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Hubble Space Telescope
COS FUV Calibration

New COS/FUV Flux-calibration for all modes

New response curves have been derived for all COS FUV modes and lifetime positions, and have been delivered to On the Fly Recalibration (OTFR). These flux calibration reference files (FLUXTAB) take into account any changes in sensitivity with lifetime position, as well as recent improvements to the G140L/1280/FUVB wavelength solution, and the correction of low-order response variations in the flat-field (see below). These new files bring the absolute flux calibration of all COS/FUV modes, except for G130M/1055 and G130M/1096, to within 5%. Currently, the flux accuracy of the G130M/1055 and G130M/1096 central wavelength settings is 20%. Data acquired recently will allow us to improve the flux accuracy of these modes in the future. The relative flux-calibration is accurate to within 2%, with the exception of a discontinuity in flux levels between data taken before and after March 23, 2012. This discontinuity is seen on segment A and appears to be correlated with a change in that segment's high voltage setting introduced to mitigate gain-sag effects. This discontinuity is ~4% for G140L data, and 2% or less for G160M and G130M data.

Users are advised to re-retrieve their data from the archive to get the most up-to-date calibrations. Please note that, as mentioned above, this reference file was updated along with a set of other reference files (flat field and dispersion solution, see below). These files should all be applied simultaneously to calibrate data. It is not advisable to retrieve just this single file and apply it to older datasets as this can lead to the introduction of artifacts.

New Flat-Field reference file for COS FUV

A new COS FUV flat field reference file has been delivered for use in On the Fly Recalibration (OTFR). This new reference file provides a correction for low-order response variations (L-flat) observed on both the FUVA and FUVB detectors. The magnitude of these variations is up to 6% for FUVA and ~2% for FUVB. The calibration of these L-flat features was previously folded in to the flux-calibration by providing a unique response curve for every observing mode. Implementing this correction in the flat-field reference file allows a more consistent and accurate derivation and implementation of the flux-calibration response curves (see above).

Users are advised to re-retrieve their data from the archive to get the most up-to-date calibrations. Please note that, as mentioned above, this reference file was updated along with a set of other reference files (flat field and dispersion solution, see below). These files should all be applied simultaneously to calibrate data. It is not advisable to retrieve just this single file and apply it to older datasets as this can lead to the introduction of artifacts.

New dispersion relation for COS FUV G140L/1280/FUVB

The low-response on the FUVB segment of the G140L/1280 central wavelength setting has previously prevented an in-depth analysis of the accuracy of the wavelength solution. The low response is due to the MgF2 cut-off in the HST optics, and before COS installation on HST, it was not known if Segment B of G140L/1280 would have any response at all below ~1080 Angstroms. The accumulation of calibration and GO observations in the archive has recently permitted a more thorough investigation to be completed. This has shown that the lack of wavelength calibration lamp lines on the G140L/1280/FUVB segment causes a fundamental uncertainty on the wavelength solution. Although this measurement cannot be made directly from G140L data, analysis of data obtained with other COS/FUV modes that contain wavelength calibration lamp lines on both segments indicates that the G140L/1280/FUVB wavelength accuracy is limited to ~220km/s due to the lack of wavelength calibration lamp lines. Utilizing some archival datasets with absorption features and high signal-to-noise, a new wavelength dispersion solution has been derived for G140L/1280/FUVB that approaches this fundamental limit; improving the wavelength accuracy from >600km/s down to 250km/s. This improved wavelength solution has been implemented in the dispersion relation reference table (DISPTAB) for use in the On the Fly Recalibration (OTFR) pipeline.

Users are advised to re-retrieve their data from the archive to get the most up-to-date calibrations. Please note that, as mentioned above, this reference file was updated along with a set of other reference files (flat field and dispersion solution, see below). These files should all be applied simultaneously to calibrate data. It is not advisable to retrieve just this single file and apply it to older datasets as this can lead to the introduction of artifacts.