The STIS NUV-MAMA objective prism…

…and looking beyond for HST UV slitless spectroscopy

Jesús Maíz Apellániz

HST Calibration worskhop

26 October 2005

Outline

• The STIS objective prism– Description– Calibration problems

• MULTISPEC• Calibration

– Flux calibration– Time-dependent sensitivity

• Slitless UV spectroscopy with HST

The STIS objective

prism

FUV

NUV

The standard star HS 2027 +0651 with the STIS objective prism

1200 2125

A crowded field with the STIS objective

prism

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

Flux calibration of the objective prism

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

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

Original measured/reference values

Sensitivity correction and TDS solution

New measured/reference values

Summary of precisions

• Fit to centered isolated stars: 1 % ( < 3000 Å) 10 % ( > 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

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)

A crowded field with ACS/SBC PR130L