Part II: STIS Data Handbook

4.2 Summary of Accuracies

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In Table 4.1 through Table 4.5, the accuracies are listed for each basic observation mode of the STIS: CCD spectroscopy, MAMA spectroscopy, CCD imaging, MAMA imaging, and target acquisition. The MAMA pixels in these tables are low resolution pixels. All accuracies quoted are 2 limits, and reflect our current understanding of STIS calibration. Any updates to these accuracies will be documented in Chapter 16.1 of the STIS Instrument Handbook. The main sources of inaccuracy are discussed in some detail in the next subsections.

We remind you that calibration data are, and have always been, made non-proprietary immediately after they are (or were) taken. Should you have a need for higher accuracy or urgent results, you may wish to consider direct analysis of the calibration data for your particular observing mode (see also Chapter 17 of the STIS Instrument Handbook for a description of our on-orbit calibration program).

Table 4.1: 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. 3Assumes use of a 2" wide photometric slit. 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 4.2: MAMA Spectroscopic Accuracies

Attribute		Accuracy1	Limiting Factors
Relative wavelength (within an exposure)		0.25-0.5 pixel	Stability of small scale geometric distortion Optical distortion Accuracy of dispersion solutions
Absolute wavelengths1 (across exposures)		0.5-1.0 pixel	Thermal stability Derivation of wavecal zero point Accuracy of dispersion solutions
Absolute photometry1,2,3			Instrument stability Time dependent photometric calibration
	L modes M modes Echelle modes4	4% 5% 8%
Relative photometry (within an exposure)2,3			Instrument stability Flat fields Echelle modes: Blaze shift correction accuracy Scattered light subtraction
	L modes M modes Echelle modes4	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. 2Assumes star is well centered in slit. 3Assumes use of a wide photometric slit. 4For 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 4.3: 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 4.4: MAMA Imaging Accuracies

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

 
Table 4.5: Target Acquisition Accuracies

Attribute		Accuracy	Limiting Factors
Guide star acquisition		1-2" 0.2-0.3"	GSCI catalog uncertainties GSCII 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