9 replies to this topic

[Itz marcus]

Hi,

Does the "HOM" need to match the star collimation results or can they be off from each other and the scope still be ok?

If the in focus collimation is perfect but the "HOM" is off does that indicate that something is off, say mechanically, and that the scope, although collimated, is not giving the best images possible, or is it possible for the scope to not be perfectly aligned mechanically but if collimated will give the best images possible?

Clear skies

Itz

Edited by Itz marcus, 18 May 2025 - 04:12 PM.

[JMP]

I had a C5 some years ago, I thought the corrector was off center because when I measured with a caliper it was offset from the physical center of the tube about 0,040" (forty thousandths). I thought I would "fix" it by moving the the secondary to the center of the tube. I actually made the view worse!

 

So I went through a process. I would collimate on a star, then the "hall of mirrors" view was noticeably off. So then I would move the secondary position to correct the HOM. Then I would touch up the collimation on a star. After a few iterations of this process I was able to have good collimation and a good HOM view at the same time. The Cassini Division showed up much better for example after I did this. I wound up with the secondary about 0,040" from the physical center of the tube, just about where the factory had placed it.

 

So I think it does make a difference. I think this is the kind of difference you got when your friend re-aligned the optics on your C8 a while back.

[Asbytec]

Here's a good read. Secondary offset - Don't Trust the Donut.

 

Basically, the nature of spherical mirrors is such that they do not have to be physically centered on each other which seems to be the case with HOM. They simply have to be tilted into collimation. The condition for collimation is the secondary and primary radius of curvature lay on the corrector optical axis. This is why the secondary can be tilted in an SCT. The secondary in an MCT is permanently collimated since both the meniscus and the secondary share the same radius of curvature and the primary can be tilted. That is not to say a good looking HOM is not collimated. It can and will be. It's simply the rare special case where the mirrors are physically aligned and collimated with the corrector. 

 

See the upper right image of an SCT that is not centered but is aligned.

 

[quilty]

I think an all spherical scope might show an imperfect HOM with a perfect star pattern.
Not quite sure of this and don't see why the HOM then can't be made staight and concentric.

I don't think that a CC or RC HOM can be imperfect at perfect collimation.

Apart from this the HOM doesn't display an astig (not completely sure neither) which the star does.

Pretty sure a perfect HOM (straight and concentric) is possible only at perfect star collimation

Edited by quilty, 18 May 2025 - 06:38 PM.

[philipdo]

Let me tell you how I proceed with my Skymax 180. I always collimate during daylight (no fiddling in the dark, no need for tracking...) on the sun's reflection off the glass insulators on a distant power pole. I start with the HOM and try to get it as perfect as I can. Then I inspect the Airy disk and the first ring through the eyepiece at high power (700 X). Once the Airy disk is perfectly centered in the first ring, I go back to the HOM which now appears slightly decentered in one direction. The dark CO-spot in the expanded doughnut is also slightly decentered. When the Airy disk is centered in the first ring, collimation is perfect and can be assessed by sharp images at insanely high powers (up to 1200 X without image breakdown). I've tried inducing miscollimation on purpose to see if the process and the results are repeatable and they are. I always end up with a perfect Airy disc and with the same slight decentering in the HOM. There can be many (mechanical) reasons for this: the primary baffle not squared tot the mirror surface, the primary mirror not perfectly centered in the OTA, decentering of the meniscus.... In my case the secondary baffle is not perfectly centered on the reflective secondary spot. The deviation is minute but enough to slightly disturb the HOM. I've learned to live with this and only trust the Airy disk and first ring. The HOM however gets you very close to perfect collimation. Mind you, this is with a Chinese Mak: very good to excellent optical quality but rather lousy mechanical execution. In this respect I suspect with the Russian Maks and their better build quality a perfect HOM would equal perfect collimation.

 

CS, Philip

[quilty]

Thinking twice now yields:
Star collimation has to match the perfect HOM. When the HOM is straght and concentric there's no way the scope's out of collimation,
Eyepiece and focusser excluded.

Maybe that's the pro and reason for the star finish: Tiny ep misalignments will be sorted out.

Edited by quilty, 19 May 2025 - 06:12 AM.

[Asbytec]

I don't think that a CC or RC HOM can be imperfect at perfect collimation.
 

I believe CC and RC, etc., each component has an optical axis centered on the figure of revolution that needs to be aligned with each other. Like a Newt with a primary optical axis at the center of a parabola, good collimation results in centered mirrors (with the exception of the diagonal flat).

 

Relying on centered and properly tilted mirrors to align optical axes seems to be the basis for Ocal collimation. I believe this is why Ocal can get an SCT or MCT without a perfect HOM close to collimation, but we still have to finish on a star.

 

In the special case of perfectly aligned mirrors and corrector, a perfect HOM will result in perfect collimation. But spherical mirrors have no discrete optical axis. Is a perfect HOM one that looks very good? You'll know how good it is when you star test it.

 

If you force an offset secondary shadow to the center, you can induce coma in the field center. (The image below, I believe, was from an AI collimation software company). 

 

 

This is the beauty of spherical mirrors being part of two larger reference spheres with their centers located on the corrector optical axis. Spherical mirrors can be displaced slightly along the surface of the reference sphere. So, when properly tilted they are always part of those reference spheres collimated on the corrector.

Edited by Asbytec, 19 May 2025 - 07:11 AM.

[tturtle]

The problem with using the HOM method is that people do not set it up with the necessary amount of precision. In order for it to be accurate you have to set up a peephole at precisely the same height as the optical center of the OTA and the OTA has to be level with the peephole something like 10ft away.  Any sloppiness in this setup will result in less than a perfect alignment and it goes without saying that you cannot just stand in front of the OTA and “eyeball” it.  I aligned my 925 in the daylight using a very precisely setup up peephole as described and there was no further collimation needed when I did a star test.

Edited by tturtle, 20 May 2025 - 06:54 AM.

[dweller25]

I have definitely achieved perfect high power optical collimation with a wonky hall of mirrors.

 

Interestingly I found - by luck - that by rotating the primary I finally achieved perfect optical and HOM collimation.

Edited by dweller25, 20 May 2025 - 08:27 AM.

[quilty]

The problem with using the HOM method is that people do not set it up with the necessary amount of precision. In order for it to be accurate you have to set up a peephole at precisely the same height as the optical center of the OTA and the OTA has to be level with the peephole something like 10ft away.  Any sloppiness in this setup will result in less than a perfect alignment and it goes without saying that you cannot just stand in front of the OTA and “eyeball” it.  I aligned my 925 in the daylight using a very precisely setup up peephole as described and there was no further collimation needed when I did a star test.

I did without any peephole