ZWO ASI 2600MC - Coma Corrector Spacing - CCDInspector

[Spaced Out]

Hi All

I hope someone is able to offer me some advice here. I am excited to have a new camera but I am having a bit of trouble spacing it with a coma corrector.

Equipment is:

ZWO ASI 2600MC

SW Aplanatic Coma Corrector

SW 130PDS

I was having trouble with non round stars around the edge of the frame. After lots of faffing with spacers I downloaded a 30 day trial of CCDInspector to see if that might help. The best result I managed is shown below, 0.4mm either side of this spacing seems to make the curvature worse.

Looking at the test image, it’s not terrible but you can see the star shapes are a little misshapen in the corners (worst on the left side), this is also reflected in the CCDI charts.

My old camera, the ASI 1600, spaced really easily with this coma corrector and gave nice round stars to the corners, in CCDI it showed 8% curvature, this 2600 is sitting at 23%.  

So...

I’m wondering if my star shape issues are down to the larger sensor of the 2600, maybe it is just harder to correct ? Or perhaps it is not CC spacing, maybe it is tilt as suggested in CCDI ?

Has anyone else had similar issues with the ASI 2600 and spacing coma correctors ?

Any experienced CCDI users able to tell me how bad these charts really are please, is 23% curvature very bad/visible in images ?

Thanks

[teoria_del_big_bang]

I am no expert but to me I would say it has some tilt.
 

Top right look pretty round.

Bottom right look fairly round

 

Top Left look  a little elongated that could be caused by incorrect BF from CC

 

And bottom left also look a little elongated.

This camera is a fair bit heavier than the 1600 so with a stock focusser could cause tilt (not that I know you have a stock focusser).

Steve

Edited April 10, 2022 by teoria_del_big_bang

[alacant]

On 10/04/2022 at 19:27, Spaced Out said:

non round stars

Hi

Does your 130 hold collimation at all tube angles? We've never seen one which did out of the box.

A good test is to inspect frames either side of a meridian flip. A gpu and a heavy camera at that distance from the tube, with the primary loose in its cell  makes loss of collimation inevitable, especially with the supplied focuser and mirror cell. Both easy fixes.

Cheers

 

Edited April 11, 2022 by alacant

[Spaced Out]

17 hours ago, teoria_del_big_bang said:

I am no expert but to me I would say it has some tilt.

This camera is a fair bit heavier than the 1600 so with a stock focusser could cause tilt (not that I know you have a stock focusser).

Steve

Thanks Steve. Yeah I think maybe tilt is contributing most to those star shapes, the CCDI charts seem to support that too, both text and the colour changes (it looks a bit off centre). The ASI 2600MC setup is actually quite a bit lighter than the old 1600, but only because the 1600 imaging train included a heavy EFW/filters and OAG/guide camera etc. I think the 2600 is actually about 100g lighter than my current DSLR, so I thought the stock focuser should cope with it OK. I will have a fiddle with the focuser and see if I can tension things up a bit better to improve the tilt.

I am still concerned about the field flatness, 23% seems the best I can get, is that down to the larger sensor being more difficult to correct or could a fair chunk of that be down to tilt ? 

Hmmmm..... 🤔

[R1k]

Is this CC correct for your scope? My understanding is that it is only for f4 newts

[Spaced Out]

29 minutes ago, R1k said:

Is this CC correct for your scope? My understanding is that it is only for f4 newts

Yes, the CC should work at f5, I had the TS version before the SW one and it worked great with the 130PDS, that was with the smaller ASI 1600 sensor tho, so perhaps less to correct.

TS blurb reads....

"Ideally suited for Newtonian telescopes with focal ratio f/4 - delivers very good results from f/3.5 to f/6"

[alacant]

9 hours ago, Spaced Out said:

concerned about the field flatness

It looks as though you're too far. We use 53mm cc to sensor distance with our 130pds'. This corresponds to the correct focal length of 650mm.

Unless the prescription has changed, the GPU needs the following spacing, depending upon focal length:

https://www.bintel.com.au/product/skywatcher-f4-aplanatic-coma-corrector/

 

Edited April 11, 2022 by alacant

[Spaced Out]

2 minutes ago, alacant said:

It looks as though you're too far. We use 53mm cc to sensor distance with our 130.

Unless the prescription has changed, the GPU needs the following spacing, depending upon focal length:

https://www.bintel.com.au/product/skywatcher-f4-aplanatic-coma-corrector/

 

Thanks for the info, it's useful. I was actually at 53.2mm, any movement either way and the curvature % in CCDI just got worse. 🙁

I think I'm going to try and improve the tilt (which I'm guessing may be down to slack focuser tensioning) then revisit this test again. Tbh if CCDI says 23% curvature I could live with it so long as the stars look better/rounder than they do atm !

 

[alacant]

1 hour ago, Spaced Out said:

slack focuser tensioning

You don't say what focuser you have, but assuming it's the original, then we'd recommend removing rubber the 'O' rings; a recipe for flexure under load if ever there was one.

But hey, we're looking at bottom of the range gear. Unless you're looking for perfection and you're going to perform all the necessary modifications, I'd stay as you are. The stars look fine to my eyes.

 

[Realtimedoctor]

1 hour ago, alacant said:

But hey, we're looking at bottom of the range gear. Unless you're looking for perfection and you're going to perform all the necessary modifications, I'd stay as you are. The stars look fine to my eyes.

 

Couldn't have said it better. 

The truth is you are looking at gear from the bottom of the food chain, they are designed and sold at the price to give decent/great results for the price. However, you can't expect them to outperform more highend scopes. 

Your stars look just fine(as a matter of fact, pretty great), unless you are aiming for APODS, I'd not frett about it. 

If you do decide to invest in something, I'd get a new focuser. I myself have been waiting on a Badder steel track for my 200p. In the meanwhile, the small amount of minutely elongated stars, don't bother me. 

Get your backfocus right, getting the tilt out of your system can become very expensive very quickly. 

Cheers, 

Nish 

[symmetal]

CCDI Curvature figure is a bit of a mystery to me. The CCDI help just quotes

Curvature: percent defocus between lowest and highest defocus points on the map

This is rather vague and no simple manipulation of the FWHM figures stated arrives at this percentage figure so I generally ignore it and just look at the FWHM variations across the image. Your 3D image indicates slight tilt, but a change in FWHM from 3.38 to 4.40 is pretty good in my opinion.

If you want to correct the tilt make a wooden test jig like these, which are fairly cheap to make and it can be done indoors during the day. That should improve your FWHM variation too. Don't try and adjust tilt on the scope looking at stars. That way madness lies. 🙂

Alan