Not all telescope autocollimators are equal.
Monday, 22 October 2012 | Admin
Ghassan 'Jason' Khadder is well known for his contribution to the design and development of telescope collimation tools. He recently posted detailed instructions at the Stargazerslounge astronomy forum on how to assess the performance of a telescope autocollimator. With his permission we have his full post here in our news section. If you are considering whether to invest in a Catseye autocollimator or one of the cheaper clones you will find his advice useful:
All autocollimators have an eyepiece case, a mirror, and all show multiple reflections of the primary mirror center spot. That does not mean much. A true autocollimator has to meet a high standard of quality and precision. If it lacks precision then it will not provide any additional collimation improvement beyond a mass produced laser collimator.Here is a detailed criteria to evaluate any autocollimator:
1) How many center spot reflections can you see with reasonable clarity? Four is excellent. Less than four is poor. Note that two of the center spot reflections might look fuzzy. That is OK. The degree of fuzziness is dependent on vision acuity and the scope focal length. The smaller the focal length the fuzzier two of the reflections. Seeing the fourth center spot reflection will enable a procedure called “CDP” introduced by Vic Menard. CDP will make it easier to align the focuser axis with high precession.
2) Rack the drawtube until the autocollimator mirror is close to the primary mirror focal plane. Unstack the center spot reflections until you see all four reflections then rotate the autocollimator in the focuser without tightening any focuser setscrew. Test it at 90, 180, and 270 degrees. To validate the test, rotate the autocollimator 360 degrees and check – repeat few times. If reflections stay close to their original location after each 360 degree rotation then this test is valid. Do reflections shift significantly at 90, 180, and 270 degrees compared to their original locations? If yes then the autocollimator quality and precision is poor. It is almost impossible for any autocollimator to pass this test with 100% accuracy. This is a highly sensitive test. Small shifts by around ¼ the diameter of the center spot is acceptable. However, if the shift amount is larger than a full diameter of the center then the autocollimator is of poor quality. This test checks the squareness of the autocollimator mirror against its casing and it checks for registration errors of the autocollimator with respect to the focuser.
3) Look for distorted center spot reflections. If some of the center spot reflections are noticeably distorted then that is an indicated of poor autocollimator mirror flatness. If some of the reflections have a slightly different size but otherwise are undistorted, that is OK. Size difference is an indication the autocollimator mirror is located away from the focal plane.
4) When the scope is collimated and the autocollimator background reflection is dark, how dark is it? The darker the better. Gray’sh background is an indication of poor autocollimator mirror flatness and possibly poor reflective surface.
5) How centered is the central pupil of the autocollimator case? Off-centered pupil will introduce errors.
6) How large is the central pupil? Around 2mm is good and reasonable. Larger pupil hole will introduce parallax errors.
7) How large is the non-reflective area around the central pupil from the autocollimator mirror side? This is important. If this area is large then a measurable error will be introduced. The reason is somewhat complicated but I will try to summarize it. A large non-reflective area around the central pupil will cause two of four center spot reflections to disappear prematurely. As all four center sport reflections start merging, two of the reflections will quickly and prematurely disappear and you will be left with two reflections. Stacking two center spot reflections is far from being acceptable. You need to stack a minimum of three reflections.
8) The reflective area of the autocollimator mirror has to be free of artifacts – especially around the central pupil. Again, the reasoning is somewhat complicated but I will try to summarize. As reflections merge, if the reflective area around the central pupil has rough surface or poor reflection then the background will suddenly brighten reducing contrast and two center spot reflections will potentially disappear.
9) How large is the reflective surface? In general, 2” autocollimator should be used when possible. The large reflective area will make it much easier to see all center spot reflections. For 2” autocollimators, it is desirable to have thinner case walls and more reflective area.
10) Is it a single or dual pupil autocollimator? Not only the dual-pupil autocollimator provides more accuracy but it will also catch errors explained in items 7 & 8 in this list.
As someone who evaluated multiple Catseye autocollimators, I can attest that all have passed the above outlined criteria.
Anyone who owns an autocollimator regardless of the vendor can evaluate their autocollimator based on the above.
Jason also added some comments regarding the way some autocollimators are marketed:
I have spent a significant time studying the autocollimator reflections and how best to interpret them. I continued the excellent autocollimator studies published by Nils Oolf Carlin and Vic Mendard. The autocollimator is an excellent collimation tool providing: