Otto Piechowski's: Collimating the INTES MK67 / ORION ARGONAUT 150

Introduction and Acknowledgements
Collimation: A Definition and a Distinction
Some Pre-Collimation Observation Results
The Collimating Method:
Some Post-Collimation Observation Results:
Caveats and Parting Thoughts

Introduction and Acknowledgements

This is a description of a technique for collimating the MK67 (also known as the Argonaut 150). This telescope is a Maksutov-Cassegrain-Rumak (Sigler) design. The MK67 has an aperture of between 150 and 152 millimeters, a focal ratio of 11 to 13 (F11 to F13), wavefront error between 1/6 and 1/8 lambda, a Strehl Ratio of around .92 with an image spot size between 13 and 15 microns. These have been produced for about ten years by a group of optical craftsmen in Russia who work for the INTES company. It is sold through Earth and Sky Adventures, Hands On Optics, Internet Telescope Exchange, APM Telescopes and by many others. However, it is no longer sold by Orion of California; the originator of the Argonaut-150 designation.

The collimating technique described here can be done with or without a clock drive. If one chooses to not use a clock drive, the telescope will need to have a mount which allows the telescope to be centered on the North Star (Polaris) and which allows regular slight directional adjustments so that Polaris can be re-centered as the mirrors are adjusted. One can collimate while using a clock drive and a mount which allows the telescope to be centered on any first or second magnitude star of one’s choice; again, with the ability to perform regular slight directional adjustments. The only tools and accessories required are a small Phillips screw-driver, two different sized Allen wrenches and a good quality high magnification eyepiece; a 7mm or 5mm will do nicely. Though not technically difficult, this method requires several hours of time, moderate attention to detail, and the ability to work in small-steps. A moderately experienced scopist-stargazer should be able to follow this collimating procedure.

I am indebted to Jeff Barbour of California, Cor Berrevoets of the Netherlands, my neighbor Roy Alcorn, to Mike Anderer of Illinois and to the opticians of INTES. The opticians of INTES have created what I lovingly refer to as a Brute-Force-Design-Telescope; consistently possessing superb optics and owner/user-friendly mechanical characteristics. They have created a telescope on which the experienced stargazer/scopist can perform all necessary adjustments. Jeff, who uses an Argonaut 150, and Cor, who uses the similar Alter M603 manufactured by Intes-Micro, have been down the collimating road I am describing, herein. They walked me through the process. I am indebted to my neighbor, Mr. Alcorn, who assists me with solving mechanical problems and who enjoys stargazing with me. I also am grateful to Mr. Anderer who permitted me to purchase his MK67 and thus obtain a fine instrument at a cost I could afford.

Brute-Force-Design is a phrase created by Frs. John Neville, O.S.C. and Myron Effing, O.S.C. in their construction of the Northern Cross Observatory at Onamia, Minnesota. From scratch they made a 30cm interchangeable cassegraine-coude/Newtonian scope. It was designed to be used a great deal by persons whose skills with telescopes varied greatly. It was designed to take years of constant use in the very harsh environment of north-central Minnesota and to sustain, without harm, some degrees of misuse. The Brute-Force-Design does not refer to a particular optical system (i.e. achromatic refractor, Newtonian reflector, etc.) but rather to the robustness of the scope in its ability to weather use and misuse. In this regard, the INTES MK67 is robust. I have the great good fortune to be associated with Cor and Jeff who have put their scopes to a great deal of use and have advised me as how to optimize the use of this optical system. I have discovered that this telescope is mechanically robust, elegant in its optical performance and aesthetically pleasing as it sits as a silent sentinel to the night sky awaiting.

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Collimation: A Definition and a Distinction

Collimation refers to the correct mechanical alignment of all the optical elements of a telescope. With well-crafted and matched optical elements, collimation means that light entering the telescope will efficiently reach the proper focus. When properly collimated by the method described here this scope attains, what Jeff Barbour has named, "the last quantum of collimation"; a point beyond which very little improvement is possible. And even when this telescope is "eye-ball" collimated in the daylight (an inexact method of collimation in which one looks through the eyepiece hole with no eyepiece in place and makes adjustments to cause all the "circles" to attain "eye-ball" accurate concentricity), it performs well. Generally speaking, an MK67, properly collimated, performs a discernible "notch" better than through eye-ball collimation.

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Some Pre-Collimation Observation Results:

I have kept a journal for over ten years, detailing my telescopic observations of celestial objects. These records indicate that the following was standard fair as regards the observational performance of this scope before collimation. These observations occurred within an urban area of nearly a third of a million people with its attendant light and aerosol pollution. The Milky Way can barely be seen on the best nights. On most occasions, the sky is bright; overwhelming faint objects (nebulae) and washing out some of the fine detail on extended bright objects (e.g. Jupiter). However, on occasion, the sky is very aerosol-clear and dry (little haze or glare), stable (one can see fine detail with good contrast) and transparent (one can see faint objects). On such a night:

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The Collimating Method:

Mechanical Characteristics and Requirements:

The MK67 has a secondary mirror attached to the meniscus lens. On the outside of the case that holds this mirror are three Phillips head screws. These are used to adjust the position of the secondary mirror.

The MK67 has three sets of two Allen screws placed in the primary mirror end of the telescope. The three outside Allen screws, which are also the larger screws, loosen the mirror so that adjustments can be made. Once loosened, the three inner (and smaller) Allen screws are used to adjust the position of the primary mirror.

Until one is certain that the diagonal (mirror or prism) is square-on to the optical train, it is necessary to do the collimating procedure without the diagonal in place. Therefore, one needs to observe and do the collimating straight through. Because the back focus of the MK67 is only 35 millimeters, it is necessary to add an extending tube of some sort to offset the loss of distance occasioned by removing the diagonal. Sometimes the tube of a Barlow lens with the negative lens removed is sufficient. Whatever is used, it is necessary to be able to view Polaris (or the star of one’s choice) well inside focus, well outside focus and in focus as well. With a 7 millimeter eyepiece, one will need perhaps ten millimeters of play on both sides of focus.

Once one knows that the telescope itself is collimated, one can then use the collimated scope to check if the diagonal is square on to the optical train; using any deviations to adjust the attitude of the diagonal.

The scope must have reached thermal equilibrium. Due to the characteristics of the maksutov design, when the scope is moved from and to places of differing air temperature, thermal equilibrium may take anywhere from 15 minutes to three hours. Often it is advantageous to be able to leave the telescope outside. However, since temperatures often fall at nightfall, even if the scope has been in a shaded area, time may still be needed for equilibrium to be reached. If collimation is attempted while the scope is still equalizing, the results will not be good.

There are two ways to determine if the scope is still not thermally equalized. On a large intra-focal image of a bright star, one will see waves slowly moving across the stellar image. On an extra-focal image one will notice a pyramidal or tear drop feature on one side of the extra-focal image. One may also notice that the central dot in the extra and intra-focal images is not exactly centered (but this is useable only if one already knows that the scope is collimated).

There are two advantages to using Polaris as the collimating star. For most residents of the northern hemisphere Polaris is sufficiently low to the horizon so that one can observe straight through the scope, necessitated by not using a diagonal. Secondly, Polaris, being only slightly more than a degree from the true North Pole will essentially stay centered without a drive being used. Of course, if one has a diagonal that is proven square-on, one can use the diagonal while collimating the scope. Similarly, if one is fortunate enough to have a clock-drive and a stable equatorial mount, one can use any bright star one chooses.

A high magnification eyepiece is useful. I have found that a 7 millimeter or 5 millimeter orthoscopic is very good. One wants to attain an intra-focal image that has at least two or more very dark and noticeable concentric rings. Similarly, one wants to obtain an extra-focal image of the star that is the same size as the intra-focal image. For this purpose, these eyepieces do nicely.

Desired Results:

It is valuable to know ahead of time what one is striving to acccomplish; what phenomena indicate that collimation has been achieved. It should be noted that in a Maksutov-Cassegrain-Rumak system, the intra and extra-focal images will appear dissimilar even when the optics are good and the optical train is well collimated. However, it must also be said that, if the optical quality is good, as proper collimation is obtained, the intra and extra-focal images become more alike, though not identical. Thus, the results one is looking for include the following:

The first set of results one is looking for include the following. One wants to see a pair of dark intra-focal rings "appear" equally and concentrically as one moves from in-focus toward a greater out-of-focus image intra-focally. Perhaps another way to say it is, is that at first you will see only a single dark ring, but then it becomes two rings. If you should see this as a single ring on one side while branching into two rings on other places of the circle, your scope is not collimated.

Extra-focally, you want to detect a similar double ring. However, this may be a visibly fainter double ring compared to the intra-focal double ring. Further, you want to see the center-dot exactly in the middle of the extra-focal image. Finally, you do not want the extra-focal disk to appear hazy or flared on any of the sides. A mutated (single to double) ring, non-centered dot and hazily flared side all indicate lack of proper collimation.

The second set of results one wants to see is the "Barbour-ring" and "Berrevoets-dot." Specifically, collimation is complete when two phenomena are seen. Intra-focally, as you near focus you see the emergence of a single nice bright ring in the intra-focal disk, which is similarly distanced from the center and uniformly bright all around. Extra-focally, as you near focus, you should see the center-dot stay in the exact center of the shrinking extra-focal disk.

The third result you want to observe is that as you approach focus, the shrinking ring (not the same as the Barbour-ring within the intra-focal disk image) is uniformly bright, distanced and thick all the way around.

The final result is that the in-focus star (assuming the night is steady) should appear as a tiny disk with a first bright diffraction ring around it that is all the way around and uniformly bright.

If you are obtaining these results, you have obtained "the last quantum of collimation."

Collimation:

As the Colonel in the Indian Army said to Tori Muirden on the way to the North Pole, as Aesop’s tortoise revealed to the hare, as was said to shortwave radio operating Eleanor Hathaway by her father in Contact, and as every person who has ever ground a mirror knows, slow and easy accomplishes the job better and faster. This principle applies a fortiori to collimation. Besides small adjustments, one is seeking progress and not perfection with each attempted adjustment. Finally, one learns by trial and error.

Though there may be other ways, this method employs using the secondary mirror Phillips head screws to adjust the intra-focal images and primary mirror Allen screws to adjust extra-focal images. The secondary adjustments to the intra-focal image are more sensitive than primary adjustments to the extra-focal image.

PLEASE NOTE: Fellow MK-67 user Harald Mertens of Germany and I (jeff barbour of lalaland) have empirically determined that Otto may have mistranslated his field notes in assigning extra-focal adjustments to the primary and intra-focal adjustments to the secondary. In fact, it is just the opposite - primary adjustments are used to set inside focus and secondary for outside focus collimation. I have done the best I can to adjust the rest of this document accordingly. Caveat emptor!

In general, adjustments to the secondary’s Phillips head screws should be no more than an eighth of a turn at a time. Also, one needs to know that it is possible to turn the secondary screws all the way out and disconnect and dislodge the secondary mirror. This would not be good at all. A technique I have used is to begin by gently tightening the secondary screws in all the way (clock-wise).

Adjustments to the primary mirror by turning the small Allen screws is less sensitive than the secondary adjustments in two regards. First, one can turn them much more with much smaller results. A general rule I use is that one should feel comfortable observing the intra-focal image transverse the entire field of view in the eyepiece with each adjustment.

One may do less, of course. Secondly, I am told by Markus Ludes of APM telescopes in Germany, that one cannot dislodge the primary by turning the small Allen screws all the way out. Of course, one would not want to do that as it might be difficult reinstalling them.

When approaching the secondary screws with a phillips screwdriver, one should use a red covered flashlight. It could prove painfully easy to scratch the meniscus by dragging the tip of a metallic screwdriver across it. Therefore, it is wise to use a light to see exactly where one is holding the screwdriver. The light will also simplify the process of finding the Allen heads screws behind the primary.

After having centered the scope on the desired collimating star, one begins by removing any dew cap or dust cap one might have on the meniscus end of the scope. Similarly, one wants to loosen (not adjust) the primary mirror by loosening well and equally all three of the large Allen screws on the rear of the scope. (These are the lock-down - not the adjustment screws.)

One then defocuses the star extra-focally (turning the focuser wheels clockwise) until the single-to-double dark ring transition is seen. On the side of the circle where the single ring is seen, collimation can be improved by finding the correct Phillips screw to turn. This is done by extending ones hand above and over (without touching) the meniscus. One will see a dark wavy shimmering image protrude over one side of the meniscus. One moves the hand until it is over the part of the ring that is single. This then is the location of the screw one wishes to turn. If one is lucky enough to have a screw at this location, you then loosen this screw (counter-clockwise) between 1/8 and ¼ turn. If the location of your hand is between two screws, you may need to loosen those two screws, the amount pro-rated to their relative distance from the exact location of the single ring in the extra-focal image.

The same can be accomplished by tightening (turning clockwise) a screw on the exact opposite side from the single sided ring (protruding wavy hand image) between 1/8 and ¼ turn. At this point you do not want to attain a perfect double circle. A little less or a little more is desirable because your upcoming adjustments to the primary will alter your extra-focal adjustments.

At this point you will need to re-center the star. Every collimating adjustment deflects the location of the star in the field of view and changes the focal point of the scope (making the out-of-focus image smaller or larger). Having already loosened the primary mirror you then de-focus the star’s image intra-focally (turning the focusing wheel counter clockwise) to about the same size as the extra-focal double-ringed image. What you want to look for now is a de-centering of the central dot or perhaps a haziness or flaring on one side of the image. You will want to move the entire image toward the haziness/flare (away from the de-centered dot). This is done through trial and error, by finding the small Allen screw (or pair of screws) that - through tightening or loosening - moves the a-focal image in the direction of the haze/flare (away from the de-centered central dot). This you want to do as you look through the eyepiece at the image. At this point in the collimating procedure, you may have to move the image all the way across the field of view - even further if you wish. (It is common to reach a point where loosening the Allen screws on one side tightens the screws on the other making it impossible to adjust those screws. When this happens to adjust the image in one direction you may need to resort to a method such as loosening all the screws equally first, or to adjust an image by loosening two screws on the opposite side rather than tightening a screw on the "correct" side.)

Now, you go back to adjust the extra-focal image. You simply repeat the process described above to adjust the intra-focal image. Probably at this point you will need to do a gross adjustment similar to the previous amount. Again, you don’t want to make a number of adjustments so that you reach a perfect double-ring a-focal image. At this point, this will almost guarantee a severely mis-collimated intra-focal image. Then you go back to the intra-focal image and do another gross adjustment of the primary in the direction of the haze or flare. Then you return to the extra-focal and continue back and forth between the primary and secondary mirror adjustments. At some point you will begin to notice that the side of the mis-collimation (the single ring or the haze/flare) is shifting around the circle. Also you will notice that the adjustments needed become very small. And finally, you will notice that adjustments to the image on one side affects the image on the other side. You are finished with this stage of adjustments when the cross-focal transition from a single to a double dark ring is smooth, happening all at once all the way around, and, when you see no haze or flare anywhere on the a-focal image. At this point, it should be noted that there is particular pair of phenomenon occurring that indicates that "the last quantum of collimation" may have already been achieved. On the extra-focal side one sees a fairly clear but faint double ring with no side haze/flares. Intra-focally, and this is the key, the double ring is not just perfectly dark and concentric and making the transition from single to double all at once all around; the double black ring almost appears to emerge from the field of view out of the intra-focal disk, to hover, to glow.

You now begin the second set of adjustments. Intra-focally you draw the image closer to focus until you notice a single bright ring within the defocused disk. If this is fainter on one side than on others, you adjust this by means of the Phillips head screws on the primary mirror.

Then you defocus the image extra-focally so that you see a bright tiny dot-disk within a rather small defocused image. This should appear quite centralized. If not, adjust the small Allen screws until the dot-disk is centralized. Of course, always remember to center the star in the field of view before attempting any adjustments.

You continue these adjustments back and forth until the ring is concentric and uniform intra-focally and the dot is centered extra-focally.

In this third set of adjustments you draw the image of the star to focus both intra-focally and extra-focally to a single bright ring around a relatively dark hole. Just before this ring collapses to the focused stellar disk (properly called, the Airy Disk), it should appear uniformly bright and circular all the way around. If it is not, tiny adjustments need to be made to the primary and secondary mirrors.

Finally, the in focus image of the star in a stable sky should appear as a tiny disk with one or more circles of light around it (the number of diffraction rings depends on stellar magnitude). You need only pay attention to the first and brightest ring around the Airy Disk. Your telescope is properly collimated if that ring completely encircles the star with no breaks or flares on any sides. At this point, you should tighten the three large Allen screws so as to tighten the primary mirror down, and you should replace the dew cap over the meniscus end of the scope.

NOTE: Tightening the lock-down allen screws on the primary can effect collimation results. In fact as you gain experience with collimating. you can anticipate this effect and use it to make the "final tweaks" needed to prefect the alignment.

The Diagonal:

Having a properly collimated scope makes it relatively simple to test the degree to which the diagonal is square-on. You center the collimating star in the scope. Remove the eyepiece and insert the diagonal. Reinsert the eyepiece and focus. If the star is not in the center of the field of view, this in itself is a sign that the diagonal is not square-on. Rotate the diagonal until the star is centered in it. (This may put your neck in an odd position.) If rotating the diagonal does not center the star, then using the declination and right ascension adjustments on your telescope mount, by using small adjustments, center the star in the center of the field of view.

Once the star is centered in the field of view. Rotate the diagonal 90 degrees. Look to see where the star is. If the star is still in the center of the field of view, then rotate the diagonal to the 180 degree position. Observe again. If the star is still centered, you are finished. Your telescope is collimated and your diagonal is square-on.

However, it is very likely that when you rotate the diagonal 90 and then 180 degrees, the collimating star will not remain in the center of the field of view. At this point you need to remove the diagonal and adjust the position of the mirror in the diagonal.

Diagonals come in many forms. Some have a plate on the back held in place by four screws. Some diagonals have a circular plate held in place by one tiny recessed screw. In the latter case you may need to purchase a specifically sized very small screw-driver to loosen that screw and remove the plate to get at the mirror. There are other designs as well. Once you loosen this plate, you will then either find a thin piece of front silvered (aluminized) glass lying on it or attached to it. Or you may find a thick elliptically shaped beveled mirror which slides into pre-cut slots. Regardless, you will need to take a tiny piece of paper; perhaps as small as 5 millimeters long, 1 millimeter wide and a half millimeter thick. You then position this piece of paper at one of the four sides of the mirror at an edge between the rear of the mirror and its backing, or at an edge between the surface of the mirror and its slot. You will now need to snugly reassemble the diagonal, reinstall it in the scope and retest the diagonal as described above. In all likelihood you will have to repeat this again to try a new position. Quite possibly, after only three or four tries you will find a very good; square-on, position. It is also possible that you will need to make the paper thinner or thicker. When you can turn the diagonal 90 and 180 degrees on a star centered in the field of view and it remains in the same position, you know your diagonal is square-on.

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Some Post-Collimation Observation Results:

As said earlier, the MK67 is a good telescope in which eye-ball collimated images are quite good. However, when fully aligned, images are recognized to be a "notch" better. In stable seeing, the image seems to iterate into an infinity of tiny details, there is a noticeable reduction in glare around bright objects, contrast between black and white is heightened, shades of gray are noticed more easily, star colors and colors of planetary features and of nebulae are somewhat improved, fainter objects become visible, and at high power images seem to "snap" into view. In particular:

Effectively then, a fully collimated MK-67 should give Jupiter views comparable to the finest 100mm apochromatic refractors. Saturn, that of apochromatic 120mm refractors. Lunar, the equivalent of such scopes of 150mm aperture. And resolve matched doubles similar to those possible through the best 7 inch apochromats. It will not however, leap over tall buildings with a single bound!

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Caveats and Parting Thoughts:

It's always nice to get somekind of "before and after" comparison whenever an effort to improve something is made. In this case, the editor used Cor Berrovoet's Aberrator software program to demonstrate how much improvement is possible when going from perceptible coma and astigmatism to the point where it is almost indetectable:

Focused Star Disparate Double Extrafocal Startest Planet Saturn Planet Jupiter

Aberrator Simulation of MK-67 Performance Under Perfect Seeing (1/6 wave SA .2 coma, .2 astigmatism, .05 pinch)

Aberrator Simulation of MK-67 Performance Under Perfect Seeing (1/6 wave SA .05 coma, .05 astigmatism, .05 pinch)

And one last thought, it seems to be a general principle of life that one has as much to lose as to gain when attempting to "improve things". For this reason, it is very important that one feel comfortable with the use of tools and can practice care and patience in implementing this or any other procedure. Be sure in advance to assemble all the needed tools, and have an easy familiarity with the scope and the methods embodied in this discussion. Run through the whole thing scope side during daylight hours as an exercise. When actually adjusting things, take small steps - and most of all don’t force anything.

Otto Piechowski,

Piechowski2@aol.com

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