Is something wrong with my reflector?

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Slavic Dragovtev avatar
I purchased a coma corrector for my ProNewton but I am not sure I'm getting better results. In fact, although on the left side of the image it seems the situation is slightly improved, on the right side it's making it definitely worse. Do I have backfocus problems? Is my mirror out of collimation? Is there anything else that you can see in the sample images?
I've made sure that there is no considerable tension from the holding screws on the primary mirror: the little rubber holds can be moved with pinky-finger kind of force.

I should, probably, add that the coma corrector is an Omegon as well. and the camera is an APS-c Canon EOS 250D.

Image one: without coma corrector
https://drive.google.com/file/d/153LIteNemmGRzdTgX_-y9HG9Em0Fjmrd/view?usp=drive_link

Image two: with coma corrector
https://drive.google.com/file/d/1G88FguHPuENST5szf2T6rAeC5gufPaPC/view?usp=sharing

Would be thankful if someone more experienced can shed some light.
Engaging
Aaron H. avatar
The uncorrected image doesn't look symmetrical - the sharpest area is to the bottom left - and the larger stars appear to be de-focused.

I'd target collimation first. The coma corrector can just make matters worse if your collimation is off.

To get accurate collimation, you need to ensure:
  • Correct secondary mirror placement.
  • Correct secondary mirror alignment.
  • Correct primary mirror alignment.

Stepping through the tools and process used for each of these will help work out where improvements can be made.

For context, Astro-Baby's guide is a great resource on how to achieve all this with inexpensive tools. At f/6, this should be sufficient.
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Slavic Dragovtev avatar
Ok, so my secondary is definitely not concentric with the focuser. It's too low inside the tube, I believe, Will investigate and update this thread with findings.
Tom Marsala avatar

Agree with Arun…That would be my first check…get the secondary right underneath the focuser and concentric with it as you look into the focuser. Get yourself a good Cheshire collimator and use it to sight down the focus tube. Then finish up collimation. Focuser being squared with the tube is also important. It seems there may be some focuser tilt because of the out of focus stars on the left, as Arun pointed out. Fix those big issues first.

Tom

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Slavic Dragovtev avatar
Back with updates.
I have centered the secondary in the OTA using the veins screws (it was almost visibly off).
I have raised the secondary to be in the center of the focuser (although it's not precision engineering, it definitely seems more center in a collimation cap now). 
I have re-collimated both the secondary and the primary using 3d-printed collimation and Cheshire caps.

This is a test without the coma corrector. Out of focus.


This is a test with the comma corrector, out of focus. I am only seeing the lack of protrusion of the focuser, as a difference.


This is a test without the coma corrector, in focus (or thereabout).

And, finally, a test with the coma corrector, in focus. At least the center area. It doesn't seem like an improvement to me.


Also, thanks for the good responses. The AstroBaby link is a treasure.
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Aaron H. avatar
It looks like you're getting there.

Are you using an extension tube between the camera's T-Ring and the coma corrector? If so, what length? If it is off, you will get that result.

For the GSO coma corrector (which the Omegon is a version of), you should require a spacing of 75mm from the end of the coma corrector to the camera sensor. Your DSLR will have 55mm already with just the camera and T-ring. So you need another 20mm.

If you're using an M48 T-ring, that means a 20mm extension tube between the T-ring and the coma corrector. If it's a T2 T-ring, then the M48 to T2 adapter that comes with the coma corrector will take up some of the space (eyeballing it, it looks like it is around 5mm, which would leave another 15mm for extension tubes).

At f/6, you probably don't need to be too precise. Within 5mm will likely get you close. So a set of extension tubes in 5mm, 10mm and 20mm lengths will probably be all you need to experiment and find the best spacing. A set with a few more options (e.g. 3mm and/or 7mm) would give you even more options.
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Slavic Dragovtev avatar
O-k, NOW we're getting somewhere. For some reason, I considered that the 75 mm spacing requirement was a "at least 75 mm". So, I acted upon that assumption. It' 94.6 mm currently 

I'm using a pretty long T2 to 2" adapter, which has threads on the inside of the telescope side, which fit the CC (I'm guessing those are M48 threads).
Aaron H. avatar
Slavic Dragovtev:
O-k, NOW we're getting somewhere. For some reason, I considered that the 75 mm spacing requirement was a "at least 75 mm". So, I acted upon that assumption. It' 94.6 mm currently 

I'm using a pretty long T2 to 2" adapter, which has threads on the inside of the telescope side, which fit the CC (I'm guessing those are M48 threads).

That explains it.

The coma corrector has M48 threads. So you just need to get a way to achieve 20mm of spacing from the coma corrector to your camera T-ring. The best bet would be using a shorter T2-M48 adapter (the coma corrector should have come with one), and some M48 extension tubes between the coma corrector and the adapter.

Ideally, just get a set of M48 extension tubes in varying lengths. They're not expensive, and they will allow some experimentation. While the stated distance is 75mm (20mm between the camera's T-ring and the coma corrector), and this will usually work perfectly, sometimes the sweet spot turns out to be a few millimetres off what is stated.

I'd also avoid an "adjustable" variable-length tube. They've always introduced tilt for me.
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Slavic Dragovtev avatar
Ordered in. Will update. I did do some experiments while I'm waiting tough, since I had another clear night. I removed the spacer entirely, only using the very short T2 adapter that came with the CC, thus going form too much distance to too little distance. The results are weird, because although the stars on the edges are better looking, there is now a heavy round glow in the middle of the image. 
Aaron H. avatar
The centre brightness can come from many sources. 

It could be a problem with calibration frames (or a lack of them), reflection, dew… I'd get the spacing sorted before worrying too much.
Slavic Dragovtev avatar

So, my spacers came in a week ago and yesterday I got to testing them. Attached is a small stack showing improved results across the whole field with slightly worse performance on the top right.

I am yet unsure whether I should try to mess around and try to improve more or not.

📷 bubble_stack.jpgbubble_stack.jpg

andrea tasselli avatar
You have way to go still, keep at it.