2.6 STIS Coordinate Systems

2.6.1 Spacecraft, User, and Aperture Frames

References to multiple coordinate systems appear in the headers of STIS data, tied to the spacecraft frame, the user frame, and the aperture frame. We briefly explain here the relationships among these coordinate systems.

The three coordinate systems of interest are the:

• Vehicle Frame (V₁, V₂, V₃): The right-handed coordinate system for the telescope, with V1 pointing in the direction the telescope is looking.
• User Frame (X_user, Y_user): This is the user frame, aligned with the detector.
• Aperture Frame (X_aperture, Y_aperture): This frame is aligned with the slit. POS-TARG movements are aligned with the aperture frame.

The angles associated with these frames that appear in the headers of STIS data files are:

• PA_V3: The position angle of the V₃ axis; the angle from North, towards East, to V₃, measured at the center of the HST focal plane (in the _spt header).
• ROLL_AVG: The average angle from North towards East to V₃, measured at the position of the STIS field in the HST focal plane (in the _jit header, computed).
• PA_APER: The angle from North through East to Y_aperture measured at the aperture reference (in the science header). PA_V3 - PA_APER is either 225 or -135 degrees, with small variations (a few tenths of a degree).
• ORIENTAT: The angle from North through East to Y_user measured at the aperture reference (in science header). It can differ from PA_APER by up to ~1.5 degrees (e.g., for the long slits with offset occulting bars). Note that this is not the same angle as the ORIENT specified in Phase II, which gives the position angle of the U₃ axis, where U₃ = -V₃. For further information on slit orientation while planning STIS observations, see Figure 3.2 and “Fixing Orientation on the Sky” in Chapter 11 of the STIS Instrument Handbook.

2.6.2 CCD Binned Pixel and Sub-array Coordinates

The STIS CCD detector supports on-chip binning by factors of 2 and 4 in each dimension. This option was often used early in the operational life of the instrument to increase the signal-to-noise for faint targets, but became less favorable as the number of hot pixels increased over time. The readout of subarrays, with less than the full dimension in Y, is also supported to reduce the readout time. These modes are discussed in Section 11.1.1 of the STIS Instrument Handbook.

Reference files for the CCD use unbinned full-frame coordinates $//<![CDATA[ \begin{array}{l}(X_\mathrm{ref}, Y_\mathrm{ref})\end{array} //]]>$ after overscan trimming. The coordinates in a trimmed science image $//<![CDATA[ \begin{array}{l}(X, Y)\end{array} //]]>$ are related to these reference coordinates by values given in keywords in the primary header:

Table 2.10 gives examples of coordinates and conversion parameters for datasets with different binnings (BINAXIS1, BINAXIS2) and array size (NAXIS1, NAXIS2). The image coordinates of the reference pixel (CRPIX1, CRPIX2) are near the center of the image.

Table 2.10: Coordinates and Conversion Parameters for Binned Data