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Old 12-07-2021, 08:43 AM
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pmrid (Peter)
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Stacking artifacts mystery

I grabbed some images last night and tried stacking them this morning only to discover that there are mysterious artifacts cropping up that I'd appreciate other greater minds than mine to ponder.
Here's a crop from 4 hours of 10minute subs of NHC346. The number of dots seem to equal the number of subs so there is clearly a drift of some kind but the image itself seems fine. What the devil is happening here?
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Old 12-07-2021, 09:47 AM
JA
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Hi Peter,

That is strange as the larger stars are not duplicated, but some of the smaller ones are in lines I make to be 19 long, with some gaps, so maybe close enough to 24 as some may be one on top of the other.

I would have a careful look through of all your 24 subexposures, flicking back from one to the other to see if you indeed have some sort of star movement, but given the larger stars and the central nebula don't appear smeared I don't think you'll find any. That then suggests some form of error in the stacking routine. Perhaps try to repeat the stacking with different stacking software and for the heck of it redo the stacking with the original software you used in case there was some form of transient glitch or incorrect setting.

Best of luck

Best
JA
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Old 12-07-2021, 11:35 AM
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Quote:
Originally Posted by JA View Post
Hi Peter,

That is strange as the larger stars are not duplicated, but some of the smaller ones are in lines I make to be 19 long, with some gaps, so maybe close enough to 24 as some may be one on top of the other.

I would have a careful look through of all your 24 subexposures, flicking back from one to the other to see if you indeed have some sort of star movement, but given the larger stars and the central nebula don't appear smeared I don't think you'll find any. That then suggests some form of error in the stacking routine. Perhaps try to repeat the stacking with different stacking software and for the heck of it redo the stacking with the original software you used in case there was some form of transient glitch or incorrect setting.

Best of luck

Best
JA
Iíll try that.
In the meanwhile, I had this thought - if I had hot pixels AND a degree of drift between subs, the darks would not take out the hot pixels because they would be in a different position - so they would show up as a series of dots in the general direction of the drift. But why is the image not streaked as well?
My PA is spot on, balance as near to perfect as I can get. The only other wildcard is perhaps the mount head has some tilt in it.
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Old 12-07-2021, 08:01 PM
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Your balance shouldn't be perfect; it should be balanced a little against the drive direction to keep everything fully engaged. I apologise if I am teaching
your mother to suck eggs, as the old saying goes.

raymo
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Old 12-07-2021, 10:49 PM
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Stacked hot pixels?

I have had that where the darks were not correctly applied.

Would the uncropped stacked frame edge would show the same distance and direction of travel as the artefacts .?
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Old 13-07-2021, 03:55 AM
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I just did a stack of the raw uncalibrated images and got the same result. So it is not a problem with the calibration frames. The artifact is in the original subs. And you see in the stacked image the corresponding shifts on the left and right edges. So there appears to me to be an extraordinary number of hot pixels plus there is some drift from frame to frame.

Might be time to consider retiring either my CCD or myself I fear. I think one or other of us has had its day.
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Old 13-07-2021, 03:34 PM
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Could there be some other defects on camera face making the problem worse?

I can also see a few grey dots in a line which could be condensation on the camera glass. The fact that calibrated subs look the same indicates some problem on the camera face and/ or with hot pixels. Have you tried taking just a couple of new darks and giving them a whirl while making sure remaining hot pixels are checked for removal? I have had bad darks before.

Quote:
Originally Posted by pmrid View Post
I just did a stack of the raw uncalibrated images and got the same result. So it is not a problem with the calibration frames. The artifact is in the original subs. And you see in the stacked image the corresponding shifts on the left and right edges. So there appears to me to be an extraordinary number of hot pixels plus there is some drift from frame to frame.

Might be time to consider retiring either my CCD or myself I fear. I think one or other of us has had its day.
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Old 13-07-2021, 04:11 PM
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I dont know Peter but that wont stop me offering an opinion...and I have not thought it thru however...could your mount be the problem? When staking the program would I expect line up using bright stars, so the object and bright stars are in place however some frames tell the truth about mount movement...it seemed like a good idea at the time...I may come back and withdraw this when I think more..but I have to go so I thought I would write it down before I forgot.
Alex
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Old 14-07-2021, 01:40 AM
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I have done fresh calibration frames but of course the weather has put a stop on anything further.

However, I did spend a couple of hours charting the positional change of some stars on the edges and lo and behold, they moved in a way that corresponded with that progressive artifact. Ergo, the artifact is hot pixels that are not being taken out by dark frames and the stacking is working on the brighter stars and so the uncorrected hot pixels do their little march.

So, it is some sort of alignment error coupled with a sensor on its last legs. I have spent a couple of hours today repositioning the mount, getting it level, doing a fresh polar align and rebalancing (yes, slightly E-heavy) so it is ready for a run if the sky clears.
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Old 14-07-2021, 12:00 PM
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Pete, are you using median combine? If not you should It does a good job of rejecting outliers.....also use a Bad Pixel Map for hot pixels instead of darks and see how that goes....not all cooling regulators do a good job.
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