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 2004 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). Over that last two years, we have modified our calibration software to take those changes into account. 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 was 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 photometry2,3
(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).
|
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 wavelengths1 |
0.5-1.0 pixel |
- Thermal stability
- Derivation of wavecal zero point
- Accuracy of dispersion solutions
|
Absolute photometry1,2,3
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 modes4 |
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 1998-18 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 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 |
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.
1A "pixel" for the MAMA refers to 1024 x 1024 native format pixels.
