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

1. 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

2. 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. HST Calibration Workshop

3. Solar System Objects Observed with the ACS/SBC (not a complete list) HST Calibration Workshop

4. Europa: STIS spectral image (1999) McGrath et al., (DPS, 2000) HST Calibration Workshop

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

6. Europa ACS/SBC PR130L image HST Calibration Workshop

7. Solar analogue 16CygB: PR130L image HST Calibration Workshop

8. Comparison of Europa and 16CygB PR130L counts HST Calibration Workshop

9. Europa disk integrated fluxSaur et al. (in preparation 2010) HST Calibration Workshop

10. Boffi et al., Technical Instrument Report ACS 2008-002 ST-ECF Current synphot Initial synphot HST Calibration Workshop

11. (21) Lutetia HST ACS/SBC F140LP Rosetta flyby on 10 July 2010 (OSIRIS image from ESA website) HST Calibration Workshop

12. (21) Lutetia: SBC and WFPC2 filters(Weaver et al., A&A in press, 2010) For F140LP only 10% of detected photons are at λ < 1895 Å. For F165LP only 10% of detected photons are at λ < 1975 Å. HST Calibration Workshop

13. (21) Lutetia: SBC differential photometry(Weaver et al., A&A in press, 2010) For F140LP−F165LP 40% of detected photons are at λ < 1675 Å, while 50% are at λ > 2400 Å. HST Calibration Workshop

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

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

16. 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! HST Calibration Workshop

17. 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. HST Calibration Workshop