Creating Master Bias, Dark, and Flat Calibration Frames

September 09, 2012  •  1 Comment

Creating Master Calibration Frames

It's a good idea to update your library of master Calibration frames every 3-6 months, even with a cooled CCD camera like my QSI 583wsg. Sensors change over time, and you may end up with sub-par calibration if you use older calibration frames.

Being a rabid PixInsight devotee, I use the process outlined by Vicent Peris for creating master Calibration frames, as outlined here. I find it simple and effective, and of course mathematically correct!

I collect 16 Bias, Dark, and Flat frames, with Bias and Dark at -15 and -20 degrees Celsius, which are the 2 common temperatures at which I run my camera. I generate Flats at -20 only, as I don't think there is much impact of temperature on the flat frames. I use a Gerd Neumann Aurora light panel for my flats. It's expensive, but I've never had luck with t-shirt or sky flats, and I don't like fussing around with those methods - the light panel is simple and works every time. Just adjust exposure until you get a consistent 1/3 to 1/2 full pixel wells, and you're good to go. Note that the panel, while very flat in its output across the panel, may very a bit in intensity for different colors, so you may need to adjust the exposure duration for L,R,G, and B.

Also note that the QSI cameras can expose flat frames at very short exposures with no problem. Some cameras with rotating shutters require a minimum length exposure in order to avoid shadowing from the shutter - this is not an issue with the QSI.

The processing steps for the master frames are straightforward:

  1. Bias and Dark master frames are created the same with, with the Image Integration process in PixInsight

  2. Don't normalize the images, and disable image weighting

  3. Use Winsorized Sigma clipping if you have enough frames - 16 is plenty, and I set the Sigma clip level to 3

  4. You can quickly process the Bias and Darks for each temperature and binning - once you have the Integration parameters set up correctly, just clear the prior raw frames and add the new ones, then rerun the process

  5. I always check the resulting Master frame with an Auto Screen Transfer Stretch (CRTL-A) to make sure everything looks OK, then I save it as a 32 bit floating point FITS.

  6. Generating the Master Flat is a 2 step process - calibration with the previously generated Master Bias and Dark frames, then Integration

  7. Run the Calibration process, and select the appropriate Master Bias and Dark that you just created

  8. Select Calibrate for both, and also select Optimize for the Master Dark. In my case, the Master Dark exposure was 300 seconds, my normal exposure time. Selecting Optimize will scale the dark current to the much shorter Flat exposures.

  9. Once the Flat frames are calibrated, they need to be Integrated to create the Master Flat. This time, use Multiplicative Normalization, and Flux Equalization in the Pixel Rejection section. Vicent does a good job of explaining these settings in his article. He uses Percentile Clipping for sky flats, but I use the same Winsorized Sigma clipping for light panel flats. Again, I save the Master Flats, one for R,G,B and L, as 32 bit floating point FITS.

Note: Be sure to save your Master frames with meaningful names - I use something like Dark-20_1x1_08282012 - the name encodes the temperature, binning, and date so that there isn't any confusion later.

Using the Master Frames

There are a couple of things to remember when calibrating light frames in PixInsight with these master calibration frames:

  1. Don't select calibration for the Master Flat - it's already been calibrated during creation
  2. Do select calibration for the Master Dark, so that the Master Bias is subtracted out. Whether or not you select Optimize is up to you.
  3. Unless you have an overscan area on your camera sensor, it doesn't matter if you select Calibrate for the Master Bias - I leave it turned off

 


Comments

Fernando Guimaraes(non-registered)
Thank you very much for the nice tutorials about PI. I really enjoy to appreciate a very well done job like yours, that in the end, helps all the community.

Thank you very much,

Fernando
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