New UVIS and IR Instrument Distortion Coefficient Tables (IDCTABs) are Available

We are distributing preliminary distortion solutions for the UVIS and IR channels of WFC3 in a form suitable for use with MultiDrizzle. These data products are based upon limited observations made during SMOV, which will be significantly improved upon during the Cycle 17 calibration program. These preliminary distortion solutions are not being used in the calibration pipeline. Nevertheless, we believe that they will be useful to observers for combining dithered and mosaicked exposures. Our testing has demonstrated that cosmic rays are accurately rejected and all images of stars are correctly aligned for both dithered and mosaicked exposure sets. However, although the distortions of the optical system are well-removed (fit residuals are 3 milliarcsecond for the UVIS channel and 10 milliarcsecond for the IR channel), errors at the 5-arcsec level in the World Coordinate System may be present. The UVIS distortion functions are derived from 30 dithered exposures of fields in 47 Tuc and the LMC through the F606W filter. The IR distortion is determined from 27 dithered exposures through the F160W filter of 47 Tuc and the LMC. Therefore, the filter and wavelength dependence of the distortion is unknown at this writing. Further information regarding the distortion solutions may be found in the reports for the SMOV proposals 11444 (UVIS Plate Scale) and 11445 (IR Plate Scale) on the WFC3 website.

The WFC3 UVIS (t982101i_uv_idc.fits) and IR (t991000i_ir_idc.fits) IDCTAB files for use with MultiDrizzle are available for download on this web page. Both channels use the standard polynomial form with terms x^m * y^n, with the maximum values of the exponents m=4, n=4, and of the sum of the exponents m+n=4, as described in the Multidrizzle Handbook. Only the coefficients for the FORWARD transformation are given. Entries are provided for all UVIS and IR filters. The distortion coefficients determined from the UVIS F606W exposures are provided in the UVIS IDCTAB as the coefficients for every UVIS filter. Similarly, the IR F160W coefficients are provided for all IR filters in the IR IDCTAB. The value of SCALE is the square root of the area of the pixel at the default pointing position for the UVIS-CENTER and IR apertures. The Pixel Area Map is normalized at that pixel.

Errors have been identified in the WFC3-specific portions of the latest public version of MultiDrizzle. Those errors include the failure to convert the units of IR image data (which causes CRs to not be rejected) and reversal of the UVIS CCD chips in calculations of the World Coordinate System parameters (which causes large errors in the computed RA and Dec.) A corrected, alpha-release version of MultiDrizzle will be made available the week of September 14th. Please check the WFC3 website for further information on how to download that version.

Improved versions of these IDCTAB files, if any, will be posted on this webpage.

We welcome comments on your experience using these preliminary versions of the IDCTAB files. Please send comments to help@stsci.edu.