18 May 2006Slide 1 of 10

STScI TAGFLASH Ground System Development

• Current ground system could support immediate COS launchCurrent ground system could support immediate COS launch• Functionality of all existing science/calibration modes has been tested and reported upon at MSRs prior to January 2004Functionality of all existing science/calibration modes has been tested and reported upon at MSRs prior to January 2004

• COS TV testing in 2003 revealed spectral drift caused by OSM motionsCOS TV testing in 2003 revealed spectral drift caused by OSM motions• Drift occurs after grating-grating changes on OSM1 or OSM2 Drift occurs after grating-grating changes on OSM1 or OSM2 • No significant drift found for small OSM motions (i.e, central wavelength changes or FP-POS motions on a grating)No significant drift found for small OSM motions (i.e, central wavelength changes or FP-POS motions on a grating)• Tests were conducted during the COS TV in October 2003 to characterize the drift behaviorTests were conducted during the COS TV in October 2003 to characterize the drift behavior

• Drift is correctable if PtNe lamps are flashed periodically during science exposuresDrift is correctable if PtNe lamps are flashed periodically during science exposures• COS CALWG met at STScI in January 2004 and recommended that a TAGFLASH observing mode be implementedCOS CALWG met at STScI in January 2004 and recommended that a TAGFLASH observing mode be implemented

• High-level requirements review completed in December 2005• Front end detailed requirements completed February 2006• STScI detailed development schedule presently being refined

• Planned completion: June 2006 • Flash duration and separation specification needed

• STScI and IDT in CALWG splinters and telecons• Test at Thermal-Vac II

• Bench Testing of Commanding (T. Wheeler)Bench Testing of Commanding (T. Wheeler)• TAGFLASH and previously developed Target Acquisition components

• Late spring 2006• Essential for SMGT-readiness certification

18 May 2006Slide 2 of 10

STScI TAGFLASH Ground System Development

• TAGFLASH ModeTAGFLASH Mode• STScI detailed development in progress and on schedule

• Planned completion: June 2006

• STScI and IDT refined flash duration and frequency specifications in CALWG splinters and telecons

• Test at Thermal-Vac II

• Bench Testing of Commanding (T. Wheeler)Bench Testing of Commanding (T. Wheeler)• TAGFLASH and previously developed Target Acquisition components

• Late spring 2006

• Essential for SMGT-readiness certification

18 May 2006Slide 3 of 10

CC BB AA CC BB AA

PtNePtNeWavecalWavecal

ExternalExternalScienceScience

NUVNUVMAMAMAMA

FUV MCP (1 of 2 segments)FUV MCP (1 of 2 segments)

External Science

Internal PtNe Wavecal

COS Spectral Layout for Simultaneous Internal Wavecals and Science Spectra

• Obtain (continuous or flashed) internal PtNe spectra at same time as science exposure

• Track internal PtNe lines and apply shifts to science spectrum (all events time-tagged) in COS data pipeline

• Scattered light from internal wavecal at position of science spectrum is negligible (<1%).

18 May 2006Slide 4 of 10

Spectral Drift Examples

G160M#03285081037

G285M#03285123358

G140L#03285101309

Drift exceeds 1 resolution element

Stee

p R

ise

Plateau

Drift exceeds 1 resolution element

18 May 2006Slide 5 of 10

Spectral Drift Correction

• Correction needed to meet Level 1 requirementsCorrection needed to meet Level 1 requirements• Spectral resolution (and perhaps more importantly, line shape)• Wavelength zero point

• Simple correction could be done with existing wavecal capabilities Simple correction could be done with existing wavecal capabilities outside science exposures if necessaryoutside science exposures if necessary• Wavecal at beginning and/or end of science exposure

• Following charts show this is not satisfactory for long exposures• Long exposures could be broken into shorter exposures with additional

overhead (~4 min per wavecal, or about 8% observing efficiency loss per wavecal assuming 52 minute orbital visibility)

• TAGFLASH would embed wavecals in science time TAGFLASH would embed wavecals in science time • No overhead for wavecals at all• CALCOS to be updated to correct for drifts

18 May 2006Slide 6 of 10

OSM Drift (G285M)Linear Drift Correction

No correction, line is offset and skewed

Better shape but still offset by almost a resolution element

Linear approximation(beginning and end)

Actualmotion

1 resel

18 May 2006Slide 7 of 10

“Optimal” approximation(4 flashes)

Actualmotion

Correction improves shape and offset

OSM Drift (G285M)Optimal Drift Correction

1 resel

18 May 2006Slide 8 of 10

Flash Frequency and Exposure Times

• Flash Frequency (after major OSM move)Flash Frequency (after major OSM move)• First orbit (4 flashes)

• Flash at beginning exposure as reference• Intermediate flash early (~600 sec) to follow fast drift• Intermediate flash after another ~1800 sec to follow change in drift slope• Flash every 2400 sec thereafter

• If CVZ, then flash again after another 2400 sec, etc

• Exposure timesExposure times• NUV settings <10 seconds if all three stripes considered in cross-correlation• FUV settings 5-30 seconds, or less if both segments considered together• Exposure time of at least 5 seconds recommended even in cases where times as

short as 1 second would suffice to be certain the lamp flashes

18 May 2006Slide 9 of 10

Lamp Lifetime

• Reduced lamp flash durations Reduced lamp flash durations • G130M will be workhorse grating (longest lamp exposures)• But other gratings might be used with more OSM motions

• e.g., extended NUV wavelength coverage• Assume 1000 COS orbits per year• Assume 4 flashes per orbit (4000 flashes per year, probably high)• Assume 15 seconds per flash• Assume 10 milli-amp current setting

• On-orbit Usage On-orbit Usage • 8 amp hr lifetime budget per lamp (2 lamps available)• In one year, use

4 x 1000 x 0.01 x 15 / 3600 = 0.17 amp hr /yr

• A 10 year mission => less than 2 amp hr of on-orbit usage

=> Lamp on-time should not be a lifetime issue

18 May 2006Slide 10 of 10

TAGFLASH Testing

• Tests during TVac-II Tests during TVac-II

• SMOV testsSMOV tests• Determine drift character on-orbit for suite of OSM motions

• Allows changes for on-orbit understanding of drift

• Early pattern will probably be conservative (more flashes)