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Hubble Space Telescope
Calibration of COS Observations of Extended Targets in Cycle 24

With the move to Lifetime Position 3 (LP3) of the COS/FUV detector, which occurred in February 2015, the calibration of COS/FUV observations of extended objects, and of COS/FUV observations using the Bright Object Aperture (BOA), has become more complex. In these cases, the standard pipeline calibration will not always produce the flux accuracy achieved for point sources observed with the Primary Science Aperture (PSA), and it is the responsibility of the user to evaluate the adequacy of the spectral extraction and, if necessary, undertake any customized extractions. For more details about changes to the spectral extraction with the move to LP3 please see COS ISR 2015-03.

1. Extended targets with COS/FUV

The 2.5” diameter circular aperture of COS was optimized for point sources and so COS spectra of extended objects have significantly lower spectral resolution. In addition, light originating outside of the central 0.4” of the PSA will be increasingly vignetted (see Section 5.9 of the IHB). While the performance of COS when observing extended targets was never optimal, the extended target flux calibration was still reasonably accurate up to the COS/FUV move to Lifetime Position 3 (LP3) in February 2015. This will however not be the case at LP3 due to its location, close to the gain-sagged Lifetime Position 1 (LP1) regions, and the new two-zone extraction algorithm required by the small LP1-LP3 separation (see February 2015 COS STAN).

For extended targets larger than ~0.6” (FWHM), the spatial distribution of incoming light on the COS/FUV detector is significantly wider than that of a point source. As a result, the light distribution of extended targets may overlap low-gain regions at LP1, located 2.5” above LP3, and may incur significant flux loss.

In addition, the two-zone extraction algorithm implemented to support LP3 science observations extracts counts in 2D profiles that tightly follow the optical footprint of the light from a point source on the detector, thus reducing the number of dark counts and avoiding large chunks of spectra from being discarded when bad or LP1 sagged pixels overlap with the wings of those profiles. This algorithm therefore provides accurate calibration for point sources, but the flux calibration of extended targets (i.e., with FWHM larger than ~0.6”) may be significantly off because the spatial distribution of their flux on the detector is wider than that of a point source.

Combined with the aperture vignetting, these effects may result in an unreliable flux calibration for COS/FUV/LP3 observations of extended targets (larger than 0.6” FWHM).

2. COS/FUV science observations with the BOA

Like the PSA, the bright object aperture (BOA) is a circular aperture 2.5” in diameter, which includes a neutral density filter to attenuate the throughput and allow observations of targets that would be too bright to be observed with the PSA. The spectral resolution of the BOA is significantly worse than the resolution of the PSA, hence this observation mode is very sparsely used.

The new extraction algorithm implemented for LP3 observations (see February 2015 COS STAN) requires templates for the profile or footprint of incoming point source light on the detector. Obtaining these template profiles for all COS/FUV modes represents a significant calibration effort, which also comes at an expensive observing cost. For instance, profiles for PSA observations at LP3 were obtained as part of program 13931, a 19-orbit observing program, which executed in September 2014. Profiles for BOA observations are expected to differ significantly from PSA profiles because their optical path is different. However, since the installation of COS on the Hubble Space Telescope (HST) in 2009, few observing programs have made use of the BOA for their science observations. Due to this low demand for COS/FUV BOA science observations and the high cost of obtaining template profiles, profiles for BOA observations at LP3 were not acquired. As a result, the new extraction algorithm implemented for LP3 operations uses profiles applicable for the PSA to calibrate BOA observations. COS/FUV BOA observations at LP3 may therefore have an unreliable flux calibration for data obtained after Feb 9 2015 at LP3.

We note that target acquisitions using the BOA are still supported and are not affected.

3. Implications for users

Users should make themselves aware of the limitations in the calibration of COS/FUV observations of extended targets or with the BOA at LP3. When preparing their phase II, users will be required to specify whether their targets are point sources or extended.