STIS Status: MSM Monthly Offsets for Echelles Echelle Wavelength Calibration Errors
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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
