May 18, 2023J. Roman-Duval et al.
High-frequency variations in the flat-field of the COS FUV detector result in fixed- pattern noise that limits the signal-to-noise achievable by the instrument. The effects of fixed-pattern noise can be reduced via spectral dithering by utilizing multiple FP-POS positions, which correspond to one motor step rotation of the optical select mechanism. There are four FP-POS offsets available for each central wavelength. Here, we mea- sure the maximum achievable S/N, (S/N)max, as a function of the number of FP-POS included in COS spectra for each segment and cenwave of the G130M, G160M, and G140L gratings. We use deep calibration observations aimed at measuring the two- dimensional spectral profiles of the different COS observing modes at LP3 to estimate the fixed pattern noise and resulting limiting S/N. As expected, we find that (S/N)max increases as the number of FP-POS used increases. (S/N)max also varies with wave- length (i.e., across a segment), grating, and cenwave. This variation can be attributed to changes in the width of the cross-dispersion profile. For settings or detector regions with wider cross-dispersion profiles, the fixed pattern noise is averaged over a larger number of two-dimensional pixels, resulting in a lower level of fixed pattern noise in extracted, one-dimensional spectra. With G130M, users can reach S/N of about 25, 30, and 35 with 2, 3, and 4 FP-POS, respectively. With G160M, users can expect to reach S/N of 22, 28, and 32 with 2, 3, and 4 FP-POS, respectively. And with G140L, a maxi- mum S/N of 23, 29, and 36 can be reached with 2, 3, and 4 FP-POS, respectively. Users are encouraged to use as many FP-POS as allowed by science requirements in order to maximize the legacy value of their data.