A pixel position on a drizzled image (drz.fits/drc.fits
) may be transformed to a position on the celestial sphere (RA, Dec) using the task pixtosky
(in the DrizzlePac
package, see the DrizzlePac website
for details). There is a corresponding task, skytopix
, also in the DrizzlePac
package, that transforms a RA, Dec position to a pixel position on a drizzled image.
The astrometric information in the header of an ACS image comes indirectly from the positions of the guide stars used during the observations.
As a result, the absolute astrometry attainable by using the image header world coordinate system directly is limited by two sources of error. First, the positions of guide stars are not known to better than about 0.3 arcseconds. Second, the mapping from the guide star to the instrument aperture introduces a smaller, but significant error.
Accurate astrometric measurements, especially for faint sources, should take into account the effects of CTE, as described in ACS ISR 2007-04
. The Institute is monitoring the variations of the linear skew terms and will continue updating the corresponding astrometric reference files described in the above-mentioned ISR.
The normal guiding mode uses two guide stars that are tracked by two of HST
's Fine Guidance Sensors (FGSs). On some occasions
, when two suitable guide stars are not available, single-star guiding is used 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 refer to the Introduction to the HST Data Handbooks
for information about jitter files.
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. Additional information is available in ISR TEL 2005-02
. For ACS with single-star guiding, the typical and maximum drift rate of the target on the detector are shown in Table 5.5
The drift over an orbital visibility period can be calculated from the values in Table 5.5
. The typical visibility period in an orbit (outside the Continuous Viewing Zone [CVZ]) ranges from 52 to 60 minutes, depending on target declination. The drifts inherent to single-star guiding are not
represented in the image header astrometric information, and have two important consequences:
Even when two guide stars are used, there is often a slow drift of the telescope up to 0.01
arcsec/orbit due to thermal effects. So, it is generally advisable to check the image shifts, and if necessary, align the images using the TweakReg
task in DrizzlePac
to improve the alignment of the exposures before running AstroDrizzle
Observers who are particularly concerned about the effect of pointing accuracy on the PSF can obtain quantitative insight using the TinyTim
software package. While this does not have an option to simulate the effect of a linear drift, it can calculate the effect of jitter of a specified RMS value.