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Basic Information on fits
Task: fits
Purpose: Conversion between MIRIAD and FITS image and uv formats
Categories: data transfer
FITS is a MIRIAD task, which converts image and uv files both from
FITS to Miriad format, and from Miriad to FITS format. Note that
because there is not a perfect correspondence between all information
in a FITS and Miriad file, some information may be lost in the
conversion step. This is particularly true for uv files.
WARNING: When writing uv FITS files, fits can handle single
frequency band, single array configuration only. Minimal
checks are made to see that these restrictions are observed!
References:
For a description of the standard, see
http://fits.gsfc.nasa.gov/fits_home.html
Key: in
Name of the input file (either a FITS or MIRIAD file name, depending
on OP). No default.
Key: op
This determines the operation to perform. Possible values are:
"uvin" Convert FITS uv file to Miriad uv file.
"uvout" Convert Miriad uv file to FITS uv file.
"xyin" Convert FITS image file to Miriad image file.
"xyout" Convert Miriad image file to FITS image file.
"print" Print out a FITS header.
There is no default.
Key: out
Name of the output file (either a MIRIAD or FITS file name, depending
on OP). If op=print, then this parameter is not required. Otherwise
there is no default.
Key: line
Line type of the output, when op=uvout. This is of the form:
linetype,nchan,start,width,step
"Linetype" is either "channel", "wide" or "velocity". "Nchan" is
the number of channels in the output.
Key: region
The region of interest. The default is the entire input image.
See the Users Manual for instructions on how to specify this.
Used when op=xyout
Key: select
Normal uv selection, used when op=uvout.
Key: stokes
Normal Stokes selection, used when op=uvout
Key: options
These options applies for op=uvin only.
compress Store the data in compressed uv format.
nochi Assume that the parallactic angle of the
telescope is a constant 0 (or that the data are
from circularly polarised feeds and have already
been corrected for parallactic angle).
lefty Assume that the FITS antenna table uses a
left-handed coordinate system (rather than the
more normal right-handed system).
varwt The visibility weight in the FITS file should
be interpretted as the reciprocal of the noise
variance on that visibility.
These options for op=uvout only.
nocal Do not apply the gains table to the data.
nopol Do not apply the polarization leakage table
to the data.
nopass Do not apply the bandpass table correctsions
to the data.
These options apply for op=xyin only.
rawdss Use the conventions for raw Digital Sky Survey FITS
files, and convert (partially!) the header. A raw
DSS FITS file has header items such as PLTSCALE,
XPIXELSZ, YPIXELSZ etc. If you are unsure if your DSS
image is raw or conventional FITS, run:
Task FITS:
in=mydss.fits
op=print
and look for those header items. Note that DSS images
retrieved using SkyView have a conventional fits header,
and do not require options=rawdss.
nod2 Use the conventions of NOD2 FITS files.
Key: velocity
Velocity information. This is only used for op=uvin,
and is only relevant for line observations. The default is
to use the information present in the FITS header. The
"velocity" parameter allows this information to be overriden or
the velocity system to be changed.
Within each line visibility data-set, Miriad stores the velocity
of the observatory wrt a rest frame. This allows account to be taken
of this when determining channel velocities.
The fits task will determine the observatory velocity either by being
given a velocity at a given channel (wrt a rest frame) or by using
a model of Earth and solar system motion (accurate to 0.005km/s).
The "velocity" parameter can be used to specify the velocity of
a particular channel. The parameter consists of three values:
the velocity system of the reference value, the reference value
and the reference channel, viz:
velocity=velsys,refval,refchan
Possible values for the velocity system are:
lsr Velocity is the radio definition wrt the LSR frame.
bary Velocity is the radio definition wrt the barycenter.
optlsr Velocity is the optical definition wrt the LSR frame.
optbary Velocity is the optical definition wrt the barycenter.
obs Velocity wrt the observatory.
The reference value gives the velocity, at the reference channel,
in km/s. If the reference value and reference channel are
omitted, a model of Earth and solar system motion is used to
determine the appropriate information.
For example:
velocity=lsr,30,1
indicates that the first channel has radio LSR velocity of 30 km/s.
The observatory velocity, relative to LSR, can then be computed.
Alternately:
velocity=lsr
indicates that fits is to determine the observatory velocity
wrt the LSR frame using an appropriate model.