We want to put raw DECam data through the stack, but in order to do that we need to add Instrumental Signature Removal (ISR) for DECam. Here are the steps that the NOAO Community Pipeline (CP) performs on the raw data. Single-frame processing is covered in section 4.3.1 (pg. 100-108) of the NOAO Data Handbook. The calibration reference files are covered in section 4.3.3.
-Description of CP processing by Frank Valdez.
-Description of DES Y1 data release.
1. Bias Correction: Overscan subtraction and Master Bias image subtraction.
2. Cross-talk correction: most noticeable from amplifiers of the same CCD (~0.01%).
3. Artifact Masking (saturation, bad pixel masking & interpolation): CRs are detected using “sharp” edges. BPM??
4. Nonlinearity correction: The DECam CCDs exhibit nonlinearity at low and high counts, but this varies from chip to chip. A look-up table is used capture the derived corrections.
5. Flat-field correction: flat field correction using a Master Dome-flat that has been corrected for large-scale effects (camera reflections and difference in response between domeflat illumination and dark sky) using “star-flats”.
6. Fringe correction: The two reddest bands (z and Y) show fringe (interference) patterns. The fringe pattern is temporally stable. A scaled fringe-pattern template is subtracted from the science image.
7. Astrometric calibration: Centroids of detected stars in the image are compared to a reference catalog (2MASS) to find corrections for each chip. Higher-order distortion terms (TPV) are kept fixed.
8. Photometric calibration: An estimate of the photometric magnitude zero-point is made by comparing the instrumental magnitudes to the calibrated USNO-B1 magnitudes.
9. Sky pattern removal: Subtract the pupil ghost/pattern (~10% of photons in dome-flats). Large-scale variations across focal plane are removed and produce a background mean of zero (i.e. sky subtraction).
10. Illumination correction: Makes the ensemble response to flux from the night sky uniform across the field of view. Made by combining long exposures (per band) and masking out sources. This dark-sky illumination calibration is divided into each individual exposure.
- Dark-correction is also not performed as the dark current is very low.
- Brighter-fatter correction is not yet implemented in the CP.
- The sky area per pixel changes with radius slightly (~1%). This is correcting in the remapping or warping of images to the tangent plane.