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 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. The accuracy of the dispersion solutions is well maintained across the spatial extent covered by the first-order modes. However, the illumination of the CCD detector by the line lamp used for wavecal exposures suffers somewhat from vignetting at the top and bottom of the detector. Fortunately, the effect of this at the location of the E1 pseudo apertures, (see Section 7.2.7 of the
STIS Instrument Handbook), has been found to be insignificant, although a few observing modes may have a slightly lower accuracy (details are presented in
STIS ISR 2005-03).
Due to the lack of repeatability of the Mode Select Mechanism (STIS's grating wheel), 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 zero point offsets using the contemporaneous wavecals (see
Section 3.4.23). The wavelength zero point in your calibrated data (the _sx2, _x2d, _x1d, and _sx1 files) is corrected for these offsets and should have a wavelength zero point 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 zero point 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.
A source can lie off-center in an aperture because of errors in the ACQ or ACQ/PEAK procedure, because of errors in the defined displacement from the acquisition aperture to the science aperture, and because of drift of the target over time. The component of error in the AXIS1 direction produces an uncalibrated shift in wavelength. The ACQ for a point source is generally accurate to 0.01 arcsec. An ACQ/PEAK can improve the accuracy, but can also degrade it if, for example, the signal to noise is poor. See
Section 5.2 for a guide to the interpretation of acquisition data, and Chapter 8 in the
STIS Instrument Handbook, for a full discussion of acquisition procedures. The error in the defined displacement from the acquisition aperture to the science aperture has generally been insignificant, except for errors in the original definition of the E1 apertures. These long-slit pseudo-apertures, which place the target high on the detector near the readout amplifier to minimize CTE effects, were introduced on 2000-Jul-03 and revised on 2003-Aug-04. The defined AXIS1 positions of apertures 52X0.2E1, 52X0.5E1, and 52X2E1 were revised by shifting them -0.73 pixels; those of the 52X0.05E1 and 52X0.1E1 were revised by shifting them -0.55 and -0.70 pixels, respectively. Before the revision, E1 aperture exposures made after an ACQ or after an ACQ/PEAK at the center of the detector were systematically miscentered by the full amount of these shifts, causing spectral features to appear at longer wavelengths. Exposures made after an ACQ/PEAK with the same E1 aperture were free of the error, and exposures made after an ACQ/PEAK with a different E1 aperture were off-centered by the relative error, which is usually ~0. Target drift is generally insignificant over the course of an orbit when two guide stars are used, but is larger and variable when one guide star is used. See
Section 5.2.5 and Section 8.1.4 in the
STIS Instrument Handbook.
