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22.3 Factors Limiting Flux and Wavelength Accuracy

22.3.1 Flux Accuracy

The accuracy to which you can trust the absolute flux calibration of your STIS spectroscopic data at slit center is limited by several factors including:

The accuracy of the absolute sensitivity calibration of the grating and central wavelength setting. The on-orbit absolute sensitivity calibration is determined by observing a standard star, with known absolute flux calibration, well centered in both wavelength and cross dispersion in a large slit. The STIS spectrum of this star is then extracted over a standard aperture extraction box and the sensitivity required to return the known flux from the star is determined as a function of wavelength. The standard aperture extraction box is large enough to be relatively insensitive to spacecraft jitter and breathing but small enough that the signal-to-noise of a typical stellar spectrum will not be degraded. STIS calibration accuracies are defined for the standard aperture extraction box; the standard boxes are mode dependent and are given in the XTRACTAB reference file.
The accuracy of the calibration of the aperture throughput for the aperture you are using for your science relative to the aperture which was used for the absolute sensitivity calibration.
The accuracy to which your source is centered in your slit.
The size of the extraction aperture you use to measure your flux and the accuracy to which the cross dispersion profile is known in the mode in which you are observing. Because the corrections for the aperture extraction can be large (e.g., 30% in the near-infrared and the far-UV) this effect can be important.
Bias and background subtraction can add considerable additional uncertainty for faint sources or spectra with significant variations in flux, particularly for the echelle modes.
The importance of scattering, which can play an important role, particularly in the G230LB and G230MB gratings. Additional uncertainties arise for flux measurements not at slit and field center. These uncertainties are relevant when, for example, you wish to determine relative fluxes in an extended source along the long slit or when you have used POS TARGs to move a target along the long slit. They include:
The variation in slit throughput along the slit. The slits have 5 micron (corresponding to ~0.2 arcsecond) variations along their widths, which for a 0.1 arcsecond wide slit on a diffuse source, would produce a 20% variation in flux. For a point source the variation would be more in the 5% range for that same slit. There are also dust specks with appreciably higher opacity along some places in some slits. These specks have not yet been cataloged.
The accuracy to which the vignetting along the cross-dispersion direction is known for your grating and central wavelength
The accuracy to which the low-order flatfield along the dispersion direction is known off of field center for your grating and central wavelength. At this early stage, the accuracies to which these factors are known are changing dramatically, as we finish analyzing the Servicing Mission Orbital Verification (SMOV) data and embark on Cycle 7 Calibration. Updated calibration information is provided on the STIS WWW page.

22.3.2 Wavelength and Spatial Accuracies

The accuracy with which the wavelength scale is known in your calibrated STIS spectrum will depend on several factors:

The accuracy of the dispersion solutions, which governs the accuracy to which relative wavelengths are known in a given spectrum
The accuracy of the wavelength zeropoint, which governs the accuracy to which relative wavelengths are known across spectra.
The accuracy to which your source is centered in the science slit (a pixel of miscentering corresponds to a pixel in absolute wavelength space). The dispersion solutions used to calibrate STIS on orbit data were derived on the ground during thermal vacuum testing. On-orbit tests confirm the applicability and accuracy of the ground dispersion solutions for on-orbit data, producing relative wavelength accuracies of 0.2 pixels across the spectrum for first-order gratings at the prime settings and appreciably better in some instances. For the echelle modes, at the prime settings, the accuracies are roughly 0.2 pixels for wavelengths in the same order and approximately 0.5 pixels for the entire format. The intermediate wavelength settings have roughly twice these errors.
Due to the lack of repeatability of the Mode Select Mechanism (STIS's grating wheel-see page 19-3), the projection of your spectrum onto the detector in both wavelength and space will vary slightly (1 to 10 pixels) from observation to observation (if the grating wheel is moved between observations). In addition, thermally induced motions can also affect the centering of your spectrum. The calstis pipeline removes the zeropoint offsets using the contemporaneously obtained auto-wavecals (see "WAVECAL Processing" on page 21-23). The wavelength zeropoint in your calibrated data (the rootname_sx2.fits, _x2d.fits, _x1d.fits, and _sx1.fits files) is corrected for these offsets and should have a wavelength zeropoint accuracy of ~0.1-0.2 pixels (better when the wavecal is taken through small slits, worse for those taken through wider slits). This accuracy should be achieved, so long as contemporaneous wavecals were taken along with the science data, distributed at roughly one hour intervals among the science exposures, and assuming the target was centered in the slit to this accuracy or better.
The accuracy of the zeropoint pipeline calibration in the spatial direction is slightly less, roughly 0.2-0.5 pixels. This is because the finding algorithm, which must locate the edges of the aperture for short slits and the edges of the fiducial bars on the slits for the long slits, is less robust. Observers need to be aware of possible offsets between spectra in the spatial direction, particularly when deriving line ratios for long-slit observations of extended targets taken with different gratings.

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