Early Results from the COS Time-Dependent Spectroscopic Sensitivity Programs

1
Early Results from the COS Time-Dependent
Spectroscopic Sensitivity Programs

• Rachel Osten
• TIPS presentation, Jan. 21, 2010

2
Overview

• 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

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NUV Gratings

• G225M, G285M gratings are coated with bare
  aluminum
• the other NUV gratings on COS (G230L and G185M)
  are AlMgF2
• 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

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NUV Grating Efficiency Test results

• G230L, G185M (MgF2Al 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

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NUV Grating Efficiency Test results
measurements at specific wavelengths appear
consistent within a given grating
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On-orbit results with external targets
WD1057719 _at_2635Å
G191B2B _at_2186Å
G191B2B _at_2617Å
stripe A
stripe A
stripe A
stripe B
stripe B
stripe B
stripe C
stripe C
stripe C (2nd order)
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summary of NUV data

• 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

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FUV TDS
G140L 1105Å
WD0947857
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FUV TDS
G140L 1230Å
WD0947857
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FUV summary

• 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

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