STIS Instrument Handbook for Cycle 25
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Space Telescope Imaging Spectrograph Instrument Handbook for Cycle 25 > Chapter 16: Accuracies > 16.1 Summary of Accuracies

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 understanding of STIS as of August 2010 and are those we expect in data that has been delivered to the archive for the STIS close-out calibration (i.e. for data taken prior to the SM4 repair mission). 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.
The absolute and relative flux accuracies quoted in Tables 16.1 and 16.2 apply only to observations using relatively large apertures, 52X2 for 1st order modes, and 0.2X0.2 for the echelons. The reliability and repeatability of throughput for smaller apertures has been less well quantified. There is also some evidence that the STIS focus relative to other HST instruments changed after about 2012. For apertures less than or equal to about 0.1" in size, this appears to have resulted in significantly increased throughput variability. Analysis of recent echelle observations taken through the 0.2X0.06 and 0.2X0.09 apertures shows that the average throughput in recent years has been only about 80% of nominal, with occasional individual exposures with these apertures showing as much as a 40% throughput loss. Note that for these apertures, throughput variations of order 10% due to telescope breathing have always been commonly encountered throughout the lifetime of STIS. For the smallest aperture, the 0.1X0.03, average throughput in recent years has typically been only half of its nominal value. Since these throughput losses vary significantly from observation to observation, it is not possible to simply update the ETC throughputs, as the ETC must also warn against observations which are too bright or which may cause saturation, and must therefore adopt the highest throughput that might reasonably be encountered.  Focus offsets can also affect the relative flux calibration as a function of wavelength within a given observation. For modes covering a wide range of wavelengths, relative flux errors of 10% over the wavelength span of E140M and E230M observations done with the 0.2X0.06 aperture are now common. If combined with small aperture centering errors, these relative throughput errors can sometimes increase to as much as 25%.
Many significant changes in pipeline calibration have been made during the lifetime of STIS; 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 CTE 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
Absolute photometry1
Relative photometrya
(within an exposure)
L modes
M modes
Assumes star is well centered in slit, and use of a 2 arcseconds wide photometric slit. See the STIS Data Handbook for a more complete description of the impact of centering and slit width on accuracies. 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 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).

Table 16.2: MAMA Spectroscopic Accuracies
0.25–0.5 pixel1
A pixel for the MAMA refers to 1024 1024 native format pixels.
Assumes star is well centered in slit, and use of a wide photometric slit.
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 1998-18 for details).
Quoted relative flux accuracies of echelle spectra assume that the time dependent shifts in the echelle blaze function are properly corrected. Currently improvements to this calibration are pending, and without these improvements the current relative flux accuracy across echelle orders is only good to between 3 - 9%.

Table 16.3: CCD Imaging Accuracies
Table 16.4: MAMA Imaging Accuracies
0.25 pixel1
A pixel for the MAMA refers to 1024 1024 native format pixels.

Table 16.5: Target Acquisition Accuracies



Space Telescope Imaging Spectrograph Instrument Handbook for Cycle 25 > Chapter 16: Accuracies > 16.1 Summary of Accuracies

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