Multiple Artifacts M42

[Alien 13]

Hello I took a few subs yesterday of M42 with the WOZS71 and Canon 650D sub length was 30s and i stacked a total of 20 subs.

 

This was a bit of a practice run so i had no flats or darks but did include 20 dark bias frames into DSS.

 

The image came out of DSS with levels adjusted as a quick check and i found it full of artifacts i know i didnt have any dark frames but i didnt expect what i saw any ideas?

 

Thanks in advance Alan

[gkec]

Hot pixels that appear as bright spots. As there was drift between exposures when stacked they appear multiple times. A dark frame subtraction before stacking would take care of it.  Did you make sure the dark bias frames were checked in DSS?  I find I have to specifically make sure that all the frames are checked for processing.

[Alien 13]

Hot pixels that appear as bright spots. As there was drift between exposures when stacked they appear multiple times. A dark frame subtraction before stacking would take care of it.  Did you make sure the dark bias frames were checked in DSS?  I find I have to specifically make sure that all the frames are checked for processing.

 

Thanks for the reply i have tried it with and without the bias frames making shure i checked them it does seem like they are copies of hot pixels because they are all the same pattern and having looked closer there are fainter copys too but i dont quite understand why its not happened like this before.

[Sam]

Using darks will get rid of those.

[Alien 13]

Ive just run a random selection of 6 subs through DSS and its completely clear of artifacts im wondering if ive got one or two rogue frames ill have to check them all..

[gkec]

Maybe take a few dark frames and select the hot pixel removal option in dark frame stacking. The temperature will probably be different but it might help removing the hot pixels.  Worth a try.

[Jannis]

Optionally, stacking with kappa sigma clipping and plenty of frames will also usually remove them more or less 100% unless your tracking is spot on.

[Alien 13]

Optionally, stacking with kappa sigma clipping and plenty of frames will also usually remove them more or less 100% unless your tracking is spot on.

 

Thanks for that i tried that and it worked ive no idea how or what it does but thanks again.

This is the new image that came out of DSS same subs the odd shaped stars near the edges are due the scope i tried it without the FF for this go.

[Jannis]

Thanks for that i tried that and it worked ive no idea how or what it does but thanks again.

Glad it worked. In short, what it does, is to look at all the frames and compare them. If it see that 5 of 6 frames are dark in that one pixel spot, but then the 6th frame have a bright red pixel, it rejects that pixel from the stacking, and your hot pixels won't be seen.

Stacking Methods

Average

This is the simplest method. The mean of all the pixels in the stack is computed for each pixel.

Median

This is the default method used when creating the masters dark, flat and offset/bias. The median value of the pixels in the stack is computed for each pixel.

Kappa-Sigma Clipping

This method is used to reject deviant pixels iteratively.

Two parameters are used: the number of iterations and the standard deviation multiplier used (Kappa).

For each iteration, the mean and standard deviation (Sigma) of the pixels in the stack are computed.

Each pixel which value is farthest from the mean than more than Kappa * Sigma is rejected.

The mean of the remaining pixels in the stack is computed for each pixel.

Median Kappa-Sigma Clipping

This method is similar to the Kappa-Sigma Clipping method but instead of rejected the pixel values, they are replaced by the median value.