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Imaging - strange star shapes


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Hi,

Can anyone advise on this problem I have. I'm using a Canon 450d at prime focus, with a Skywatcher Coma Corrector, on my 250mm f/4.8 Newt. I've noticed that the stars on the bottom of my images have a 'partial eclipse' appearance. This image shows it. The first is a single full-frame shot, and the second is a detail from the bottom right:

lukebl-albums-luke-s-help-images-picture8622-image1.jpglukebl-albums-luke-s-help-images-picture8620-image1b.jpg

Any idea what's going on here? I wonder if something is obscuring the light path somewhere, or maybe I need a better Coma Corrector. Perhaps if I rotate the camera 90 degrees that might change things. Does the angle of the camera make any difference. Unfortunately, there have been so few clear spells for me to test the alternatives.

Edited by lukebl
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I may be wrong, but I think it is the focusser tube extending into the main tube.

Not sure. I've looked at some images I took last year and they don't have this problem, so I don't think that is the reason. On the other hand, I think the camera was oriented differently in the focuser then, so maybe if I try the camera in different positions it might correct the problem.

Trouble is, I've had so few opportunities to do any imaging at all for the past few months due to the weather that I think I've lost the knack!

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I had this problem with my 150PDS. I removed +- 15mm from the end of it(the focus tube), which sorted it out.

I think it would be more noticeable in a smaller diameter, like the 150PDS, and less in your 250PDS. I'm sure different camera rotations would also make it invisible at times.

And the weather hasn't been nice at all. Seems the forecast for the next two weeks will be better though.

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In the words of Homer Simpson: doh....doh....doh....doh....dohhh!

I'm big enough to admit when I've been stupid, particularly if a lesson has been learnt and it might help others. I've realised that the strange star shapes were caused by my makeshift cardboard dewshield not being aligned correctly and blocking part of the light path. Tried some imaging this evening with it off, and the stars are perfect!

Lesson learnt! here's the silly dewshield:

lukebl-albums-luke-s-help-images-picture8066-my-dewshield.jpg

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In the words of Homer Simpson: doh....doh....doh....doh....dohhh!

I'm big enough to admit when I've been stupid, particularly if a lesson has been learnt and it might help others. I've realised that the strange star shapes were caused by my makeshift cardboard dewshield not being aligned correctly and blocking part of the light path. Tried some imaging this evening with it off, and the stars are perfect!

I've been getting these kind of star shapes on random imaging sessions and thought it might of been my dew shield being too long and the end was interfering with the light path.

I too imaged last night, only for an hour so I left the dew shield off and now all the stars are good.

It's so good when the light bulb comes on and it's a simple inexpensive solution ;)

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