Old 17-02-2016, 06:26 PM
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pmrid (Peter)
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What am I looking at here?

This is a center crop from a stack of 5x10 minute luminance subs of NGC4038.

I'd add crops from the RGB as well but they just repeat the same sequence.

Here's my guess. It's a hot pixel that isn't taken out by the usual darks etc because it is moving from sub to sub. There are 5 bright spots with a gap between 2 and 4 (counting from the lop right) which corresponds to the fact that I ditched sub 3 because of a big jump. So I stacked 1,2,4,5,6. And this sequence shows that from one sub to the next the image has moved in a steady and regular pattern from top right to bottom left. The RGB subs continue the same pattern and take up from where the lum subs finished. So there is a pattern of regular and consistent movement that is occurring BETWEEN subs. i.e. one sub finishes, there is a pause for the download and guider settle - up to 20 seconds or so on my QSI CCD and in that period between subs, something happens to allow this drift.

I am guiding with MaximDL and imaging with it also.

Is this a sign of polar alignment that is off, tracking rate error, .... I'm out of guesses.


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Old 17-02-2016, 07:14 PM
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codemonkey (Lee)
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Seems likely. Are you guiding with an OAG or a scope? Could be flex.
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Old 17-02-2016, 07:24 PM
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billdan (Bill)
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Hi Peter,

It is very unusual, firstly the dots are too bloated to be hot pixels and it can't be tracking error otherwise the star bottom right would be doing the same thing.
It could be a slow moving satellite or some other object that was in the FOV.


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Old 17-02-2016, 07:44 PM
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ZeroID (Brent)
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Does the same little pattern repeat elsewhere on the image ? Just that I saw a faint visual 'echo' on your image.
I've got a water damaged camera that still works but shows a repeating pattern over the whole sensor on long exposures.
Still ok for normal pix and movies if you know how to get round the limitations but long exposures show the pattern. And longer the exposure , the longer the pattern.
I'm guessing my camera has some electronics damage from the water. Maybe something similar on yours.
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Old 17-02-2016, 07:46 PM
ericwbenson (Eric)
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Hi Peter,
Almost certainly hot pixels moving along between subs due to OTA vs guider flex. Redo your darks or make sure you are using a sigma clip combine, and you should find it goes away.
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Old 17-02-2016, 07:49 PM
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The fact that the artefacts are repeated, but fainter, at top left, adds to
the mystery. Maybe one of the sets of dots is a reflection of the other set.
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Old 17-02-2016, 08:45 PM
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tempestwizz (Brian)
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I have this phenomenon often with my dslr, but the pixels are coloured. I believe it is hot pixels. The trail shows flexure. Did you do hot pixel removal on individual subs before aligning and stacking? If you do a stack of the subs without alignment and the hot spots show on top of each other it should confirm the source. It may be a case of 'quite warm' pixels helped along with thermal effects in the sensor rather than true 'hot' pixels. From the difference in intensity between two rows shown in your example indicates there is a time relationship for intensity. I am surprised they are not removed by dark subtraction. Is the dark frame you used current, or one taken some time ago?

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Old 17-02-2016, 10:08 PM
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RobF (Rob)
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Hi Peter

What software did you stack with?
(In PI, Cosmetic Correction should help zap this sort of thing)
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Old 18-02-2016, 07:04 AM
cfranks (Charles)
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I'm with Bill, they are much too large and nicely round to be hot pixels. The dimmer 'reflection' at top left blows any theories I had away.
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Old 21-02-2016, 01:44 PM
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PRejto (Peter)
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Rather than a hot pix (too large!) maybe it is an artifact not taken out by proper flat? The size almost looks about right for a spec of dust on the CCD window. If the flat is not linear the artifact will still show up but I think inverted in colour. Dark spot becomes a white spot. Given drift between frames aligning the stars will cause the spot to move. Anyway, just a guess!

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Old 21-02-2016, 02:26 PM
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theodog (Jeff)
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Originally Posted by pmrid View Post
........ occurring BETWEEN subs. i.e. one sub finishes, there is a pause for the download and guider settle - up to 20 seconds or so on my QSI CCD and in that period between subs, something happens to allow this drift.

Peter, I think they are hot pixels (due to the repeat pattern above left).

Are you using a dithered guide? One where the camera is moved several pixels to move hot pixels between frames. If not, than it maybe flexure -don't see how though, perhaps in the focuser unit (very slight sag).
Check your software for any ability to change the strength of hot pixel removal -sometimes dark-frame subtraction just doesn't seem to get all the hotties.
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Old 21-02-2016, 02:52 PM
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multiweb (Marc)
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+1 .Just two hot pixels trailing in your stacked image.
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