ISR 2024-08: Simulating the 600s flash for SPLIT-wavecals at LP6
July 08, 2024K. Rowlands et al.
The majority of COS science observations have used concurrent TAGFLASH Pt-Ne lamp exposures for wavelength calibration, whereby the wavelength calibration lamp is flashed at certain intervals during an exposure. However, the COS wavelength calibration lamp cannot be flashed when the aperture block is at a position greater than 113 steps, which corresponds to +5.4” arcseconds on the sky relative to Lifetime Position (LP) 1, due to a light leak through the Flat-field Calibration Aperture (FCA). Therefore at LP6, located at +6.5” above LP1, the aperture block must be moved to a different position such that the lamp can be flashed and the wavelength zero point can be correctly accounted for, a process called ‘SPLIT-wavecals’. To reduce overheads due to SPLIT-wavecals, we undertook a study to determine whether a lampflash could be effectively removed without significantly increasing the uncertainty on the wavelength calibration or smearing the line profile resulting from uncorrected Optics Select Mechanism (OSM) drift. This ISR first describes the tests of four methods used to simulate observations without the 600 s lampflash while correcting for OSM drifts, and then presents a description of the adopted method. The chosen method minimizes the uncertainty on the FUV wavelength calibration. We found that the along-dispersion shift (in pixels) at 600 s was best replicated by a fractional value of the shift at the end of the exposure, with an additional dependence on exposure time. This correction was implemented in CalCOS starting with version 3.4.0 for LP6 observations.