he pixel position on the drizzled images (drz.fits
) created by multidrizzle
which corresponds to an undistorted pixel position on a tangent plane projection of the sky.
A pixel position on a drizzled image (drz.fits
) may be transformed to a position on the celestial sphere (RA, Dec.) using the task xy2rd
(in the stsdas.toolbox.imgtools
package). There is a corresponding task, rd2xy
, that transforms a RA, Dec. position to a pixel position on a drizzled image. (These tools are only meant for drizzled images; they cannot be used for raw.fits
files as they do not include the very large effects of geometric distortion).
The transformation between pixel positions on a distorted image (raw.fits
) and drizzled (drz.fits
) image may be performed using the dither
package tasks traxy
; these tasks implement the same geometric mapping as drizzle
, respectively. For more details on how to use them, the Multidrizzle Handbook
or the dither
package help files should be consulted.
However, there is a more convenient high-level wrapper for determining pixel
position transformations between distorted and undistorted images. The tran
task in the dither
package, which was released as part of the late-2004 version of STSDAS, allows the mapping of X,Y positions between flt.fits
images. This task requires the following to enable processing:
|A set of geometric distortion coefficients files (“coeff
”). These are created when multidrizzle
is run to combine a set of flt.fits
images on your home machine. (“coeff”
files are not available from the archive or from the STSDAS Web site; they can only be created by running multidrizzle
on the flt.fits
files on your local machine.)
|In order to run multidrizzle
, you need to retrieve the IDCTAB
distortion reference table (idc.fits
) and the DGEOFILE distortion correction ref
erence image (dxy.fits
) associated with those images. These filenames are given by the IDCTAB and DGEOFILE image header keywords, and they can be downloaded
from the ACS Reference Files Web page
Work is now underway at STScI to substantially simplify and improve the way
astrometric information is represented and used by MultiDrizzle. This should allow users to more easily handle astrometry, and align images to each other and to external catalogs. Please check the ACS Web site
and STSDAS MultiDrizzle Web site
Example: run tran
to translate the x,y position on a flt.fits
file to its location in a drz.fits
file. This task requires that multidrizzle
has processed the flt.fits
images on your local machine, in order to use the coefficient files (coeffs*.dat
) generated by multidrizzle
|# List of input flt.fits images for multidrizzle, and resulting drizzle-combined image.
The reverse operation, translating a position on a drz.fits image to a flt.fits image,
can be applied as follows:
Finally the xytosky
task, which is part of the dither
package (as of May 2011), will convert a pixel position on a distorted flt.fits
file directly to a sky position, by applying the distortion correction from the IDCTAB
reference table and using the world coordinate information from the header. It may be used as follows:
This task doesn’t require the coefficient files but, like multidrizzle
, it requires a copy of the IDCTAB to be available. Both tran
have options for lists of positions to be supplied as text files to allow multiple positions to be efficiently transformed. The task xytosky
does not currently support DGEOFILE reference images
. If accuracy at a level better than 0.1 pixels is needed, then we recommend the following:
to get the corresponding X,Y pixel position on the drizzled (drz.fits
) image, followed by xy2rd
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.
Although absolute astrometry cannot be done to high accuracy without additional
knowledge, relative astrometry with ACS 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. This is discussed in detail in the Multidrizzle Handbook
. With the recent inclusion of a time-dependent skew into the model of the ACS distortion used by MultiDrizzle, typical accuracy in aligning two ACS images is a few hundredths of a pixel or better.
Accurate astrometric measurements, especially for faint sources, should take into
account effects of CTE, as described in ACS ISR 2007-04
. The institute is monitoring the variations of the linear skew terms and updating the corresponding astrometric reference files derived from 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.
In single guide star guiding, typical gyro drift rates produce a roll about the guide
star of 1.0 - 1.5 mas/sec., which in turn introduces a translational drift of the target on the detector. This roll is not reset and continues to build over multiple orbits and reacquisitions, until the next full guide star acquisition.
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 5.7
The drift over an orbital visibility period can be calculated from the values in Table 5.7
. 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:
| There will be a slight drift of the target on the detector within a given expo
sure. 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 arcsec, 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 MultiDrizzle products from the pipeline because it depends on the header WCS (predicted) positions to determine image offsets when combining dithered images. Therefore, the structure of sources in the image will be degraded during the cosmic ray rejection routine. This problem can, however, be addressed during post-processing by running multidrizzle
, off-line, using manually-determined image offsets.
Even when two guide stars are used, there is often a slow drift of the telescope up to
arcsec/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 multidrizzle
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 ACS data, provided that the shifts are measured post-facto and multidrizzle
is re-run offline using these shifts. However, we do not recommend single-star guiding for the following applications:
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.