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Old 04-12-2009, 02:32 PM
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Star Elongation begone!

Well not quite..

There are stars elongated from top-right to bottom-left corners appearing and I'm not sure why. the main stars look round; but fainter stars are elongated right through the image but only from one corner to its opposite crnr.

I've seen this both with and without the focal reducer.
I've checked weight, balance, changed PHD settings.

I'm thinking maybe this is focuser tilt? But how could this be?

I turned tracking and sidereal off and took a 2min exposure to see if it was drift, but the stars drifted from bottom-right to top-left; the opposite of the direction of these elongated stars.

I'm running VC200L; ST80 piggybacked(DSI Pro guided).

When I slewed the controller in DEC and RA; the stars on the screen would move parallel NSEW, so the camera is straight.

Has anyone come across this before ?

http://i611.photobucket.com/albums/t...s/ngc-2516.jpg

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Old 04-12-2009, 02:52 PM
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That's weird indeed. You can eliminate a few things though. It's not your guiding and your camera/focuser is square. What's left is the optics. It doesn't look like it's coma. I've never see coma only on opposite corners and not the others. I have no idea what it could be. Astigmatism maybe? But then again it should show in other parts of the picture and pretty much all your stars are in focus. Something's not straight somewhere but hard to tell. Slight mirror misalignment maybe? (as in optical axis not matching)
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Old 04-12-2009, 02:54 PM
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Read this post.
http://www.iceinspace.com.au/forum/s...ad.php?t=53356
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Old 04-12-2009, 03:07 PM
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Hmm read the other post.

But if it is focuser tilt, I'm baffled why it causes this affect from one corner to the other; opposite the motion of star drift.

P.S I guess one option is to play around with the focuser tilt screws, and see what changes occur.
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Old 04-12-2009, 04:03 PM
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Quote:
Originally Posted by leinad View Post
Hmm read the other post.

But if it is focuser tilt, I'm baffled why it causes this affect from one corner to the other; opposite the motion of star drift.

P.S I guess one option is to play around with the focuser tilt screws, and see what changes occur.
The other post was a problem I got by taking the camera off the focuser and putting it back. Something was not square. In your case I don't think it's a tilt because usually you get the whole side of the field good and the opposite elongated. In your case it's like one diagonal is ok the other elongated.
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Old 04-12-2009, 04:21 PM
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Maybe the scope needs collimation,
Looks like coma showing?
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Old 04-12-2009, 05:25 PM
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This looks like out of focus combined with astigmatism to me (pinched optics?). Could be collimation as well, but not likely, as the effect goes diagonally all the way through the image.
I had this problem with cheaper lenses..
Also with my MTO1100-A, before mirror relaxing exercise.

I do not think this is guiding.. guiding was OK. If it was, you would have the problem everywhere on the frame. This is a problem with optics.
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Old 04-12-2009, 07:22 PM
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In this image from the same night, it can be seen throughout the image.
http://s611.photobucket.com/albums/t...nt=canopus.jpg

Ok it probably isn't focuser tilt. I checked my old photos and it's still there; this leads me to believe the mirror may actually be pinched too tight; either by the mirror clips; or by the collimation push/pull screws.

I guess I could start by adjusting the push/pull screws.. loosening them up and imaging at same time to see what affect it has.

Then if nothing changes; pull the mirror out, and loosen the mirror clips. Guess I should give the mirror a quick bath at the same time.
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