space telescope science institute operated for nasa by aura cos tagflash system requirements review...

SPACETELESCOP

ESCIENCEINSTITUTEOperated for NASA by AURA

COS TAGFLASH System Requirements Review

COS Lamp Lifetime Estimates for TAGFLASH Scenarios

Steven Penton Center for Astrophysics and Space Astronomy

University of Colorado at Boulder14 December 2005

University of Colorado - Boulder

COS Lamp Lifetimes

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December 14, 2005 COS Lamp Lifetime Estimates

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• RequirementsRequirements

• COS Calibration Optics ReviewCOS Calibration Optics Review

• Lifetime Estimates from FOS and GHRSLifetime Estimates from FOS and GHRS

• Lifetime Estimates from COS 2003 Thermal Vacuum TestingLifetime Estimates from COS 2003 Thermal Vacuum Testing

• Implications for COS TAGFLASH OperationsImplications for COS TAGFLASH Operations

• Detector Issues ?Detector Issues ?

• RecommendationsRecommendations

Presentation Overview


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Requirement : The calibration lamps must support a 5-year on-orbit mission (but the goal is 8 years).

COS contains two internal Platinum-Neon (PtNe) wavelength calibration (wavecal) lamps, and two flatfield Deuterium lamps on the calibration platform.

Additional Lamp Requirement


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COS Calibration Subsystem


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COS Calibration Subsystem


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COS PtNe Wavecal Lamp - Side View


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COS PtNe Wavecal Lamp - Front View


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• Based upon George Hartig’s analysis of the FOS (PtCrNe) and Dennis Ebbets’ recollection of GHRS (PtNe) lamp performance, concludes that the COS PtNe lamps should degrade at 15% per Amp-Hour (A-Hr).

• After 8 A-Hr, estimates that the COS lamp output would be down to 27% of its original output.

• Concludes that COS has 2x8 = 16 Amp-Hrs of lifetime wavecal exposure available, a safety margin of 2:1 (Based upon 1000 orbits/yr x 5 yrs x 6 min/orbit.)

• As pointed out by Ken Sembach on December 15, 2005. New Estimates by Florian Kerber, et al. (http://www.stecf.org/newsletter/stecf-nl-39.pdf) indicate GHRS degradations on the order of 25% / (50 hrs * 10mA) = 50% per Amp-Hr.

Initial Lamp Lifetime EstimatesBall SER No. SYS-039

http://www.stecf.org/newsletter/stecf-nl-39.pdf


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DRM Scenarios(Design Reference Mission)

Science Operations Requirements Document (OP-01)

Lifetime. The wavelength calibration subsystem shall be designed to provide the required data for the entire lifetime of the COS instrument. The COS shall be designed for a minimum of five years on-orbit operating life and a minimum of ten years calendar life (CEI Sec. 5.1.2). The pre-launch testing and calibration may represent the equivalent of one year of operations. The Design Reference Mission estimates that COS may be prime instrument for up to 10,000 orbits, with as many as four wavelength calibration exposures per orbit. The DRM also estimates that approximately 500-1000 individual targets will be observed with COS over its lifetime. Each on-board target acquisition may require at least one exposure of the calibration lamp. The above estimates imply approximately 40,000 wavelength calibration exposures. If each exposure requires one lamp to be on for 3 minutes the requirement is for 120,000 minutes (2000 hours) of use. These estimates imply approximately 40,000 turn-on cycles.

10,000 orbits x 4 wavecals/orbit x 3 min/wavecal x 0.01 Amp = 20 A-Hr10,000 orbits x 6 wavecal mins/orbit x 0.01 Amp = 10 A-Hr, 5 A-Hr per lamp


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Looks OK, BUT there is one under “appreciated” aspect and one Looks OK, BUT there is one under “appreciated” aspect and one assumption (15 % per A-Hr) that complicate matters.assumption (15 % per A-Hr) that complicate matters.

Estimates of the actual lamp degradations (% per A-Hr) haveEstimates of the actual lamp degradations (% per A-Hr) have

been measured from Appendix B “Repeatability Monitor” data been measured from Appendix B “Repeatability Monitor” data and will be presented later. First, we need to consider the obvious and will be presented later. First, we need to consider the obvious implication of lamp degradation, one needs to constantly increase implication of lamp degradation, one needs to constantly increase

the exposure time and/or lamp current to maintain a constant the exposure time and/or lamp current to maintain a constant signal-to-noise in the wavecal spectra.signal-to-noise in the wavecal spectra.

Assume 1,000 orbits/yr, 6 wavecal mins/orbit, split between lamps.

Complications of Previous Estimates


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15% per A-Hr + 6 min/orbit =

~ 7 years lifetime


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25% per A-Hr + 3 min/orbit =

~ 7 years lifetime


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• 5 Repeatability Monitors (Test #850) : Sequenced observations of all gratings with one of the PtNe lamps + FUV and G185M flatfield observations.

• Each observation fully corrected:•Background subtracted•FUV : Thermally and Geometrically Corrected

• Each extracted spectrum cross-correlated to the first observation to ensure only identical spectral regions were tracked.

• ALL COS exposures examined to determine actual A-Hr used for each lamp (included those observed in pseudo TAGFLASH).

The output of the lamps over the 2003 thermal-vac was tracked to quantify actual lamp The output of the lamps over the 2003 thermal-vac was tracked to quantify actual lamp degradation in % per A-Hr.degradation in % per A-Hr.

Appendix B - Lamp Degradation Analysis


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tt

G130M Example


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G185M Example


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Results (1000 orbits/yr x 6 min/orbit)


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Results (1000 orbits/yr x 3 min/orbit)


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PtNe #1 PtNe #2

NUVSpectral Stripe Spectral Stripe

A B C A+B+C A B C A+B+C

G185M 12 32 20 64

G225M 70 105 55 230

G285M 180 5 65 250

G230L 560 900 30 1490

FUVSegment Segment

A B A B

G130M 80 50

G160M 200 580

G140L 800

COS Wavecal Count Rates (counts/sec)


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COS Wavecal Exposure Times (seconds to 1,000 cts)

PtNe #1 PtNe #2

NUVSpectral Stripe Spectral Stripe

A B C A+B+C A B C A+B+C

G185M 83.3 31.3 50.0 15.6

G225M 14.3 9.5 18.2 4.3

G285M 5.6 200.0 15.4 4.0

G230L 1.8 1.1 33.3 0.7

FUVSegment Segment

A B A B

G130M 12.5 20.0

G160M 5.0 1.7

G140L 1.3


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The NUV/FUV detectors exhibit efficiency reduction with usage (local gain sag). The figure of merit is total lifetime counts in any spot (fluence).

•Count Rates•< 100 per sec for G130M < 600 per sec for G160M

•< 800 per sec for G140L/A•Max Count Rate per line•< 2 per sec G130M/A ; < 1.2 per sec G130M/B

•< 2 per sec G160M/A ; < 20 per sec G160M/B (2/3 of max allowed)

•< 6 per sec G140L/A•Max Counts per year per line :•1,000 orbits/yr X 6 min/orbit x 60 sec/min x 20 cts/sec

•= 7.2 million cts/yr per line (1/3 of max allowed)

Detector Issues ?


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Item FUV NUV Rationale

Minimum Count rate:Minimum of 5 lines

in each exposure0.33 cts/sec/line 0.33 cts/sec/line Count rate integrated over emission

line : 100 counts in 5 minutes.

Maximum global count rate (Time Tag)

< 21,000 cts/sec global Time-tag

< 21,000 cts/sec global Time-tag

Maximum time-tag count rate limited by on-board data processing

Maximum count rate in one pixel 5 cts/sec/pixel 200 cts/sec/pixel Charge replenishment +

uniform image illumination

Maximum count rate in one emission line 75 cts/sec/resel 800 cts/sec/resel

Local bright object protection, maxrate derived from CARD limits:

FUV: 1500 cts/sec/resel / 20NUV: 300 cts/sec/pixels x (2x2)

optimistic resolution

Accumulated counts in one emission line

per year

2.2 x 107 cts[1000 five min exposures @

75 cts/sec/]

2.4 x 108 cts[1000 five min

exposure @ 800 cts/sec]

Threshold for onset of gain sage:FUV 109 counts/mm2

NUV 1010 counts/mm2

No obvious Detector Issues

Count Rate Limits


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•Don’t Panic

• The lamp degradation look problematic, but are measured over very small baselines (0.033-0.113 A-Hr)

• We are in the process of organizing an in-depth lamp analysis with non-flight lamps to determine the actual lifetime/degradation rates of COS PtNe lamps

• Use shorter exposure times for each wavecal (1,000 cts)

• Use all NUV stripes in cross-correlation

• Strive to keep lamp usage to < 3 min/orbit

Recommendations


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• Separate wavelength calibration templates are needed for each lamp (PtNe #1, PtNe #2). The lamps are unique, have different usage, and follow different optical paths

• Wavelength calibration templates MUST evolve with time (at least annually). The line ratios of the lamps change with usage history and current setting

• Minimize the wavecal local rate check exposure times (1 min x 1,000 orbits x 0.01 mA x 5yrs = 1 mA = -25%) (6 sec x 1,000 orbits x 0.01 mA x 5yrs = 0.1 mA = -2.5%)

Recommendations


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• Investigate whether unique exposure times are needed for (some) NUV FP-SPLIT positions (1 step = 50 p)

• Investigate whether the lamps can be run at the ‘LOW’ current settings (3mA for PtNe#1, 6mA for PtNe#6) to conserve lifetime (Thermal Vac II, Appendix C)

• ??

Future Tasks

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space telescope science institute operated for nasa by aura cos tagflash system requirements review...

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