Time Dependent Sensitivity
STIS CCD-Modes
Currently
Ralph Bohlin
Scott Friedman (PI)
Paul Barrett
Previously
Nolan Walborn
Ed Smith
Ivo Busko
STYS - TIPS ‘04
TDS: Time Dependent Sensitivity
• Monitor the sensitivity of each CCD grating mode
to detect any changes due to contamination or
other causes.
• Prior to analysis we first account for temperature
dependencies as well as losses due to CTE.
• Create reference files for use in the STIS datareduction pipeline to correct extracted 1D spectra
for time dependent sensitivity losses.
Observations
Cycle Proposal
7
7672
8
8418
9
8856
10
8914
11
9627
12
10030
Frequency
Every two
months
Every three
months
1.5 months
• AGK+81D266 is a flux standard sub-dwarf
whose position allows for observations
throughout the year.
• CRSPLIT = 2 for cosmic ray rejection;
CCDGAIN=1 (default)
• G750L and G750M observations have
contemporaneous fringe flat exposures for
fringe removal.
G230LB
• 1700-3000 angstroms
• 173 sec. exposure
• 34 observations
Net Count Rate
AGK+81D266
Wavelength (A)
G430L
• 2900-5700 angstroms
• 173 sec. exposure
• 35 observations
Net Count Rate
AGK+81D266
Wavelength (A)
G750L
• 5500-9900 angstroms
• 432 sec. exposure
• 35 observations
Net Count Rate
AGK+81D266
Wavelength (A)
Temperature Dependency
Mode
%/C
G230LB
0.34
G430L
0.25
G750L
0.07
The Problem
Net Count Rate
G230LB
Wavelength (A)
t0 ~ 1997
t1 ~ 2004
The Ratio
t1/t0
Obsn/Reference
0.91
Wavelength (A)
t0 ~ 1997
t1 ~ 2004
Avg. G230LB Trends
1%/yr - 3%/yr
1.10%/yr +/- 0.19 sig = 0.23%
-1.79%/yr +/- 0.10 sig = 0.59%
-0.43%/yr +/- 0.43 sig=0.59%
G230LB - all wavelengths
Avg. G430L Trends
0.2%/yr
Avg. G750L Trends
Typically 0%/yr
Pipeline Implementation
G230LB
t0 ~ 1997
t1 ~ 2004
Flux
mean(ratio)
0.995
• A TDS reference file contains MJDs and slopes at
each wavelength bin.
• calSTIS uses linear interpolation to correct each
observation based on wavelength and epoch.
• The result is an extracted 1D flux spectrum
corrected for both TDS and CTE.
G230LB
Uncorrected Net Count Rate
Corrected FLUX
Side 2
Electronics
1.10%/yr +/- 0.19 sig = 0.23%
-1.79%/yr +/- 0.10 sig = 0.59%
-0.43%/yr +/- 0.43 sig=0.59%
G430L
Uncorrected Net Count Rate
Corrected FLUX
G750L
Uncorrected Net Count Rate
Corrected FLUX
Summary
• STIS sensitivity has been monitored since activation and is
dependent on time, temperature and wavelength.
• Sensitivity losses in the CCD modes range from 0%/yr
(G750L) to ~3%/yr (G230LB)
• M-mode sensitivities mimic the trends found in the Lmodes. We apply the L-mode corrections to the M-modes
since we only observe medium dispersion modes at
selected wavelengths.
• TDS reference files are available for first-order MAMA
and CCD modes. In the future, imaging and echelle modes
will also be corrected for TDS.