The Medium Deep Survey Key Project team announces the availability of super-sky flat fields and other calibration files for WF/PC-1 which result in improved photometric accuracy. Exposures of random sky fields have been taken in parallel mode, and used to construct super-sky flats.
The goal of the calibration effort undertaken by the MDS team has been to improve quantitative measurement of faint images. Several modifications to the standard calibration and image combination procedures have been introduced, including new bias and dark frames, CTE correction for non preflashed data, and super-sky flat fields, with internal pixel-to-pixel rms errors of about 2.4% in $ I $ and 2.0% in $ V $. Overall, these modifications have improved the quality of faint images by about a factor of five in photometric accuracy and about 0.3 mag in sensitivity.
Detection and quantitative study of faint objects in WF/PC-1 images has been possible by applying the appropriate effort in the calibration process. The camera is sufficiently stable for good photometric performance (at the 3% level or better). In order to achieve this performance in both sensitivity and photometric stability, we have augmented the standard STScI pipeline calibration with some additional procedures. Noise in the bias and dark frames has been reduced by judicious filtering, which includes removing known patterns and smoothing over scales where nothing but noise appears to be present. An even more significant improvement is achieved by the use of so-called super-sky flat field images, obtained from a combination of the sky background at many different pointings, instead of Earth flats. The latter suffer from streaks and non-linearity, and also from large-scale inhomogeneities at the 15--20% level which are difficult to eliminate. raw sky flats are shown to be repeatably flat at the 1% level over scales of several tens of pixels, and are better suited for faint objects because of the lower level of illumination compared with the Earth flats. Earth flats are useful in the definition of high-frequency pixel-to-pixel variations, and have been used in combination with raw sky flats to generate the super-sky flats.
The MDS calibration procedures result in significant improvements in both photometric accuracy and sensitivity for faint images. Photometric accuracy is at least 0.03 mag, or a factor of 5 better than that obtained with standard procedures, mainly because of the use of super-sky flat fields. Sensitivity is improved by about 0.3 mag., owing to the reduced noise in the calibration files, especially bias and dark.
A full description of the methods used to generate these calibration files is contained in Ratnatunga et al (1994) - to be published in the proceedings of the STScI Calibration Workshop held in November 1993.
The archived calibration images are as follows:
e2m1054hw.r0h Mask: Static Mask from skyflats.
e2m10543w.r2h Bias: Smooth version of d8c08261w.r2h
e2m10560w.r3h Pref: Improved CTE correction for non preflash data
e2m1054aw.r5h Dark: Aug'92 to Dec'92 restack 8 image
e2m1054dw.r5h Dark: Mar'92 to Jul'92 restack d3m1553fw.r5h
e2m1054iw.r5h Dark: Jul'91 to Dec'91 restack d3m1553qw.r5h
e2m1054mw.r5h Dark: Aug'92 to Jul'93 Super Stack
e2m1055lw.r6h Flat: F785LP Restack of Earth flat c1d1456nw.r6h
e2m10554w.r6h Flat: F555W Restack of Earth flat c191513jw.r6h
e2m10558w.r6h Flat: F785LP Raw sky flat: 79 image stack
e2m1055dw.r6h Flat: F555W Raw sky flat: 56 image stack
e321016fw.r6h Flat: F785LP Super-sky flat
e321016hw.r6h Flat: F555W Super-sky flat
-Kavan U. Ratnatunga, Richard E. Griffiths, Stefano Casertano, Lyman W. Neuschaefer, Eric W. Wyckoff