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Old 16-06-2008, 03:35 PM
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Is this field rotation. coma or what?

Attached is a single image from Sat nights run of the Keyhole via the C9.25 suing the 0.63 reducer.

Stars in every corner appear to be elongated as if the frame has turned slightly. Any ideas what causes this? Will field rotation show up like this from guiding on a star that is not in the field of view.

There is some "slop" in the canon t-ring adaptor that I though might contribute, but I tape this up to keep it steady.

Anything else
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Old 16-06-2008, 03:52 PM
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I think it is focal reducer.
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Old 16-06-2008, 04:26 PM
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Quote:
Originally Posted by bojan View Post
I think it is focal reducer.
Aah, hadn't thought of that. Not that it's a big concern it's only a small amount to have to crop out... but good to know why it's there.

I'd have a lot less image if I didn't use the reducer so that's ok with me
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Old 16-06-2008, 04:38 PM
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Yep, I'd have to agree with master bojan. While you may read that you can use a 6.3 meade/celestron reducer with DSLR and SCT the realities are that both those reducers are really meant to be used with smaller chipped CCDs. It's not easy to get a flat field with an SCT across the full width of a DSLR chip.
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Old 16-06-2008, 04:44 PM
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Quote:
Originally Posted by Robert_T View Post
There is some "slop" in the canon t-ring adaptor that I though might contribute, but I tape this up to keep it steady.

Anything else
Why have you got slop in the t-ring adaptor ?
You know there's a little grub screw in there that you should be able to tighten - at least there is in mine.
Check it and see if it's a little loose.
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Old 16-06-2008, 05:32 PM
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Quote:
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Why have you got slop in the t-ring adaptor ?
You know there's a little grub screw in there that you should be able to tighten - at least there is in mine.
Check it and see if it's a little loose.
The slop is actually in the canon EOS mount fitting into the camera... poor workmanship maybe
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Old 16-06-2008, 09:03 PM
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Wild guess check your reducer and see if the lens cell is tight and secure, my Meade example comes loose sometimes and I'm sure I had some similar symptoms one time after a nights imaging
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Old 17-06-2008, 10:22 AM
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I would have thought field rotation. If it was the focal reducer, I would have expected the star to be elongated away from the centre, rather than in arcs around the centre.

Last edited by Geoff45; 17-06-2008 at 06:20 PM.
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Old 17-06-2008, 10:29 AM
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I would have thought field rotation. If it was the FR, I would have expected the star to be elongated away from the centre, rather than in arcs around the centre.
Thanks - how can one limit field rotation?
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Old 17-06-2008, 01:03 PM
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You have field rotation.
As Geoff said, your stars are arced around the centre star, thus "Rotation".

You need to look at your pointing accuracy (Polar alignment) on your mount, as it looks like your out by some bit.

Theo.
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Old 17-06-2008, 03:34 PM
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Sloppy adaptors

I've got an Olympus to EOS adaptor which works well but had a small residual slop. Managed to tighten it up by puting a bit of masking tape ( small pads x 3) on the face surface. Worked a treat! Also used a similar solution ( but used a thin velvet stick on pad - I think its sold for protecting the bottom of glass ornaments?) on a MOgg Olympus to M42 delrin adaptor.
Worth trying.
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Old 17-06-2008, 04:33 PM
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Quote:
Originally Posted by Merlin66 View Post
I've got an Olympus to EOS adaptor which works well but had a small residual slop. Managed to tighten it up by puting a bit of masking tape ( small pads x 3) on the face surface. Worked a treat! Also used a similar solution ( but used a thin velvet stick on pad - I think its sold for protecting the bottom of glass ornaments?) on a MOgg Olympus to M42 delrin adaptor.
Worth trying.
Thanks Merlin, I'd thought to do that but was afraid it might come lose and rattle around in side the camera body. So far I've just applied some tape to the outside.... oh and just ordered a new lumicon adaptor from my astro shop... I need a second and who knows maybe it won't slop about
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Old 17-06-2008, 06:20 PM
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Field rotation is caused by improper polar alignment. Everything rotates about the guide star. You can limit it in two ways:
1. Improve polar alignment
2. Shorten exposures
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Old 17-06-2008, 06:46 PM
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Try with and without the focal reducer.
If it is field rotation (due to bad alignment), it will be the same on both images.
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Old 17-06-2008, 06:55 PM
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Is it a combination of both?
A larger image would be a big help.
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Old 17-06-2008, 07:15 PM
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The reducer has nothing to do with it.
Its rotation, plain and simple.
Distortion from a reducer (Most obvious astigmatism) would be in the direction of the middle. Not arc around the middle.

Do a search on field rotation and compare you "Classic" image pattern.

Theo.
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  #17  
Old 18-06-2008, 07:36 AM
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Thanks Guys, given my lack of care with polar alignment I'm quite prepared to buy that this is field rotation
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Old 18-06-2008, 08:21 AM
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This is weird Robert. I get the same thing sometimes - what looks like rotation in the corners. I use a f/6.3 reducer on my C8 too. I know I've had great alignment because I sat there with StarTarg looking at a star - no guiding and without movement in my reticle for some 15 minutes or more. That to me indicates pretty darn good drift alignment. My next night out will see me do some shots with and some without the reducer. I'm still leaning toward it being a form of spherical aberration rather than rotation, but I guess we'll see.
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Old 18-06-2008, 10:07 AM
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Meade & Celestron 6.3 focal reducers were designed in the days of film. DSLR sensors are smaller than a 35mm film frame so IMO it would not be the reducer.
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Old 18-06-2008, 10:38 AM
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I agree with the field rotation diagnosis, but also agree it's likely the reducer would be introducing some distortion, but i'd expect it to be making stars ilongated from the centre out, as other have said.

If you try with & without the focal reducer I would expect even if it's field rotation it will look less noticable without the focal reducer because you'll be looking at a narrower FOV. I might be wrong, but that's what I'd expect.

Roger.
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