Lack of signal in blue channel (ASI533, Sharpcap 3.2.6383.0)

[encelade18]

Hello Robin,

I have a problem whose origin i maybe sharpcap, or not, i want to have at least your opinion about that please.
The problem is the following:

I can't have a correct debayerized image after dark correction (i use Siril 0.9.12) (target Messier M56)
I use a master dark since a long time and i never had any problem with it (acquisitions for this dark with Sharpcap 3.2.6248.0)
If i apply the dark into one raw image, i have a lack of signal in blue channel and then, after alignment/ stacking, i can't equilibrate the color.
I never had a similar problem before.

And looking for where the problem came from, i saw that :
- I updated the version of SharpCap from Sharpcap 3.2.6322.0 to Sharpcap 3.2.6383.0
- I simply debayerized a raw fits image with Siril and saw into the statistics of the image that:

for Sharpcap 3.2.6322.0:
the min/ max values for all channels are 0/ 65535
(example for Messier M92 as target which is not problematic)

all chanels: 65535 max values

cfg_Siril_M92_Sharpcap-3-2-6322-0_statistiques.png (18.72 KiB) Viewed 2315 times

for Sharpcap 3.2.6383.0
the min/ max values are 0/ 65535 only for red and green channels, and 57068 into blue channel
(For Messier M56 as target which is problematic)

blue channels not have 65535 max value

cfg_Siril_M56_Sharpcap-3-2-6383-0_statistiques.png (27.68 KiB) Viewed 2315 times

Remarks:
- For my acquisitions i use generally a ASI533 camera, FITS format, RAW16, 300s exposure, Binning 1, capture area 3008x3008. This is the configuration used for the M92 and M56 targets described above.
- I saw many of my previous raw files for other acquisitions with this same above configuration for other targets with Sharpcap version older than 3.2.6383.0 and all time i have max values = 65535 for the 3 channels and all time he have no problem.
- I didn't updated Siril from many months (Siril 0.9.12). I also tried a stacking with DSS 4.2.3 and i have the same problem.

[admin]

Hi,

I guess the first question to ask is what the colour balance settings are set at for your camera when you are capturing – the ZWO cameras do allow colour balance adjustment to take effect in raw mode, so any changes to the colour balance settings could have the sort of effect that you are seeing. I can make the same sort of thing happen (the maximum value of the blue channel is significantly less than 65535) by setting the White Bal (B) control to less than 50 when I test with one of my colour ZWO cameras. This is clearly visible if you put the SharpCap built-in histogram into logarithmic mode. I
Cheers, Robin

[encelade18]

Hello Robin,

Thanks a lot for your answer,
However i don't understand that white balances informations could modify original raw informations into a raw file.
This is because i dont remember and i didn't recorded the white balance configuration i used on sharpcap for this acquisition set.
In my undestand, even if additionnal informations are added into the raw file, all the original captured informations are not modified or deteriorated.
And i dont use white balance informations with Siril.

Additionnaly, into the Siril settings, on debayer options, i choosen CFA FITS to 16 bits. Into the comment of this option, we can see that the normalisation is applied if the normalisation is stored in the FITS header. My question is: the informations was stored for red, green colors channels and not for blue ?

cfg_Siril_Settings_OtherDebayer.png (29.27 KiB) Viewed 2242 times

Regards
Olivier

[admin]

Hi,

the colour balance controls for ZWO cameras do affect the pixel values of the different colours when the camera is running in raw mode. My preferred approach is to leave them at their neutral values of 50 and do any colour balancing in postprocessing, but other people like to get the colour balance approximately right when capturing. These are the controls I am referring to

Capture.PNG (8.04 KiB) Viewed 2229 times

Whether you could call the image strictly raw if you have been able to adjust the colours in this way is debatable. I do know that some models of CMOS camera have separate analogue gains for the R/G/B channels in which case adjusting them separately would be perfectly legitimate. If however the colour balance adjustment is being done in digital space after ADC readout then I'm not convinced that it is true raw imaging any more.

SharpCap doesn't write any fits keywords that I can imagine would help Siril perform normalisation for any of the colour channels, so I can't really help you much with how that piece of functionality might work – may be asking the developer of Siril exactly what that option does and what headers it looks for would help explain what is going on.

Cheers, Robin

[encelade18]

Hello Robin,

Thank you very much for your answer,
I confirm that your description is coherent with what i saw on my previous acquisitions.

While searching other old problematics acquisitions in term of color balancing on the treatment, i found others series and all time i had either red bal <50 or blue bal < 50 (and i know it because generally i keep a screenshot of sharpcap config when i start acquisition (unfortunately screenshot not done for the acquisitions on M56...)).

In addition of your explanations related to set red and blue col = 50, i precise that if both red and blue col >=50, it seems like i have no problem to equilibrate the colors.
At last, i suggest that the informations of white balancing could appear in the exif field of the raws in the case of a effect will be done to the signal (as well as gain, exp time...) ? it will be helpfull to analyse problematics raw files.

Regards
Olivier

[encelade18]

Hello Robin,
For information, i remade a series of darks by trial and error with a white balance modified.
R=50 B=40 leads to a good result
I published on my website the result on my M56 aquisitions: http://encelade18.free.fr/photos/M56.html

Thank you very much Robin

[admin]

Excellent news, and thanks for sharing the results - hopefully it will help others who encounter the same problem.

Robin