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  #1  
Old 06-11-2011, 08:48 AM
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peeb61 (Paul)
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Help with coma? - UPDATED.

Hi All,
I was wondering if anyone could give me good advice on how to fix this problem.

Up until now, I have had good success using my QHY8L with my current set up which is...

Bintel f/4 200mm Newt, QHY8L with Baader Coma Corrector with I assume, the correct spacing from chip to the back of the CC of 56mm and an Orion Deep Sky Filter on the end to curb light pollution in my area.

Just recently the images taken are showing Coma? on the edge of field, see attached. (This is a 1 minute exposure at -15 taken last night.) I mean would this be coma or bad collimation? The scope was collimated a while back with the Cats Eye and I will get it checked again as soon as possible.

The cropped image is taken from the bottom left hand corner.

Many thanks for looking.

Paul
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Last edited by peeb61; 19-11-2011 at 06:41 PM. Reason: update with a cropped image
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  #2  
Old 06-11-2011, 09:38 AM
adman (Adam)
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That doesn't look like coma. The elongation of the stars is all in the one direction, unevenly distributed and not radial. Is most likely an unsquarely seated MPCC that makes the sensor not parallel to the image plane or collimation issues. Have a look at your MPCC in the focuser to see whether it looks square, or better still have someone (or yourself) run one of the subs through CCD Inspector - gives lots of good info and cuts out the guesswork

Adam
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  #3  
Old 06-11-2011, 01:56 PM
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peeb61 (Paul)
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Thanks Adam for getting back to me, it is much appreciated.

I have ran an image through CCDInspector and this is one of the outcomes, I am not so sure how to read it but it doesn't look good but it is pointing me in the right direction.

Does this mean, obviously the image is not flat so the Coma Corrector must not be in the correct spot??

Paul
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  #4  
Old 07-11-2011, 04:07 PM
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Looking at that image isn't easy to see what is actually happening, its too far zoomed out so to speak.

From the CCDIS image it
A. Looks like you have a decent amount of tilt on the imaging plane
B. Coma in the corners is struggling.

So look at
A. the seating of the MPCC in your focuser. If you are using the standard focuser, it is probably not really upto holding the load for AP accurately.
B. the Baader MPCC isn't really made for a F4 type of correction in saying that it will kind of work but just not perfectly so work with the spacing you may need to increase the spacing from the sensor you may find a middle ground of under over and properly corrected image plane.
C. Look at a wynne corrector...
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  #5  
Old 07-11-2011, 04:53 PM
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Thanks Brendan,
You have made a lot of sense, the standard focuser is just that...standard as a starter goes, it has two retaining screws, maybe these are pushing the imaging plane out.

As for the Baader....I will try the 'push in and pull out' and see if this can be fixed if not then I will look into this Wynne Corrector.

Many thanks
Paul
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  #6  
Old 07-11-2011, 09:03 PM
adman (Adam)
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Yep - I'm no expert at interpreting CCDInspector results, but to me that looks like sensor tilt and coma thrown in for good measure despite the MPCC. Try and find some way of getting it square and work on the corrector spacings.

Adam
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  #7  
Old 09-11-2011, 03:39 PM
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the wynne corrector will scare all but the most seasoned or hardend astrophotographer they start somewhere around the 1000 dollar mark
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  #8  
Old 09-11-2011, 07:22 PM
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Thanks Brenda, I did notice that and it was a good job I was sitting when I found out! I managed to dig out a test shot I did of the Jewel Box when I first got the camera, going off this shot I think the distance from the chip to the Coma Corrector might be okay? So I think I may need to check the collimation and tilt of the camera.

Paul
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  #9  
Old 09-11-2011, 07:27 PM
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what are you using to collimate the scope?
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  #10  
Old 09-11-2011, 08:03 PM
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Brendan, at first it was the off the shelf laser but now its the Cats Eye.
As previously mentioned this has to be rechecked, f/4 scopes and faster, as you know are renown for losing collimation.

Paul
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  #11  
Old 09-11-2011, 09:04 PM
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well not loosing collimation but they are a little bit fiesty.

Be really really sure that your stacking the right reflection as i can barely see the correct one with my 4.7 at f4 it gets even harder! bright light pointed dead on at the hot spot will give you reasonable results.!

Brendan
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  #12  
Old 10-11-2011, 08:57 AM
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Thanks for the tips Brendan, a mate of mine is coming over on the weekend to lend a hand. (It is of course his Cats Eye setup) so we will the check the tilt at the same time, it is always good to have that second set of eyes.

I will post what i find here.

Paul
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  #13  
Old 12-11-2011, 06:46 PM
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Just an update on my little problem.

I have checked the colimation with the Cats Eye (Thanks Charlie) and found it was out a bit, took the focuser apart and gave it a good clean out and a bit more grease, got it running nice and smooth, checked the distance from the Chip on the QHY8L to the back of the Coma Corrector and made sure all was square in the focuser.......Now for the star test...before I can do that I need clear skies!

Paul

Last edited by peeb61; 12-11-2011 at 07:47 PM.
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  #14  
Old 12-11-2011, 08:20 PM
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Good luck Paul , let us know how it goes

Adam
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  #15  
Old 19-11-2011, 04:08 PM
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Hi All,
Thought I'd give an update to my woes. I have revisited the colimation using the Cats Eye (Thanks Charlie) and it was out a bit, all adjusted including the secondary, the camera in the focuser looked plum' and we did a test image last night of NGC 1365, this is a full frame (Just reduced to upload) 5 minute shot at -15 and apart from the obvious, noise etc I'm still having troubles with the star shapes.

I'm now thinking...distance from the chip to the coma corrector? Because I have ruled everything else out.

Any ideas??

Paul
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Last edited by peeb61; 20-11-2011 at 01:57 PM.
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  #16  
Old 19-11-2011, 04:15 PM
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Can you give us some crops of the 4 corners and the centre - its a bit hard to tell what the stars are like on that image

Adam
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  #17  
Old 19-11-2011, 05:02 PM
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Thanks Adam for your time,
I have uploaded as requested, four corners plus center. I hope this is much clearer.

Paul
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Last edited by peeb61; 19-11-2011 at 05:19 PM.
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  #18  
Old 19-11-2011, 05:17 PM
adman (Adam)
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Sorry should have been a bit clearer - can you do some 100% or bigger crops of the original image. Your stars don't look too bad on those, but there's not enough pixels to be sure

Adam
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  #19  
Old 19-11-2011, 05:19 PM
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Sorry Adam,
I did realise what was going on and was working on new uploads and now I'm having problems uploading. Stay tuned.

Paul

Adam, I hope these are better.
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  #20  
Old 20-11-2011, 09:15 AM
adman (Adam)
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Well, coma-wise I don't think they look too bad, so youve probably got your corrector distance nearly right. There is still something going on there though that shows up more in the mis-shaped stars in one corner, and my best guess would be collimation - but it would be only that - a guess.

Can anyone else chime in and have a look at these?

Adam
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