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Originally Posted by alpal
The long subframes even at 1200s have over exposed the center of the Tarantula.
The center is very bright.
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Good point, I guess I'll have to take some 400-600s exposures to even out the core.
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Originally Posted by Paul Haese
I think star shapes would be more influenced by PA, focus and tilt rather than PE, so long as your PE is corrected out via guiding. It's always good to experiment and find out what works best with your equipment.
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It is in the same direction as PA (I had my guider fail during the night, USB issues to which I will be trying another one tonight). I had an aspect of 17% on the 1200s, 16% on the 2400s but 28% on the 3600s so I am guessing it was a guiding issue more than anything.
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Originally Posted by Slawomir
Master Bias will inject some minor noise, while Super Bias will not induce any noise. It only takes one or two mouse clicks to generate a super bias from master bias...
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I was thinking that it may have been the opposite with a 200 frame bias, opening it up and running the script is dead easy to do though.
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Originally Posted by Placidus
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.
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I have done a fair bit of tightening and fiddling with the focuser and I think I have managed to remove near all of the tilt, I probably should do some testing tonight to see how it has gone, or whether it is gone!
It is a nice and rigid focuser, it almost feels like I get more flop in the mount (mounting plate stuff) than the focuser
My report said that MA was spot on and ME was 18" off. Now, it was just a 23 point model so maybe it could be off by further in both directions as a 23 point model could very well be too small. I'll have to keep that in mind, keeping a bit west of SCP if imaging near the meridian. Food for future thought.
The KAF-8300 with its 25,500 shallow wells doesn't take TOO much before they're full. In the future I would definitely consider getting a 11002 sensor, more real estate and same size wells as the 16803.
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Originally Posted by RickS
Nice capture, Colin! Loads of nebulosity there.
Wrt darks, most of the folks here that don't take darks are using low noise Sony sensors. With a KAF-8300 you may find it is worthwhile.
Cheers,
Rick.
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More nebulosity than I was expecting! It was your image from a month or so ago that inspired me to give this a shot
I am definitely thinking that I'll need to take darks for the hour subs. The vast majority of the "stars" within the image are just hot pixels.
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Originally Posted by Andy01
Thats very cool -loads of depth there. Should be a great shot when finished
I purchased my belt modded and hypertuned EQ6 from Al Sam, because he regularly did 1 hr exposures with it. I'm personally not brave enough to go that long with so many clouds and satellites about, and the likelihood of losing subs, but after seeing yours maybe I can go to 30mins on the next project.
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I started off purely as a test to not only see how the 3nm filters would perform but also to see what the mount was capable of. I bought it second hand (well 4th hand really) off of IIS, it had been belt modded by the previous owner but I think he only used it once or twice after that.
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Originally Posted by strongmanmike
Looking good Colin, quite amazing what can be stretched out of the Tarantula region, huh?
1hr subs hey?...Oooooh scary I haven't ventured past 15min, not because my rig can't handle it but rather I'm just time greedy and perhaps a little paranoid about losing bigger chunks of data.
Mike
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I am pretty sure that anything over ~12 minutes for me doesn't bring in any more nebulosity, hitting my sky limit for that night. I am still really in two minds as to how worthwhile it is pushing well past the sky limit and heading for read noise limit. Realistically I hit read noise limit at 90-120m (so happy to stick at 60m) but trying to figure out whether it is better in the long run to go longer just to grab the extra signal. Looking at the raw subs, mathematically it would appear that I would be getting a better SNR with 3x20m subs than a 1x60m.
That is calculated by sqrt(2)*3*SNR and then comparing the two. I did blend the 1200 & 2400 together and then compare that against the 3600. The single 3600 was still less noisy and better to process. Decisions decisions!
As I mentioned earlier my guider did drop out during the night while I was fast asleep waiting for predawn to shut down
The first image shows what I got (full frame) while the second is just a crop on a star, it stopped working about half way through the previous image so this is an hour long sub without any guiding.
I assume that the oscillating is the PE on ~7.5m cycles but would I be correct to assume that the streak length is longer than what I would get with an 18" offset from SCP? Working at 1.5"/pix, 741mm focal length.
