New Developments for the Cosmic Origins Spectrograph
Rachel Osten, osten@stsci.edu, for the COS team
Those of us who are committed spectroscopists know the unparalleled value of a spectrum. The
results of the Cycle 18 proposal selection process affirmed that many Hubble Space Telescope
users feel similarly: in Cycle 18, orbits with the Cosmic Origins Spectrograph (COS) account for
23% of GO prime orbits allocated. The overwhelming majority of these orbits use COS farultraviolet (FUV) spectroscopic configurations. COS/FUV spectroscopy also accounts for about
29% of Snapshot (SNAP) orbits.
The decline in sensitivity of the COS FUV spectroscopic configurations continues, but the
behavior has changed. Discovered midway through Cycle 17, the phenomenon was described in
the instrument science report (ISR), COS 2010-15, “Early Results from the COS Spectroscopic
Sensitivity Monitoring Programs,” by Osten et al.
(http://www.stsci.edu/hst/cos/documents/isrs/ISR2010_15(v1).pdf).
The monitoring program uses repeated observations of spectrophotometric white-dwarf standard
stars to measure any time dependence to the spectroscopic sensitivity. The main finding in the
ISR for the FUV detectors was that the decline increased for longer wavelengths, down by about
4–6% per year at short wavelengths and by 12% per year at the longest wavelengths.
Observations taken since mid March 2010 have exhibited a flattening of the slope at the longest
wavelengths. As a result, the FUV data are now consistent with a value almost independent of
wavelength: a weighted mean of –5.1 ± 0.2 %/year (see Figure 1). This result suggests that the
sensitivity decline of the FUV detector has two origins: one is producing a more or less constant
level of sensitivity decline, and another—now past—causing the initial decrease. The updated
time-dependent sensitivity reference file reflects this new information, with a change in the FUV
slope occurring on the date 2010.2.
The COS team at the Institute is setting up routine monitors of the data quality, enabling
automated checking of the instrument’s performance. Examples include monitoring the accuracy
of acquisitions; automated monitoring of the dark current on both the FUV and near-ultraviolet
(NUV) detectors; routine searches for short time-scale, high-voltage transients; weekly images of
the accumulated photons falling on the detector; and monitoring the decline in the brightness of
the lamp. The results of these monitors will be available from the main COS web pages.
The aim of the Cycle 18 calibration program for COS is to maximize the scientific performance
of the instrument. Nevertheless, with four science instruments now operating on Hubble, there is
a need to minimize internal and external calibration orbits. The team took a critical look at
routine monitoring programs established in Cycle 17 and cut back on programs where previous
monitoring revealed stable behavior, or where configuration usage in Cycle 18 was dramatically
lower than in Cycle 17. In these cases, we judged that we could afford reduced monitoring
frequencies. Continued regular monitoring includes measurements of the NUV Multi-Anode
Micro-channel Array fold distribution, detector dark rates, spectroscopic sensitivity, and
internal/external wavelength scales. We continue a special program designed to characterize
geocoronal Lyman-alpha airglow in blank fields for users interested in modeling this effect in
their data. A new program aims to improve the accuracy of FUV sensitivity characterization to
better than the 5% now possible. A companion calibration program for the Space Telescope
Imaging Spectrograph will investigate the use of a particular DB white-dwarf for flux
calibrations in the COS/G130M mode. Unlike the DA white dwarfs that are typically used, DB
white dwarfs do not have strong photospheric Lyman alpha absorption lines and their flatter
spectral energy distributions provide a more uniform signal-to-noise ratio across the COS UV
detectors. The object under consideration, WD0308-566, could be used for flat-field monitoring
or additional flux calibrations with an enormous savings in time compared with the standards
currently in use. The STIS observations will ensure that the target can be modeled sufficiently
accurately (<1%) to serve as a flux calibrator.
In time-tag mode, the FUV detector records the pulse-height amplitude (PHA), in addition to the
time and location of each detected photon. The PHA gives a measure of the gain of the detector,
which means the number of collected electrons after one photon strikes the detector at a given
position. The COS team has determined that the peak of the PHA distribution has been steadily
shifting to smaller gain values (see Figure 2). This “gain sag” is most apparent in regions of the
detector subject to the largest photon accumulations. On FUV detector segment B, these regions
are where geocoronal Lyman-alpha emission from the most used aperture (the Primary Science
Aperture) and central wavelengths of grating G130M are located. Since the data are filtered by
PHA, events falling below the current threshold of PHA = 4 are removed from the events that
contribute to extracted spectra. Several artifacts due to gain sag—“absorption features”—are
now apparent in science data. In late December the team delivered a new reference file which
has a reduced PHA lower threshold of PHA=2. Application of this new reference file to data
will remove the absorption features. It additionally raises the dark count rate by 7% compared
with the PHA=4 threshold, although the dark rate is still low, between 3 and 4 x10-6
counts/sec/pixel.
There is no relation between the COS gain sag and the time-dependent decline in spectroscopic
sensitivity. Although a reduction in the number of photoelectrons produced by a photon occurs,
if the gain is high enough the probability of detecting the photon is not affected. The testing of
short-term strategies includes marking the low-gain regions with data quality flags and
performing position-dependent pulse height filtering. The COS team is investigating long-term
options, including raising the voltage back to its original value or moving to a new position on
the detector.
Results from early in the COS on-orbit validation revealed that the instrument has sensitivity
below 1150 Å (McCandliss et al. 2010 ApJ 709, L183). Two supplemental calibration programs
executed in Cycle 17 explored this wavelength region further. One program utilized two new
central wavelengths of the G130M grating (G130M/1096 and G130M/1055) to obtain higher
spectral resolution and sensitivity in this region, compared to the G140L grating. Figure 3
compares the effective areas. The spectral resolution of the new G130M central wavelength
settings is greater than about 2000 (=/) below 1050 Å, and also exceeds the spectral
resolution of the G140L/1280 setting, by up to a factor of two in this wavelength range. The new
central wavelengths allow users to exploit this wavelength region, while avoiding the large
increase in effective area at longer wavelengths. This increase is a factor of roughly 30 in
effective area, and may cause a local rate violation above 1150 Å, depending on the spectral
energy distribution of the source. These new modes will be available for Cycle 19 proposers.
Another calibration program will improve the flux calibration of the G140L/1280 segment B
configuration to 3% in the 900–1150 Å region and explore the sensitivity below 900 Å.
Figure 1. The sensitivity trends of the COS FUV detector since mid March 2010. Symbols
indicate grating and segment. With the exception of G140L/segment B, all data are consistent
with a nearly wavelength-independent sensitivity decline of 4–6 %/year. The scatter in the data
does not show any trends with detector segment or grating, in contrast to the results from early in
Cycle 17.
Figure 2. The evolution of the gain versus x position on the COS FUV detector at a variety of
times since COS was installed on Hubble. The “modal gain” is the peak of the pulse height
distribution for events in the spectral extraction region. The large jump between the top (green)
curve and the lower (blue) curve is the result of a lowering of the high voltage of the FUV
detector on August 15, 2010. Since then the gain has been steadily decreasing, and pulse-height
minima are appearing at regions of enhanced geocoronal Lyman-alpha emission. In the most
affected regions, standard screening through CALCOS produces “absorption features,” when valid
photon conversions produce pulse heights of photoelectrons that fall below the threshold.
Figure 3. The effective area of two COS FUV gratings below 1200 Å. The B detector segment of
the G140L/1280 and G130M/1096 and G130M/1055 (both segments A and B) are sensitive to
FUV light. The G130M settings are more sensitive in the range below 1050 Å. These modes will
be available to proposers in Cycle 19.