specpsf -- Form a longslit spectrum for a point source from many point spread function images


specpsf refima psfwavs psfims sliwid pixwid subsam cenoff psfpos outpsf


refima = "" [file name]
The name of the input 2-dimensional reference image to supply the dimensions and header information for the output longslit Point Spread Function (PSF) spectrum image.
psfwavs = "" [table file name]
The name of the STSDAS table file listing the X (or wavelength if CRVAL1, CD1_1 were present in the header of the reference image) values corresponding to the Point Spread Functions. The values in the table must be contained within the range of X (or wavelength) values for the input image.
psfims = "" [file name template]
The list of the names of the Point Spread Function images. All the images must be of the same dimensions.
sliwid = "" [float]
The width of the slit in arcseconds.
pixwid = "" [float]
The size of the pixels in the Y (i.e. cross-dispersion) direction for the output longslit Point Spread Function (PSF) spectrum image.
subsam = "" [float]
The number of sub-sampled pixels per pixel of the input Point Spread Function images.
cenoff = "" [float]
The offset of the centre of the slit from the peak of the Point Spread Function in the X (i.e. perpendicular to the longslit axis) direction in arcseconds.
psfpos = "" [float]
The pixel position for the output Point Spread Function spectrum in the Y (cross-dispersion) direction.
outpsf = "" [file name]
The name of the ouput longslit Point Spread Function spectrum image.


This task forms a longslit spectrum of a point source by interpolating in the wavelength direction the cross-dispersion profiles formed by integrating the signal across a slit placed over the Point Spread Functions. A two-dimensional reference image supplies the header and format for the output longslit spectrum image. The centre of the Point Spread Function in each input image is determined by centroiding and the slit is placed over the image. An offset of the slit from the centre of the PSF image is accommodated by the cenoff parameter. This offset is perpendicular to the long axis of the slit and is specified in arcseconds and scaled to the pixel size in arcseconds. The flux is integrated across the slit onto the output pixels, which may be subsampled with respect to the pixels of the Point Spread Function images. Interpolation is performed using bicubic splines. The resulting 1-dimensional slit profiles are then cubic spline interpolated and extrapolated onto the output X (or wavelength) grid using the X (or wavelength) values of the individual Point Spread Function images and the X pixel (or wavelength) scale of the reference image.


1. To form the longslit Point Spread Function spectrum image from three input stellar point source images:

       refima = "testref.fits"  Name of input reference file
      psfwavs = ""   Name of table of wavelengths of the PSF's to be
       psfims = "@psflis1.lis"  List of names of the PSF images for interpolati
       sliwid = 0.5             Width of slit (arcsec)
       pixwid = 0.05            Width of pixels (arcsec)
       subsam = 1.              Number of (subsampled) pixels in PSF image per 
       cenoff = 0.              Offset of centre of slit from X peak of PSF (ar
       psfpos = 128.            Required Y value for PSF centre
       outpsf = "test1.fits"    Name of output PSF image

The 256x256 reference image has a CRVAL1 of 7751.0A and a CD1_1 of 4.882A with a wavelength coverage of 7101.6 to 8346.5A. The three PSF images have wavelengths specified by the input table as:

The slit width is 0.5arcseconds and the pixel size is 0.05arcseconds, giving a slit width of 10 pixels. The output Point Spread Function spectrum is sampled on the same pixel size as the input PSF images and the slit is centred on the PSF's. The spectrum of the Point Spread Function in the output longslit spectrum image is centred at Y pixel 128.0.


For 3 PSF images with Gaussian PSF's of FWHM 4.0pixels, slit width 10 pixels, the task takes 99 sec on a Sparc Ultra 10 with 256Mbyte of memory. The processing time scales linearly with the number of PSF's, with the number of pixels in the diameter of the input PSF image (viz. the FWHM) and with the subsampling factor. There is a weak inverse relation between processing time and slit width in pixels.


World coordinates are not supported.


mk2dspec, specholucy, specinholucy