Image Items

In this Appendix we will describe the items that can be present in an image dataset. Miriad utilities such as prthd, imlist, itemize, puthd, copyhd and delhd may be used to browse or modify these `header variables'.

Note that the units used in Miriad are not always FITS-like, e.g. frequencies are stored in GHz in Miriad, whereas in FITS one finds Hz. For example, Miriad keeps velocity in km/s, sky coordinates in radians and frequencies in GHz.

Table: Item names in Miriad image datasets
Item Type Units Description/Comments
bmaj,bmin real radians Beam major and minor axis FWHM.
bpa real degrees Beam position angle, measured
      east from north.
bunit character   The units of the pixels.
btype character   The type of the image. Possible values:
      intensity - Normal map.
      beam - Point spread function.
cdelt1,cdelt2,... double See ctype The increment between pixels.
cellscal character   Latitude/longitude cell scaling
      convention with frequency/velocity.
      Possible values are:
crpix1,crpix2,... real pixels The pixel coordinate of the reference pixel.
crval1,crval2,... double   The coordinate value at the reference pixel.
    radians if ctype represents a celestial coordinate.
    km/s if ctype VELO or FELO
    GHz if ctype FREQ
ctype1,ctype2,... character   The type of the nth axis (as FITS CTYPE).
datamin,datamax real See bunit The minimum and maximum pixel values.
epoch real years The epoch of the coordinate system.
history text   A text item containing the history of
      processing performed to the data set.
image real See bunit The pixel data.
llrot double radians Eastward rotation relative to north of the
      sky grid relative to the pixel grid.
lstart,lstep,lwidth real    
ltype character   The linetype used in making the map.
mask integer # Bitmap used to determine which pixels in
      the image have been blanked.
mostable binary   Mosaic information table.
naxis integer # Number of dimensions.
naxis1,naxis2,... integer # Number of pixels along each dimension.
niters integer # The total number of deconvolution iterations
      performed on the image.
object character   source name
observer character   observers name
obsra,obsdec double radians The apparent RA and DEC of the observation
obstime double Julian date The mean observing time.
pbfwhm real arcsec Gaussian primary beam size (deprecated).
pbtype character   Primary beam type. Standard values include:
restfreq double GHz The rest frequency of the observed data.
telescop character   Telescope name. Standard values include:
vobs real km/s Velocity of the observatory
      with respect to the rest frame,
      during the observation.

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