Early Results from the COS Time-Dependent Spectroscopic Sensitivity Programs Rachel Osten TIPS presentation, Jan. 21, 2010

✓ Absolute flux calibration of NUV, FUV channels done during SMOV ➡ Cycle 17 calibration programs (11896 & 11897) monitor time-dependence of sensitivity using external observations of standard stars to capture e.g. contamination ➡ Low- and medium-resolution gratings are observed roughly monthly and quarterly, respectively  exceptions are two of the NUV gratings which are observed monthly Overview

 G225M, G285M gratings are coated with bare aluminum  the other NUV gratings on COS (G230L and G185M) are Al+MgF 2  bare Al quickly develops a thin oxide coating  the expectation was that this coating would stabilize once in the vacuum of space  ground-based Grating Efficiency Tests (GETs) done from 2003 onward demonstrated a degradation of grating efficiency for the G225M and G285M gratings  similar GET was performed during SMOV NUV Gratings

NUV Grating Efficiency Test results G230L, G185M (MgF 2 +Al coated) appear stable G225M and G285M show efficiency decline: 1.6% per year (G225M) and 4.5% per year (G285M) data point in red is from SMOV SMOV data consistent with ground trends

measurements at specific wavelengths appear consistent within a given grating NUV Grating Efficiency Test results

On-orbit results with external targets G191B2B stripe A stripe B stripe C stripe C (2nd order)

observations spanning ~5 months so far G230L stable, with 6 observations spanning 5 months G185M also appears stable, only 2 observations Bare-Al gratings appear to indicate continued sensitivity degradation on-orbit  ~3% per year for G225M, 10-12% per year for G285M G225M grating is used for 0.7% of time in COS Cycle 17 (by exposure time); G285M is used for 2.7% of time results were confirmed by manual extraction of spectra: not a pipeline issue repeatability in NUV estimated to be 1-2% no good current explanation for the on-orbit degradation summary of NUV data

G140L 1105Å FUV TDS WD

G140L 1230Å WD FUV TDS

observations span 4 months, 5 data points for G140L using the standard error of the mean as the uncertainty, TDS slopes are large if systematic errors are larger (±1.5% repeatability), then uncertainty on slopes is larger there is a systematic behavior in G130M & G160M gratings, where second data point is down by 2-3%; not enough data points for a given grating/cenwave to see a trend confirmed trends using manual extraction of spectra no flat-field is currently applied to data need more data to conclude whether this is a physical effect FUV summary

STIS TDS results nothing apparent from STIS -- no OTA effect

COS TDS summary