Odd star shapes at edges of image

[Andyb90]

Hi all,

I've taken some 5 minute guided subs with my 80mm ED refractor, with a 0.8x reducer on the end. I have a standard T-adapter screwed into the reducer and I attach that to my 600d.

I'm getting some odd star shapes at the edge of my images. I've created crops of the corners, which show the stars.

Does anyone have an idea of what could be causing this? The star shapes in the central part of the image seem okay. Can post a central crop too if it would help.

I thought maybe it is field rotation, but the scope focal length with reducer is only 400mm. This coupled with only 5 minute subs I didn't think field rotation would be a problem.

Also in my last session I drift aligned using the DARV tool in APT. I used 5 minute exposures, checking both az and alt. I couldn't see any hint of a v shape with the star trails, so I think my polar alignment was pretty accurate.

Top left corner

Top right corner

Bottom left

Bottom right

[Thalestris24]

Hi

Is that the SW .85 flattener/reducer you  have? It looks like you still have field curvature which suggests a spacing problem.

Louise

Edit: Sorry, just realised you have Altair Astro kit, doh.

[michael8554]

The Altair site says the DSLR T2 adapter needs to be 10mm thick to give 55mm spacing.

Michael

[Uranium235]

As mentioned above, it could be a spacing problem. But it also depends on how well matched the reducer is to your telescope. Not all reducers suit all telescopes, thats why (when possible) its better to go for a reducer that is specifically designed for that telescope (the Altair is a bit of a "catch all", so it probably never will be perfect). And its probably unlikely that its designed for a corrected field large enough to cover an APS sized chip (you need to find some specs).

For now you will have to fiddle with it slightly, or failing that - crop the image. With your sensor size I guess you can afford to lose a certain percentage of the frame.

Usually the case is:

Coma distortion: Spacing too short

Radial distortion: Spacing too long

You could be talking about sub-millemeter accuracy in spacing, so try some delrin spacers - they work quite well.

[ollypenrice]

I think ths scope should handle the chip when all else is right. I've used it with a 15mm square chip.

Olly

[Owmuchonomy]

To exclude some potential causes take some higher ISO shorter duration subs, say 20 seconds of a star field. If it is still apparent then its curvature/spacing or related matters and not field rotation.  It looks like the former to be honest.  Let me know because I have considerable acute experience of your particular dilemma.

[Andyb90]

Thanks for the responses.

I'll try some high iso short exposures, to see if I get a similar effect.

I'll order the delrin spacers too. I see FLO have them.

I'm using a celestron T ring for canon eos. It looks to be between 10 and 11mm thickness.

I did think the lightwave reducer was matched to my scope, but after doing a bit more research online I found a very similar (if not identical) looking orion 0.8x reducer. Also to be fair it doesn't specifically state on the Altair website that the reducer is matched for my scope. Looks like it's a general reducer for refractors.

If I have no luck is it worth trying a more premium reducer, the Televue trf 2008 0.8x, for example? I guess there is no guarantee, but I'm thinking Televue would hold its value if sold on.

Andy.

[ollypenrice]

To my eye this is not trailing due to rotation, but optical. The stars are not neat lines made by steady rotation and otherwise OK. They are distorted with an in and out of focus zone for each. I, too, will bet on spacing.

Olly

[Andyb90]

Hopefully if its spacing and a bit too short the delrin spacers will work.

If the spacing is too long would something like this work ?:

http://www.teleskop-express.de/shop/product_info.php/info/p2463_TS-Optics-ultrashort-Adapter-from-T2--M42x0-75--to-Canon-EOS-EF-Bayonet.html

I think its a low profile T-ring. I'm guessing I could then add something like a short extension tube and the delrins to get the correct spacing.

Andy.

[Waldemar]

Rotation is out of the question because you use an equatorial mount...

A reducer is not always also a flattener. Because the distortion in the stars all relate to the middle of the image, this is definitely curvature: a spacing problem or a mismatch between scope and reducer or just a reducer... not a flattener.

[ollypenrice]

Rotation is out of the question because you use an equatorial mount...

A reducer is not always also a flattener. Because the distortion in the stars all relate to the middle of the image, this is definitely curvature: a spacing problem or a mismatch between scope and reducer or just a reducer... not a flattener.

This would only be correct if polar alignment were perfect. If there is a PA error and the guidestar is fairly central to the main image then the main image will show fairly symmetrical rotation. The rotation becomes more asymmetrical when the guide star is off axis. I saw this quite recently when one of our fixed mounts must have been disturbed somehow. The guidescope is on axis and the rotation in long subs was symmetrical.

However, I still think, as you do, that this is a spacing issue.

Olly

[Waldemar]

This would only be correct if polar alignment were perfect. If there is a PA error and the guidestar is fairly central to the main image then the main image will show fairly symmetrical rotation. The rotation becomes more asymmetrical when the guide star is off axis. I saw this quite recently when one of our fixed mounts must have been disturbed somehow. The guidescope is on axis and the rotation in long subs was symmetrical.

However, I still think, as you do, that this is a spacing issue.

Olly

Thanks Olly, a good look at the pictures shows a specific distortion of the stars in all corners:  out of focus and with emphasis on pointing to the middle of the image.

Can that be rotation? In my humble opinion rotation always points to a side, not to the middle, but maybe I am wrong about that?

[ollypenrice]

Thanks Olly, a good look at the pictures shows a specific distortion of the stars in all corners:  out of focus and with emphasis on pointing to the middle of the image.

Can that be rotation? In my humble opinion rotation always points to a side, not to the middle, but maybe I am wrong about that?

I don't normally worry about aligning my guidescopes on axis with the imaging scope but with our dual rig, for reasons I don't understand, it seems to be necessary. (I may be wrong about this but it seemed to be true during testing.) So when the dual rig mount suffered a bump we found circular trailing (rotation) building up in long subs. This disappeared when we reset the PA.

Now I notice you have a DDM so probably don't guide? If your PA is out and you're not guiding then I guess the behaviour of your trailing will be different from that of a misaligned mount being corrected by a guider.

Olly