Time Dependent Sensitivity

STIS CCD-Modes

STYS - TIPS ‘04

Currently

Ralph Bohlin

Scott Friedman (PI)

Paul Barrett

Previously

Nolan Walborn

Ed Smith

Ivo Busko

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 data-reduction pipeline to correct extracted 1D spectra for time dependent sensitivity losses.

Observations

• 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.

Cycle Proposal Frequency

7 7672 Every two months8 8418

9 8856Every three

months10 8914

11 9627

12 10030 1.5 months

G230LB

• 1700-3000 angstroms• 173 sec. exposure• 34 observations

Wavelength (A)

Net

Cou

nt R

ate

AGK+81D266

G430L

• 2900-5700 angstroms• 173 sec. exposure• 35 observations

Wavelength (A)

Net

Cou

nt R

ate

AGK+81D266

G750L

• 5500-9900 angstroms• 432 sec. exposure• 35 observations

Wavelength (A)

Net

Cou

nt R

ate

AGK+81D266

Temperature Dependency

Mode %/C

G230LB 0.34

G430L 0.25

G750L 0.07

The Problem

Net

Cou

nt R

ate

Wavelength (A)

t0 ~ 1997

t1 ~ 2004

G230LB

The Ratio

t 1/t 0

Wavelength (A)

t0 ~ 1997

t1 ~ 2004

Obsn/Reference

0.91

Avg. G230LB Trends

Sensitivity loss

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

Sensitivity loss

0.2%/yr

Avg. G750L Trends

Sensitivity loss

Typically 0%/yr

Pipeline Implementation

• 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.

Flu

x

t0 ~ 1997

t1 ~ 2004mean(ratio)

0.995

G230LB

G230LB

1.10%/yr +/- 0.19 sig = 0.23%

-1.79%/yr +/- 0.10 sig = 0.59%

-0.43%/yr +/- 0.43 sig=0.59%

Uncorrected Net Count Rate Corrected FLUX

Side 2Electronics

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 L-modes. 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.