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Savage234

Advice adjusting coma corrector

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So I have been out of the hobby for a while. During college I got burnt out having to set up/pack up every night/morning. I am now able to set a scope up for as long as the weather allows and am getting back into the hobby. 

I set everything up the other night just to troubleshoot any tracking/mount/alignment problems before attempting to actually get images and noticed that the stars come out distorted. They look more like triangles with rounded tips. I messed around with the back-focus on my Baader Rowe Coma Corrector F/3.5 -F/6 but couldn't seem to get the distortion to go away, it got worse/better but was still there.

My scope is an F/3.9 and has worked in the past with this coma corrector. (I unfortunately did not mark the back-focus that it was set at ? ) 

Since my scope is so close to the limit of focal length do I need to adjust the back-focus rather drastically from the recommended 36.5mm (for a total of around 96mm of back-focus)? The directions say "adjust +- 3mm if necessary". So will I need to adjust to ~39.5mm or ~33.5mm? Is there a rule of thumb like; A higher f/ ratio = more back focus,  or vice versa? 

Thanks for any assistance/advice

 

Edited by Savage234
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Knowing what I know about coma correctors- it is going to be a case of trial & error. Going through the steps- f3.9 is a fast scope, is the collimation spot on? Check this before doing anything else. Then try different back focal lengths. I have found that the manufactures recomendations are usually not that far off give or take 1mm.

 

 

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I am using the same Rowe corrector on an f4 Newt and get pretty tight rounded stars over the whole CCD.

The corrector back focus distance should be around 96mm as you say. This is from the last lens to the imaging chip surface so is plenty for my CCD, filter wheel and OAG, with enough to spare that I need an extension tube to make up the difference.

From your description, I would look to collimation first or possibly there is some astigmatism present. I have never experienced it, but possibly pinched optics...

Good luck.

 

Gordon.

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Thanks for the responses ( I was not notified via my email so I didn't realize there were responses)

Last night I started at the -3mm low end the company advised and worked my way up to the +3mm half MM by half MM. Around 36 it seemed to be best (just about where it says it should be) , but the stars were still very triangular. I figured I'd just forget the corrector and try taking pics without it, and that's when I realized that the distortion was there without the corrector as well. 

The secondary mirror is straight, scope is culminated with a laser culmination tool and there are no light leaks. As far as i can tell my scope is working fine. Tonight I am going to image with another reflector (with and without the corrector) scope and a small refractor to try and determine if it is my primary reflector or possibly my camera (its a very old DSLR). Although, I don't see how camera damage could cause this kind of distortion. 

The pinched optics problem came to mind after I sat on the problem for a while. But where would the optics be "pinched" in a reflector? Nothing in the corrector is tightened any more than necessary (and the mirrors are aligned). 

I tried uploading a picture but the upload kept failing ?

I'll post an update after I troubleshoot with other scopes tonight.   

Thanks!                

Edited by Savage234

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Slight update: I took apart the primary mirror of my main scope and re-tightened everything. The culmination adjustment screws were nearly completely tightened, I think that is what was causing the "pinching" on the mirror. I took some test shots to see how it looked and it seemed okay, tonight I am taking ~3 hour image to see how it looks when a few hours are stacked. Didn't get a chance to test other scopes, bad weather, though after seeing the difference after fixing the tightness on the screw I am confident it is not a camera issue. 

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