Color Dependent Flat Field Tools

As detailed in ISR-99-02, the pipeline flatfield data for NICMOS is made with the use of onboard flatfield lamps which have an intrinsically blue color over most of the NICMOS sensitivity range. Sources with extreme colors do exist and broadband images of such sources may require special treatment.

A common image processing technique for images that don't flat well is to use flats taken through filters of different (but nearby) color. We have expanded on this technique for NICMOS observations by producing two routines and associated calibration files to produce linear combinations of existing flatfields: interflat and bandflat.

The script bandflat.cl is an IRAF/STSDAS script that will take an input spectrum and camera and filter and produce an average of narrowband flats weighted by the expected contribution of the source.

The script interflat.cl is an IRAF/STSDAS script that will accept the NICMOS camera number, the desired wavelength for the interpolated flat (in microns), and the name of the output file. It will do the linear interpolation by multiplying the slopeN file by the wavelength and adding the interceptN file to that product, where N is the camera number.