1.1.2 Spectral Filters

The WFPC2 contains 48 filters mounted in 12 wheels of the Selectable Optical Filter Assembly (SOFA). These include a set of broad band filters approximating Johnson-Cousins UBVRI, as well as a set of wide U, B, V, and R filters, and a set of medium bandwidth Strömgren u, v, b, and y filters.

Narrow band filters include those for emission lines of Ne V (3426Å), CN (~3900Å), [OIII] (4363Å and 5007Å), He II (4686Å), H (4861Å), He I (5876Å), [OI] (6300Å), H (6563Å), [NII] (6583Å), [SII] (6716Å and 6731Å), and [SIII] (9531Å). The narrow-band filters are designed to have the same dimensionless bandpass profile. Central wavelengths and profiles are uniformly accurate over the filter apertures, and laboratory calibrations include profiles, blocking, and temperature shift coefficients.

There are also two narrow band "quad" filters, each containing four separate filters which image a limited field-of-view: the UV quad contains filters for observing redshifted [OII] emission and are centered at 3767Å, 3831Å, 3915Å, and 3993Å. The Methane quad contains filters at 5433Å, 6193Å, 7274Å, and 8929Å. Finally, there is a set of narrow band "linear ramp filters" (LRFs) which are continuously tunable from 3710Å to 9762Å; these provide a limited field-of-view with diameter ~10".

At ultraviolet wavelengths there is a solar-blind Wood's UV filter (1200-1900Å). The UV capability is also enhanced by control of UV absorbing molecular contamination, the capability to remove UV absorbing accumulations on cold CCD windows without disrupting the CCD quantum efficiencies and flat field calibrations, and an internal source of UV reference flat field images.

Finally, there is a set of four polarizers set at four different angles, which can be used in conjunction with other filters for polarimetric measurements. However, due to the relatively high instrumental polarization of WFPC2, they are best used on strongly polarized sources (>3% polarized). Sources with weaker polarization will require very careful calibration of the instrumental polarization.