Calibration of the WFPC2 HeII and [SII] Filters.

Observations of NGC 6720 {the Ring Nebula} will be used to determine the calibration constants for the important emission-line filters that isolate nebular HeII {F469N} and [SII] {F673N} emission. The pre-launch calibrations are inadequate because of possible temporal changes and the fact that these interference filters are used in a different configuration from that of the ground calibration. The Ring Nebula is a nearly ideal reference source as multiple 2.4"x4.0" samples have been accurately measured spectro-photometrically and five of the six samples can be imaged with one pointing of the HST.The method of derivation of the calibration constants will be the same as previously employed to calibrate the primary emission-line filters for the WFPC2 {F487N, F502N, F656N, F658N} and ACS {F502N, F658N, F660N} using the Orion Nebula as a reference source. However, Orion cannot be used for this calibration because the targeted lines are weak {[SII]} or absent {HeII} and the scattered-light continuum is strong. The Ring Nebula has strong HeII emission in its middle, strong [SII] emission in its main ring, and a weak {atomic only} continuum.