Quote:
Originally Posted by Atmos
That's a good point, could end up being PA. A 23 point model was saying I was within 18" of SCP so I figured that would be close enough for hour long subs... Maybe not!!
I think I'll have to do some shorter subs to be able to check for tilt, hour long is a bit long!
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Hi, Colin,
Rigid camera tilt will show up in a tenth of a second. It is independent of frame length. Camera tilt will show up as stars that are in focus along a line (which may be vertical, horizontal, or diagonal) across the image, but elongated 90 degrees to the line by progressively greater amounts away from that line.
Camera flop is different - the camera can be on square in some parts of the sky but move under gravity in other parts of the sky if the attachment is rickety. You can again test that with very short exposures on a bright star - just check different parts of the sky. The direction of flop can change across the sky.
Incorrect polar alignment can be spotted because the pattern of star enlargement is that of field rotation. Little star trails rotating a tiny bit about your guide star (which might be at the edge of the frame, not necessarily in the middle). I see no evidence of this in your shot.
18 seconds of the SCP is superb. Ten times that is fine, so long as the HOUR ANGLE of the error is roughly 90 degrees (say within 45 degrees) of the hour angle where you are photographing. We (and many others) routinely put our polar axis about 150 sec arc WEST of the pole, and we then photograph between hour angles of about +/- 3 hours of the meridian. It helps with guiding but causes negligible field rotation.
Out of focus stars will of course be blurry but symmetrical on-axis. On very large chips, out-of-focus stars in the corner can look like flying saucers.
Other things that can cause funny stars: wind buffet, momentary loss of guiding due to tiny clouds, and grit causing static friction (grabbing) in bearings and gears. Personally I don't think you have a problem that needs fixing.
Trish and I routinely do 1-hour subs. They give relatively clean noise-free backgrounds at our dark site. Our stars aren't exactly round, but they're not exactly round in a 1 minute shot. It's nothing to do with the long exposure, it's more about hanging 80 Kg of scope and cameras off a not-so-expensive mount. We don't ever get burn-out in nebulosity, but that's because the camera chip (16803) has sufficiently deep quantum wells. 3nM filters have no effect on nebulosity because they let 90% through, but they help prevent star burn-out.
Very best,
Mike