Routine monitoring of the COS spectrographic sensitivity
has shown that the throughput of a number of modes is
declining over time. This includes all COS FUV modes as
well as the bare aluminum NUV G225M and G285M gratings. In
contrast, throughputs for the MgF2 coated G230L and G185M
NUV gratings appear not to have changed. The mean trends
derived for a number of routinely monitored central
wavelength settings are shown in the figure.
The G225M and G285M gratings are uncoated bare Aluminum
gratings. Some degradation in the throughput of these gratings
had been observed prior to launch and had been attributed to the
formation of a surface oxide layer. It had been hoped that this
degradation would cease on-orbit. However, periodic monitoring
observations of external standard stars show that the on-orbit
rates of throughput decline appear to be as big or bigger than
the prelaunch estimates. The throughput of the G225M grating is
dropping at a rate of about 3 %/yr, while the G285M's throughput
is decreasing at about 10 %/yr. For each of these gratings, the
throughput decline is roughly constant with wavelength. In
contrast, the throughput of G185M and the first order parts of
the G230L spectra have been changing at a rate of less than
1%/yr. Uncertainties for the stripes that contain the 2nd order
G230L spectrum are larger.
For the FUV modes, the rate of change appears to depend
primarily on wavelength. The throughput is declining at
about 3%/yr for wavelengths shorter than 1400 Angstroms,
and by as much as 11%/yr at 1800 Angstroms. The possible
causes of this decline are still being investigated, but
the trend with wavelength appears to be consistent with
some kind of degradation or contamination of the CsI
photocathode that makes it more difficult for lower energy
photoelectrons to make their way out of the photocathode
material and into the micro-channel plate pores. The COS
FUV detector, unlike the STIS FUV or ACS SBC detectors uses
an unsealed windowless tube, and this may expose the
photocathode to additional sources of contamination.
Monitoring of these modes continues, but, given the limits
on the visit-to-visit repeatability imposed by pointing and
focus uncertainties, it is not yet possible to say whether
the rate of throughput decline for any of these modes is
slowing with time or not. A more detailed description of
the throughput measurements and the derived trends will
be shortly reported in a COS Instrument Science report
by Osten et al (2010).
Version 18.2 of the COS Exposure Time Calculator (ETC) has
been updated under the assumption that these trends can
be extrapolated to the middle of Cycle 18 (March 2011).
While corrections for these time dependent sensitivity
changes are not yet included in the CALCOS pipeline,
in the near future, updated reference files will be delivered
that to allow CALCOS to apply these corrections to the extracted
fluxes.
Note that even taking into account these projected
decreases in throughput, the COS FUV sensitivity is still
excellent, and COS should remain the instrument of choice
for most spectroscopic observations of very faint FUV
targets.
Figure 1: The mean rate of throughput change for each of
the routinely monitored COS central wavelength settings is shown
here. The vertical error bars show the uncertainty in the
rates, while the horizontal bars show the range covered by
that spectral segment or stripe.
