1. Mscimage: The mim files.

We will now use the WCS information to combine the eight single extensions into a single image, where the geometrical distortions have been removed and there is a uniform plate scale across the iamge. We want to do this in such a way that all of the images of a galaxy (all filters, all ditherings) have been corrected to the same scale, orientation, and tangent point. For the galaxies in which a single field is used, we can constract a file containing all of the images of that field and then run mscimage; it will use the first image in the file as the reference image to set the scale, orientation, and tangent point. For the multiple-field galaxies (M33, M31) we need to explictly set the scale, orientation, and tangent point so that overlapping fields match. Here a mim*p.fits file refers to a single image with pupil removed.
We have found that a sinc interpolation produces a detailed, precise image. While sinc (default 31x31) is time intensive, for purposes of 1% photometry, this is likely the best choice. Sinc17 (approximately half of sinc31) is twice as fast, but aperture photometry, when compared with sinc31 images, develops an uncertainty of 0.02 instrumental magnitudes.For our purposes, there appears to be little difference between the fitting geometry parameters of 'general' vs 'rxyscale'. An example of the parameter file:


PACKAGE = mscred
   TASK = mscimage
    
input   =             @todom31  List of input mosaic exposures
output  =         @todom31.mim  List of output images
(format =                image) Output format (image|mef)
(pixmask=                  yes) Create pixel mask?
(verbose=                  yes) Verbose output?

                                # Output WCS parameters
(wcssour=                image) Output WCS source (image|parameters)
(referen=                     ) Reference image
(ra     =                INDEF) RA of tangent point (hours)
(dec    =                INDEF) DEC of tangent point (degrees)
(scale  =                INDEF) Scale (arcsec/pixel)
(rotatio=                INDEF) Rotation of DEC from N to E (degrees)

                                # Resampling parmeters
(blank  =                   0.) Blank value
(interpo=                 sinc) Interpolant for data
(minterp=               linear) Interpolant for mask
(boundar=              reflect) Boundary extension
(constan=                   0.) Constant boundary extension value
(fluxcon=                   no) Preserve flux per unit area?
(ntrim  =                    8) Edge trim in each extension
(nxblock=                INDEF) X dimension of working block size in pixels
(nyblock=                INDEF) Y dimension of working block size in pixels

                                # Geometric mapping parameters
(interac=                   no) Fit mapping interactively?
(nx     =                   10) Number of x grid points
(ny     =                   20) Number of y grid points
(fitgeom=              general) Fitting geometry
(xxorder=                    4) Order of x fit in x
(xyorder=                    4) Order of x fit in y
(xxterms=                 half) X fit cross terms type
(yxorder=                    4) Order of y fit in x
(yyorder=                    4) Order of y fit in y
(yxterms=                 half) Y fit cross terms type


(fd_in  =                     )
(fd_ext =                     )
(fd_coor=                     )
(mode   =                   ql)

The output mim*p.fits and cooresponding mim*bp.pl (bad pixel maps) care now single images with no more extensions. Sky values may be viewed subtracted using 'imexam'. The average can be subtracted using 'imarith'.

A note on the 'mscimatch' & 'mscskysub' tasks...

We do not use `mscskysub' as this "fixes" gradients in the sky. Being that there is a huge gradient in the case of M31 (it is a gradient!), applying 'mscskysub' would be a no-no.

'Mscimatch' has not yet convinced us of it's accuracy with determining the scale factors necessary to match the intensity of same galaxy same filter images. These factors can be done by hand, i.e. photometry of stars on each of the dithered frames (yuck!) or usoing the automated method we are developing.