Tim,
Sorry for the delay, I was off work yesterday.
> All have [1,1,1] set at 05:20:04.000 +89.59.00.000 6.914730e+11 Hz
> [The spectral headers are not liked by AST v3 so I need to look into
> that].
The problem is the "-LSR" in the CTYPE3 value. The final 4 characters
should indicate the algorithm used for projecting frequency onto pixel
number. I guess your data is linear, so CTYPE3 should be just "FREQ".
The standard of rest should go in the SPECSYS keyword which should be set
to "LSRK" or "LSRD".
> You'll be pleased to hear that GAIA does correctly report the
> RA/Dec for CAR as the pole is crossed but the axis overlay is not quite
> correct since it stops at +90 and does not draw in the top half of the
> grid. I assume this is because of the RA discontinuity. Any chance that
> could be fixed?
Interpreting your headers according to the published FITS-WCS paper II
should result in circular grid lines, not a rectangular grid. For the
reasons, see section 2.8 of FITS-WCS paper II. You need to decide if you
are going to stick with the old AIPS conventions or go with the new
FITS-WCS conventions - in the case of a CAR projection they give very
different results unless you have CRVAL set to (0,0).
If you decide to stick with the AIPS conventions (which GAIA assumes,
although I really think this should be a user-selectable option), I'm
not sure what sort of plot you would expect in the case of your data.
Would you expect dec values greater than 90 to be displayed in the top
half of the plot?
> Also, the projection information is not reported in the
> title (it's written as <null>) for all of the test images.
This I think must be a bug in the version of AST used by GAIA. I have no
record of consciously fixing this bug, but CONVERT built with an earlier
release of AST V3 shows the bug whereas CONVERT built with my current
AST system does not.
> Jamie reports that the MER projection seesm to fail completely. FV
> displays it fine but it seems to display as a SIN projection in GAIA. Is
> AST meant to support MER?
I think this is the same point as for CAR. If you are expecting to see a
rectangular grid, you wont. If you use MER, CAR, etc, the thing which is
rectangular is *native spherical* coords (that is, a spherical
coordinate system which has origin (0,0) at the reference point spcified
by CRVAL). Final celestial coords (RA,Dec) will only be rectangular if
they coincide with native spherical (i.e. if CRVAL=(0,0)). Otherwise, the
(RA,Dec) gridlines will reflect the spherical rotation which occurs from
native spherical to celestial.
David
----------------------------------------------------------------------
Dr David S. Berry ([log in to unmask])
STARLINK project | Centre for Astrophysics
(http://www.starlink.ac.uk/) | University of Central Lancashire
Rutherford Appleton Laboratory | PRESTON
DIDCOT | United Kingdom
United Kingdom | PR1 2HE
OX11 0QX
|