Non-Linearity Correction
Introduction
NSFCam uses an H2RG detector. Like all such arrays, the signal from the detector does not have a perfectly linear relationship to the incoming flux density. Furthermore, the non-linearity varies from pixel to pixel. As such, precision relative photometry requires correcting for the non-linearity of the detector on a pixel-by-pixel basis.
To this end, we have measured the non-linearity of the NSFCam detector. We used the thermal glow of the bottom of the telescope as a steady source of light, and took flats over a range of exposure times to sample the dynamic range of the detector. This was done with high well depth, 3 microsecond pixel sampling, and both in double and raw readout modes.
The non-linearity vs. ADU for each pixel was fit with a 4th order polynomial, yielding 5 coefficients per pixel. We made 5 arrays, one per coefficient, to be used for the non-linearity correction. Our non-linearity correction code reads in these 5 FITS files, as well as the science image, and outputs a linearity corrected science image.
How It Works
Step 1: Download the IDL-based non-linearity correction program (it's a simple text file), and the 5 coefficient arrays.
Step 2: Put these files into the same directory as your data.
Step 3: Start IDL, and compile the program with this command: compile linearity_correct.pro
Step 4: Run the program with the following command: go2, 'filename.fits'. Here, filename.fits is the science file that will be linearity corrected. The program will produce a FITS file output called filename.corrected.fits
The Files You'll Need for Double Readout Mode, High Well Depth
The IDL program to correct for the detector's non-linearity is here
Coefficient 0 is here
Coefficient 1 is here
Coefficient 2 is here
Coefficient 3 is here
Coefficient 4 is here
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Last modified 23 September 2013