about galaxy "velocity cubes"

[Anita M. S. Richards]

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

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Dr. A.M.S. Richards, UK ARC Node, Room 3.135
Jodrell Bank Centre for Astrophysics, Alan Turing Building, 
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and
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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)
>