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Old 05-10-2015, 12:33 PM
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Tunnel Vision

AlexN is offline
Join Date: Mar 2008
Location: Brisbane, Australia
Posts: 6,708
I dont feel that its breathing as the image scale of the light frames does not change, it just seems that the vignetting pattern changes..

I might try again tonight with the focuser disconnected and see how that goes. Just rig it up manually.

The focuser is a home made arduino powered stepper motor focuser with a belt drive around the focus ring of my 300mm f/2.8 camera lens. The camera lens is mounted on a DSLR camera.

The flats were done off a laptop LCD monitor with a fully white screen image with the lens hood of the 300/2.8 just off touching the monitor to minimise stray light in a dark room (no lights on, at night time)

No bias subtracted from the flat frames, and no flat darks

Im using ICNR with the DSLR (In Camera Noise Reduction) that essentially takes a light frame, then immediately takes a dark and subtracts it from the light in camera. This is not optimal most of the time, but because the temperature changes night after night I find that darks of this method work best with a DSLR

I might take a series of flat darks (that will obviously contain the bias information) and subtract them from the flats to see if that helps at all.

The one thing that I want to test but is currently impossible for me is this.

This lens is reportedly (by someone VERY experienced with astro imaging using this lens) extremely sensitive to backfocus distance. Now you might think "well that shouldn't be a problem with a DSLR lens on a DSLR camera.. Right?" The problem lies in the fact that I've modified this DSLR myself by removing the AA filter and the UV/IR hot filter completely from the camera, and as a result, there is a discrepency (the focal point is 1.3mm further back than was originally intended.. As such, the spacing for the lens is wrong and I am suffering coma in the corners as a result (which I know for a fact did not happen pre-modding the camera. and I know for a fact isn't the sensor being tensioned out of shape etc because I have no visible differences in images taken with my 8" newt...

I've added some images to show the problem.

Next thought, I was thinking about wrapping the rear of the lens and camera in Al Foil as a light trap incase somehow during my modification I've induced a light leak somewhere...
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