Instrument Science Report 2009-04:
Wavelength Stability of the WFPC2 Narrow Band and Linear Ramp Filters
J. Biretta and P.L. Lim
May 11, 2009
As part of the WFPC2 close-out calibrations, we test for long-term changes in the wavelength calibration of the narrow band and linear ramp filters. Relative wavelength measurements are made by crossing each narrow band filter with one of the linear ramp filters and taking a VISFLAT, and then noting the position of the resulting bright spot in the field of view. We test the stability of the central wavelengths by using this procedure on data from 1995 and 2008, and then compare the results. Twelve pairings of narrow band + ramp filter were tested in this way, and most were found to be highly stable. Ten showed a central wavelength change less than 1.1± 0.6 Å. The largest change was for FR868N+F953N of 3.8 Å, and the second largest change was for FR680N+FQCH4N-C of 1.8 Å. In general, the wavelength changes are a small fraction of the filter bandwidths (7% or less) and should not impact the vast majority of science observations. The four narrow band filters most often used for science: F502N, F656N, F658N, and F673N, were noted to be especially stable.
The Complete Paper (PDF 4.3 MB) is available.