16.1 Summary of Accuracies

In Table 16.1 through Table 16.5, the accuracies are listed for each of STIS's basic observation modes: CCD spectroscopy, MAMA spectroscopy, CCD imaging, MAMA imaging, and target acquisition. The pixels in these tables are low-resolution pixels. All accuracies quoted are 2

limits. The accuracies reflect our current understanding of STIS as of June 2003 and are those we expect the pipeline calibration or recalibration of archive data to achieve during Cycle 13.

The accuracies quoted below do not include the effects of time-dependent changes in sensitivity (See Section 7.1.4 and Section 7.3.3). We modified our calibration software in September 2002 to correct 1st order MAMA spectroscopic modes for these changes in sensitivity over time. Work to include similar modifications for other modes is in progress. Likewise the effects of CTE losses on accuracies are not included in the estimates in this chapter (see Section 7.2.6). Correction of extracted CCD spectra for CTE and time-dependent sensitivity is expected to be implemented during Cycle 12.

The flux calibration of the echelle modes is adversely affected by the monthly offsetting of the spectrum on the MAMA detectors (see Section 7.5), and this can cause wavelength dependant flux errors of as much as 15%. Software changes to minimize these effects on the flux calibration of extracted spectra have been implemented for the primary echelle wavelength settings. Note that starting early in Cycle 11, the monthly offsetting of all echelle modes was stopped. For data obtained after this date, this change will ameliorate the uncertainties in flux and wavelength calibration that resulted from the offsetting. Implementation of improvements in the calibration software to correct earlier data for this effect will continue.

We remind you that calibration data are immediately non-proprietary and should 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 wavelengths-within an exposure

0.1-0.3 pixel

Stability of optical distortion
Accuracy of dispersion solutions

Absolute wavelengths-across exposures

0.2-0.5 pixel

Thermal stability
Derivation of wavecal zero point
Accuracy of dispersion solutions

Absolute photometry2^,3^,4
L modes
M modes

5%
5%

Instrument stability, evolution of charge transfer efficiency, and photometric calibration

Relative photometry ² ^,3
(within an exposure)
L modes
M modes

2%
2%

Instrument stability, evolution of charge transfer efficiency, and photometric calibration

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).

Attribute	Accuracy1	Limiting Factors
Relative wavelengths-within an exposure	0.1-0.3 pixel	Stability of optical distortion Accuracy of dispersion solutions
Absolute wavelengths-across exposures	0.2-0.5 pixel	Thermal stability Derivation of wavecal zero point Accuracy of dispersion solutions
Absolute photometry2^,3^,4 L modes M modes	5% 5%	Instrument stability, evolution of charge transfer efficiency, and photometric calibration
Relative photometry ² ^,3 (within an exposure) L modes M modes	2% 2%	Instrument stability, evolution of charge transfer efficiency, and photometric calibration

Table 16.2: MAMA Spectroscopic Accuracies

Attribute

Accuracy1

Limiting Factors

Relative wavelengths-within an exposure

0.25-0.5 pixel

Stability of small-scale geometric distortion
Optical distortion
Accuracy of dispersion solutions

Absolute wavelengths ¹

0.5-1.0 pixel

Thermal stability
Derivation of wavecal zero point
Accuracy of dispersion solutions

Absolute photometry ¹^,2,3 L modes
M modes
Echelle modes ⁴

4%
5%
8%

Instrument stability and photometric calibration

Relative photometry
(within an exposure) ² ^,3 L modes
M modes
Echelle modes ⁴

2%
2%
5%

Instrument stability and flat fields

Ripple correction accuracy, scattered light subtraction

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 wide photometric slit. See the HST Data Handbook for a fuller description of the impact of slit width on photometric accuracy.
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).

Attribute	Accuracy1	Limiting Factors
Relative wavelengths-within an exposure	0.25-0.5 pixel	Stability of small-scale geometric distortion Optical distortion Accuracy of dispersion solutions
Absolute wavelengths ¹	0.5-1.0 pixel	Thermal stability Derivation of wavecal zero point Accuracy of dispersion solutions
Absolute photometry ¹^,2,3 L modes M modes Echelle modes ⁴	4% 5% 8%	Instrument stability and photometric calibration
Relative photometry (within an exposure) ² ^,3 L modes M modes Echelle modes ⁴	2% 2% 5%	Instrument stability and flat fields Ripple correction accuracy, scattered light subtraction

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

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 and external illumination

Table 16.5: Target Acquisition Accuracies

Attribute

Accuracy

Limiting Factors

Guide star acquisition

1-2 arcseconds

Catalog uncertainties

Following target acquisition exposure
Point sources
Diffuse sources

0.01 arcsecond 0.01-0.1 arcsecond

Centering accuracy plus plate scale accuracy to convert pixels to arcseconds
See Chapter 8

Following peakup acquisition exposure

5% of the slit width

Number of steps in scan and PSF

Attribute	Accuracy	Limiting Factors
Guide star acquisition	1-2 arcseconds	Catalog uncertainties
Following target acquisition exposure Point sources Diffuse sources	0.01 arcsecond 0.01-0.1 arcsecond	Centering accuracy plus plate scale accuracy to convert pixels to arcseconds See Chapter 8
Following peakup acquisition exposure	5% of the slit width	Number of steps in scan and PSF

16.1.1 Flats

We are in the process of updating all library flats to a signal-to-noise of at least 100:1 per pixel.1 The CCD flats have temporal variation of < 1% per year. The MAMA flats have shown some evidence for variation at the 1-2% per resolution element level over roughly year timescales. Due to the limited calibration-lamp lifetimes, we expect to take MAMA flats once per year per detector. As our knowledge grows, we will provide updates on the web pages.