Space Telescope Imaging Spectrograph Instrument Handbook for Cycle 17

16.1 Summary of Accuracies

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In this chapter we describe the accuracies for STIS photometric, spectral, and astrometric calibration achieved in the close-out calibration of data obtained up to the suspension of STIS operations early in Cycle 13. It is anticipated that similar accuracies will be obtainable with a repaired STIS.

Table 16.1 through Table 16.5 list the accuracies for each of STIS' basic observation modes: CCD spectroscopy, MAMA spectroscopy, CCD imaging, MAMA imaging, and target acquisition. The pixels in the tables for the MAMA detectors are low-resolution pixels. All accuracies quoted are 2-sigma limits. The accuracies reflect our current understanding of STIS as of August 2008 and are those we expect in the data that has been delivered to the archive for the STIS close-out calibration. The sources of inaccuracy are described in Chapter 4 of the STIS Data Handbook, which includes in-depth discussions of instrumental phenomena and the creation of reference files that characterize those phenomena.

Many significant changes in pipeline calibration have been made since the last edition of this Handbook was published in October 2003; see Chapter 3 of the STIS Data Handbook for details. Extracted spectra and rectified spectral images from all STIS detectors are now corrected for time-dependent and temperature-dependent variations in sensitivity (see also STIS Data Handbook 5.4.1). Extracted CCD spectra are corrected for CTI losses and are adjusted for the formerly neglected interdependence of grating and aperture throughputs. Time-dependent rotation of spectral traces is applied to the most commonly used first order modes during spectral extraction and spectral image rectification. The blaze shift correction has recently been substantially improved for echelle spectral extractions, and echelle flux calibration has also recently been improved. Improvements were made to flat field reference files (STIS Data Handbook Section 4.1.4).

We remind you that calibration data have always been immediately non-proprietary. If you have need for extreme accuracy or urgent results, you may wish to consider direct analysis of the calibration data for your particular observing mode. (See also Chapter 17 for a description of our on-orbit calibration program.)

Table 16.1: CCD Spectroscopic Accuracies

Attribute		Accuracy1	Limiting Factors
Relative wavelength		0.1-0.4 pixel	Stability of optical distortion Accuracy of dispersion solutions
Absolute wavelength (across exposures)		0.2-0.5 pixel	Thermal stability Derivation of wavecal zero point Accuracy of dispersion solutions
Absolute photometry2,3,4			Instrument stability Correction of charge transfer inefficiency Time dependent photometric calibration Fringe correction (for > 7500 Å)
	L modes5 M modes5	5% 5%
Relative photometry2,3 (within an exposure)			Instrument stability Correction of charge transfer inefficiency Time dependent photometric calibration Fringe correction (for > 7500 Å)
	L modes5 M modes5	2% 2%

1All accuracies refer to prime wavelength settings and directly calibrated special secondary settings. Intermediate settings have roughly a factor of two less accuracy. 2Assumes star is well centered in slit. See the HST Data Handbook for a more complete description of the impact of centering on accuracies. 3Assumes use of a 2" wide photometric slit. See the HST Data Handbook for a fuller description of the impact of slit width on photometric accuracy. 4Photometric accuracies referenced are for continuum sources; equivalent width and line profile measures are subject to other uncertainties (such as spectral purity and background subtraction). 5This accuracy excludes the G230LB and G230MB modes when used with red targets, for which grating scatter can cause large inaccuracies in the flux calibration; see Gregg et al., (2005 HST Calibration Workshop) available at URL http://www.stsci.edu/hst/ HST_overview/documents/calworkshop/workshop2005/papers/gregg.pdf).

Table 16.2: MAMA Spectroscopic Accuracies

Attribute		Accuracy1	Limiting Factors
Relative wavelength (within an exposure)		0.25-0.5 pixel2	Stability of small scale geometric distortion Optical distortion Accuracy of dispersion solutions
Absolute wavelengths1 (across exposures)		0.5-1.0 pixel2	Thermal stability Derivation of wavecal zero point Accuracy of dispersion solutions
Absolute photometry1,3,4			Instrument stability Time dependent photometric calibration
	L modes M modes Echelle modes5	4% 5% 8%
Relative photometry (within an exposure)3,4			Instrument stability Flat fields Echelle modes: Blaze shift correction accuracy Scattered light subtraction
	L modes M modes Echelle modes5	2% 2% 5%

1All accuracies refer to prime wavelength settings and directly calibrated special secondary settings. Intermediate settings have roughly a factor of two less accuracy. 2A pixel for the MAMA refers to 1024 × 1024 native format pixels. 3Assumes star is well centered in slit. 4Assumes use of a wide photometric slit. 5For 0.2X0.2 arcsecond slit. These are typical accuracies which can be 2 to 3 times better or worse as a function of wavelength (see STIS ISR 98-18 for details).

Table 16.3: CCD Imaging Accuracies

Attribute	Accuracy	Limiting Factors
Relative astrometry within an image	0.1 pixel	Stability of optical distortion
Absolute photometry	5%	Instrument stability
Relative photometry within an image	5%	External illumination pattern

Table 16.4: MAMA Imaging Accuracies

Attribute	Accuracy	Limiting Factors
Relative astrometry within an image	0.25 pixel1	Small scale distortion stability
Absolute photometry	5%	Instrument stability and calibration
Relative photometry within an image	5%	Flat fields and external illumination

1A pixel for the MAMA refers to 1024 × 1024 native format pixels.

Table 16.5: Target Acquisition Accuracies

Attribute		Accuracy	Limiting Factors
Guide star acquisition		1-2" 0.2-0.3"	GSC1 catalog uncertainties GSC2 catalog uncertainties See Section 1.2.1
Following target acquisition exposure		0.01" 0.01-0.1"	Signal to noise Source structure Centering accuracy plus plate scale accuracy to convert pixels to arcseconds See Chapter 8 of the STIS Instrument Handbook
	Point sources Diffuse sources
Following peakup acquisition exposure		5% of the slit width	Signal to noise Source structure Number of steps in scan and PSF