Strange artifacts

[robindurant]

This is a heavy cropped area of a  20 x 20 second stack of the night sky with a Conon 5D M2 and a 20mm lens non tracked .  These artifacts sometime occure in colour, can anyone explain what Im doing wrong

 

[almcl]

They look like hot pixels.  

If you are not tracking, the stars progressively move across the frame and when each frame is aligned the hot pixel appears to move.

You might be able to eliminate them with a Kappa-Sigma clip stacking algorithm, but if you're using DSS you will probably need a good deal more than 20 frames for this to be effective.

Edited March 29, 2022 by almcl

[Shibby]

11 hours ago, almcl said:

They look like hot pixels.  

If you are not tracking, the stars progressively move across the frame and when each frame is aligned the hot pixel appears to move.

You might be able to eliminate them with a Kappa-Sigma clip stacking algorithm, but if you're using DSS you will probably need a good deal more than 20 frames for this to be effective.

I believe 16 is the "magic" number, so stacking 20 should work OK with a sigma clip stack.

[almcl]

1 hour ago, Shibby said:

I believe 16 is the "magic" number, so stacking 20 should work OK with a sigma clip stack.

Unfortunately that's not been my experience.  I have had stacks of 25 in which DSS failed miserably to remove hot pixels, despite quite large dithers every frame and just the other night it failed to Sigma clip a single satellite trail from a stack of 70.

Of course, other stacking software may perform differently...

[Shibby]

15 minutes ago, almcl said:

I have had stacks of 25 in which DSS failed miserably to remove hot pixels, despite quite large dithers every frame and just the other night it failed to Sigma clip a single satellite trail from a stack of 70

This surprises me greatly. I've never had this problem.

[vlaiv]

20 minutes ago, almcl said:

Unfortunately that's not been my experience.  I have had stacks of 25 in which DSS failed miserably to remove hot pixels, despite quite large dithers every frame and just the other night it failed to Sigma clip a single satellite trail from a stack of 70.

Of course, other stacking software may perform differently...

Sub normalization is very important for sigma clip to work properly (called background calibration in DSS).

Unfortunately - DSS version is not the best way to do this as it only considers mean / median pixel value of the sub, but should instead account for both LP and atmospheric extinction, effectively performing linear fit between two subs.

Small number of subs and background calibration turned off - or even large difference in transparency between frames will make sigma clip algorithm fail as pixel values will have high dispersion and statistics will fail to isolate hot pixels.

 

[almcl]

3 hours ago, Shibby said:

This surprises me greatly. I've never had this problem.

Well, here's an example.  I count 25 hot pixels in the stacked image (grey scale) and they all stem from one in the individual light (bluish image):

 

Not now sure how many lights were stacked, there were 30 altogether, but at least 25 must have been used.

(Sorry to the OP for hi-jacking the thread, but this is moderately relevant to the question.)

 

Edited March 30, 2022 by almcl