Halpha Continuum-Subtracted Images
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Aligned R and Halpha continuum-subtracted images are provided for each
galaxy. The R image is the longest exposure image. In most cases, it
was necessary to convolve the images to the same seeing before the
continuum subtraction. This was done using the psfmatch routine in IRAF, 
based on 1-2 well-exposed star(s) as close to the galaxy as possible. 
Cosmic rays were removed before the convolution using the IRAF task 
cosmicrays. The unconvolved image(s) with the best seeing is (are) provided 
for reference. Halpha continuum-subtraction scale factors are initially 
derived from a comparison of foreground stars in the R and Halpha image 
and adjusted as necessary to reduce diffuse emission and minimize central 
continuum subtraction problems. The headers contain the subtracted background 
sky values, standard deviations, the seeing, and the best-fit Halpha continuum 
subtraction factors. Sky background values should be rechecked before 
photometry.

For some galaxies, the center was saturated for the long R
exposure. For those galaxies with available short R images, the image
provided has been corrected for saturation by replacing saturated
central galaxy pixels with scaled intensities from the short R
exposure. The region of substitution for central saturated pixels is 
given in the header. The aligned short R exposure is provided for these
galaxies. There are two Halpha continuum subtracted images provided
for these galaxies: (i) the Halpha image with continuum measured from the
corrected long R exposure and (ii) the Halpha image with continuum
measured from the short R exposure (please note that the scale factor for 
these images has not been optimized).  These images should be used with
caution, and the error analysis of the central regions should be done
carefully. 

Stellar (R) and stellar residual (Halpha) mask images will be added as
completed.  Stellar residual masks are needed for the Halpha images
because of imperfect subtraction due to differing psf's and because of
saturated stars in the R image.  Mask images are given in the IRAF pixel
list format and have the same root name as the corresponding
image. Bad pixels are marked with a value of 1 and good pixels with a value
of 0. The IRAF imcopy command can be used to produce a fits image
from the pixel list (imcopy name.pl maskname.fits).  The IRAF/STSDAS task
ellipse accepts masks in the pixel list format (but the image name must
have a '.fit' suffix.

A list of cosmic rays (corrected before convolution) is provided in pixel
list format for each image. 

NAMING CONVENTIONS:
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ROOT NAMES:  

nXXXXr600: long R   (XXXX = NGC number)
nXXXXryy:  short R  (yy = short R exposure time)
nXXXXha:   Halpha 
nXXXXhacs: Halpha continuum-subtracted 


ENDINGS ON R AND HALPHA ROOT NAMES:
r:  background-subtracted, cosmic-ray corrected, unconvolved
rc: same as r, but convolved to same psf
f:  image had a saturated central region, but was 'fixed'.
    applies to the 'fixed' R image and the Halpha continuum-subtracted 
       image obtained using the fixed R image

cos.pl: pixel list of cosmic rays found in IRAF cosmicrays task

.pl: stellar or stellar residual mask corresponding to the root name image


ENDINGS ON HALPHA CONTINUUM-SUBTRACTED ROOT NAMES ONLY:
lr: continuum-subtracted using long R image
sr: continuum-subtracted using short R image

HEADER PARAMETERS
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SKY:      sky background value subtracted
SKYSIGMA: standard deviation in background
SEEING:   FWHM
HASF:     Scale factor applied to long R image in continuum subtraction
REPLACEB: Rectangular box region containing replaced (saturated) pixels
          in long R image
SRSF:     Scale factor applied to short R image in replacement of saturated
          pixels
REPLACEC: Additive correction applied to the REPLACEB region in case of poor
          automatic correction