[Issues] Poor astrometry on SUBARU test field

Philippe Heraudeau heraud at astro.uni-bonn.de
Thu Aug 21 14:55:24 CEST 2008

Dear Tim,

You should certainly run "Gastrom" at some point to derive the global 
astrometric solution on your observations if they
have enough overlapping.
Astrom gives you a first solution whose accuracy should be about the one 
of the reference catalogue i.e. about 0.3" for USNO.
Thus, 1" rms seems already too large for Astrom and I leave Erik and 
John comment on it but it should be improved by
Gastrom which will give you the final astrometric solution.

Note however than GAstrom cannot handle more than 64 sourcelists (or 
chips) at the moment as indicated in
astro/main/AssociateList.py:            raise Exception, 
'__append_sourcelist: Cannot handle more than 64 SourceLists at the 

As I can see, your data set has 100 chips, so you should make a 
selection of max. 64 chips for Gastrom...

Maybe John or Erik can tell us when this limit will be removed since it 
represents only 2 omegacam pointings!

All the best,

Tim Schrabback wrote:
> Dear John and Erik,
> during the Leiden tutorial I randomly picked a multi-color SUBARU data set 
> to work with. The results for the astrometry (Astrom) were poor, with
> an RMS exceeding 1" for some chips/exposures. Looking at the frames they 
> visually look fine. I think it would be very valuable to understand the 
> cause for such poor results in order to further improve the Astro-WISE 
> system.
> If you want to check this yourself, this is how I ran the astrometry 
> task:
> pars=Pars(AstrometricParameters)
> pars.AstrometricParameters.process_params.MAX_RMS=1.1
> pars.AstrometricParameters.preastromconf.RMS_TOL=5.0
> dpu.run('Astrometry', i='SUP', o='deep16_0', C=1,pars=pars.get())
> We are currently performing extensive astrometry tests (also using 
> different reference catalogs) using CFHTLS-Wide MEGACAM data outside of 
> Astro-WISE. It would be good to get these data fully supported in 
> Astro-WISE at some point, so we can do cross-checks.
> Thanks and best regards,
> Tim
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Philippe Heraudeau
Argelander-Institut für Astronomie Email: heraudeau at astro.uni-bonn.de
Auf dem Hügel 71                   Tel.: +49 (0)228 736729
D-53121 Bonn                       Fax.: +49 (0)228 731775

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