[vsnet-chat 2054] Re: Supernovae estimates using USNO SA2.0

["Fraser Farrell" <fraserf@dove.net.au>]

G'day Taichi,

On Tue, 29 Jun 1999 10:32:26 +0900 (JST), Taichi Kato wrote:
> We have been recently receiving CCD observations of SNe based on USNO
> SA 2.0.  What would be the systematic difference between A1.0/A2.0 and
> SA1.0/SA2.0?  Will USNO SA series photometrically as uniform (nonuniform?)
> as A series?  Any suggestions are welcome.

I'll go for the long explanation for the benefit of other readers...

According to the documentation on my SA1.0 CD-ROM, there's no 
photometric differences between it and the "full" A1.0.  The SA1.0 is 
just A1.0 with about 90% of the stars omitted; and was intended 
primarily as a single-CD astrometric reference to faint magnitudes.  

SA1.0 doesn't pretend to be "complete" in the sense of producing a 
realistic star chart; and in some crowded fields there seems to be a 
deliberate bias against really faint stars.  SA2.0 does the same job for 
A2.0.  Therefore the Kato Equation for V magnitudes still works for 
either SA, although on fewer dots per image ;-)

A2.0/SA2.0 used an improved coordinate reference frame, changed all the 
star designations, and added a few million more stars over A1.0/SA1.0.  
So it's vital to specify which version you're using.

The SA1.0/SA2.0 CD-ROMs have become quite popular among us unpaid 
astronomers because the US Naval Observatory very generously gives them 
away for _free_ and pays the postage too.  By contrast, the ten CD-ROMs 
of the full A1.0/A2.0 are available only to selected recipients; and I 
get the impression that some money has to change hands.

Downloading the full A1.0 or A2.0 is frankly impossible in my part of 
Australia - it would take about 60 full days (~$1500) online - so it's a 
shame that I'm not allowed to buy the discs.  Getting those little 
extracts from the USNO or ESO servers can be frustrating if you discover 
your object isn't in the square after all.  Or you need the next square 
as well to finish a chart...

But I've used SA1.0 for astrometry of faint asteroids and variables, and 
to help (roughly) calibrate CCD images taken at the local observatory. 
It can sometimes be difficult to match an image to the (obviously 
incomplete) SA1.0 star chart, but this problem is eased considerably by 
the use of astrometric tools such as Guide's Charon utility.

Incidentally, the USNO are also giving away free copies of their AC2000 
and ACT Reference Catalog [sic] CD-ROMs.  The latter is ideal for 
nailing down those brightest SNe and galactic novae, improving asteroid 
occultation predictions, or showing what the night sky used to (or will) 
look like at remote epochs.


cheers,
Fraser Farrell

http://vsnet.dove.net.au/~fraserf/

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