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help needed with this image


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Hi,

Last night I tried to image M51 again to see how it would go with my recently purchased orion mini guider.

I also had my ST80 mounted which makes it to heavy for my mount, but I wanted to test it.

Only one sub looked reasonable and even in that sub there were some weird looking stars.

I have a coma corrector in place and my PA is acceptable.

Does anyone know what the reason could be of those weird looking stars in the corners of the sub?

I really would like to improve my subs, I am getting more and more frustrated not finding the reason of this..

Thanks!

post-24961-0-22424800-1367152042_thumb.j

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nothing wrong with the image it needs more data to reduce the noise, depending on how big your chip is in the dslr the coma corrector on covers so much of the chip ( i think) you could just crop the outer 5% of the image and it would be fine

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Thanks for responding. I never realized the posibility that my 2" CC could miss some areas of my eos 1100d chip.

Thats probably because I have seen so many images, from other people, where all the stars were perfect.

(hench assuming that I have some drift in my system, maybe I have but this gives me some more insight..)

Is there a way to figure out what area the CC covers of the chip?

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The diagnositc for coma is elongated stars at the edges of the frame pointing outwards from the centre (especially the corners because the are furthest from the optical axis). Yours mainly looks to be on the right side of the frame, not the left:

- Check the spacing from the CC to the face of the sensor in your camera to make sure it is as close to optimal as possible. You can usually find the optimal distance for a given CC online or in the instructions. If you are too close or too far away, you won't get the optimal correction. Even so, with a large chip like a DSLR, you may not be able to get perfect correction right in to the corners.

- If the coma is worse in a given direction (like yours is) it may be because the chip is off-centre from the optical axis, so you'd end up with more distortion on the side furthest from the optical axis. Hard to deal with that problem since you can't really move the chip closer to the centre.

- You can use a flat to get some idea of the coverage of your CC. If you stretch it in some image processing software you should be able to see a clear circular vignette showing the positioning of the CC relative to the scope. (May be helpful to take one without the CC and compare it).

- If you can freely rotate the camera with respect to the CC, or the camera and CC with respect to the scope, you may be able to frame your target so you can crop out the worst effects.

- Equally, it might be that the camera chip and/or the CC is slihgtly tilted relative to the optical axis. This may happen if the focusser tube is drooping under gravity or if you haven't got things screwed together properly. (E.g. I had a problem with my Flattener/Reducer after I added a new LP filter in to the mix; quickly discovered that the ring fixing the filter in to the holder wasn't screwed in properly and everything was tilted by about 0.5mm off centre as a result).

- Check all your screw fittings, etc. to make sure they are OK. I found a cheap digital caliper was really helpful as I can check that everything is square to everything else when putting my kit together, or maybe make some kind of 'feeler gague' that you can slot in to the gaps between rings and adapters, it is usually pretty easy to tell if there is a significant difference between once side and the other, but hard to tell just by looking.

- Try to minimise the effects of gravity on the focuser/camera train. If you can arrange the OTA so that the camera is pointing downwards in to the scope when you are on target that might be better that having it off to one side, though it would be a pain on a GEM as the ideal position would change through the night.

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excellent write up, thank you.

A number of the things you mentionned I did checked and some are new to me so thats good :)

So I am going to check those aswell.

Will post a new image with the corrections when I am able.

thanks again.

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Ian's guide is excellent and I don't have anything to add really. I might be able to help eliminate a couple of things, though!

I have the same scope and corrector; I find that the corrector covers the sensor no problem. Also, the corrector and M48 adapter are designed to give you perfect spacing, so I doubt this is the problem either. So, some sort of tilt or other miscollimation seems to be the most likely cause.

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I discovered that my collimation was way of, this is a bit weird since I check it often.

I corrected it and it seems a bit better.

The image attached is made with a different camera orientation so the coma is on a different spot.

In this case there was a lot wind so it didnt turned out really well..

I also added a flat file and I think its just a bit off..

post-24961-0-01259100-1367511034_thumb.j

post-24961-0-56561400-1367511290_thumb.j

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