1. The STIS NUV-MAMA objective prism … … and looking beyond for HST UV slitless spectroscopy Jes ú s Ma í z Apell á niz HST Calibration worskhop 26 October 2005

2. Outline The STIS objective prism –Description –Calibration problems MULTISPEC Calibration –Flux calibration –Time-dependent sensitivity Slitless UV spectroscopy with HST

3. The STIS objective prism FUV NUV

4. The standard star HS 2027 +0651 with the STIS objective prism 1200 2125

5. A crowded field with the STIS objective prism

6. Calibration issues Sources –Difficulty of wavelength calibration –Lack of “ repeatability ” (~10%) Diagnostic –Coupling between wavelength and flux calibration: positioning and geometric distortion are crucial –Strong count gradient @ 3300 Å: PSF effects –TDS effects Solution –Flux recalibration and geometric distortion solution –Ad-hoc solution for > 3000 Å –TDS correction

7. Flux calibration of the objective prism

8. Calibration issues Sources –Difficulty of wavelength calibration –Lack of “ repeatability ” (~10%) Diagnostic –Coupling between wavelength and flux calibration: positioning and geometric distortion are crucial –Strong count gradient @ 3300 Å: PSF effects –TDS effects Solution –Flux recalibration, separation by settings (1200 and 2125), and geometric distortion solution –Ad-hoc solution for > 3000 Å (low precision) –TDS correction

9. MULTISPEC Slitless-spectra automatic-extraction IDL code Profile fitting (with or without residuals) Requires: –Spectral exposure (objective prism, grism, grating) –Image of the field (two filters) –Image photometry (using e.g. DAOPHOT) –Optional multiple orientations –Calibration files (ready for STIS objective prism) See http://www.stsci.edu/~jmaiz

10. Original measured/reference values

11. Sensitivity correction and TDS solution

12. New measured/reference values

13. Summary of precisions Fit to centered isolated stars: 1 % ( 3000 Å ) Flat-field and variations of the intensity profile across the detector: 3-5 % (lower for isolated stars and dithered data) Detector: only Poisson (no read noise) Background: case-dependent but negligible for bright stars Contamination from nearby spectra: case- dependent

14. Slitless UV spectroscopy with HST STIS: –Slitless and 52x2 datasets in the archive (prism, G140L, G230L) –Will STIS become a born-again instrument? ACS: –HRC (1 objective prism): PR200L –SBC (2 objective prisms): PR110L + PR130L Possible problems: –Wavelength and flux calibration: yes for prisms (especially PR200L) –Strong count gradient: minor issue for PR200L –TDS effects: yes for SBC, not for HRC (but CTE) Ongoing calibration programs: –Observations of standard stars with very different spectral slopes and emission line objects (10391,10743) –Observations of crowded, UV-bright clusters (10722, 10736)

15. A crowded field with ACS/SBC PR130L