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Just been looking at some of my images have noticed a common theme in all of them. The stars all have a nibbled corner, this is across the whole field of view and always in the same direction.

I'm using a skywatcher 150p with a 1000d DSLR. Could it be scope collimation? The focuser tube protruding? I have to rack the focuser in almost all the way to achieve focus.

Here is a close up example to demonstrate.

Your thoughts and advice are very much appreciated.

post-18298-133877436753_thumb.png

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It could be collimation - the pic suggest your primary mirror is not aligned.

It could also be some kind of obstruction in the tube. Does the 'nibble' align with where the focuser is ?

How does collimation look on the scope ?

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Yep, AB's got it I reckon. Looks as though your focuser is impinging into the lightpath. The reason could be very poor collimation, but I think it is due to the dslr sensor being too far away from the image plane. You probably need a low profile focuser if you are going to be doing a lot of prime focus imaging with the DSLR.

Ron.:(

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Yup - I should have said there may be other reasons which are specific to imaging -0 thats outside my know how.

If I saw that visually I would say primary collimation is out of whack OR theres some kind of obstruction.

Thanks Ron - I wouldnt have thought about imaging stuff causing it.

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Would the collimation need to be totally way off for this effect to happen, or would a slight misalignment cause it?

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I assume these images are taken slightly out of focus? In which I'd agree with above posters...

No the images were in focus.

Here is the image from which the star above is a close up crop.

4441295521_c4d8a946fe.jpg

Edited by johnrt

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Yep, AB's got it I reckon. Looks as though your focuser is impinging into the lightpath.

Same here. Next time you're on an imaging session, make a note of where your focus point is and then look how far the focuser tube goes inside the OTA. Try having the focuser fully racked out and take a couple of test shots, I reckon your problem would disappear.

Tony..

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Here is a pic of how far the tube protrudes in to the scope body at the point where the dslr achieves focus.

Looks like a low profile focuser for me then.

post-18298-133877436883_thumb.jpg

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I'd agree - if it were a collimation issue as well/instead of a small tweak to the primary should do it but you'd need a Chesire to be sure.

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its a partial eclipse.

ha, erm, i found that when taking afocal images through my 8inch newtonian (namely with a digital camera or mobile phone) then i had similar results. i think in my case that it was because the lining up of the imager with the eyepiece hole wasn't quite perfect i.e. the sides of the imager or focuser unit were interfering with the light path.

that's the only thing i can say.

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Here is a pic of how far the tube protrudes in to the scope body at the point where the dslr achieves focus.

Looks like a low profile focuser for me then.

Or a Hacksaw and an extension tube for when you want to use EP's? :(

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Or a Hacksaw and an extension tube for when you want to use EP's? :(

EEEK!

I'm liking the idea of a low profile focuser though, as it will give me some extra travel for a coma corrector or filters at a later date.

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Ah. Just noticed the skywatcher focuser is quoted to fit Newtonian scopes from 9 - 16 inches. At 6 inches, it isn't going to fit my 150p.

Back to the drawing board.

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Just been to telescope house where the very friendly chaps sorted me out with a laser collimator.

The collimation was out, the primary mirror was much further out than the secondary - the laser dot was half way across the bullseye on the collimator after I had aligned the dot in the center of the primary mirror.

Pic before any adjustments

4487155048_63d29662b1.jpg

Fingers crossed this will help!

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I think replying to your own posts may be he first sign of madness, but in other good news the collimating of my scope has fixed the problem!

2, 5 minute subs of M51 and not a nibble in sight.

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