Dark-Sky + "Correction Map"

The first and perhaps simplest alternative is to continue to flat-field images using current dark-sky technique, and then construct a photometric correction map and apply that as a secondary flat-field correction. The correction map is a measure of the photometric response variations across the focal plane produced by measuring a bright star or group of stars at finely spaced intervals around the array. The brightness of the star would be measured at each position, and a smooth surface would then be fit to the observed brightness variation pattern. This can be thought of as a "super" ROUND map such as that shown in Figure 1. Figure 2 shows such a map that was constructed for a NICMOS 3 equipped camera on the Steward Observatory 1.55m telescope using a 10x10 sample of observations of a single bright star.

Advantages:

This process would use the existing 2MAPPS design, but would use only a single secondary flat field image for each night. The construction of the correction map would be done once per night, or less frequently if it is found to be stable, and would use existing pieces of the pipeline.

Disadvantages:

This process still uses the night sky to flat-field so still has unknown color effects. The frequency of recalibration of the correction map is unknown. Finally, the in scan-resolution of the correction map is limited to 1/6 of the array size given the geometry of 2MASS scans, so higher frequency photometric response variations will remain uncorrected.

  
Figure: Photometric response correction map constructed for a NICMOS 3 camera on the Steward Observatory 1.55m telescope. A bright star was images in a 10x10 grid pattern on the focal plane, and a smooth 2-d surface was fit to the resulting matrix of photometry to create this image.