Hot pixel stacking artefacts?

[Peter Reader]

Hi all,

Managed to process a decent image of Pinwheel in the end (thanks to those who helped me). I notice some weird red, green and blue pixels in streaks that looks like some sort of artefact from stacking.

Is this something I can only remove with dark frames? Is there any processing that can be done to remove? Raw stack attached 

Thanks,

Pete

Pinwheel DSS output autosave 32 bit.tif

[vlaiv]

These can be handled with DSS hot pixel filtering as well as using sigma clip stacking.

In either case you need to restack your data.

[Peter Reader]

22 hours ago, vlaiv said:

These can be handled with DSS hot pixel filtering as well as using sigma clip stacking.

In either case you need to restack your data.

Thanks for the suggestion, unfortunately this didn't seem to help at all...

[vlaiv]

2 minutes ago, Peter Reader said:

Thanks for the suggestion, unfortunately this didn't seem to help at all...

Which one?

Sigma clip stacking is sure to get rid of them provided that data is dithered - and yours appears to be.

put Kappa between 2 and 3

(2 noiser result but removes more outliers, 3 smoother result but removes less outliers)

number of iterations - 3 is enough

 

[Peter Reader]

On 16/03/2022 at 22:10, vlaiv said:

Which one?

Sigma clip stacking is sure to get rid of them provided that data is dithered - and yours appears to be.

put Kappa between 2 and 3

(2 noiser result but removes more outliers, 3 smoother result but removes less outliers)

number of iterations - 3 is enough

 

Would you recommend median Kappaa-Sigma or the one you highlighted? Median seems to be working for me but the other one not so much...

Edited March 19, 2022 by Peter Reader

[vlaiv]

5 minutes ago, Peter Reader said:

Would you recommend median Kappaa-Sigma or the one you highlighted? Median seems to be working for me but the other one not so much...

That is quite interesting.

I think that there is not much difference between regular and median if data is normalized.

What sort of background normalization did you select in DSS?

[Peter Reader]

I currently have bothRGB and Per Channel background callibration unchecked. Would you recommend?

[vlaiv]

20 minutes ago, Peter Reader said:

I currently have bothRGB and Per Channel background callibration unchecked. Would you recommend?

It really depends how much post processing you want to do, or rather - what kind of it.

From DSS technical documentation:

Quote

Background Calibration
The Background Calibration consists in normalizing the background value of each picture before stacking it.
The background value is defined as the median value of all the pixels of the picture.

Two options are available.

• With the Per Channel Background Calibration option the background for each channel is adjusted separately to match the background of the reference frame.

• With the RGB Channels Calibration the three red, green and blue channels of each light frame are normalized to the same background value which is the minimum of the three medians values (one for each channel) computed from the reference frame. On top on creating compatible images (stacking wise) this option is also creating a neutral gray background. A side effect is that the overall saturation of the stacked image is quite low (grayscale look).

It is important to check one of these options when using Kappa-Sigma Clipping or Kappa-Sigma Clipping Median methods to ensure that the pictures being stacked have all the same background value.

I personally prefer third option - not implemented in DSS which deals with both background LP and transparency differences (version that I use also deals with gradients) - which is best described as linear fit (above is "constant fit").

Per channel background will normalize frames against reference frame - keeping color balance of reference frame

RGB one will normalize frames against reference - but will neutralize background (which might not be a good thing since it skews other colors if not done carefully - and above warns about that - possible loss of saturation being an issue).

Go with Per channel as your first choice - you can always restack with other option and see differences for yourself.