Quote:
Originally Posted by RickS
Well done, M&T! Great colour and I love those faint spiral arms. ...
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Thanks Rick.
Quote:
Originally Posted by Joshua Bunn
... What's going on with the green stars and green at 8 o'clock on some of the brighter stars? ...
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Quote:
Originally Posted by RickS
... Not sure about the green blobs but I think it's unlikely to be something on the filter since it affects bright stars all across the field.
Cheers,
Rick.
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I think I might have figured out what the green blobs are. They are residual or ghost images in the green sub from the immediately previous red sub.
Examining the raw green sub, I can see that they are perfectly formed stellar images, all offset by precisely the same amount across the field on any given sub. They are not smears, nor diffraction spikes. The amount of offset varies from sub to sub.
We use a poor-man's dithering between subs. We do a focus run between subs, and rely on the scope's pointing accuracy being only of the order of 11 sec arc standard deviation. We don't explicitly dither by a large but controlled amount. We did something like 20 luminance frames, and statistical rejection effortlessly gets rid of after-images, especially after-images from the usually brilliant focus star. But with RGB, there are only three subs in each colour, and that is not enough to do anything meaningful.
Dr Mike proposes the following cure:
(1) Stop looking for wasps nests (there was one, and a big one too, but it wasn't the culprit).
(2) Dither by a larger and more controlled amount.
(3) Do more RGB frames, perhaps 6 shorter ones rather than 3 longer ones. Then our usual Winsorized mean statistical rejection technique will get rid of the blobs.
Some folk use infrared pre-flash to get rid of after images. That is important if you are doing spectra at a chip temp of -100C, but close to useless on our chip. At -30C the pre-flash hugely increases the background noise. We care much more about grit-free faint spiral arms and distant faint fuzzies than about green blobs next to the odd bright star.
Thanks everyone for their thoughts on The Blob.
Mike