[Sticky] A Light frame calibration workflow using a Bad Pixel Map, darks, flats, bias for a Nikon D750 DSLR camera
This is a workflow example for correctly calibrating light frames using bias, flat & dark frames.
Data is courtesy of Michel.
The first step is to create a Bad Pixel Map if you don't have one for the sensor of your camera yet:
The Bad Pixel Map can always be used to calibrate your light and flat frames.
Read this if you haven't already:
Okay let's create the master calirbation frames:
Michel supplied me with the following frames:
5 bias of iso 100 exposure was 1/4000th second
5 flats of iso 100 exposure of 6 seconds
5 darks of iso 800 exposure 120 seconds
5 lights of iso 800 exposure 120 seconds
I used the flats and darks to make a Bad Pixel Map of Michel's sensor
In the frame list panel you'll see all details of all frames.
Using these frames, we can perform an optimal data calibration of the lights:
- the flats can be calibrated with bias frames of the same iso 100 (rule 1 in FLAT frame calibration in data-calibration-priciples-must-read ) and the BPM
- the lights can be calibrated with dark frames of the same iso and exposure as the light frames ( rule 2 in DARK frame calibration in data-calibration-priciples-must-read ) and the BPM.
Since the Bias frames have a different ISO than the lights, the masterbias will NOT be subtracted from the darks. So the Masterdark will contain the bias pedestal.
This calibration workflow meets the requirements that the bias pedestal is removed once and only once ! from both the light frames and the flat frames:
By subtracting a masterbias from the flats, the bias pedestal is removed from the flat frames.
By subtrating a masterdark (which isn't bias calibrated) from the lights, the bias pedestal is removed from the light frames.
In the menu 2) CALIBRATE, I set all integration modes to median since I only have 5 frames of each to work with. If you have more than 20 frames, average is the preferred method with sigma of winsorized sigma outlier rejection. (Never use Linear Fit Clipping for outlier rejection of calibration frames)
I now click on "calibrate" and wait for APP to finish. After completion of creating the master calibration frame, the file list panel is updated:
At the bottom of the frame list panel, you'll see the added master calibration frames.
And notice the information in the frame column for the light frames and flat frames:
For the light frames we see the marks D, F, BPM, CA, meaning
- D a master Dark is found and will be applied in light frame calibration
- F a master Flat is found and will be applied in light frame calibration
- BPM a bad pixel map is found and will be applied in light frame calibration
- CA , means "are CAlibrated" and ready for further processing from 3) STAR ANALYSIS
For the flat frames we see:
- B a master Bias is found and will be applied in flat calibration
- BPM, a Bad Pixel Map is found and will be applied in flat calibration
Since the master flat is already created the flat information actually means "was applied" instead of "will be applied".
Let's load one of the images now with both the linear(l) and l-calibrated image viewer modes to visually verify that calibration is working, always try to verify this before continuing. Left frame is uncalibrated, right frame is calibrated.
Data calibration is working like it should 😉 !
(I created the last 2 images with the save button and linear background neutralization (N-BG) on and stretch on, below the histogram)
Main developer of Astro Pixel Processor and owner of Aries Productions