MARCH 5, 2019
COS NEWSLETTERS

March 2019 STAN

In this newsletter we provide updates on COS to users preparing a program for the Cycle 27 HST Phase 1 deadline.


Reminder: New FUV Cenwaves, G140L/800 and G160M/1533, Available for Use Since Cycle 26

Two new central wavelengths (cenwaves) were released in Cycle 26 for COS/FUV observations, named G140L/800 and G160M/1533, and remain available for Cycle 27.

Cenwave 800 allows for continuous coverage of the entire spectral region from 800 to 1950 Angstroms on a single detector segment (FUVA) with a low spectral height below 1150 Angstroms, allowing higher S/N for background-limited observations. This setting is particularly useful for background-limited observations of faint targets at low wavelengths.

Cenwave 1533 extends coverage at the short-wavelength end of G160M by 44 Angstroms to overlap with the longest wavelengths covered by cenwave G130M/1222, and otherwise has similar spectral resolution and sensitivity to G160M/1577. This allows a broad range of FUV wavelengths to be covered at good spectral resolution and at high S/N (with four FP-POS positions) by just two cenwave settings (1222+1533) without placing Lyman-alpha on the detector, allowing the full FUV to be covered with an efficient use of orbits.

The coverage of these modes compared with the existing COS modes is shown at: Wavelength Tool. For full details, see the COS Instrument Handbook. Both modes are used in multiple Cycle 26 GO programs and we encourage their use in Cycle 27.

 

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Use of the G285M Grating Is Discouraged

Because of declining throughput, NUV observations with the G285M grating are discouraged. The remaining NUV gratings, G185M, G225M, and G230M, continue to be recommended for use and are described in Section 13.3 of the COS Instrument Handbook. Users interested in medium-resolution spectroscopic coverage of the wavelength region from 2500 to 3200 Å are encouraged to use STIS instead.

 

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Dark Rate Behavior Near the Edges of COS/FUVA and Impact on Spectral Extraction

The dark rate of the COS FUV detectors varies across the detectors and over time. FUVA experiences increased dark counts along the edges of the detector segment which lead to an increased overall average detector dark rate. Dark images and collapsed count plots composed of five 22-minute dark exposures from Program 15533 are shown below:

Collapsed Dark Image
Figure 1. Dark images of FUVA with pulse heights 2 to 23. Each dark image is composed of five 22-minute exposures from a single visit for a total of ~6650 seconds. The dark counts were collapsed in the along dispersion and cross dispersion direction to provide a projection of the two-dimensional data as dark counts per second for each XCORR or YCORR position.

The dark rate monitor has been updated to track the dark rate versus time of these edges individually and remove them from the calculation of the overall (inner region) average dark rate for each segment. The mean dark rate over time of FUVA for the following regions – inner, bottom edge, top edge, left edge, and right edge is shown below:

Dark Rates for each edge
Figure 2. COS/FUV dark rates on FUVA as a function of time since COS installation. The first 5 subplots show the measured dark rate in 25 second increments throughout every observation for each region on FUVA. The red dots represent dark rates that were observed close to when HST was passing over the South Atlantic Anomaly. The bottom panels display the 10.7cm emission tracking the solar cycle.

When calibrating FUV observations, the background is removed by measuring the background count rate in regions below and above the spectra. The lower background extraction region for FUVA LP4 observations overlapped with the lower dark-enhanced region of the detector, leading to an artificially inflated background. In order to avoid this, the XTRACTAB and TWOZXTAB reference files for selecting background extraction regions have been updated. Both the upper and lower background boxes for all FUVA LP4 modes are now located between the WCA extraction region and below YCORR=660, with an extraction height reduced to 31 pixels in order to avoid the edge effects.

The updated XTRACTAB and TWOZXTAB reference files, 2bj2256il_1dx.fits and 2bj2256nl_2zx.fits, respectively, were activated 2018-11-20. The FUV dark rate had been fairly consistent since the move to LP4 (October 2, 2017), but has begun increasing along the edges since April 17, 2018. Users interested in data obtained in April 2018 or later are encouraged to re-retrieve the data from the Mikulski Archive for Space Telescopes.

 

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Please Contact the HST Help Desk with any Questions

https://hsthelp.stsci.edu.