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Hubble Space Telescope
March 2015 STAN

| STScI Analysis Newsletter (STAN) 
| Cosmic Origins Spectrograph (COS)
| Space Telescope Imaging Spectrograph (STIS) 
| March 2015

In this Newsletter we provide some updates of interest to 
COS and STIS users who will be drafting and submitting Phase I 
HST programs over the next weeks:



Observing Background Limited Targets with the STIS FUV MAMA
and the COS FUV XDL Detectors
The detector background rate of the STIS FUV MAMA detector varies by large factors both over time and as a function of position on the detector. The STIS Exposure Time Calculator (ETC) adopts a conservative value for this detector background to ensure that proposed observations will be feasible under most observing conditions. However, for certain background limited observations this can result in prohibitively large predicted exposure times. In this article we discuss strategies for working around the highly variable STIS FUV MAMA detector background and also include instructions for recalculating the exposure time estimate when adopting an alternate background rate estimate. The COS ETC adopts a large extraction box in its calculations to ensure that all the flux is included. However, for background limited observations this can include unnecessarily large amounts of detector dark background. This is especially true when using the G140L grating as the cross-dispersion width of the profile varies strongly as a function of wavelength. We include an example of how to recalculate the exposure time to better estimate the S/N at a particular wavelength. -------------------------------------------------------------- Limitations on the Calibration Accuracy for COS/FUV Observations
of Extended Targets and for COS/FUV BOA Observations in Cycle 23
With the move to lifetime position 3 of the COS/FUV detector, the calibration of two observing scenarios, COS/FUV observations at LP3 targeting extended objects and COS/FUV science observations of either point or extended sources at LP3 that use the Bright Object Aperture (BOA), will become more complex, and the standard pipeline calibration will not always produce the flux accuracy achieved for point sources observed with the Primary Science Aperture (PSA). For these cases it will be the responsibility of the user to evaluate the adequacy of the spectral extraction and, if necessary, undertake any customized extractions. In this STAN article, we describe the limiting factors in the calibration of such modes, and the impact for users. -------------------------------------------------------------- Exposure Time Calculator 23.1 Updates for COS and STIS The Exposure Time Calculator (ETC) has been updated and revised in support of the Hubble Space Telescope Cycle 23 call for proposals. In this STAN article we report on the newly supported ND slits, available in the STIS spectroscopic ETC in the MAMA UV echelle slit pulldown menu, as well as in the STIS target acquisition ETC under the ACQ/PEAK options. Additionally, several minor changes to the STIS and COS ETC pages have been performed to enhance the clarity of the calculation results as well as reflect changes in detector behavior and instrument throughput. -------------------------------------------------------------- OSM Drift and NUV Wavelengths For COS, spectral light is transferred to the NUV channel by a mirror on the FUV Optics Selection Mechanism (OSM1) and then dispersed by a grating on the NUV OSM2. With usage, these mechanisms can drift in the dispersion direction causing a small change in the wavelength ranges observed, particularly for the NUV. This article describes such changes and alerts users to check their NUV observations to ensure that they cover the wavelengths of interest. +----------------------------------------------------------+ | Need help? +----------------------------------------------------------+ | To subscribe or unsubscribe to the STAN, send a message | to with a blank subject line and | only the following in the body: [un]subscribe cos_news | or [un]subscribe stis_news. +----------------------------------------------------------+ | The Space Telescope Science Institute is operated by the | Association of Universities for Research in Astronomy, | Inc., under NASA contract NAS5-26555. +----------------------------------------------------------+