About the draft itself Re: WD-SIA-2.0

[Arnold Rots]

Hm, TT would be infinitely preferable as a timescale - after all, we are
doing astronomy.

For the spectral band TOPOCENTER may make the most sense when we are dealing
with wide bands - and there it does not matter too much.
High-resolution spectra are a different matter. My sense is that people are
going to give
you whatever they have and will not go through the exercise of transforming
their values
- basically, because whatever they have is likely to make the most sense
for the data.
BARYCENTER might be the preferred one, but I doubt its enforcement will be
feasible.
And don't call it barycentric wavelength.

I trust BAND refers to the spectral coordinate, not the
redshift/Dopplervelocity coordinate.
There you have the argument that I made above on spectra in spades.
TOPOCENTER-GEOCENTER-BARYCENTER is one thing, but LSR and Doppler
definitions
provide an additional twist and forcing extragalactic conventions on
Galactic data would be
unwise.

However, considering that TIME is a much easier case for settling on a
common reference
position, and that in that case you accept "UNKNOWN", why not just throw up
your hands
and do the same for the spectral coordinate?

Altogether pretty unsatisfactory, that's true. Personally, I think that's
why the Simple
protocols are a bit of a curse for the IVOA. As I predicted, when we
started out with
them, we will never manage to get beyond the assumptions in the SxAPs.

I have always wondered why meters was chosen if it isn't anyone's
first,...,nth choice,
besides wavelength being a quantity that is inferior to Hz or keV,
physically speaking.

Cheers,

  - Arnold

-------------------------------------------------------------------------------------------------------------
Arnold H. Rots                                          Chandra X-ray
Science Center
Smithsonian Astrophysical Observatory                   tel:  +1 617 496
7701
60 Garden Street, MS 67                                      fax:  +1 617
495 7356
Cambridge, MA 02138
arots at cfa.harvard.edu
USA
http://hea-www.harvard.edu/~arots/
--------------------------------------------------------------------------------------------------------------



On Mon, May 5, 2014 at 8:07 PM, Patrick Dowler <
patrick.dowler at nrc-cnrc.gc.ca> wrote:

>
>
> On 14/04/14 01:00 PM, Arnold Rots wrote:
>
>> And what is "barycentric wavelength"? Conversion from frequency to
>> wavelength as applied at the gravitational potential at the barycenter?
>> Or does it mean to reduce the spectral coordinate to the barycenter only
>> dynamically? That's pretty inconvenient for people who are doing
>> Galactic work.
>>
>
> The intent here is that the BAND parameter only supports one unit (m) and
> one reference position/time/scale/hard-to-remember thing.
>
> Since we have to pick one, can you tell me the one most suitable that
> would apply to BAND and TIME parameters. Note that the draft currently just
> refers to the same restricted time metadata as DALI uses for literal values
> (time scale UTC and unknown reference position). What should we use for
> wavelength values in the BAND parameter, knowing that implementers will
> actually just query the wavelength(s) they have in files, filter lookup
> tables, or spectral wcs already.
>
> PS-Yes, wavelength in meters - no one's first choice :-)
>
> --
>
> Patrick Dowler
> Canadian Astronomy Data Centre
> National Research Council Canada
> 5071 West Saanich Road
> Victoria, BC V9E 2E7
>
> 250-363-0044 (office) 250-363-0045 (fax)
>
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