[vsnet-campaign-v4641sgr 133] (fwd) Re: V4641 Sgr: observations from last night

[Taichi Kato <tkato@kusastro.kyoto-u.ac.jp>]

From: aah@nofs.navy.mil
Date: Sat, 13 Jul 2002 12:26:06 -0700
Subject: [vsnet-chat 5381] Re: V4641 Sgr: observations from last night

Gianluca posted an image that identifies his comp stars:
http://vsnet.bellatrixobservatory.org/v4641sgrcomp.gif
If I have identified these stars correctly, I get:
RA      (J2000) DEC        V      B-V   U-B    V-R    R-I
18:19:21.64  -25:24:25.6 13.420  0.345  0.153  0.234  0.208  var
18:19:17.24  -25:24:38.5 13.756  1.653  1.907  1.030  1.053  C1
18:19:25.12  -25:24:40.8 13.433  1.182  1.011  0.689  0.592  C2
(It is a crowded field, and it looks like Gianluca's image is
unfiltered, making it difficult to identify properly).
  If these are the correct comparison stars, then there is a
very large color mismatch.  You should look at Sumner's
sequence or my .dat file to pick something closer.  This
will be of particular importance for such a low altitude,
high airmass field.
  Regarding C2, there are some close companions that may
be impacting the photometry at the 0.1mag level.
Arne

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