[vsnet-chat 4548] Re: UCAC1 / first impressions

["Berto Monard" <lagmonar@csir.co.za>]

Hi, 

In case you are interested in my (rather preliminary) findings with UCAC1 (to derive starsequences), read on. 

This only applies to the published southern sky.
As you know the UCAC1 mags are somewhere between V and R.

Field to field calibrations agree quite well / zero offset is reasonably constant and small!  
The main problem with UCAC1: there is no indication of the star colour. Therefore the colour factor adjustment cannot be done properly and the V deriving accuracy varies with the unknown (B-V) factor. 
As can be expected, blue objects (ie B=V=R=UCAC1) need no (or small) adjustment, red targets require corrections of + 0.3 and higher, depending on magnitude and B-V factor. 
The USNO-A2.0 R and B values would come in handy here to make the proper corrections. It is not difficult to get a 'feel' to derive acceptable B-V factors from the USNO-A2.0 B and R data. It will be quite obvious after a while that the conversions differ from one starfield to the next. 
With Loneos and GSPC data handy to provide field calibrations for different starregions, nothing will stop you deriving acceptable V (and visual) sequences. 

A catalogue combining UCAC1 and USNO-A2.0 data would go far in helping to produce V values ... for the amateur.

Regards,

Berto Monard
PS I will be off e-mail for the next 10 days

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