STIS Status:
MSM Monthly Offsets for Echelles
Outline:
Echelle Wavelength Calibration Errors
Echelle Flux Calibration Errors
Disabling MSM Monthly Offsets
Aug 15, 2002 TIPS
Jeff Valenti
Echelle Wavelength Calibration
Calstis 2.10 Methodology
Shift canonical solution to match auto-wavecals
Relative precision goal: 0.25 - 0.5 pixel
Absolute accuracy goal: 0.5 - 1.0 pixel
Limited by thermal stability, geometric distortion, etc.
Effect of MSM Monthly Offsets
Preserve MAMA sensitivity throughout mission
Spectra shifted by a few pixels each month
Monthly offsets also distort wavelength solution
Relative precision goal not always achieved
Achieving Calibration Goals
Old data: Calstis 2.12 includes Lindler dispersion algorithm
New data: No offsets for echelles as of Aug 5, 2002
ECF working on physical model of STIS
Echelle Wavelength Errors – Calstis 2.10
Wavelength errors depend systematically on Y shift of echelle format
Echelle Wavelength Errors – Calstis 2.13
Lindler dispersion algorithm often reduces wavelength errors
Echelle Flux Calibration
Calstis 2.10 Methodology
Sensitivity curve for each grating, order, and CENWAVE
Sensitivity shifted by amount inferred from auto-wavecal
Relative precision goal: 5%
Limited by blaze function and scattered light
Effect of MSM Monthly Offsets
Preserve MAMA sensitivity throughout mission
Spectra shifted by a few pixels each month
Blaze function shift not the same as wavelength shift
Relative precision goal not always achieved
Achieving Calibration Goals
Old data: Calstis 2.13 includes Lindler blaze shift algorithm
New data: No offsets for echelles after Aug 4, 2002
ECF working on physical model of STIS
Echelle Blaze Shift Correction
Calstis 2.10
10% errors
Calstis 2.13
Disabling MSM Monthly Offsets for Echelles
MAMA Detector Lifetime
Quantum efficiency (QE) degrades as photons detected
Monthly offsets prevent localized sensitivity degradation
Monitored by Charles Proffitt and James Davies
Imaging and first order spectroscopy are main culprits
Maximum cumulative QE loss is <1% (typically 0.0001%)
Safe to disable MSM offsets for echelle modes
Implementation
Update MSM table in SCIOPSDB (Paul Goudfrooij, Jinger Mo)
Create dither ID “02” from “01”
Use December MSM positions (no offset) for all months
Preserve monthly offsets for first order MAMA modes
Update target X and Y locations for G140L and MIRFUV
New MSM table took effect on Aug 5, 2002
Update TRANS rules
MAMA Cumulative Images
FUV
NUV
Geocoronal Ly-α
First order
3” Above wire
Vignetting
First order
Echelle Line
3” Below wire
Bright
star
(Imaging)
Echelle order
Images clipped at 125,000 detected photons
Cumulative Decrease in Quantum Efficiency
QE loss is less than 1% for all pixels
0
