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Drift Scanning at the 4-m (1Mar94)
(from KPNO, NOAO Newsletter No. 37, 1 March 1994)

In 1986 we implemented a scan-table for the Prime Focus CCD camera that
allowed much "flatter" (more uniform) images to be obtained.  In this
mode a scanning table on which the CCD is mounted is stepped by a few
rows while the charge is shifted simultaneously by the same number of
rows; the procedure is continued until the requested number of rows and
total integration time have been achieved.  Since a given point in the
resulting image has actually been observed with a number of different
pixels of the detector, the non- uniformities in the chip have been
effectively averaged in producing the final image.  The flat-fielding
job is therefore easier, as you are starting with intrinsically much
flatter data.

We have recently upgraded the system to work with our large Tektronix
2048 x 2048 devices and to make it considerably easier for users to
both obtain and reduce their data.  In addition to the new scan-table
itself, the data acquisition software ICE has been modified to do
basically it all for you: all the user needs to specify is the number
of rows he or she wishes to scan in addition to the integration time.
The integration time is divided by the number of rows one wishes to
average over, and the table is stepped (and the charge clocked) that
often - although care must be taken not to exceed the rate of a few
Hertz! In typical use, one might short-scan in order to reduce the
effects of the night-sky fringing in the I-band, as Marc Postman
(STScI) has done in the accompanying pair of images.  After a little
experimenting he choose to scan 60 rows to smooth over the higher-
frequency (and higher amplitude) fringes apparent in the unscanned
image.  The low-frequency stuff still needs to be removed by normal
de-fringing techniques, but the fact that he is starting with something
much flatter to begin with makes this job far easier.

		      [Figures not included]

In addition to updating the instrument hardware and software, we have
also prevailed upon Frank Valdes of the IRAF group to modify the CCD
reduction code to check for and process scanned images.  If one
attempts to reduce a scanned image with an unscanned flat-field, the
software will now do the numerical smoothing of the flat-field image to
match what would have been obtained if the flat-field had been scanned
in the first place.  (Thus the user need only obtain unscanned flats.)

In the words of one user, "Why would anyone not scan?"

Phil Massey

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