Master dark frame problems?

[Fukinagashi]

I've followed without any issue until now the procedure for frame pre-processing on Light Vortex Astronomy.
I do use a QHY163M COLDMOS camera and as I have understood bias frames are not necessary for CMOS.
So my master dark is obtained without calibration.
Dark frames have been taken using same Gain, Offset, Temperature and Exposure time as for light.
Problem is that calibrated light frames are completely dark (apart from some hot pixels) until I do not check Optimize in Master Dark section.
In my previous processing I did never use Optimize as I do use dark frames taken at the same conditions as lights (if I understand Optimize rescales dark frames to match lights conditions).
Same if I simply try to make difference of a light frame and the master dark using pixelmath.
Any suggestion?

[bulrichl]

The use of a NOT calibrated MasterDark (same temperature, same gain, if applicable same offset, same exposure time) is just fine for cameras with CMOS sensors. In ImageCalibration
- uncheck the MasterBias section,
- check the MasterDark section, select your MasterDark, uncheck both 'Calibrate' and 'Optimize',
- if you use a MasterFlat (highly recommended), check the MasterFlat section, select your MasterFlat, unchek 'Calibrate'.

For the preparation of a MasterFlat you need either a MasterBias (or a MasterFlat-Dark) that are made by integrating the corresponding calibration frames. The individual flat frames have to be calibrated with the MasterBias (or MasterFlat-Dark). Integrate the calibrated flat frames to make the MasterFlat with following settings:
Image Integration
- Combination: Average
- Normalization: Multiplicative
- Weights: Don't care (all weights = 1)

Pixel Rejection
- Normalization: Equalize Fluxes

Have alook at my guide: https://pixinsight.com/forum/index.php?topic=11968.0 if you need the explanation in more detail.

Bernd

[Fukinagashi]

Dear Bernd. It is the same procedure I followed and that I follow since I started using PI.
I build a master dark_flat then I calibrated flats with master dark_flat building a master flat.
Then I built a master dark (same gain, offset, temp and exposure).
If I calibrate lights without checking optimize master dark I obtain completely black calibrated frames...

[Fukinagashi]

In case let's try to subtract (pixelmath) from this light (NGC7000 in Ha):

https://www.dropbox.com/s/hmhtijvzae3gz8s/NGC7000.fit

the following master dark:

https://www.dropbox.com/s/oxc219yxjxmnw44/master_dark.xisf

Why should the result of subtraction a completely black image?

[bulrichl]

Maybe you have to apply an 'Output pedestal' (in the 'Output File' section of ImageCalibration) when calibrating the lights. It's late here, so I will have a look at your files not before tomorrow.

Bernd

[Fukinagashi]

Thank you Bernd (it's late here too).
In the meanwhile I'll try some pedestal values...
See you tomorrow!

[bulrichl]

These are the statistics of your MasterDark and your light frame:


               NGC7000          master_dark
count (%)      100.00000        100.00000
count (px)     16398432         16398432
mean           1586.1           4967.681
median         1536.0           4936.616
stdDev         670.9            316.664
avgDev         272.0            91.974
MAD            224.0            59.077
minimum        16.0             4541.334
maximum        65504.0          65504.000

No wonder that the difference (NGC7000 - master_dark) is a black image: the light NGC7000 has a mean of about 1590 and the master_dark of about 5000. The application of a pedestal in the light frame calibration is not going to fix this issue. The MasterDark is totally wrong.

The settings for the integration of the dark frames shall be:
- Combination: Average
- Normalization: No normalization
- Weights: Don't care (all weights = 1)

Pixel Rejection
- Rejection algorithm: Winsorized Sigma Clipping
- Normalization: No normalization

Take care to integrate only dark frames.

It is important to use the same acquisition software (and same settings of gain and offset) for light and dark frames. If you changed the camera driver, old calibration frames will not work anymore.

Please send a link to a few (maybe 4) dark frames.

Bernd