What is the status of the Linear Ramp Filters?
WFPC2 contains four “linear ramp filters” which provide a narrow band imaging capability (bandpass FWHM ~ 1.3% of central wavelength) at all wavelengths in the range 3710 to 9762 Å. These filters, known as FR418N, FR533N, FR680N, and FR868N, are essentially narrow band filters whose central wavelength varies as a function of position on the filter. To use these filters, observers merely specify filter and aperture names “LRF” and the desired central wavelength in their proposals. Scheduling of LRF observations by STScI requires an accurate mapping from desired wavelength to target placement in the WFPC2 field of view. This wavelength / aperture position calibration was completed in early May 1995. The results are based largely on pre-flight JPL tests, which give the run of central wavelengths on the individual filters, and on March 1995 on-orbit observations where flat fields were taken through linear ramp filters crossed with narrow band filters, so as to define the registration
