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Updates to the COS ETC

Significant updates to the ETC for COS have been implemented for ETC version 21.2. In particular, dark rates for all FUV modes are more accurately calculated, the G130M/1055 and G130M/1096 modes have been updated to reflect their improved performance at a new focus position, and a new option has been added to the NUV ACQ/Imaging ETC.

COS FUV Dark Rates

Previous versions of the ETC code assumed a single dark rate for the entire FUV detector. At times, the dark rate between the two segments has differed by as much as a factor of two, needlessly making certain observations longer than necessary. The current FUV detector dark rates appear stable over the past year, with a significant decrease occuring for Segment A. Therefore, the current ETC version incorporates the ability to distinguish between detector specific dark rates and reflects the drop in dark rate currently observed. The previous value of the dark rate was 4.37x10-6 cts/pixel/s, whereas the current values are 3x10-6 cts/pixel/s for Segment A and 2x10-6 cts/pixel/s for Segment B.

COS/FUV G130M/1055 and G130M/1096 Modes

The G130M/1055 and G130M/1096 modes, the so-called Blue Modes, were newly implemented in ETC Version 21.1, but did not reflect significant improvements to expected performance due to a focus change that optimized each modes' resolving power. As a result, the extraction heights and resel sizes needed to be updated to reflect the optimized performance of the Blue Modes. ETC Version 21.2 includes these updates. One caveat is the wavelength dependence of the resel sizes--currently the ETC assumes a resel size of 8 pixels for G130M/1055, and 9 pixels for G130M/1096, which correspond to their optimal resolving powers. Users who wish to make S/N or exposure time calculations based on a resel size at a different wavelength are encouraged to visit the ETC News and Known Issues Page for correction factors to ETC calculations.

COS Imaging Target Acquisition

Finally, the COS Imaging Target Acquisition ETC now has an additional option to calculate acquisitions using the ACQ/SEARCH option. The main differences between ACQ/SEARCH and ACQ/IMAGE calculations are the S/N requirements and the calculation of the background. For ACQ/IMAGE calculations, a checkbox with a size of 9x9 pixels is assumed, whereas the ACQ/SEARCH option simulates a 220x470 pixel subarray used in this acquisition algorithm. For all ACQ/SEARCH acquisitions and ACQ/IMAGE acquisitions using the Primary Science Aperture (PSA) the recommended S/N is 40, while for ACQ/IMAGE acquisitions using the Bright Object Aperture (BOA) the recommended S/N is 60. These requirements are selected as default choices within the required S/N box for each acquisition option in the ETC. Users who anticipate using the ACQ/SEARCH mode and who made Imaging Target Acquisition ETC calculations of faint targets during Phase I may discover a significant increase in exposure time due to the larger ACQ/SEARCH background. In this case, users are encouraged to contact their Contact Scientist (CS) for further consultation.