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WFPC2 Astrometry FAQs

NOTE: Due to limited resources, these pages may not have been regularly updated. It is possible that the information provided below and/or in the links given may be outdated or inaccurate. If you come across conflicting information or are confused by the answers given, please contact the STScI helpdesk at: help@stsci.edu>.

Q: How can I measure accurate relative positions from WFPC2 images?

A: The IRAF task xy2rd can be used to transform pixel coordinates into RA, Dec. If high precision (better than 0.5 arcsec) is desired, we recommend using XY2RD after running MULTIDRIZZLE (which will account for known geometric distortions in the pixel coordinate system). Tasks such as METRIC are no longer recommended due to the geometric distortion they use being out of date (there are currently no plans to update this task to use a more appropriate distortion correction). Note that this will still offer an accuracy of about 0.5 arcsec, due to the uncertainties in the actual coordinates of the guide stars, but the relative precision of the position of different points in different chips should be better than 0.1''.

One possibility for your image alignment is to use MULTIDRIZZLE, which will rebin the HST image to a common frame for all four chips, removing known geometric distortions and relative rotations of the chips. This image can be used for accurate photometry or astrometry, and can also be used profitably for cross-identification. Please note that this will degrade the PC images, since they will be resampled to the resolution of WF2.

Note that xy2rd and MULTIDRIZZLE (like all IRAF/STSDAS tasks for WFPC2) can be used with images either in GEIS or multi-extension FITS (MEF) format. By default, WFPC2 data are now delivered in (MEF) format, while Waivered FITS (WFITS) data can be requested from the archive and converted to GEIS format using the STSDAS task strfits before running either of the tasks mentioned above.

Q: I need to determine orientations (within measurement errors). Is the ORIENTAT keyword in the header a global value or specific to each chip?

A: The ORIENTAT keyword in the header accounts for all the inter-chip rotations. It is distinct for each individual chip.

Q: How can I correct the geometric distorsions due to the F160BW and F130LP UV filters in the WFPC2 images? Could you please provide the up-to-date cubic distortion coefficients, as described in "The performance and calibration of WFPC2 on the HST" (Holtzman et al., PASP 1995) for those two specific filters?

A: We do not have specific solutions for the geometric distortion of WFPC2 for individual filters. As you know, the cubic solution published by Holtzman et al was derived for F555W, but it should be reasonably good for all filters. The shifts (coefficients C1, D1) and rotations (C2-C3 and D2-D3, expect possibly for an overall plate scale factor) are expected to apply exactly, independent of wavelength. The plate scale and the geometric distortion within each chip could have a small wavelength dependence due to the field flattener, which is a refractive element. We plan to derive geometric solutions for three filters (F300W, F555W, and F814W) from observations to be taken after the refurbishment mission.

If you would like to estimate the possible wavelength dependence of the geometric distortion, you might want to use the solution derived by Trauger, et al., and published in the Proceedings of the workshop "Calibrating HST: Post Servicing Mission" held at STScI in May 1995. This solution is based on ray tracing, rather than on measurements of stars, and has an explicit wavelength term.

Q: I have an error in my WPFC2 header which is causing a position error.

A: Prior to April 19, 1994, there were errors in the WFPC2 headers which resulted in incorrect positions by about 3". A task in stsdas, uchcoord, can be used to fix the headers.

Note that if you retrieve data from the WFPC2 static archive at the Space Telescope Science Institute, the headers will be correct (you will not need to run uchcoord on these images).