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Old 09-10-2020, 06:05 AM
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PKay (Peter)
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Coma Aberration - Persist or desist?

After spending another few hours testing my 'home made' behemoth,

I have reached a point of turning it into a shaving mirror.

Even after installing a coma corrector, it just made things worse.

The collimation was set using a laser. Didn't help much...

Guiding was atrocious (it was a little windy).


Should I persist in trying to get it to work, or bite the bullet and buy something decent?

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Old 09-10-2020, 07:46 AM
glend (Glen)
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Looking at your image it seems you have Coma in the centre, which is unusual for traditional Coma produced by the mirror, but is fairly easy to produce in Cassegrains if the Primary mirror is not perfectly aimed. Can you provide some details on your scope: aperture, focal length, mirror source, etc.?
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Old 09-10-2020, 07:54 AM
PaulSthcoast (Paul)
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Hi Peter,

My thoughts are yes !

Last night wasn't the best night, I was out for a few hours as well.

Wind was on the wild side, even with my small refractor.

I'm sure it would have given you more than a challenge ?

Keep your patience and choose your times and I'm sure it
will come together for you.

Plenty of support and advise to be had from other members
I'm sure !

Paul.
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Old 09-10-2020, 08:13 AM
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PKay (Peter)
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thanks for responding Glen & Paul

Aperture: 305mm
F/L 1500mm
Primary mirror has been re coated.
Sec. mirror is new.

I am starting to think:

1: There is too much play in the focuser.
2: There is too much flex in the truss/ tube.
3: The whole rig is just not suited to high accuracy astro photography.

In essence, trying to push 'ship' uphill...
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Old 09-10-2020, 09:53 AM
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ChrisV (Chris)
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I reckon there's also some tilt that's making it hard to correct coma - as the coma differs across the corners. If you could get the tilt + coma good then focusing would end up a lot better. I had similar issues with my newt. I spent a few nights adjusting tilt with the coma corrector in. I eventually got there (well almost). I just kept adjusting tilt, taking a shot and looking at the corners to see what changed in which direction. But I suppose you have to be sure collimation is good before starting all that

But I suppose the remaining issue would be - does the scope flex etc. If so, then it will change the aberration depending on where you scope is pointing
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Old 13-10-2020, 08:36 AM
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mental4astro (Alexander)
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Peter,

Of course I am not familiar with how you built your scope, nor the pedigree of the optics (so I am assuming the primary is a good one), I have a couple of questions first.

How is your primary mirror held in its cell? Not glued into place I hope at any point.

Did you make the primary's cell too?

My thinking is not necessarily a coma issue. My thinking is two possible sources from the way the aberration in the stars shows itself.

My first thought from the image you've presented suggests a pinched primary mirror. Either the primary is squeezed in by the cell from the sides, but more from overtightened clip or other retention system being employed. If the primary has any glue holding it in place, there is your source of strain.

If coma is "worse" rather than improving, this suggests distorted optics. This is because the light is not going where it is supposed to if the optics were not under strain. Nor is the distribution of the aberration seen in the star a uniform pattern of being evenly distributed radially. The strain in the mirror is not evenly distributed, and this shows in the way the aberration comes off the stars - the primary mirror is not sitting passive in its cell. And it really does not take very much force at all to distort a primary by overtightening the clips! It is one thing I have come to learn very quickly.

Tilt has been suggested by Chris could be the second cause, to which it could be from the focuser not set square, or flex in the OTA, not the optics. If the focuser is square, then the collimation process fine tunes the optics, eliminating tilt. This could be a cause for persistent aberration, which again is not coma per se, but from a more fundamental misalignment. However, this does not necessarily explain the appearance of the aberration pattern we are seeing. If the OTA is a solid tube that has been perforated along its entire length, this could be introducing torque into the OTA because of the way Newts are loaded unevenly across their length - heavy primary, the coupling point to the mount, the focuser and camera assembly, and then it is rotated across three axis during its use meaning the loads are shifting and the way the OTA is experiencing the loads changes as does the way it flexes. Flex is diabolical and because we are talking about wavelengths of light, it does not take much. One easy we to figure out tube flex is to check collimation with the scope in many different positions across the sky. If it is flex then the collimation will change when the OTA is pointed to different directions, like to zenith and shifting the OTA from one side in RA and then over to the other side in RA. A laser may be enough to expose this flex. A perforated solid tube can be a good idea, but it needs to be done with consideration of the loads at play with the thickness of the tube.

My first suspicion remains with the primary mirror being under strain. I have seen the same thing with my scopes. The strain still allows you to collimate the scope, but when focused, the image of the stars is distorted, and not evenly across the FOV. It is the same as a poorly figured mirror - you can collimate it and achieve the same collimation quality as a finely figured mirror, but the final image reflects the poor figure. In this case it is showing the primary mirror is under strain. The primary must sit passive in its cell and not have overtightened retention clips or source of strain. Last point also relates to the cell's build in case the cell is not pushing evenly in its contact points on the primary. This can mean that the primary is under strain from flexing over the cell. These are all aspects that need to be examined, not just tweaking the collimation screws and cussing the coma corrector.

Alex.

Last edited by mental4astro; 13-10-2020 at 06:29 PM.
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Old 13-10-2020, 10:54 AM
sunslayr (David)
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I was having a problem with coma recently and I wasn't able to solve it until I disassembled my imaging train. Turns out the 0.1mm spacer I used to align the OAG with the chip was enough to give significant coma in the corners. Any chance the coma corrector spacing is making it look worse than it is?
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Old 13-10-2020, 02:51 PM
garymck (Gary)
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You can't collimate a newt just with a laser.....It needs to be properly set up with sight tube, a cheshire or barlowed laser, and ideally with an autocollimator..........

FWIW
Gary
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Old 13-10-2020, 03:25 PM
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strongmanmike (Michael)
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I'm with Alex, triangular stars tends to point to astigmatism, could also be caused by how the secondary mirror is attached too..?

Mike
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Old 13-10-2020, 06:15 PM
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mental4astro (Alexander)
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Quote:
Originally Posted by garymck View Post
You can't collimate a newt just with a laser.....It needs to be properly set up with sight tube, a cheshire or barlowed laser, and ideally with an autocollimator..........

FWIW
Gary
I completely agree, Gary!

A laser will not do it all. It is only a tool for part of the collimation process, and not a mandatory or necessary one too. Very handy though.

The final tick of approval for collimation shouldn't be left to the laser either. A star test is the final check. Professional observatories will use lasers to tweak collimation, but the final check and tweaking is done by star testing.
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Old 14-10-2020, 02:08 PM
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PKay (Peter)
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Bloody hell, you guys are really into it.

And I mean that in a nice way :-)

Thank you very much for entering and the invaluable input.

It will take some time to make sense of this information.

In the mean time, the behemoth is hanging from the workshop roof and I am looking at a Planewave 12.5 CDK...
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