STIS SMOV Science Report
Tuesday, August 4, 2009
1
NUV MAMA Recovery Status
• Final full voltage ramp-up for NUV recovery on day 212 succeeded,
but dark current is 4-5 X higher than predicted.
• This was despite using a model that takes the effects of long cold safing
on window glow into account
• Details on later slides
• Normal NUV HV ramp-up on day 214 failed due to dark current
exceeding 15000 c/s during final stages of HV ramp-up.
• During HV ramp-up, red limit set at 15000 OR c/s, equivalent to
about 11500 detected c/s;
– limit is reset to much higher value (770,000 OR counts) after ramp
• Tube voltage OM2MCPC had reached full -2050 V
• Photocathode OM2PCV had reached -300 out of -700
• Tube was warmer than it was on day 212, so expected higher dark rate.
• NUV MAMA commanding was inhibited
– Two visits of 11392 (NUV format verification) and two of 11402 (NUV dark
monitor) failed
• Attempt to clear flag on Monday, allowed LVPS to come back on, but HV
commanding was still inhibited (MCE flag also needs to be cleared)
2
11391 NUV MAMA Recovery, visit A4
•
•
Final visit of 11391, NUV
MAMA HV recovery
succeeded
Dark rate was much higher than
predicted
– cool tube temperature due to
late LVPS turnon prevented
ramping limits from being
exceeded
•
•
Latter stages of visit near SAA,
but this made only a modest
addition to the dark rate
Later dark away from SAA
showed similar rates
3
11391 NUV MAMA Recovery, visit A4 - cont
•
Fold analysis in family with
previous results
– Measures size of charge cloud
at bottom of MCP
– Micro channel plate appears to
be in good condition
•
Dark (below left) looks normal
except for high count rate
– Shape similar to older darks
•
Detected HITM1 lamp flux
(below right) consistent with
extrapolation of previous trends
4
History of fold flat field tests
•
•
Previous history of HITM1 flats
at full voltage of -2050 V
• Diamonds give measured rate
• +’s are rate after subtracting dark
New dark rate much higher, but lamp
flat brightness after subtracting dark
consistent with past trending
•
Implies throughput probably OK
5
NUV Window Phosphorescence
2) Thermal excitation
ΔE
Meta stable state
Can’t radiatively decay
1) Excitation via
cosmic ray impact
3) Immediate radiative decay
UV photon emitted
Ground state
6
STIS NUV MAMA Dark Current
•
NUV MAMA dark current dominated by de-excitation of meta-stable states in window.
–
–
–
–
Cosmic ray impacts during SAA populate these states
Depends both on number of such states that are occupied and the probability of de-excitation
Dark rate ~ Nexe -(ΔE/(T+273.15))
dNex/dt = Γ (Ntot – Nex) – [Ae -(ΔE/(T+273.15))] Nex
•
•
•
•
•
Ntot is the number of metastable states
Nex is the number of those states currently populated
Γ is rate at which cosmic rays populate empty states
Ae -(ΔE/(T+273.15)) is the probability a trapped state will de-excite due to thermal transition to an unstable
state
Timescale for Nex to change is several days to a few weeks.
7
Model fits to old data:
•
In pre-failure data, our current model properly accounts for extended safings
–
–
–
–
–
Use OM2TUBET telemetry value and then integrate model to find the population of excited states as a
function of time
– Dark rate ~ Nexe -(ΔE/(T+273.15))
Found a set of parameters that fits old side1 and side2 data
Model slightly underestimates very first darks during SM2 (left below)
Good fit to observed dark after 40+ day safing in 1999 before SM3a
Dotted line gives model, symbols give observed data
8
New NUV observations
•
Visit 3A had ramped tube to -1750 V on day 210
–
Effective QE of -1750 observation uncertain, but can compare flat and dark to those in visit 4A and scale the gain for visit 3A
assuming lamp constant
•
•
Visit 4A ramped tube to full voltages on day 212
–
–
•
Both visits included a time-tag exposure over the ramp, a flat and a dark.
Measured global rate of 8373 c/s in visit 4A (day 212)
Ordinary ramp-up on day 214 failed due to high dark
•
•
Day
Measured 512 c/s in 3A; scale to ~ equivalent 13447 c/s at full HV
During HV rampup, red limit set at 15000 OR c/s, equivalent to about 11500 detected c/s; limit reset much higher after ramp
Scaling to full PC voltage suggests dark at almost full rate
VE equiv. dark rate
OM2TUBET
210
13447
31.05
212
8373
27.91
214
~11900
30.21
9
Fitting 2009 NUV Dark Data
•
•
•
New results much brighter than predictions
– Solid line shows model used previously
– Diamonds give “measured” values
NUV dark current about 4 - 5 X brighter than expected
– Standard model accounted for long cold soak
– dotted line gives “old model” scaled up.
– Data may be consistent with such simple scaling, but physical cause unclear
Working assumption is that there is a different population of traps that became slowly populated at
low T between 2004 & 2009, dramatically increasing the window glow.
–
Time scale to depopulate these extra states now that NUV MAMA is at a higher T is uncertain
10
STIS Activity
PropID
STIS SMOV Program Title
Analysis
Lead
Visits
Done
Total
Visits
STIS02
11347
STIS03
Status
Memory Load and Dump
BS
1
1
Complete
11348
Science Data Buffer Check
BS
3
3
Complete
STIS04
11349
Mechanism Mini-Functional
TW
1
1
Complete
STIS05
11399
CCD Anneal (two anneal iterations)
MW
6
6
Complete
STIS06
11382
CCD Functional
DL
4
4
Complete
STIS07
11404
CCD Dark & Bias Monitor
MW
90
105
Ongoing
STIS08
11383
Aperture Wheel & Lamp Functional
WZ
2
2
Complete
STIS09
11384
STIS-to-FGS Alignment
CP
1
1
Complete
STIS10
11385
CCD Spectral Format Verification
WZ
1
1
Complete
STIS11
11386
External Focus Check
CP
3
3+1
STIS12
11387
Corrector and Focus Alignment
CP
-
8
Contingency
STIS13
11388
CCD External Spectroscopic Quality
TG
1
1
Complete
STIS14
11400
CCD CTI Check
PG
1
1
Complete
STIS15
11401
CCD Spectroscopic Throughputs
DL
3
3
Analysis
STIS16
11389
CCD Image and Pointing Stability
TG
1
1
Complete? LVPS changes?
STIS17
11350
FUV MAMA HV Recovery
TW
4
4
Complete
STIS18
11351
NUV MAMA HV Recovery
TW
2 (+2F)
4 (+2F)
Analysis
STIS19
11390
FUV MAMA Dark Measure
CP
5
10
SMS 215 (Aug 7-8)
STIS20
11402
NUV Dark Monitor
CP
3 FAILED
20
SMS 208 +
STIS21
11391
FUV Optical Format Verification
WZ
2
2
Analysis
STIS22
11392
NUV Optical Format Verification
WZ
2 FAILED
2
FAILED
STIS23
11393
FUV MAMA Spectroscopic Quality
TG
1
SMS 215 (Aug 4-5)
STIS24
11394
NUV MAMA Spectroscopic Quality
TG
1
SMS 215 (Aug 5)
STIS25
11403
MAMA Spectroscopic Throughputs
DL
3
SMS 215 (Aug 5-6)
STIS26
11395
MAMA Image Stability
TG
2
SMS 215 (Aug 3)
11
Near term activities
•
208 SMS
–
NUV MAMA recovery
•
–
NUV Darks
–
NUV Spectra Format (Internal wavecals)
•
•
•
•
113514A 212:18:15:00 - 212:20:45:22
1140201 214:18:18:39 - 214:18:48:09 FAILED
1139201 214:18:48:09 - 214:19:11:43 FAILED
1139202 214:19:11:43 - 214:19:46:23 FAILED
215 SMS
–
NUV Darks - 5 visits
•
–
Focus check (repeat after secondary move; uses CCD only)
•
–
1139402 217:14:15.34 - 217:17:34.41 - external
MAMA Sensitivity (both detectors)
•
•
•
–
1139302 216:22:01:40 - 217:00:37:57 - external
NUV External Image Quality
•
–
1138611 215:05:59:04 - 215:06:56:28
FUV External Image Quality
•
–
215 (FAILED), 218, 221
1140302 217:17:57:39 - 217:19:29:39
1140303 217:20:51:48 - 217:23:17:24
1140305 218:14:38:05 - 218:16:20:25
FUV Dark measure
•
1139011,12,13,14,15
219:14:17:43 - 220:00:25:00
12
Plan forward
For Wednesday’s and next week’s visits an intercept
SMS will reset the ramping limit from 15000 OR
counts/s to 25000 OR counts/s for the NUV MAMA
•
–
–
–
Should prevent additional HV ramp up failures and lost
observations
Will continue to monitor dark current
May be requesting extra NUV dark visits
13