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February 2017 STAN

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STScI Analysis Newsletter (STAN)

Cosmic Origins Spectrograph (COS)

February 2017

In this STAN we announce the release of an updated NUV DISPTAB as well as recommendations for optimal NUV extraction box heights.


Updated COS NUV Wavelength Dispersion Solution Reference File Delivered

A new COS NUV wavelength dispersion solution reference file (DISPTAB) has been delivered for use in the COS calibration pipeline--12e1345gl_disp.fits. This file includes updated wavelength zero-points for all G185M cenwave/stripe modes and specific G285M, G225M, and G230L modes that were previously not optimally calibrated. The new updates lead to overall uncertainties in the wavelength calibration of approximately 0.3 Å (36km/s) for G185M and 0.17 Å (15km/s) for all other medium-resolution gratings. G230L now has an overall uncertainty of ~1.8 Å (~175km/s). Users interested in an improved wavelength calibration for any of the updated NUV modes are encouraged to retrieve their data from the HST archive.

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COS NUV Extraction Box Heights for Faint Targets

The COS team studied the effect of varying the HEIGHT parameter in the XTRACTAB reference file for COS/NUV data. An extraction height of 8 pixels is used when estimating the signal-to-noise ratio in the COS exposure time calculator (ETC), while CalCOS uses a height of 57 pixels to create the final spectra. This is because the flux calibration for bright sources is performed using a spectrum extracted with a box large enough to encompass all the flux, while for faint sources, the maximum possible S/N is obtained by minimizing the dark rate and thus reducing the extraction height. We recommend that PIs observing faint targets consider optimizing the extraction height to the specifics of their targets. We also recommend that extraction heights smaller than 13 pixels should not be used to ensure that no signal is removed. PIs considering customizing their extraction height should also make sure that the B_SPEC value for each stripe is well centered on the 2D spectra While spectra extracted with a 57-pixel HEIGHT are not susceptible to centering errors, vignetting may occur if spectra extracted with a small height are not well centered.

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