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Hubble Space Telescope
New Flat-Field reference file (FLATFILE) for COS FUV

Prior to September 18, 2013, independent grid wire flat field reference files were applied to the G130M and G160M gratings to correct the grid wire shadows. For the G140L grating, however, the grid wire shadow regions were flagged as bad and excluded from final data products. New analysis of the grid-wire shadows shows them to be independent of both grating and lifetime position. The newly delivered flat field reference file will now be applied uniformly to all FUV observations, including those using the G140L grating. The application of this reference file to G140L data obtained at both lifetime positions will result in better signal to noise in multiple FPPOS datasets, and fewer gaps in coverage with a single FPPOS.

In addition to the grid-wire shadows, a 2D empirical correction has been derived and implemented for a selection of deep features on the edges of the FUVB detector. These features are similar in appearance to grid-wire shadows, but are due to residuals in the geometric distortion correction and therefore change in strength, shape, and x-location as the y-location of the spectrum varies. Previously, these features were treated as grid wire shadows and the correction from lifetime position 1 was applied to lifetime position 2, leaving the deep features uncorrected and adding in additional features. The flat field reference file delivered on September 18, 2013 corrects these features properly.

Users interested in G140L data obtained at either lifetime positions 1 or 2, as well as users interested in data near the edges of Segment B, obtained with any grating at lifetime position 2 (i.e. after July 23 2012) , are advised to re-retrieve their data.