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 B.1: 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.
			depolarisation_ratio
			fractional_polarisation
			optical_depth
			polarised_intensity
			position_angle
			rotation_measure
			spectral_index
cdelt1,cdelt2,...	double	See ctype	The increment between pixels.
cellscal	character		Latitude/longitude cell scaling
			convention with frequency/velocity.
			Possible values are:
			CONSTANT
			1/F
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
			centre.
obstime	double	Julian date	The mean observing time.
pbfwhm	real	arcsec	Gaussian primary beam size (deprecated).
pbtype	character		Primary beam type. Standard values include:
			HATCREEK, VLA, ATCA, WSRT
restfreq	double	GHz	The rest frequency of the observed data.
telescop	character		Telescope name. Standard values include:
			HATCREEK, VLA, ATCA, WSRT
vobs	real	km/s	Velocity of the observatory
			with respect to the rest frame,
			during the observation.