Characterization question

[Tim Jenness]

2011/11/12 Laszlo Dobos <dobos at pha.jhu.edu>

> Hi group,****
>
> ** **
>
> How do I specify overlapping axes in characterisation?****
>
> ** **
>
> Let's say I have a spectrum and want to give the spectral axis in both
> angstroms and velocities. Do I just add two axes and mark them both
> SPECTRAL and assume that they overlap?****
>
> **
>

You should start by taking a look at the FITS formalisms for world
coordinate systems. Paper III covers spectral coordinates and that shows
how you can specify a single wavelength frame that includes details of how
to convert to velocity.

The FITS papers also show to specify multiple coordinate systems but I'm
not suire how this is implemented in the IVOA data model.


> **
>
> What if I have an image with two spatial axes? Do I add two SPATIAL axes
> then or just one and assume that the resolution etc. is the same in both RA
> and Dec?****
>
> ** **
>
> What if I have overlapping spatial axes, let's say ra,dec and (for a not
> too good example) also Mpc? How do I associate one Mpc axis with Ra and the
> other with Dec.****
>
> ** **
>
> I'm sorry, if I misunderstood something about the whole thing. (Samples on
> the CDS server are unfortunately not available).****
>
> ** **
>
> What I'm actually working on is a generic toolkit to characterize data
> cubes with any number of dimensions, and I want to allow using multiple
> scales along the axes.****
>
>
>
The Starlink GAIA and SPLAT software are applications that know how to
convert from wavelength to velocity (and many other schemes).

-- 
Tim Jenness
Joint Astronomy Centre
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.ivoa.net/pipermail/dm/attachments/20111112/13ee2a23/attachment.html>