about galaxy "velocity cubes"

[Adam Brazier]

Yes indeed. The radio astronomers with whom I am working are excited 
about the prospect of their data cubes being easily and directly 
embeddable in VO protocols and any suggestion that they are "horribly 
wrong" or the like is going to be met with reactive hostility and a 
complete lack of compliance with whatever else we suggest. The whole 
point is not to remake astronomy but to make it easier to do, it seems 
to me, which means accepting the pre-existing preferences, expertise and 
protocols and the field and working/extending from there.

Also agreed on RA!

Cheers

Adam

On 12/7/2010 9:12 AM, Anita M. S. Richards wrote:
>
> Yes, unfortunately we are stuck with what astronomers have, not what 
> they ought to do! (otherwise RA could go for a start... ;-)
>
> As there is already a full STC description (and also the Greissen et 
> al. papers defining FITS standards in great detail) it should not be a 
> problem to use RA, Dec plus velocity
>
> best wishes
>
> Anita
>
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> Dr. A.M.S. Richards, UK ARC Node, Room 3.135
> Jodrell Bank Centre for Astrophysics, Alan Turing Building, University 
> of Manchester, M13 9PL
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> and
> MERLIN/VLBI National Facility, Jodrell Bank Observatory, Cheshire SK11 
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>
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>
> On Tue, 7 Dec 2010, Juan de Dios Santander Vela wrote:
>
>> I think that trying to put "spatially local" coordinates makes it much
>> worse, because that "size" is a function of distance. Plus, you are
>> having a lot of effects due to the angular inclination of the object.
>> You are indeed observing ra,dec, and frequency, and that is what you
>> actually have. The conversion to velocities is bidirectional, no
>> losses, and depends on the selection of a suitable 0 value for the
>> velocity.
>>
>> So, we might argue if having them as ra, dec, and frequency is a more
>> natural representation, but the velocity one is equivalent to that.
>>
>> On Tue, Dec 7, 2010 at 07:46, Igor Chilingarian <chil at sai.msu.ru> wrote:
>>> Hi Arnold,
>>>
>>> Following our short discussion after Jose Enrique's talk.
>>> Unfortunately, since we astronomers are neither mathematicians, nor
>>> physicists, sometimes we tend to do quite weird things (here I would 
>>> also
>>> recall to my last ADASS talk).
>>>
>>> The "galaxy velocity cubes" being a standard practice in 
>>> radio-astronomy is
>>> a very good example of such a thing. I have to admin that sometimes I'm
>>> using it myself and now we even have a service of providing such an 
>>> output
>>> from the results of simulations in the GalMer database which I 
>>> implemented
>>> in order to fulfill the demand from radio-astronomers.
>>>
>>> The problem with this data type is that the two *observable axes* 
>>> (RA and
>>> Dec) are mixed with the velocity axis (or redshift if you wish) 
>>> which is a
>>> data analysis result. My impression is that conceptually it is horribly
>>> wrong. If one is using radial velocity for the Z-axis, then he/she 
>>> should be
>>> using real spatial coordinates in kpc or other physical units (e.g. 
>>> meters)
>>> for the other two axes. On the other hand, if obervables are used (e.g.
>>> RA/Dec, l/b or whatever) for coordinates, than the spectral coordinate
>>> (wavelength/frequency/energy) has to be used for the 3rd dimension.
>>>
>>> What do you think?
>>>
>>> With best regards,
>>>                                                Igor
>>>
>>
>>
>>
>> -- 
>> Juande Santander Vela
>> Applied Scientist, Archive Management Group
>> Archive Department, Data Management & Operations Division
>> European Southern Observatory (Germany)
>>