Issues with hot pixels.


New member
Mar 27, 2020
So I'm new to the forum and PI in general so this is all very new to me. I need some help to better understand issues I'm having with hot pixels in my final stacked images. My image is comprised of 30 light frames, 20 dark frames, 40 flat frames and 40 bias. The following screenshot is a zoomed in master light of M51, the one on the left is without bias frames and the one on the right is with the bias frames. I have ran both with the BatchPreprocessing script using default values, average combination and winsorized sigma clipping for all frames. I'm not understanding why bias frames would have any effects with hot pixels, why wouldn't the dark frames take care of this? The camera is a ZWO ASI533MC PRO, the light and dark frames were taken at 3 minutes with a temperature of -20c no dithering. Thanks for any help in advance, Dan.


Well-known member
Nov 2, 2016
La Palma, Canary Islands
The right settings in ImageCalibration depend on the approach that you are using for the preparation of the master calibration files and whether you use dark frame optimization or not. Please disclose this information. Please also post a screen shot of your complete current settings for the calibration of the light frames and indicate which of the two results above correspond to it.



New member
Mar 27, 2020
Thank you for your replies,

Dave the only time I have tried the CosmetricCorrection was on the final stacked image and it made no difference, I never thought to try it beforehand. Just for future reference are you supposed to run CC before you run BPP? Maybe that's why I didn't notice a difference in the image.

Bernd, these images where created by only using BPP I did not use ImageCalibration or any other processes. I'm just lost why bias frames would correct my issue with the hot pixels and wanted to better understand why.

On a positive note I did get some clear skies and was able to get back on M51 but this time I enabled dithering for the first time and not surprising my issue with the hot pixels has been corrected without the need of bias frames.


Active member
Mar 6, 2020
CC always gets run first before you do anything else.
If CC is run on an uncalibrated image it will remove all hot and cold pixels. If you then follow that up with dark frame calibration you will have an overcorrection.

Cosmetic correction should only be used to remove the hot and cold pixels that image calibration has failed to remove.

My normal workflow is to use WBPP to calibrate only, then use Vicent’s scripts to remove line defects from the calibrated images and then finally to use CC to remove any remaining problem pixels.


Well-known member
sorry my bad, I did mean after calibration but before you start any other processing, although I didn't say that.....did it in a rush. I never use either BPP or WBPP.

Over the last few months I have been using a remote telescope and getting pre-calibrated frames from it, so the first I do is CC because they are already calibrated........and I forgot that LOL....DOH
Last edited: