TIPS presentation 19 July 2007Charles Proffitt COS-STIS TeamPage 1 STIS FUV MAMA Dark Current...
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Transcript of TIPS presentation 19 July 2007Charles Proffitt COS-STIS TeamPage 1 STIS FUV MAMA Dark Current...
TIPS presentation 19 July 2007
Charles Proffitt COS-STIS Team Page 1
STIS FUV MAMA Dark Current
Charles Proffitt
COS-STIS Team
TIPS presentation 19 July 2007
Charles Proffitt COS-STIS Team Page 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
TIPS presentation 19 July 2007
Charles Proffitt COS-STIS Team Page 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
TIPS presentation 19 July 2007
Charles Proffitt COS-STIS Team Page 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.
TIPS presentation 19 July 2007
Charles Proffitt COS-STIS Team Page 5
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131 darksApr 1997Aug 1998178809 s
Mean 0.21Glow 0.37D.C. 0.161x 10-5 c/p/s
(hi-res-pixel)2048 x 2048
126 darksAug 1998Nov 1999173880 s
Mean 0.45Glow 1.08D.C. 0.165x 10-5 c/p/s
141 darksDec 1999May 2001194580 s
Mean 0.56Glow 1.24D.C. 0.161x 10-5 c/p/s
125 darksMay 2003Aug 2004172500 s
Mean 0.65Glow 1.65D.C. 0.154x 10-5 c/p/s
Dark Corner
Glow region
TIPS presentation 19 July 2007
Charles Proffitt COS-STIS Team Page 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.
TIPS presentation 19 July 2007
Charles Proffitt COS-STIS Team Page 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.
TIPS presentation 19 July 2007
Charles Proffitt COS-STIS Team Page 10
Effect of cooling on glow• NUV safing event in 2001 resulted in
FUV MAMA running about 4 C cooler for about 10 days.
TIPS presentation 19 July 2007
Charles Proffitt COS-STIS Team Page 11
Cooling reduces the rate at which the glow increase with HV on-time.
• Glow brightness during weeks surrounding cooling episode
o Glow brightness during cooling episode
(This effect originally reported by Brown & Roberts, TIR STIS 2002-02)
TIPS presentation 19 July 2007
Charles Proffitt COS-STIS Team Page 12
Glow region brightness over time.
• Correlates with HV on time• Rate at which glow brightens has been increasing over time
TIPS presentation 19 July 2007
Charles Proffitt COS-STIS Team Page 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)
TIPS presentation 19 July 2007
Charles Proffitt COS-STIS Team Page 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?
TIPS presentation 19 July 2007
Charles Proffitt COS-STIS Team Page 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).
TIPS presentation 19 July 2007
Charles Proffitt COS-STIS Team Page 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.
TIPS presentation 19 July 2007
Charles Proffitt COS-STIS Team Page 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
TIPS presentation 19 July 2007
Charles Proffitt COS-STIS Team Page 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.