README: SM3A ERO observation of NGC 2392

This directory contains the drizzled images created for the SM3A ERO
obervation of NGC 2392.  The images are:

Image File    Drizzle Weight File    Filter    Exposure Time (s)
F469N.fits         F469Nw.fits       F469N        1400
F502N.fits         F502Nw.fits       F502N         400
F656N.fits         F656Nw.fits       F656N         400
F658N.fits         F658Nw.fits       F658N        1400

The nebula was centered on chip3 of the WFPC2, and only this image
(of the four group image) was processed.  The flat used in this
analysis was created before the servicing mission.  The "delta" dark,
used to correct pixels signficantly different from those in the "super"
dark, was created from dark images taken with 24 hours of the ERO
observations.   Each final image was created from four dithered
images, where the dither pattern was the standard WFPC2 four-point
"box" dither.  The weight files used in drizzling had values equal
to either 0 or the exposure time of the input image, depending on
whether a particular pixel was masked.  As the image did not closely
approach the edge of the chip, and the shifts of the "box" pattern
are relatively small, no weighting for the curvature of the flat 
was used.   Cosmic rays and other image defects were located and 
masked using the techniqe described in the Fruchter and Hook
drizzling paper (see link in parent directory). The final output 
images have a pixel scale one-half that of the original WFPC2 images, 
or about 0."05.  The user of these images should remember that because 
the noise in drizzled images is correlated, the true image noise over 
an area of N (>>1)  pixels is greater than root(N) times the image r.m.s.  
In particular, with the drizzle parameters used  here, scale=0.5 
and pixfrac=0.6, the final the true image noise is about 1.66 times 
greater than that predicted by root(N).

Note for users wishing to work on individual images:
The dark calibration image used to process this data has been placed
in the archive.  It is now the "recommended" darkfile for these datasets,
and will be used in calibration-on-the-fly processing by the archive
of these datasets.