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[atnf-data-reduction] Miriad supports the FITS spectral coordinate standard

From: <mcalabre_at_email.protected>
Date: Wed, 21 Mar 2012 14:37:38 +1100

Greetings to Miriad users,

ATNF's Miriad now supports the FITS spectral standard as well as
maintaining backwards compatibility with the older AIPS spectral

In practical terms, this means that you should be able to read
any FITS image with a standard spectral coordinate header, or
an older AIPS-style header, and have the spectral coordinates
interpreted correctly. That includes IR, optical, UV, X-ray,
and gamma ray spectra that conform to the FITS standard.

Task 'velsw' can also now switch the spectral axis to any of the
new spectral types such as redshift, wavelength, or relativistic

If you are not familiar with the FITS spectral standard, here is
a brief summary of the main differences:

  1) The Doppler frame is recorded in a separate SPECSYS header
     record in FITS - now copied to the 'specsys' header item in
     Miriad and listed by 'prthd'. In all cases the spectral
     axis is Doppler shifted to the frame specified by specsys.
     The old method recorded the Doppler frame as part of
     CTYPEi, e.g. 'FREQ-LSR', 'VELO-HEL', etc. and these are
     still written by 'invert'.
  2) You may see a three-letter code in place of the old Doppler
     frame, for example 'VOPT-F2W'. The 'F2W' bit is a non-
     linear algorithm code analogous to the codes used for
     celestial projections. 'F2W' indicates that the axis is
     linear in frequency but is being expressed in terms of a
     variable (VOPT) that is linear in wavelength. If the code
     is absent, the axis is linear, e.g. 'FREQ'.

While there are dozens of possibilities, the following table
lists some of the new spectral types you may encounter:

    Old New
   ctype ctype specsys
  -------- ------------------
  FELO-LSR VOPT-F2W LSRK ...optical velocity
     n/a ZOPT-F2W (any) ...redshift (radio spectra)
     n/a ZOPT (any) ...redshift (optical spectra)
     n/a WAVE-F2W (any) ...wavelength (radio spectra)
     n/a WAVE (any) ...wavelength (optical spectra)
     n/a VELO-F2V (any) ...relativistic velocity
     n/a VELO-W2V (any) ...relativistic velocity
     n/a ENER (any) ...photon energy (e.g. X-ray)

A linear wavelength axis, 'WAVE' as above, might come from an
optical spectrum or a regridded radio spectrum. Notice that the
old VELO-{OBS,HEL,LSR} is radio velocity, but in all other cases
VELO now indicates relativistic velocity.

You should be aware that, historically, Miriad adopted an
unconventional interpretation for FREQ-HEL and FREQ-LSR. In a
Miriad header and FITS files written by Miriad, these axes are
actually topocentric, not barycentric or LSRK as it might
appear. The help for task 'velsw' explains how this is handled,
and 'impos' also has a new option related to it.

Mark Calabretta
Received on 2012-03-21 14:38:56