UVIS Calibration Update Greg Holsclaw July 10, 2006

Outline FUV Calibration history Approach to deriving a time variable sensitivity Spica variation Fomalhaut variation Spica vs. Fomalhaut Continued work

Calibration History 1997 – original ground calibration 1999 – update reflecting improved lab measurements of PMT reference detectors and initial Spica observations 2004 – update of 1999 calibration to match the spectral shape of the lunar albedo from SOLSTICE observations 2005 – (red patch) update of 1999 calibration to match observed change of Spica observations from Jan 1999 to Oct 2005

Approach to Deriving a Time Variable Sensitivity Need to find consistent observations of the same target over a broad time range Several rows should be illuminated to avoid row-to-row variations Selected all Spica observations using the occultation slit (Jan. 1999 – Jan. 2005): filename Fslit nx ny nz int odcid x1 x2 y1 y2 FUV1999_016_19_47_15 2 1024 64 2 60 12 0 1023 0 63 FUV1999_016_22_46_15 2 1024 64 2 60 12 0 1023 0 63 FUV1999_016_23_16_15 2 1024 64 2 60 12 0 1023 0 63 FUV2001_093_08_35_28 2 1024 64 29 60 93 0 1023 0 63 FUV2001_093_16_52_33 2 1024 64 99 36 95 0 1023 0 63 FUV2001_299_19_41_34 2 1024 64 29 60 93 0 1023 0 63 FUV2002_155_18_10_03_UVIS_C32ST_DECON001_ISS 2 1024 64 24 60 36 0 1023 0 63 FUV2005_029_12_55_04_UVIS_00CST_ALPVIR001_PRIME 2 1024 64 2 45 135 0 1023 0 63 FUV2005_029_13_16_34_UVIS_00CST_ALPVIR001_PRIME 2 1024 64 2 45 135 0 1023 0 63

Spica Spectra Average count rate, summed over central 16 rows (24-39) Each spectrum shifted to match wavelength scales Increase in response above ~155nm, decrease below

Spica Spectral Ratios No offset corrections, which would include: RTG background Scattered light

Spica Curve Fit Fit an exponential curve to each pixel of the form:

Fomalhaut Spectra Fomalhaut observations provide an independent dataset in which to validate the sensitivity variation derived from Spica Different spectral distribution and lower count rate

Fomalhaut Spectral Ratios Fomalhaut spectra show a change in response similar to that from Spica

Fomalhaut Curve Fit Fomalhaut spectra show a change in response similar to that from Spica

Spica and Fomalhaut Spectra Spica is much brighter across the entire FUV wavelength range Increased Spica signal toward shorter wavelengths contributes scattered light to long wavelengths Fomalhaut long wavelengths less sensitive to scattered light but more sensitive to Lyman-alpha and RTG background

Spica and Fomalhaut Fit To place both datasets on the same scale, multiply Fomalhaut curves by the Spica fractional change at the initial Fomalhaut observation Fomalhaut spectra show a faster rate of change than that expected from Spica Discrepancy thought be due to neglect of offset subtraction

Lyman alpha estimation Lyman alpha can be estimated by scaling an IPH scan This shows that the contribution is large at shorter wavelengths, but fractionally small at longer wavelengths for Fomalhaut

Continued work PSF estimation for scattered light removal Lyman alpha, RTG background estimation Backgound and scattered light subtraction will increase the current values of the sensitivity at long wavelengths Consider datasets without the occultation lens Goal: Time variation of the 2D calibration