RADESYS and EQUINOX, FK5 vs ICRS

[Maik Riechert]

Hi,

On http://montage.ipac.caltech.edu/docs/headers.html the following is written:

"RADESYS defaults to IRCS unless EQUINOX is given alone, in which case it defaults to FK4 prior to 1984 and FK5 after 1984."

In the wcs header of astrometry solutions, there is only the EQUINOX value written. Should I then assume it is FK5? Or is it really ICRS? I'm using the Tycho-2 index.

Cheers

Maik

[Dustin Lang]

Hi,

Good question!  We do NOT precess the positions from the reference catalog -- we pass them through unchanged -- so the index files and the final astrometric solutions are in whatever system the reference catalog is in.  For Tycho-2 (as I understand it) that is ICRS.  I believe that for most purposes, J2000 and ICRS are nearly indistinguishable.

cheers,
--dustin

[Maik Riechert]

I see, so technically the WCS headers then contain incorrect data. They should not include EQUINOX if the system is ICRS. Wouldn't it make sense to find out which system each catalog uses and then store this information somewhere (inside the catalog files?) such that astrometry can write it to the WCS header? On the other hand, if you can verify that all catalogs which astrometry offers are in ICRS, then for the moment it would probably be reasonable to just assume ICRS by default and remove EQUINOX.

Cheers
Maik

[Dustin Lang]

So your proposal is:

1. RADESYS = 'ICRS'
2. remove the EQUINOX header

I believe that is consistent with WCS "paper 2", and I also believe that both Tycho-2 and 2MASS are on the ICRS system.  But confirmation of that wouldn't hurt.

--dstn

[Maik Riechert]

Tycho-2 is definitely ICRS (see http://tdc-www.harvard.edu/catalogs/tycho2.format.html) and 2MASS should be as well, but I only found this for 2MASS:

http://books.google.nl/books?id=FhKfr3ZPX6kC&pg=PA171&lpg=PA171&dq=2mass+reference+frame&source=bl&ots=F37D5GaV_F&sig=pwHryVVNv9H-HYK6EoGmgiNg9vs&hl=de&sa=X&ei=sbn0UvLTFbHM0AXnn4H4DQ&ved=0CFIQ6AEwBA#v=onepage&q=2mass%20reference%20frame&f=false

where it says that the reference frame is based on the Tycho-2 stars. We have to find a more definite source.

[Dustin Lang]

Right, I remember, 2MASS was calibrated to Tycho-2 stars so should be on the same system.

--dstn