[vsnet-chat 3444] [Fwd: nova astrometry ...]

[no name <crawl@zoom.co.uk>]

-------- Original Message --------
Subject: nova astrometry ...
Date: Fri, 14 Jul 2000 14:46:31 -0400 (EDT)

John,

Because novae are generally distant objects, their actual proper
motions should in general be very tiny.  Any differences between
outburst and quiescent positions should be due to other factors,
such as:

1) Image saturation.  If the brightest parts of the outburst image
   exceed the dynamic range of the CCD, the image centroid may be 
   thrown off.

2) Different comparison stars used in the astrometry, or errors 
   in the alignment of various comparison star sets with the 
   ICRS.  (By the way, coordinates don't have an epoch any more ...
   see http://aa.usno.navy.mil/AA/faq/docs/ICRS_doc.html for 
   an explanation of the ICRS).

This last problem can be quite severe, and leads me to make this 
proposal of nova observers:  Take direct images of novae during their
fade from outburst (to avoid saturation), and publish not only the 
RA and dec but the *X-Y pixel coordinates* of the nova and a number 
of stars on your picture (that is, your raw data).  The larger the 
image scale, the better of course.  That way, later workers can 
reconstruct very precise positions for the ex-nova, without the 
possible systematic errors of alignments in the reference frames 
(it's the difference between a differential and an absolute measure).  
This should make identification of the ex-nova in crowded fields 
much less troublesome.  

			John Thorstensen

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