Red Leak Effects in Observations of Solar System Objects with ACS/SBC

Paul D. Feldman, Johns Hopkins UniversityHarold A. Weaver, JHU Applied Physics Laboratory

Joachim Saur, University of CologneMelissa A. McGrath, Marshall Space Flight Center

HST Calibration WorkshopSTScI, July 21-23, 2010

Overview

• Following the failure of STIS in August 2004, attempts to obtain ultraviolet spectroscopy and photometry of solar system objects shifted to the Solar Blind Channel (SBC) of the ACS.

• Initial estimates of long wavelength ("red") contamination of the data due to impurities in the FUV MAMA detector suggested that these observations were feasible.

• Subsequent analyses produced better sampled, more reliable response curves (Boffi et al., TIR ACS 2008-002) that showed the long wavelength response to be much worse than expected.

• Analysis of differential photometry of asteroid (21) Lutetia (Weaver et al., A&A, in press) shows an effect 2.5 times larger than the published data.

• A PR130L spectrum of the solar analogue star 16CygB was used for the modeling of the Europa emission spectrum.

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Solar System Objects Observed with the ACS/SBC

Comet C/2002 T7 (LINEAR) June 2004 F140LP, F165LP

Comet 9P/Tempel 1 July 2005 PR130L F140LP

Europa June 2008 PR130L

(21) Lutetia November 2008 F140LP, F165LP

Mars Lyman-α imaging

Jupiter FUV auroral imaging

Saturn FUV auroral imaging

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(not a complete list)

Europa: STIS spectral image (1999)

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McGrath et al.,(DPS, 2000)

Initial ETC Prediction for Europa/PR130L

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Input spectrum assumes thatthe OI emissions uniformlyfill Europa’s disk at opposition.

Europa ACS/SBC PR130L image

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Solar analogue 16CygB: PR130L image

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Comparison of Europa and 16CygB PR130L counts

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Europa disk integrated fluxSaur et al. (in preparation 2010)

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Boffi et al., Technical Instrument Report ACS 2008-002

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ST-ECF

Initial synphot

Current synphot

(21) Lutetia

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Rosetta flyby on 10 July 2010(OSIRIS image from ESA website)

HST ACS/SBC F140LP

(21) Lutetia: SBC and WFPC2 filters(Weaver et al., A&A in press, 2010)

04/18/23 12HST Calibration Workshop

For F140LP only 10% of detected photons are at λ < 1895 Å.

For F165LP only 10% of detected photons are at λ < 1975 Å.

(21) Lutetia: SBC differential photometry(Weaver et al., A&A in press, 2010)

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For F140LP−F165LP 40% of detected photons are at λ < 1675 Å, while 50% are at λ > 2400 Å.

(21) Lutetia: derived F140LP throughput(Weaver et al., A&A in press, 2010)

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(21) Lutetia: geometric albedo(Weaver et al., A&A in press, 2010)

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Summary

• ACS/SBC observations of some solar system targets were more difficult than expected because of an underestimate of the detector red leak.

• For Europa, a PR130L spectrum of solar analogue 16CygB is used to model the extended source background present at FUV wavelengths.

• Throughput curve at λ > 2000 Å remains uncertain. For the Lutetia photometry, in order to match the observed count rate in F140LP and F165LP, and additionally match the difference (F140LP−F165LP), we need to increase the system throughput at λ > 2000 Å by a factor of 2.5 from current synphot values.

• Validity of this conclusion will be tested by analysis of the FUV spectra of Lutetia obtained by the Alice spectrograph on Rosetta. Good data were obtained during the fly-by on July 10, 2010. Stay tuned!

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Thanks

• Thanks to many individuals at STScI for their help in both scheduling the moving target observations and understanding the “red leak” problems: Tony Roman, Alison Vick, Max Mutchler, and Jennifer Mack.

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