fits op=xyin in=hco+_4-3.fits out=hco+_4-3.sky
To compare the emission with observations made at, e.g., the JCMT, it is convolved to a 14-arcsecond beam using `convol':
convol map=hco+_4-3.sky out=hco+_4-3.cnv fwhm=14 scale=4.5e-3
The scale factor f ensures that flux is conserved in this process. It is calculated from the full width at half maximum of the telescope main beam B and the pixel size p using
f = [ 1.13309 (B/p)2 ]-1
The factor pi / 4 ln 2 = 1.13309 makes up for the difference between a round beam and square pixels.The convolution program keeps the pixel size and velocity coverage and resolution of its input. Instead of specifying the FWHM of a Gaussian beam, a self-constructed, assumed or measured beam pattern can be used through the the keyword beam=, followed by the Miriad image containing the pattern.
The task `imspec' extracts synthetic spectra:
imspec in=hco+_4-3.cnv region='arcsec,box(20,10,20,10)' log=hco+_4-3.msp
In the example above, a spectrum at an offset of 20 arcsec East and
10 arcsec North from the image center is written to the file specified
as ``log''. This file can be read by a plotting program, to compare the
result to observation. The quotes in the region= keyword
protect
the parentheses from interpretation by the C-shell. Velocities are
written to 0.1 km/s precision; use the command imspect if higher spectral
resolution is needed.
The velocity-integrated intensity can be obtained from the spectrum using `moment':
moment in=hco+_4-3.cnv out=hco+_4-3.int
Task `imstat' gives the image minimum & maximum and other information:
imstat region='arcsec,box(0,0,0,0)' in=hco+_4-3.int
and `maths' is useful to add, subtract, multiply or divide images. This example calculates a line ratio, where the <> signs protect the + in hco+ from interpretation by maths, and the quotes prevent the C-shell from interpreting the <>.
maths exp='<hco+_4-3.cnv>/<hco+_3-2.cnv>' out=hco+_ratio.cnv
If dust emission was included in the calculation, it should be taken out before extracting line fluxes. The first method to do this uses a combination of Miriad and C-shell commands. After convolving the image to the desired resolution, store the number of channels and the channel width (km/s) in shell variables:
set nchan = 100
set chw = 0.2
then derive the continuum temperature:
set tc = `imspec in=hco+_4-3.cnv |& tail +12 |head -1 |awk '{print $3}'`
and subtract the continuum from every channel:
imspec region="arcsec,box(0,0,0,0)" in=hco+_4-3.cnv |&
tail +12 | head -$nchan |\
awk '{s+=$3-tc}END{print s*chw}' tc=$tc chw=$chw
The second method subtracts the first channel from the others with Miriad, starting from SKY output:
imsub in=hco+_4-3.sky out=hco+_4-3.ctm region='image(1)'
maths exp='<hco+_4-3.sky>-<hco+_4-3.ctm>'
out=hco+_4-3.sub
options=grow
followed by beam convolution and spectrum extraction.