|WFC3 Data Handbook v.4|
9.2 There are three coordinate systems applicable to WFC3 Images. First, there is the position of a pixel on the geometrically distorted flat-fielded images (FLT) after pipeline processing through calwf3. Second, there is the pixel position on the drizzled images (DRZ) created by AstroDrizzle, which corresponds to an undistorted pixel position on a tangent plane projection of the sky. Third, there is the corresponding world coordinate (RA, Dec) position on the sky.There are utilities to transform between these coordinate systems that are built on the WCSLIB, a C library that implements the FITS standards for World Coordinate System (WCS) information. For transformations involving (FLT) images, the distortion information in the FITS header is used to correct for the very large effects of geometric distortion. Drizzlepac and astropy WCS are two commonly used Python interfaces to this library that implement these transformations.For example, consider an object found at x,y pixel position (152,156) on UVIS chip 1 on an FLT image (test_flt.fits). The position on the celestial sphere, and the corresponding pixel position on a distortion-corrected DRZ file (test_drz.fits) can be found as follows.
astropy.wcs.all_pix2world does all three transformations in series (table lookup distortions, SIP, and core WCS) from pixel to world coordinates.
The x,y pixel position in the DRZ frame corresponding to a coordinate in the FLT frame can also be found.
The astrometric information in the header of a WFC3 image is derived, in part, from the measured and catalog positions of the particular guide stars used. As a result, the absolute astrometry attainable by using the image header world coordinate system is limited by two sources of error. First, the positions of guide stars are not known to better than about 200 mas. Second, the calibration of the FGS to the instrument aperture introduces a smaller, but significant error, approximately 15 mas. Although absolute astrometry cannot be done to high accuracy without additional knowledge, relative astrometry with WFC3 is possible to a much higher accuracy. In this case the limitations are primarily the accuracy with which the geometric distortion of the camera has been characterized. Typical accuracy of the distortion correction in the pipeline with the standard fourth order polynomial solutions is 0.1 pixels (4 mas for the UVIS and 10 mas for the IR).The guiding performance and pointing stability of HST are described in the HST Primer. The normal guiding mode uses two guide stars that are tracked by two of HST’s Fine Guidance Sensors (FGSs). However, sometimes two suitable guide stars are not available and single-star guiding is used instead with the telescope roll controlled by the gyros. These observations will suffer from small drift rates. To determine the quality of tracking during these observations please review Chapter 6 of the Introduction to the HST Data Handbooks. The gyros have a typical drift rate of 1.5 mas/sec. This causes a rotation of the target around the single guide star, which in turn introduces a small translational drift of the target on the detector. The exact size of the drift depends on the exact roll drift rate and distance from the single guide star to the target in the HST field of view. For WFC3, the roll about the guide star produces a translation of 7 mas (0.2 UVIS pixel, 0.05 IR pixel) in 1000 sec and 38 mas (1.0 UVIS pixel, 0.3 IR pixel) per orbit.The Tweakshifts task may be used to measure and correct for such shifts between successive exposures. The drift over an orbital visibility period can be calculated from these numbers; the typical visibility period in an orbit (outside the Continuous Viewing Zone [CVZ]) is in the range 52-60 minutes, depending on target declination (see Section 6.4 of the HST Primer). The drifts inherent to single-star guiding are not represented in the image header astrometric information, and have two important consequences:
• There will be a slight drift of the target on the detector within a given exposure. For the majority of observations and scientific applications this will not degrade the data (especially if the exposures are not very long). The drift is smaller than the FWHM of the point spread function (PSF). Also, the typical jitter of the telescope during an HST observation is 0.003-0.005", even when two guide stars are used.
• There will be small shifts between consecutive exposures. These shifts can build up between orbits in the same visit. This will affect the AstroDrizzle products from the pipeline, since these rely on the header astrometry, hence the structure of sources in the image will be degraded during the cosmic ray rejection routine. This can however be addressed during post-processing if the user first measures the shifts and then runs AstroDrizzle off-line, using the measured shifts.Also, even when two guide stars are used, there is often a slow drift of the telescope up to 0.01"/orbit due to thermal effects. So, it is generally advisable to check the image shifts, and if necessary measure them to improve the alignment of exposures before running AstroDrizzle off-line to perform the cosmic ray rejection and image combination.In summary, for most scientific applications, single-star guiding will not degrade the usefulness of WFC3 data, provided that the shifts are measured post-facto and AstroDrizzle is re-run offline using these shifts. However, we do not recommend single-star guiding for the following applications: