Vignetting - any help?

Hi all, unfortunately for me, I have run into a wall regarding vignetting in my images. I have had precious litle time using my william optics 110 FLT over the past few months, and have finally had time to try to process this orion nebula shot. Ok - the tracking isn't so good, because it's unguided, but I am constantily being held back by trying to get rid of the vignetting, simply as my Canon 40D's chip is too big

Any tips on how to eliminate this problem? I have tried a few tutorial from the web, but no success
I have attached the file here as a jpeg, my original file is a TIFF file, and I can use photoshop

www.flickr.com/photos/79652716@N00/5676458460/in/photostream

Thanks,

Keith..

Hi Keith

I presume you are taking and using flats. I have the same problem with my C11 at F6.3. The vignetting is so bad that I can only take subs about 1 minute long before it is noticable. Taking flats really reduces it when processing though. I normally focus the scope on a distant light, then point it at a blue part of the sky (before darkness), set the camera to Av and take about 10 flat exposures. It doesn't eliminate the problem completely but enough for levels and curves to remove anything left over.

Hi Keith
Have you considered using a focal reducer or specifically a field flattener, I think this may help your situation if I am not mistaken. The distance placement of the field flattener in relation to the image plane of the CCD chip is very important and this can be calculated out when certain factors are known to obtain optimal distance. I think the main issue is the size of the imaging chip though and you need to let as much of the light cone through the optics as possible, are you using any camera adapters that may be restricting light throughput? I think the combination of using a high quality filed flattener, letting as much light throughput as you telescope system allows and taking good flats should see this problem very much reduced so as not to cause concern.

I had a similar problem on my C8 SCT when I tried to use a x0.63 telecompressor with my large format SBIG STL camera, vignetting was awful, the light fall-off from centre towards edge of FOV was quite noticeable (50%+). In the end I commissioned Optec to build an special FR optical adapter which was based upon a Ted Agos design. Now vignetting is vastly reduced and taking good quality flats greatly helps as an additional step in reducing vignetting so as not to detract from a good astro image.

Best wishes and clear skies
Mike

Hey John and Mike, thanks for your thoughts, I think I have to rethink my flats, John - when you set to Av, how long exposure is it, and I take it you take them using the same ISO as what you are going to be shooting at?

Mike - yes the chip size is quite large, all i'm using is the standard t-ring and t-adapter. Thanks for the tip on the field flattener !

Keith..

Hi Keith

At the Av setting on my 450D the exposure is taken automatically by the camera until it reaches a certain level of saturation (around half the histogram). I keep the ISO the same. The frames are usually a fraction of a second on a twilight patch of sky with the C11.

Thanks John, as I thought, the exposure is quite short and taken automatically. Next clear night i'll do this .......

Keith.