STIS FUV MAMA Dark Current

1. STIS FUV MAMA Dark Current Charles Proffitt COS-STIS Team Charles Proffitt COS-STIS Team

2. STIS FUV MAMA Darks • FUV MAMA - background • Optimized for FUV 1140 - 1700 Angstroms • Micro Channel Plate (MCP) with “solar-blind” opaque CsI photocathode deposited on the surface of the tube • 1024 x 1024 format, subsampled to 2048 x 2048 for readout (lo-res and hi-res formats respectively) • To avoid excessive counts during SAA passage, MAMA HV power supply turned off during any SAA impacted orbit. Only usable for one 5 - 6 orbit block per day. • Due to optical isolator problem, LV power supply also turned off during deepest SAA passages; causes daily temperature cycling of detector with 4 - 5 C amplitude Charles Proffitt COS-STIS Team

3. STIS FUV MAMA Darks • Reasons to characterize FUV darks • Better subtraction of darks from existing observations and identification of any artifacts • Provide predictions for after SM4 to aid planning of cycle 17 observations • Give indication of health of detector • For STIS NUV MAMA, intrinsic MCP current hidden by window glow • Similar effect seen in ACS SBC, but SBC has much less data to characterize effect Charles Proffitt COS-STIS Team

4. Data for analysis • Each week during normal ops, STIS dark monitor took two 1380 s exposures • Additional darks during SMOV periods • 629 FUV MAMA dark exposure where telemetry allows it to be determined how long HV and LV power supplies on. Charles Proffitt COS-STIS Team

5. 131 darks Apr 1997 Aug 1998 178809 s Mean 0.21 Glow 0.37 D.C. 0.161 x 10-5 c/p/s (hi-res-pixel) 2048 x 2048 126 darks Aug 1998 Nov 1999 173880 s Mean 0.45 Glow 1.08 D.C. 0.165 x 10-5 c/p/s Glow region Dark Corner 141 darks Dec 1999 May 2001 194580 s Mean 0.56 Glow 1.24 D.C. 0.161 x 10-5 c/p/s 125 darks May 2003 Aug 2004 172500 s Mean 0.65 Glow 1.65 D.C. 0.154 x 10-5 c/p/s Charles Proffitt COS-STIS Team

6. Charles Proffitt COS-STIS Team

7. STIS FUV MAMA Temperatures • Two telemetry temperatures • OM1CAT measures FUV MAMA electronics • OM1TUBET measures FUV MAMA tube itself • Tom Wheeler has found an excellent correlation between micro-channel plate current and OM1TUBET, which suggests it correlates well with the mean temperature of the microchannel plate. Charles Proffitt COS-STIS Team

8. Charles Proffitt COS-STIS Team

9. Glow vs Temperature • FUV dark glow has a only a loose correlation with tube temperature or LV on-time. • Glow correlates best with HV on-time. • Glow is getting brighter over time. Charles Proffitt COS-STIS Team

10. Effect of cooling on glow • NUV safing event in 2001 resulted in FUV MAMA running about 4 C cooler for about 10 days. Charles Proffitt COS-STIS Team

11. Cooling reduces the rate at which the glow increase with HV on-time. • Glow brightness during weeks surrounding cooling episode • Glow brightness during cooling episode (This effect originally reported by Brown & Roberts, TIR STIS 2002-02) Charles Proffitt COS-STIS Team

12. Glow region brightness over time. • Correlates with HV on time • Rate at which glow brightens has been increasing over time Charles Proffitt COS-STIS Team

13. Dark rate in “dark corner” • Dark corner shows little change with time, with median rate remaining near 1.6 x 10-6 counts/hi-res-pixel/s • (6.4 x 10-6 counts/lo-res-pixel/s) Charles Proffitt COS-STIS Team

14. Possible causes of glow? • Changes to photocathode? • Separation of CsI in photocathode and migration of cesium into pores? • Clumping of CsI? • Surface contamination leading to easier emission of electrons? • Related to red leak? • Increasing number of defects in MCP tubes? • Why better correlation with HV than T? Charles Proffitt COS-STIS Team

15. Observation Strategies • Effects on observers • Dark current at is worst was still low; observers of bright targets won’t notice. • In future, COS to be used for faint point sources. • Will primarily affect observations of targets that are both faint and extended. • Mitigation strategies • Use only first orbit of each SAA free period for observations that need low dark current. (only 1 orbit per day). • Place target on darker part of detector. • Keep FUV HVPS off when detector not in use (ops change). • Cool detector (NUV MAMA off). Charles Proffitt COS-STIS Team

16. The count rate summed in each column over a seven pixel high region of the mean dark image covering the period between May 2003 and August 2004. The dotted line gives the results for a region near the standard 1st order spectral location, and the solid line gives the results at the new D1 position located near the bottom edge of the detector. Charles Proffitt COS-STIS Team

17. Data Reduction Strategies • STScI does not attempt to subtract the dark glow in the pipeline • Coadded darks, hot pixel masks, and glow images for different epochs available on STIS web pages • Suggest users • Remove hot pixels • Scale dark using part of the image without external data • Subtracted smoothed image of glow Charles Proffitt COS-STIS Team

18. Future • Post repair operations • Unclear what will have happened to glow during hiatus in STIS observations. • With extra heat from COS, STIS MAMAs expected to be 3 to 4 degrees warmer. • As baseline guess, assume FUV glow will increase 1.4x as quickly after HVPS turn-on as during latter part of Cy 12. • Future calibrations • Take more dark monitor observations at opposite ends of same SAA free window. Charles Proffitt COS-STIS Team

19. STIS FUV darks in 2004 • Dark current during latter part of STIS operations. (Units of counts/lo-res-pixel/s). • Red line shows SBC values during Dec 2006 test. Charles Proffitt COS-STIS Team