where I was initially - the 35mm unable to achieve focus with the OIII filter installed and quite possibly the smaller "flat-field" region. (30 arc-minutes.)

You may be wondering how my "wingnut" adjustment mechanism worked. Awesome. All it takes is a twist of the nut between index finger and thumb and everything tweaks beautifully. No longer do I have to pull out a phillips head scewdriver and wave it's dangerous tip in the general direction of the coated meniscus. No longer do I have to concern myself with the ever more "well rounded" phillips head blade edges in the 3mm screws. No longer is there the risk that I will install too long replacement allen head screws and pop the secondary mirror off the adjustment block. And most importantly, now issues incidental to the overstrong tensioning springs are banished. The new screws don't turn inside the aluminum backing plate. (They are fixed in position by the glue.) The wingnuts now take the combined tension of rotation and springs instead of the threads in the plate.

With the collimation roughed in, (remember all those concentric images reflecting around inside the OTA?) all I need now is a bright star to guide me by.

That too will come...

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Next Pass at Collimation

Unexpectedly, the weather took a turn for the better yesterday (Wednesday, March 7, 2001). By mid-afternoon, Argo was fully assembled and "cold collimated". About 5:00PST, the Moon made it's appearance over the treeline west of town. The view was somewhat disappointing. Between the brightness of the sky, the moons very gibbous phase, and the roughness of Argo's collimation, the view was seriously short of being inspirational. Later Jupiter could be picked out in the darkening (but still blue) sky. Only the two equatorial belts were visible, and these lacked any sense of detail. Much of this problem was poor stability (3/10) but part of it was definitely due to serious misalignment. (In traversing between in and out of focus, Jupiter's disk was seriously offset to the north while out-focal image glow maintained its place.) I then hoisted the scope (stand and OTA as a single unit) and moved it to a position where I could sweep for Sirius to the south. After a few minutes, the Dog Star revealed itself through the finder and I shifted to the main tube.

As expected Sirius' image showed excessive coma. Removing the yepiece I sighted down through the diagonal. The obstruction was visibly centered. Light from Sirius bathed the entire meniscus. What could be wrong? It took me only about five minutes of fussing with the adjustment wingnuts to really screw it up! The image of the central obstruction actually "kissed" the tube wall (from the perspective of the diagonal). Luckily after detecting this I screwed on my thinking cal and returned the secondary obstruction back to the center of the tube. A few more minor adjustments and I had a nice, fat, circular, outfocus image of the star showing the accustomed four or five equally-spaced and concentric rings that I have come to know and appreciate.

The rest of the evening was spent in "high energy ecstasy" as I rushed around getting caught up with my hugely-delayed observation plan. During the course of that evening, much (but not all) of my faith and appreciation for Argo was restored.

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Should I? or Shouldn't I?

This morning, I reflected on how the secondary housing could have been so precisely centered in the tube and yet still give such a comatose image. The answer wasn't long in the coming. The secondary floats around on those three adjustment screws. Under certain combinations of spring pressure, the whole secondary shifts laterally. The linear relationship between the center of the actual secondary is lost, even though the appearance of the fixed central obstruction is maintained. Slapping myself on the forehead, I realized that I had forgotten to extend the setscrew that captures the center of the aluminum backing plate and acts to fulcrum adjustment.

Well, I hadn't actually "forgotten" to do this. I was concerned that the screw was perhaps a little too short to make contact and that it might "fall through" into the secondary mechanisms interior. So now, I have four choices.

1. Don't plant the setscrew. Live with the floating backing plate (along with the possibility of it eventually fouling up some evenings observing session due to one mishap or another.)

2. Plant the setscrew and if it falls through, fix it later during bad weather.

3. Plant the setscrew and if it falls take the meniscus apart this evening -- irrespective of the weather.

4. Use the "If it ain't broke don't fix it." rationale to repress the whole issue.

What would you do?

NOTE: Sometime later, the secondary mirror managed to "unglue" and I dissassembled the entire secondary mechanism. After carefully re-gluing the mirror to the backing plate, I ensured that the central set screw properly meshed against the detent in the center of the aluminum plate...

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In the (Star) Field

Last night (Saturday, March 11, 2001) I had the opportunity to take Argo out China Ridge for an extended observing session. A quick star test showed that Argo is holding collimation well. The scope's performance was quite acceptable in many ways. The problem of "star haze" continues to persist - but may ultimately prove to be an atmospheric phenomenon. Jupiter and Saturn gave acceptable 180X images (despite only fair atmospheric stability). Tests using Saturns image across the field of view confirmed that there has been no real improvement in field flatness (still at 30 arc-minutes) - but may ultimately prove to be inch and a quarter sized eyepiece related. On the basis of the last nights performance, I've pretty much decided to leave Argo untouched and move ahead with my observing plan. So nows a good time to review the original shortcomings of the instrument and go on from there:

• Excess Light Scatter in the Field of View.
This may ultimately prove to an artifact of the cold, wet, high humidity, conditions of the winter months that predominate here along the Northern California coast. Late spring should prove this one way or the other.
• Low Detail Contrast on Features of Jupiter's Main Belts.
Although the 150mm MCT is a fine general purpose instrument, it really is not in the same class as a comparably figured 5 or 6 inch refractor. Views of Jupiter are quite pleasing, and when viewed through a dry, clear, stable sky the planet can positvely explode with intricate detail. But it is extremely rare for this combination of atmospherics to manifest in these parts. If you are into planets - get a high cost, high quality, low portability, refractor of at least 125mm in aperture. (Or 140mm+ if you also want to use it for "deepsky".)
• Poor Field Flatness.
Research has shown that long focal-ratio scopes tend to have more difficulty in this area of performance. To offset the problem may require special 7 and 8 element wide-view 2 inch barrel eyepieces at a cost several hundreds of dollars. If the issue becomes that important to me, I can allways consider either purchasing a field flatening focal length reducing multi-element lens - or go to 2 inch ultra wide angle field flattened eyepieces. Option two no longer appeals to me since I now have in my possession an F5 80mm achromatic rich field refractor ("The Pup"). Option one (the field flattener) may become an option in dealing with the next issue to be reviewed.
• Focal Length is Too long
As discussed above, Argo's long focal ratio means that field flatness suffers. Even more importantly, long focal length (1800mm) means that even the longest focal-length 1.25 barrelled eyepieces can only achieve a too high magnification of 50X. Since image contrast is eroded by excess magnification, certain classes of deepsky objects (such as the eastern and western Veil and the North American Nebulae) can not be easily viewed, nor properly appreciated in their proper scale. One step taken to remedy this has been the inclusion of "the Pup" in my astronomical toolkit. Another could be the acquisition of the aforementioned focal reducer.
• Visible Mismatch of Extra/Intra-focal Star Test Images
Internet postings indicate that most MCTs (irrespective of manufacturer) suffer slightly from this particular issue. However, I must remark that the Pup (even if it is a refractor) does not show this particular optical anomaly. Again most internet postings generalize to say that it is really "in-focus" performance that makes or breaks a telescope. With this I heartily agree. Interestingly enough, despite the perfection of the Pups out-focal star test images (better than 80% Strehl - one quarter wave) it is completely incapable of performing half as well as Argo in terms of planetary performance. (This even when a 3x barlow is used to extend it's effective focal ratio to F15.) So the issue is moot. Argo should not be knocked from suffering a minor imperfection inherent to his breed.

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Field Flatness Issue Update

Over the last few weeks, despite some nice views of Jupiter and acceptable deepsky results, I was still plagued by the fact that I had not really accomplished my main hopes of noticeably improving Argo's field flatness. Also in collimating the scope I noticed that I had to seriously "tweak" one of the wingnuts - to the point of binding. There was also a need to add a few flat washers to prevent the the wingnut surface from making contact with the front plate of the secondary. Installation of the washeres under the wingnuts would require removal of the meniscus and dissassembly of the collimation mechanism.

So yesterday evening (Saturday, March 31, 2001) I pulled apart the secondary. In doing so the source of wingnut binding became apparent - the secondary mirror had partially separated from the backing plate. This changed the presentation of the mirror. This, in turn, made it necessary to tweak the wingnut to the limits in order to align the secondary. I carefully reglued the secondary and added the missing washers beneath the wingnuts. Reassemblily of the scope went very quickly and soon I was outside adjusting the collimation.

Before actually tweaking the wingnuts I decided to have a look at Jupiter - to compare views later. Without getting into details, what I saw reminded me of poor seeing conditions (3/10 stability). Set the scope up where I could catch Sirius, then began adjustments in earnest. Within five minutes the job was done. The wingnuts were accessible to my left hand even as I followed the a-focal image of the star across the field of view. What a delight to get instant feedback as you make adjustments! In no time I was seeing something that I had never really seen before. I could begin with an extrafocal Sirius, slide to focus and watch the "ring of light" collapse into a perfect "white hole" without the least comatic shift. Passing through focus, I was able to see a perfect ring of light form and expand into a multiplicity of annuli. (Tube currents were very obvious along with a great deal of general instability.)
I moved Argo over for a view of Jupiter - no real improvement. The sky was, in fact highly unstable and tube currents remained an issue - even if collimation wasn't. However, as I toured the heavens throughout the evening I couldn't help but notice how perfect and round the spurious images of stars were. The Cor Coroli Double! La Superba Carbon Star! All were perfect.

Tonight, Deos Concedente, I hope to repeat the tests on Saturn that originally prompted my field flatness concerns. Will I have to refocus anywhere inside the 40 arc-minute 25mm eyepiece field of view? Will I see astigmatism anywhere inside the 35mm eyepiece?

NOTE: I've now had several opportunities to observe using super-multi-elemental 65 and 85 degree apparent field 22mm Panoptic and 17mm Nagler eyepieces. Both eyepieces showed a virtually flat field displaying pointillistic stars throughout their respective 45 plus arc minute sized fields.

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On (Orion Ultrascopic) Eyepieces

Received the following email from fellow amateur Robert Derouin:

Hi Jeff!,

It's me again! I'm concerned about your fondness for ORION optics Ultrascopic oculars.You and another Amateur observer,for whom I have much respect,both like those Orion Ultrascopics.Just because they come in these 'silly' extremely high magnifications.I not too long ago purchased a finder scope for my 6"f/12 apo refractor.(8x50).This finderscope was purchased from Orion.The optics are fair.Coma resides at the field edges.Yeah,the central image is fine.But,coma resides at the edges!!Ok,ok,I am satisfied enough not to send it back for a refund.I need it primarily for sighting lunar and planetary targets 'dead on!'.It works for me,and the price was right. So,till this day,I suspect Orion optics!!!Ok, you've accomplished great things with your 6" MCT.Beautiful photos.!! You and Mr.Tom Back(refractor king),both use Orion Ultrascopics oculars!! Jeff,I NEED TO KNOW!!! IS IT ALRIGHT TO START LOVING ORION OPTICS!!!!!?????? Do their eyepieces really 'shine' while their finderscopes really suck!??? Jeff, Please talk to me about Orion Ultrascopics,and would you suggest buying one over a Meade Series 4000 ocular or a TeleVue plossl?????????

Sincerely, Bob D.

My initial response to Bob was very straightforward: "The answer is simple - I don't know any better!" Of course, I also had to send him the straight answer too:

The Ultrascopics -like most Orion products - are a decent value. I have a pretty limited budget (but have already spent more than 2K on scopes and accessories). The MK-67 was a very good investment (not perfect, it has field curvature issues that are probably exacerbated by the Ultrascopics).

What do I look for in an eyepiece?

Absolute / Need to have:

• Comes to focus.
• Flat center 50% of field.
• Free of coma 75% of field.
• Free of astigmatism 90% of field.
• Don't notice light transmission problems.
• Rarely notice ghosting/internal reflections.
• Minimum 50 degree apparent field.
• Less than $100 per ocular.
• Don't notice any light splintering before planets enter FOV.

Ultrascopics meet above requirements.

Relative / Nice to have:

• Enough eye relief that I could wear my glasses (if I had to).
• Ease of eye positioning (no blackouts/kidneying).
• Parfocal with other eyepieces in kit.
• Flat center across 90% of field.
• Coma only outside of 90% field.
• Completely free of astigmatism.
• Sense of sparkling clarity.
• Minimum 65 degree apparent field.
• Complete freedom from internal reflections and ghosting.
• Can't detect any light splintering before moon/planets enter FOV.

25mm/15mm/10mm Ultrascopics fine in these areas:

• Ease of eye positioning (no blackouts/kidneying).
• Parfocal with other eyepieces in kit.
• Flat center 90% of field.
• Coma outside of 90% field.

35mm/25mm/15mm Ultrascopics fine in these areas:

• Enough eye relief that I could wear my glasses (if I had to).

Frankly, until I come into some serious money, I just can't rationalize going to $400 Naglers, Panoptics etc. Once this is possible, I'll probably take the plunge then realize how sadly deficient the Ultrascopics are. (But I'll still use them on the ShortTube 80.) To be sure field flatness issues with Argo relate to its current eyepiece set (as described earlier)...

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Telescopic Limiting Threshold Magnitude

It's now Saturday April 28, 2001, 4:15 in the afternoon. I've just made a changes to this document by adding the results of a few observations to some of the more positive features of this scope. Unfortunately, I now have to balance the positives with a negative. Basically, Argo runs out of breath when it comes to revealing dim stars. As a result, I've extracted the following note from the list of positives. And have amplified on it here for future reference.

• Minimum Threshold Magnitude (Original statement:) On a 5.5 unaided limited magnitude zenith night, I have directly seen 13.1 magnitude stars in the vacinity of the Ring Nebula at 180X. Revised Statement: Recent testing using M44 (Praesepe) on a 5.5 ULTM night has shown that Argo reveals 12.7 magnitude stars directly about 50% of the time under steady gaze. On a 5.3 ULTM night, 13.5 magnitude stars could only be detected using rapid eye movement (REM). These two facts point toward a 6.0 ULTM direct visual magnitude limit of 13.0 (at 180X). This LTM rating is barely adequate (in my estimation) for a 150mm objective telescope. (A good refractor will display stars of magnitude 13.8 directly under similar circumstances, while a good newtonian will show stars of magnitude 13.4.) All things being equal, such a refractor performs as well as a 200mm SCT in terms of light grasp, while the newtonian is comparable to a 7 inch SCT, and the MCT is comparable to that of a 6 inch SCT.

  NOTE: Since documenting this particular concern, extensive limiting threshold magnitude tests (using the region around M57 in Lyra) has revealed that seeing stability is a significant factor impacting telescopic limiting magnitude across all magnifications. (For instance on a night when unaided limiting magnitude approached 6.0, was able to hold a 13.4 magnitude star in the vacinity of the Ring at 120X.) Previous thinking was that seeing conditions "capped" the highest magnification possible under a given sky. It is now established that good stability also improves magnitudinal reach of lower magnifications as well. Finally, by way of comparison, on that same 6.0ULM and 8/10 stability night, a 200mm Meade SCT held a 14.1 star at comparable magnifications. Despite this new insight, I personally continue to encourage those who purchase MCT's to take advantage of any high reflective coatings available - since more light is more light. (Although investing in new coatings for Argo is now a lesser priority.)

  Corrective Action: To bring Argo up to snuff would require that all aluminum surfaces be re-done with high reflectivity coatings. In addition, the meniscus would require high transmissivity coatings on both faces. Doing so might add .4 magnitudes to Argo's reach. The net result would no doubt enhance Argo's overall performance noticeably. Before re-coating, it is also possible to have all optical surfaces re-worked. This would transform an instrument of decent performance into one of exceptional virtue. Many of the issues cited above might very well be resolved.

  NOTE: The manufacturer of this scope offers a "Deluxe" version. That version comes with high-efficiency coatings and 1/7th wave-rated optics. I emphatically suggest that potential buyers consider upgrading their orders to the Deluxe version. This recommendation is based on the need for "more light" - not improved optical refinement.

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Low Power Field Astigmatism

During a recent star party, tried out my 35mm Ultrascopic eyepiece in a 200mm Meade LX200 SCT. I also tried the 200mm SCT's 35mm Ultrascopic in my scope. In both test scenarios I noticed image-focus traversal astigmatism! What do I mean by this, you ask. Well do the experiment yourself. Install a low power eyepiece (50X is fine). Start with the image of a star (or better yet - a planet) outside focus then rack it in. Watch the out-focus "halo of light" converge toward the star (or planet) and collapse into focus. Continue racking past focus - intra-focally. Does the halo shift? Does it take a right or left turn? If it shifts from one side to the other, or at an angle, you would assume the scope is not properly collimated. So, if you dare, re-collimate your scope and get rid of it. Ahah! No go. You just end up screwing up your collimation!

Now repeat this whole experiment at say, 180X. Does the glow collapse and pass through focus the same way? Or does it behave as you would expect it to behave? On Argo, it behaves normally. So what's up? Where's the astigmatism coming from?

Could be your eyes...

Or it could be something very strange about low power eyepieces (such as my 35 and 25mm Ultrascopics). These things have huge lenses. They only seem to work well when the image is right in the center of the field of view and your eye is strategically positioned directly above them. I don't know about you but I'm going to get a better handle on this phenomenon. Personally, my sense of optical excellence is offended by this behavior. Anybody out there have a fix?

To determine if the problem lies in your own "native optical train" try rotating your head while viewing a star in the middle of the field. If the "distortion" follows your head then well...

NOTE: "Fixes" for low power astigmatism include the wearing of astigmatically correct eyeglasses while observing or the purchase of a superwide field eyepiece that effectively shows as much apparent field at a higher magnification. (For instance a 22mm Panoptic at 81x rather than a 25mm Ultrascopic at 70x.)

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A Statement of Comparative Worth

Argo and I have now been observing together for more than a year. Over that time, we've both grown in many ways. Efforts to test Argo's mettle against the widely disparate and difficult Delta Cygni pair have forced me to refine the collimation to the point where Argo can now regularly resolve the subtle Equatorial Belt (EB) and other low contrast detail in Jupiter's atmosphere. Saturn's Encke Minima and Crepe Ring are visible without special tricks of eye or magnification. Stars to magnitude 14.1are regularly seen (with some aversion) at commonly used deepsky magnifications (120x). A poorly sky-positioned Mars offers up significant maria detail even while sub-ten arc seconds in apparent size. Uranus and Neptune both show visible discs. (Uranus with central brightening.) Merope nebulosity in the Pleiades is no longer confused with garden variety star haze. Add to this all the well-resolved views of Messier globular clusters, and numerous gemlike NGC planetaries and you get the sense that this scope delivers!

There is no longer any doubt in my mind that I made the right decision when acquiring Argo. A quality Maksutov-Cassegrain dollar for dollar and inch for inch is perhaps the best scope available today in the six to nine inch aperture range.

Now that's a pretty stong statement and can (and should) be contested by owners of other scope types. But let me go on to say that it is my personal belief that a high quality achromatic refractor offers better value in apertures less than 150mms. While the SCT may offer a good alternative up to 300mms. Beyond that the Newtonian design may very well become preferable for its combination of low cost and exceptional light grasp.

And let us not forget the high end scopes either. Surely, there is not an observer in the astronomical community who doen't "lust in their heart" after a truly apochromatic refractor in the 100 to 200mm aperture range. But the best 100mm APO's - though they may be thrice as expensive as a 150mm MCT - can not outperform one even on lunar-planetary observing.

So my choice in a scope finally comes down to this: How much can I afford and what do I want to look at? If galaxies were at the top of my list, I'd go for that 300 plus millimeter newtonian on a Dob mount BUT, it had better be "optically correct" (ie. truly diffraction limited) and hold decent collimation. If lunar-planetary were my exclusive calling, and I had the bucks, I'd go for that 125mm apochromat. If general observing were my theme and I had limited resources. (Hey, that sounds like me!) I'd get a Mak!

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Intes Replacement Kit and Primary Removal

Received the following email from fellow MK-67 User Vincent:

Jeff,

Re-read your argo's perilous journey and am wondering if you ever got the primary mirror separated from the central baffle.

I just recently replaced my menisus, secondary and primary mirrors on my Argonaut. As you are probably aware Orion does not even carry the scope anymore and offers no support. I bought a complete MK67 optical assembly from ITE for $365 including shipping. The optics are from Intes and are essentially the same except the menisus coatings appearance change with light conditions and are not as readily apparrent as the purple coatings on the Argo. I actually like the new optical train better it is possible that the standard intes broadband coatings are a little superior to those provided with the Argo? I don't know this for certain, and it could just be my imagination. I do no the std. intes MK67 without mount cost more than the Argo, and since they are identical in all respects it seems except for tube color I figure there must some difference in the optical train.

If you haven't gotten your primary mirror detached from the argo it really is not to hard. Before unscrewing the back cell/push pull plate, release the allen screws on the focuser, take it off, and the loosen the lock nut holding the central baffle to the push/pull plate. It is on very tight. I use a rubber mallet and a flat screwdriver placed in one of the slots in the nut head. A few taps and it was loose enough to remove by hand. I then took out all the screws around the perimeter of the rear cell/push pull plate, and from the front with meniscus removed pushed the mirror cell out the back using the central baffle. Once removed from the tube I completely unscrewed the baffle from the rear cell and removed the mirror. There is a fiberglass washer and three small rubber pads glued to the mirror cell. Upon reinstalling all the components I could never get the baflle hold down screw as tight as it came from the mfr. and there was a little play in the mirror parallel to the baffle. I fixed this by adding thicker rubber pads behind the mirror. It is only hand tight in place to keep from stressing it, and can be rotated with a little bit of effort. This does not seem to be a problem as the scope maintains collimation. The only problem I have had once assembled is that I had trouble getting enough outside of focus for star testing. I fixed this by adding a 1/4" spacer between the 2" to 1-1/4" adapter. All my eyepieces now reach focus easily, whereas initially my 8mm radian only barely reached focused all the way racked out.

Thought I would pass this on in case you ever take the scope apart and want to give the mirror a good cleaning separate from the baffle.

Clear skies.

vincent

Thank You Vincent!

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