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Anyone identify this imaging issue?

samtheeagle

By samtheeagle
in Imaging - Discussion

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samtheeagle

samtheeagle

Posted
Posted

Hello all,

Well finally I saw some clear sky on Saturday night! As it was a rush setup job I decided to try and get some unguided images of M45. All in all it went pretty well, but I've noticed some issues with the subs I've been capturing, during this and other sessions... The example below comes from the top right of one of the subs, as you can see the stars seem to be flaring towards the centre somewhat.

post-16299-133877406635_thumb.jpg

My question is what might be causing this? Poor guiding? I think not, as this issue seems to be worse on the right hand side of the image, particularly at the top. It's evident to some extent at the edges elsewhere, but nowhere near as bad.

Other guesses: Collimation, I guess this might be an issue on a refractor. Or coma? General incurable lens imperfection? Any ideas?

This was using my C102 4" refractor btw ;) Thanks in advance...

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steppenwolf

steppenwolf

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It would be useful to see a full frame (reduced for posting would be fine) to see what is going on - field curvature could be at play here but it would be unusual for it to be offset. If it is offset then either collimation or sensor misalignment could cause this.

Can you post a full frame at, say, 800 pixels wide so we can analyse the field for you?

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Sam

Sam

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I agree it looks like it's not lined up properly, I used to get this with my 8" scope and the canon. The focuser had a bit of movement when the camera was hanging of it and the grip screws on the focuser didn't help - maybe that's worth checking on yours too.

Sam

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jgs001

jgs001

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Sam, I had horrendous focuser slop on my Vista 80s. It was so bad, that with a 2x barlow and a webcam, the target I was trying to focus on would shift out of view... not fun... I looked into replacing the focuser with a crayford, but as that would involve paying nearly twice what I paid for the scope, it didn't seem worth it. I found some teflon tape (self adhesive on one side) and stuck it in layers, built up till it fit on the drawtube.. then in tests, using a 5x barlow and the webcam, I saw virtually no slop when adjusting focus. It appears the online shop I bought from is no longer trading, but I did find

oven liner, mouse mat, teflon tape, teflon tapes, uhmw tape, mouse mats, baking sheet, zone tape, oven mesh, baking sheets

I know nothing about them, just found them.

HTH.

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jgs001

jgs001

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I actually removed the draw tube altogether... Fixed the tape, in several layers, where the felt runners inside the focuser are. I guess you could use some ordinary tape to build up, then the teflon tape on top to finish... I just did layers of teflon, till it fit... didn't even think of using other tape first... or you could, I guess, wrap the drawtube in a couple of layers of sticky back plastic then the teflon tape in strategic places... This is beginning to sound more and more like a Blue Peter episode ;)

Exactly how far you need to pack it out will depend on how much slop you have.

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