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Passive Tool Collimation and the Newtonian
Discuss this article in our forums
With illustrations/pictures courtesy of Jim Fly and Jason Khadder
→ Please read the updated version of this article: Collimation And The Newtonian Telescope (version 4)
“If I have seen further it is by standing on the shoulders of giants.” Isaac Newton
The telescope most likely to need collimation is the Newtonian reflector (which includes Dobsonians) because it is most likely to have its mirrors knocked out of alignment by handling, travel, and/or reassembly at an observing spot.
Collimation is the alignment of the optical parts of a telescope. Though lining up the secondary under the focuser is essential for uniform illumination of the field of view, there are only two alignments in collimation: the focuser axis (done by adjusting the secondary mirror), and the Primary Axis (done by adjusting the primary mirror). We all understand that these must be lined up to produce high-quality images, but the necessity of doing so becomes more critical the shorter the focal ratio of the scope.
To wit, the tolerance for primary mirror axial error (PAE) is usually given as .005mm x the f/ratio cubed. Since that cubed figure goes down rapidly as the f/ratio diminishes, you can see that excellent collimation becomes a necessity with shorter f/ratio scopes because of rapidly decreasing tolerances for miscollimation. At f/6, that tolerance calculates to 1.08mm at the focal plane, but at f/4.5 only 0.46mm. Techniques that work fine on the loose tolerances of long f/ratio scopes simply aren’t good enough to provide the best images possible in short f/ratio scopes. Improvements in technique are necessary, and that is one of the purposes for this article. Fortunately, the same techniques for collimating the short focal ratio telescope can be equally well-used on a longer focal ratio, so the techniques you learn for one can be used for the other.
Though the purpose of this article is not to discuss coma, but collimation, it is illustrative to note that the coma-free field of view in a Newtonian reflector is .0007 inches times the f/ratio cubed (e.g. 0.0875”, or 2.22mm, on an f/5 scope)*, and a very slight misalignment of the optical axis can have a devastating effect on the very presence of any coma-free field in the eyepiece!
[*Everhart’s less stringent calculation is .022mm x the f/ratio cubed. It still results in a small coma-free field, but slightly larger than the one quoted above from Sinnott, et.al.]
The reason I mentioned coma is that if a TeleVue Paracorr coma corrector is added, the maximum allowable tolerances for miscollimation of the focuser axis (Focuser Axial Error, or FAE) reduce to 1/6th the allowable miscollimation without the corrector [FAE tolerances are usually given as .03 times the diameter of the primary in millimeters--this becomes .005mm times the primary diameter with a Paracorr], so learning the techniques for collimation of the focuser axis to a couple hundredths of an inch is critical with shorter f/ratio telescopes, which typically do use a coma corrector like the Paracorr.
What happens if the scope is not collimated? At the very least, the position of best focus will not be located in the center of the field of view. At worst, every star in the field will display a flaring away from a poorly focused image that resembles a small comet. Extended objects, like the Moon or planets, will have details “smeared” out and focus will be nearly impossible to achieve. A very small miscollimation can introduce visual effects exceeding ¼ wavelength of aberrations, the maximum allowable error before the image deteriorates to less than the aperture can normally display.
So many other factors can reduce the quality of the image in a telescope (there is a veritable “stack” of image-blurring aberrations) that are beyond our control that it makes no sense not to address and fix the presence of miscollimation when it can so easily be eliminated. Since miscollimation may occur during the night due to sag or movement in the various telescope components, learning to collimate accurately insures that the degree to which the telescope un-collimates in use will never cause the actual miscollimation to drift outside of the envelope of allowable tolerances of collimation errors. That means you can observe from dusk to dawn without worrying about collimation during your observing hours, whatever the f/ratio of your scope.
What follows is a description of how to collimate accurately to a level where collimation errors no longer influence the image quality.
The most commonly used (there are other exotic, do-it-yourself tools that also work, but they are uncommon) Newtonian collimation tools are:
simple peep-hole, though not good enough for scopes below f/10. I
don’t recommend these except, perhaps, as a quick check to see
if the optical elements are at least in gross alignment at the start)
(not useful unless perfectly collimated itself, possessed of a small
beam diameter, and not accurate enough for primary mirror collimation
unless used with a Barlow lens, but quite useful in the dark. Mfrs.:
Glatter, FarPoint, etc.
(Cylinder with peep hole at one end and crosshairs at the other.
Tectron (1.25”), Catseye (2”). Tectron is no longer in
business, but the tools are common in the used market.
(Cylinder with interior reflective surface and blackened center area,
producing a reflection from the primary mirror that is a bright ring
with a dark center. Tectron, Catseye.
Simple Definition and Step-By-Step Procedures of Collimation:
Collimation is the aligning of the optical axes of both mirrors (which may not be the geometric axis of the secondary) to the focuser axis, so we start out assuming the focuser is tight on the tube and relatively square to the tube axis. The entire procedure can and should be done in daylight. A bright sky is useful to allow us to see all the necessary reflections.
[Note: I have no monetary interest in Catseye Collimation (http://www.catseyecollimation.com ), and I use examples of their tools (with permission) to illustrate the techniques of collimation simply because I feel they are the best tools that money can buy to achieve collimation in today’s Newtonian reflectors, whatever the focal length. All these tools could be homemade by any decent machinist, but why reinvent the wheel?]
First, start by centering the secondary in the telescope’s tube. Use a piece of cardboard or some other indicator to make sure the secondary is equidistant from the tube walls in both the focuser and perpendicular-to-the-focuser axes.
Then follow the steps described below:
THESE STEPS ARE DONE IN STRICT ORDER:
Aligning the Secondary mirror under the focuser: The tool used is the Sight Tube, a long cylinder with crosshair wires at one end and a peep-hole at the other. The Sight Tube is inserted in the focuser until the outline of the inside diameter of the sight tube appears to surround the outside diameter of the secondary mirror, and then is fastened tightly in the focuser with the setscrew. The Catseye TeleTube (pictured above) is adjustable in length to help you do just that, as well as guarantee the proper alignment without parallax-induced errors. If you can’t tell where the edge of the secondary mirror is, hold a piece of white paper against the inside of the tube opposite the focuser. This will surround the secondary mirror with a white background, making it easy to see where the edge of the mirror is. The goal is to make the outside edge of the secondary mirror appear concentric with the inside diameter of the end of the sight tube, as seen through the peep hole.
Start out by rotating the secondary on its center bolt until it appears round to the eye when viewed through the peephole of the sight tube. It may be necessary to reach inside the tube and gently grasp the outside of the secondary holder to do this. If the secondary is merely cemented to a stalk and is not in a holder, to avoid the risk of loosening the cement bond, you may want to slightly loosen the 3 collimation screws on the secondary mirror first so it is not so difficult to turn.
If the secondary appears too far up the tube, away from the primary, its center bolt will need to be loosened and the secondary lowered in the tube toward the primary, or vice-versa. Do this with the tube nearly horizontal to avoid the possibility of dropping a tool on the primary mirror.
Once the mirror is in the center in the up-tube/down-tube direction, it will need to be moved until it is centered in the side-to-side direction. This does not have to be exact, but if it appears oval, even slightly, rotate the secondary until it appears round. Then, carefully use the screws on the secondary to make the round image of the secondary appear concentric with the inside diameter of the sight tube. [This last assumes the secondary center bolt is already at the center of the tube, as measured from each side. If it is not in the center of the tube, you should adjust the spider vanes to center the mirror before you start on Step A.]
This will only have to be done once, though in initial collimation you may have to repeat it a couple times after doing the next step, so take your time to make it right.
If moving it sideways to center its outline in the sight tube makes the secondary appear non-round or oval, rotate the secondary on its center-bolt until it appears round again.
When you are done, the secondary will appear concentric to the sight tube, and round in outline.
See the following diagram of the appearance of a properly positioned secondary in a collimated scope as seen through the sight tube. This is what you will achieve:
Here is the same view with a slightly smaller secondary in the view to emphasize the lack of concentricity of the secondary mirror reflection’s outline to the other images. The dot is centered, and the sight tube’s crosshairs are removed to make it clearer that the crosshairs do not cross the image of the secondary reflection in exactly the same way as the reflected spider vanes. This is a slight exaggeration of the appearance through the focuser in the “New Model” collimation (more on that later), but serves to illustrate the secondary’s outline does not appear concentric in a properly collimated telescope:
B Aligning the Secondary mirror to the focuser axis: The tool used is the Sight tube, the point-source red laser collimator, or the Combination Sight Tube/Cheshire tool. The laser must itself be collimated or using it will misalign the scope. Many low-cost lasers come out of the box miscollimated. It is primarily for this reason I prefer the sight tube instead. Insert the sight tube and fasten the setscrew tight. Look at the crosshairs through the peep-hole and note the position of the distant center dot on the primary mirror*
[*if the primary mirror does not have a center mark, you will have to remove it and put one on the mirror. If you don’t know how to do this, have a local shop or astronomer friend help you out. You cannot successfully collimate without one. A transparent mirror center-spotting template such as the ones available from Catseye and FarPoint allows a home user to perfectly position the center dot to less than 0.01”, which is accurate enough for perfect collimation. If the mirror already has a center dot, it should be checked—some come misapplied and off-center as much as ¼”.]
Carefully adjust the collimation screws on the secondary until the center dot/mark appears lined up exactly behind the crosshairs’ intersection. What you’re doing in this step is to tilt the secondary mirror to point the reflected focuser axis directly at the center of the primary mirror. That’s why we’re adjusting the secondary mirror. This may move the round image of the outline of the secondary out of concentricity with the inside diameter of the sight tube. If so, repeat Step A and then Step B again. Each iteration brings the secondary closer to exact alignment. If you have to choose between having the center mark and crosshairs line up, or having the secondary be centered under the focuser, pick the center mark’s line up with the crosshairs. The centering of the secondary mirror in the focuser is only to provide even illumination of the image all the way around the edge of the field of view—less important than correctly adjusting the optical reflection of the secondary mirror with regard to the optical axis.
[If you have a lot of light in the tube, or the sky light illuminates the bottom of the Sight Tube’s crosshairs, a distant reflection of the underside of the crosshairs will be visible, but will appear a lot smaller than the near-to-the-eye crosshairs in the tool (which appear dark because they are not lighted on the side nearest the eye). When the telescope is collimated, this distant reflection of the crosshairs will be hidden behind the near-field crosshairs. This distant reflection of the underside of the crosshairs appears in the BEFORE image that follows.]
Some people have trouble focusing on the center mark and the crosshairs at the same time. It helps to use glasses in that case, or back the eye up far enough to allow both to be in focus. Even if the crosshairs are slightly out of focus, the diffraction put up by the crossing of the hairs creates a “dot” in the vision that can be lined up with a center mark that has a hole in it. If you back away from the sight tube, hold on to the scope or focuser with one hand—this steadies the body and the eye so you won’t be bobbing back and forth trying to see the image through the peephole.
If a laser is used, the secondary mirror’s tilt is adjusted until the laser beam hits the center of the Primary’s center dot. It should be noted that this is adjusting the secondary to the focuser axis, but it does not adjust either rotation or centering of the secondary relative to the focuser. To accomplish this, it’s still necessary to use a sight tube. Since adjusting the tilt may move the secondary off-center relative to the focuser, you may need to check again with the Sight Tube. This step essentially aligns the secondary to the focuser axis, so that moving the focuser in and out (as in focusing) will make no difference in collimation.
BEFORE ALIGNMENT OF CENTER MARK WITH CROSSHAIRS
AFTER ALIGNMENT OF CENTER MARK WITH CROSSHAIRS
AN ASIDE ON SECONDARY MIRROR OFFSET AND THE OLDER “CLASSICAL” VERSION VERSUS THE “NEW MODEL” FOR OFFSET
You may have read that the secondary mirror has to be “offset” in the tube in order to center the optical system on the mirrors and result in uniform illumination of the field of view in the telescope. This is automatically accomplished by following the collimation steps in this article, so the discussion of Offset that follows is an aside for those curious about how offset is accomplished, and how the two “models” differ:
If the focuser is not perfectly perpendicular to the tube, it doesn’t matter for collimation except that the optical axis may not EXACTLY coincide with the mechanical center-line of the tube. Indeed, this is one of the consequences of secondary mirror “offset” when using the “New Model” protocol (where the secondary mirror is offset from perfectly centered only in one direction--the up-tube/down-tube direction--to center the secondary mirror under the focuser wherever it points); and it can be when using “Classical Offset” (which entails moving the secondary away from the focuser slightly as well) if the focuser is not exactly “square” to the tube. In fact, if Classical Offset is used, the Optical Centerline and Mechanical Centerline can coincide only if the focuser is perfectly perpendicular (“square”) to the tube. With the New Model Protocol, the Optical axis will be ever-so-slightly tilted toward the focuser. This will make literally no difference at the eyepiece, but MAY make a difference to certain brands of digital setting circles (DSC) used in certain conditions. Note this offset is very small, and is less deleterious to the accuracy of a DSC than inaccurate centering of the alignment stars. Nonetheless, it’s real and may affect DSC accuracy.
The advantage of the “New Model” is that the secondary remains centered in the tube and appears centered under the focuser. Both can be achieved at the same time, and easily.
Here is an illustration with 2 different versions of “Classical Offset”. Note that the “New Model”, which is the technique described in this article, does not require this mechanical offsetting of the secondary, yet results in the same full offset when the scope is collimated. Since it is simpler to accomplish, it is the “New Model” which has been described in the collimation procedures in this article.
ILLUSTRATIONS OF FULL “CLASSICAL” OFFSET VERSUS THE “NEW MODEL” METHOD:
Note that in Classical offsetting of the secondary mirror it is dropped toward the primary mirror and moved slightly away from the focuser. This keeps the mechanical axis of the tube coincident with the optical axis of the telescope. The secondary will no longer be centered in the tube.
THE MODERN FULL OFFSET “NEW MODEL” COLLIMATION PROTOCOL (Exaggerated)
Note that in the “New Model” offsetting of the secondary mirror is only done toward the primary mirror and this results in a slight tilt of the optical axis away from the centerline of the tube. It is a tiny amount and *barely* makes a difference compared to Classical Offset. But it is much easier to accomplish for the user because the secondary mirror stays centered in the tube. Since both techniques result in full offset of the secondary mirror, yet one is easier to achieve, it is the New Model that is described in the text.
Aligning the Primary mirror: The tool used is the Cheshire eyepiece, the barlowed laser, the focused Krupa collimator, or the combination sight tube/Cheshire tool. A Cheshire eyepiece is either a cylindrical tool with a hole in the side of it and an internal 45-degree mirror to reflect light from the sky down onto the primary mirror and back or an even-simpler tool with a bright ring on the bottom and a simple peep hole on the other end (see the picture of Catseye Black Cat below).
Be careful when you insert a 45-degree Cheshire like the Tectron or most combination tools on the market (the type with an open window on the side) that you do not cover the side hole with the bill of a cap so the 45 degree surface can reflect the bright light of the ceiling or sky; for the simpler bright ring type (pictured), the eyepiece should be fully inserted. What this tool provides is a reflected bright ring with a dark center. We will use the collimation screws on the Primary mirror to move the reflected image of the center dot into the dark center of our bright ring of reflected light. It may be necessary, if a lot of movement is required, to repeat Step B for the Secondary mirror, and then repeat Step C. In a properly collimated telescope both steps will agree at the same time. If a combination tool is used, the crosshairs, center dot, and dark center and reflected crosshairs will all line up at the same time.
If a barlowed laser is used, the reflected red beam will have a dark shadow of the reflected center dot that you will center in the open hole at the bottom of the Barlow. This alignment is just as accurate as a Cheshire, and some would argue less intrinsically plagued by parallax—presuming, of course, that you can even see the bottom of the Barlow from the bottom of the focuser. I will not describe the Barlowed Laser Technique, here, even though it is as accurate as a Cheshire used properly because you cannot see the image on the bottom of the Barlow on-axis and because a typical Barlow does not enter the focuser deeply enough to have its bottom visible (except in large scopes with low-profile focusers).
When this step is done, the primary’s optical axis will be pointed at exactly the same point on the secondary mirror the eye sees the reflection of the primary mirror’s center mark. If a perfectly collimated single point laser is used, the return beam will accurately track the outward beam to its source. However, a single beam laser can return its beam to the source with errors in secondary placement and tilt, so if a laser is used, the Barlowed laser technique is necessary for primary mirror alignment.
If you have been careful, any Newtonian will be well collimated at this point in the procedure, but, for the users of f/5 scopes and shorter, or for the users of longer focal ratio scopes who want absolutely perfect collimation, some improvement can yet be made:
Eliminating all residual errors with an Autocollimator (see the picture of the Catseye Infinity above): No, it doesn’t do the work for you, despite its name, but it allows you to see tiny misalignments of the mirrors and correct them to a high degree. On short f/ratio scopes, where the allowable tolerances on miscollimation are in the thousandths of an inch, such a tool is essential. This tool is so sensitive to misalignment that simple mechanical flexure will be visible (i.e. in the focuser, tube, mirror spider, primary springs, etc). If your secondary spider vanes are too loose, the telescope will not hold collimation to a few thousandths through all the altitude changes the telescope goes through, so the first thing to do is to tighten the screws that hold the secondary spider to the tube. Be careful! Though they should be quite tight, most of them are small screws that can be stripped if applying too much force. Most commercial scopes come with these screws quite loose, however, and tightening is almost always called for. Be careful that tightening the spider screws does not start to pull them through the tube. I had to add fender washers under my spider’s screw heads in order to get the spider vanes tight enough. If the spider is tight enough, you will not see any collimation changes with a change of altitude of pointing on the scope’s tube.
The Autocollimator is inserted and tightened in place. This is a tool with an internal flat mirror that faces the secondary mirror. As you can guess, when alignment is achieved, there is no light in the peep-hole and the whole field goes dark. Because 4 reflections of the mirror’s center dot will be visible against this darker background, you are essentially collimating the telescope at multiple focal lengths of your telescope. If done carefully, collimation will be accurate to a small number of thousandths of an inch—accurate enough for even an f/4 telescope. This accuracy rewards the observer with the best possible images that can be had with the scope.
When looking in, there will be 4 images of the primary’s center mark, each one slightly fainter than the next. The 4th reflection is often difficult to see if you do not have enough light in the tube. This is why I recommend using a daylight sky as your collimation “target” when doing initial alignment.
If your previous steps were carefully done, these 4 images will be overlapped or nearly “stacked” on top of one another. But, if all 4 are seen, better alignment can be achieved. By carefully moving the secondary collimation screws, the four reflected images of the center mark will stack tightly on top of one another.
Two of them disappear (if a triangle is used the ones that point the opposite direction than the brightest and sharpest image, so you will see only one triangle when all images are stacked).
It is imperative to return to the Cheshire at some point and check alignment of the primary. If it’s off, correct it in the Cheshire (turning the primary collimation screws), and return to the autocollimator. After a couple times back and forth, all four images of the center mark will “stack” tightly on top of one another, and your telescope will be perfectly collimated. Tiny changes in secondary tilt affect the primary’s optical axis, so it’s essential you go back and forth between the autocollimator and Cheshire. Both tools have to agree at the same time to achieve collimation.
AFTER STACKING—NOTE HOW THE DOWN-POINTING TRIANGLES HAVE DISAPPEARED
The next time the telescope is taken out, Step A will not need to be done. It’s probably not a bad idea to check the alignment of the secondary with the sight tube, but it is unlikely that more than a minor tweak of one screw will be necessary. Then, proceed to Steps B and C. If you use an autocollimator, skip Step A and start with Step B. The autocollimator will perfectly align the secondary in Step D.
The final step of stacking the 4 images of the center mark can be made easier if the “Carefully Decollimated Primary” (CDP) protocol is followed. In essence, when the images are nearly stacked, one collimation screw on the primary mirror is purposely loosened to decollimate two of the 4 stacked images, i.e. they swing away from the “stack” into the outer parts of the field of view in the autocollimator. The two that remain nearly stacked are easily stacked perfectly with minor adjustments of only the secondary mirror’s collimation screws, while the two separated images are brought into the stack by adjustment of only the primary mirror. This technique is described in great detail on the web at:
[Go down the page to VIC MENARD'S CDP PROTOCOL to see a description of this process with pictures]
MECHANICAL IMPROVEMENTS TO HELP HOLD COLLIMATION FIRMLY:
All your efforts will be for naught if the mechanical structure of your scope is incapable of holding the collimation (you’ve achieved) as the scope changes in altitude or attitude. If you notice a change in collimation with the movement of your scope, here are some things to check:
Mechanical tightness of the secondary spider. Since the secondary hangs below the spider when the tube points up, and beside it when the tube is horizontal, this off-center weight is quite able to impart a twist to the secondary’s spider vanes, which causes changes in collimation. To fix this one, make sure the spider is VERY tight (it should emit a high pitch if tapped with a finger). If tightening the spider vanes this much would cause the spider attachment screws to pull through the tube, then install large diameter fender washers under the heads of the screws to distribute the force over a larger area.
Mechanical movement of the primary mirror. If the spring stiffness is insufficient to hold the primary mirror in place, then the primary mirror may tilt forward when the tube points at low altitude. Some telescopes compensate for weak springs by installing locking bolts that hold the mirror in place once collimation has been achieved. The problem with these bolts is that tightening them will change the collimation, so final collimation must be done with the locking screws rather than the collimation screws. I strongly recommend eliminating the locking screws and changing the springs to very stiff springs capable of holding the weight of the mirror without movement. Not only will this make collimating easier, but it will insure that your collimation will hold. Of course, for any spring to be able to hold the mirror in place, it should be compressed nearly all the way as well as be very stiff on its own. Springs like this are available at a lot of better hardware stores or from McMaster-Carr online.
Sag in the focuser. Needless to say, if putting an eyepiece into the focuser causes the focuser drawtube to change angle, then collimation is immediately off. At the very least, the focuser should be adjusted to provide as close to zero “slop” as possible. A good focuser will not change the collimation as it moves in and out. If yours does, then an overhaul or replacement of the focuser is called for. Quality focusers (as of this writing) start under $100, so if you intend to keep your scope for a while, this expenditure is quite justifiable.
There are several other mechanical issues that affect collimation: flexure in the tube or truss tubes, temperature changes that cause contraction of mechanical parts, insufficient rigidity in mechanical attachments, sag in mirror cells, focuser drawtubes not cut or drilled accurately, etc. But I won’t go into all of them here because the first three points seem to represent 99% of the mechanical reasons why collimation changes in a scope.
A NOTE ON LASER COLLIMATORS AND CATSEYE TOOLS:
Because there are only a couple brands of lasers that come to the purchaser collimated, I do not recommend the use of a laser unless you are willing to assume the responsibility of collimating the laser once it arrives. Even if collimated, the stock single-beam laser is only truly effective at collimating the secondary mirror to the focuser axis. A simple Cheshire is more effective at achieving primary mirror alignment because the return beam of the laser is too large (and unlikely to have hit the center of the primary with sufficient accuracy to make the return beam precise enough) to allow the same precision of alignment as passive tools [special exception: the Barlowed Laser Primary Alignment not described here, which is as accurate as the Cheshire tool previously described]. And since the sight tube is necessary to initially align the concentricity of secondary mirror and focuser, I fail to see the necessity of a laser in collimation unless you have no choice but to set up in complete darkness. Even then, a bright red LED flashlight aimed at the primary mirror’s center mark will make collimation possible and easy with the three passive tools described above.
There are some people who have great difficulty focusing on the near-field crosshairs of the sight tube and the primary center mark at the same time. They find the use of a laser easier. Indeed, if you have a truss-tubed scope, you can get your eye close enough to the primary to see the beam hit the mirror from up close, and that will greatly increase the accuracy of your positioning of the laser beam dead center in the primary’s centermark, but if you have a full-tubed scope you cannot do so. Plus, most lasers’ beams do not hit the mirror with a tiny point, but have beams shaped like an oval or dash. Estimating the center of the primary mirror’s centermark to better than 1/50th of an inch (and even less if you wish to use the return beam of the laser for collimation) is just, simply, highly unlikely with such a beam.
If you have an XLK autocollimator (a newly designed AC with 2 holes in it for different points of view of the stacked images—see illustration below) then either method (passive or laser) will get you close enough to refine the alignment of the secondary-to-focuser axis closely enough for the AC to eliminate residual errors. But if using a standard, single pupil, autocollimator, I still prefer to start by using the passive tool (sight tube) instead of a laser. If you choose to use a laser, start with one that has a small beam spot (small aperture stop) and will require no collimation on your part to be useful as a collimator—at this writing, Howie Glatter and FarPoint lasers satisfy that requirement.
Catseye collimation has recently introduced an upgraded line of collimation tools with a smaller peep-hole (only 2.5mm (0.099”), in order to enable a better centering of the eye when looking through the tools (to reduce parallax--the change of viewpoint when the eye can shift back and forth when looking through the tools, which can give different perspectives and make exact collimation more difficult). These tools, labeled “XL” are a little easier to use for those of us with small daytime pupil diameters and who don’t wear glasses to focus at the distance of our mirrors from the eyepieces.
Outwardly, they are similar (though the Black Cat XL and Infinity XL are a bit longer and insert a little deeper in the focuser), but they do function a little differently. I, as one user, have found the new XL tools easier to use and achieve perfect alignment with. I find the Infinity XL (autocollimator) requires me to do less fiddling when stacking the 4 reflected images of my primary’s center mark because the TeleTube XL and Black Cat XL get the alignment a little closer before I switch to the Infinity XL to eliminate the residual errors.
The autocollimator (Infinity XL) has been improved even further with the addition of a second hole to allow viewing the 4 stacked images as two stacks of 2. It is called the Infinity XLK. This tool is a step up in accuracy from the standard autocollimator, and I recommend it. I find that what appears as a “perfect” stack of the 4 centermark images in the standard autocollimator can be refined even more by using the XLK version.
Here is a picture of the images of the “stacked” center marks in the two pupils of the XLK autocollimator, courtesy of Ghassan “Jason” Khadder, whose influence inspired the XLK tool:
You may find (I certainly do) that stacking the two sharp images on the left in the offset pupil of the 2-pupil tool is easier to do than to achieve a perfect stack in other ways. This eliminates any residual FAE (focuser error). If you use the 1-pupil autocollimator, you will see the center image when you are done.
I wholeheartedly recommend the Catseye XL tools if purchasing them new from the maker.
I repeat that I have no commercial interest or business association with Catseye Collimation. I simply recommend them as the best passive tools currently available, whichever version you may use. Their usage will result in collimation to tolerances previously unachieved and which have become a necessity in the current market of super-short f/ratio scopes.
Donald E. Pensack
Los Angeles, CA