Quote:
Originally Posted by gregbradley
Very nice. You are having some good results with those 1 hour subs.
How do find them in terms of sharpness and star size?
Greg.
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Hi, Greg,
Last night seeing was good but not brilliant. The FWHM of a 10 second H-alpha focus frame was 4.0 pixels, or 2.4 sec arc. The FWHM of a non-burned-out star in a 1 hour sub was 4.7 pixels, or 2.6 sec arc. Thus all the accumulated bumps and glitches of hitting bits of grit or tight spots or rough teeth, field rotation, differential flexure (the focuser is still a teensy bit floppy), bats flying through the field, etc, only made things about 17% worse.
It's a long time since I did the maths and I've lost all the source documents, but from memory, going any longer than 1 hour is not justified because the contribution of read noise to the total image noise is already very small.
The biggest disadvantage is that if you've only got two one-hour H-alpha subs (as opposed to say a stack of 12 ten minute subs) you can't do any automatic cosmic ray and satellite trail rejection. We just leave the cosmic rays in - they are, after all, perhaps the most spectacular deep sky objects in the image!
The second biggest disadvantage is intermittent cloud. We set up at dusk, and leave the system to run a fairly complicated script, going to a suitable focus star and refocussing after each frame, changing filters, returning to the object, reacquiring guide stars on the two guide cameras. If a small cloud comes, we lose one hour. But that happens quite rarely. And we can program the beast to do two objects in the one night. Last night it did the Emerald Firebird in NB until 1am when the moon went down, then went on to NGC 5128 in RGB, and finally rattled off some darks.
Best,
Mike