During Cycles 17 and 18, dispersed-light target acquisitions with the FUV detector could be performed using only grating G130M, because the spectra produced by other gratings led to large uncertainties when the PEAKXD algorithm (which centers the target in the cross-dispersion direction) was applied. Earlier this year, we implemented a new PEAKXD algorithm designed to combat the effects of Y walk on the FUV detector. The new algorithm works with all of the COS FUV gratings, and observers are invited to use them all in Cycle 19 Phase II proposals.
The new algorithm considers only data from segment A of the FUV detector and requires a signal-to-noise ratio of 25 to compute an accurate centroid. To estimate the exposure time necessary to achieve this S/N, use version 19.2 (or later) of the COS Exposure Time Calculator (ETC) and request the exposure time needed to obtain a S/N ratio of 25. On the results page, look for the line, "Time Required for Requested SNR in Segment A only." This is the exposure time required for a PEAKXD exposure. Each PEAKXD should be followed by a set of PEAKD exposures to center the target in the dispersion direction. PEAKD exposures employ both segments of the FUV detector and require a S/N of 40; their exposure time should be estimated in the usual way.