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STIS Data Handbook 6.0 May 2011
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STIS Data Handbook > Chapter 4: STIS Error Sources > 4.2 Summary of Accuracies

4.2
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 2σ 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
Stability of optical distortion
Accuracy of dispersion solutions
Absolute wavelength
(across exposures)
Thermal stability
Derivation of wavecal zero point
Accuracy of dispersion solutions
Instrument stability
Correction of charge transfer inefficiency
Time dependent photometric calibration
Fringe correction (for λ > 7500 )
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 λ > 7500 )
L modes5
M modes5
2%
2%

1
All accuracies refer to prime wavelength settings and directly calibrated special secondary settings. Intermediate settings have roughly a factor of two less accuracy.

2
Assumes star is well centered in slit.

3
Assumes use of a 2 wide photometric slit.

4
Photometric accuracies referenced are for continuum sources; equivalent width and line profile measures are subject to other uncertainties (such as spectral purity and background subtraction).

5
This 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
Relative wavelength
(within an exposure)
Stability of small scale geometric distortion
Optical distortion
Accuracy of dispersion solutions
Absolute wavelengths1
(across exposures)
Thermal stability
Derivation of wavecal zero point
Accuracy of dispersion solutions
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%

1
All accuracies refer to prime wavelength settings and directly calibrated special secondary settings. Intermediate settings have roughly a factor of two less accuracy.

2
Assumes star is well centered in slit.

3
Assumes use of a wide photometric slit.

4
For 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
Table 4.4: MAMA Imaging Accuracies
Flat fields
External illumination pattern
Table 4.5: Target Acquisition Accuracies
1-2
0.2-0.3
GSCI catalog uncertainties
GSCII catalog uncertainties
See Section 1.2.1


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
5% of the
slit width
Signal to noise
Source structure
Number of steps in scan and PSF

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