Red Leak Effects in Observations of Solar System Objects with ACS/SBC Paul D. Feldman, Johns Hopkins University Harold A. Weaver, JHU Applied Physics Laboratory Joachim Saur, University of Cologne Melissa A. McGrath, Marshall Space Flight Center HST Calibration Workshop STScI, 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 ) 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. 5/17/20152HST Calibration Workshop

Solar System Objects Observed with the ACS/SBC Comet C/2002 T7 (LINEAR)June 2004F140LP, F165LP Comet 9P/Tempel 1July 2005PR130LF140LP EuropaJune 2008PR130L (21) LutetiaNovember 2008F140LP, F165LP MarsLyman-α imaging JupiterFUV auroral imaging SaturnFUV auroral imaging 5/17/20153HST Calibration Workshop (not a complete list)

Europa: STIS spectral image (1999) 5/17/20154HST Calibration Workshop McGrath et al., (DPS, 2000)

Initial ETC Prediction for Europa/PR130L 5/17/20155HST Calibration Workshop Input spectrum assumes that the OI emissions uniformly fill Europa’s disk at opposition.

Europa ACS/SBC PR130L image 5/17/20156HST Calibration Workshop

Solar analogue 16CygB: PR130L image 5/17/20157HST Calibration Workshop

Comparison of Europa and 16CygB PR130L counts 5/17/20158HST Calibration Workshop

Europa disk integrated flux Saur et al. (in preparation 2010) 5/17/20159HST Calibration Workshop

Boffi et al., Technical Instrument Report ACS /17/201510HST Calibration Workshop ST-ECF Initial synphot Current synphot

(21) Lutetia 5/17/201511HST Calibration Workshop 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) 5/17/201512HST 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) 5/17/201513HST Calibration Workshop For F140LP−F165LP 40% of detected photons are at λ 2400 Å.

(21) Lutetia: derived F140LP throughput (Weaver et al., A&A in press, 2010) 5/17/201514HST Calibration Workshop

(21) Lutetia: geometric albedo (Weaver et al., A&A in press, 2010) 5/17/201515HST Calibration Workshop

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, Stay tuned! 5/17/2015HST Calibration Workshop16

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. 5/17/2015HST Calibration Workshop17