[vsnet-chat 1629] USNO-A2.0 field sequences

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

Hi interested observers,

just for those that want field calibrations a bit better than GSC,
there is a way to do it, but it costs time and effort. It will help
you for those fields that haven't got any proper CCD surveys, which
are IMO far superior, especially if they are done competently. 
I use USNO-A2.0 as a transfer medium for nearby GSPC measures, which
can be assumed to be near photometric. This certainly applies to those
GSPC fields that are listing stars as A, B, C and not those with R, S,
T which have questionable accurracies.

I have done the conversion for several (hundred) Southern fields and
it costed me several Sundays on which I might not have had to do
better things than those that increase the cholesterol...

I hope these efforts result in some general conclusions as to the
inherent value of those B and R measures from the USNO-A2.0 data base,
which is btw slightly better (based on more recent surveys) than
USNO-A1.0. 
I have compared several fields for both of them and for final values I
have used an indicative weighting factor 3 to1 in favour of the A2
version... this in transfer mode with GSPC calibration, sorry for the
confusion..

I used V = (B+R) /2  and calibrated vs the V mags of GSPC.

What did I found for those values?
The following is a conversion that is typical for such Southern
fields. Individual fields do differ up to 0.6 (+ or -) from this mean
value, often neighbouring fields show differences of that size! Which
of those to apply to the variable field? only telescope observations
will guide in this...

Typical conversions of Southern fields (with V = (B+R)/2 ...
USNO-A2.0)
USNO V / actual V  /  correction

11  /  10.9  /  - 0.1
12  /  12.1  /  + 0.1
13  /  13.3  /  + 0.3
14  /  14.1  /  + 0.1
15   /  14.9  /  - 0.1

I wished the field to field calibrations were more repeatable. I just
wonder if those fields with the same entrance numbers (0525 etc..)
have been based on the same plates of those surveys, or have they been
catalogued for convenience under the same number despite resulting
from different surveys???? Such knowledge might help understand local
discrepancies. 

Another weakness of USNO-A2.0 (at least for sequences) is the
inclusion of some '1954' measures which are interesting from an
astrometric point. They give confusion and do often not agree with the
more recent measurements...


Regards,

Berto Monard
Pretoria

VSNET Home Page

Return to Daisaku Nogami

---

vsnet-adm@kusastro.kyoto-u.ac.jp