IC4665 photometry

[Patrick Wils <patrickwils@yahoo.com>]

I have compared the TASS photometry of stars from the open custer
IC4665 with the values given by Brian Skiff recently (see the AAVSO
discussion forum
http://mailman.mcmaster.ca/mailman/private/aavso-discussion/0209/msg00006.html).
 
The cluster was observed on three nights (from the July data set), with
between 10 and 52 observations per star per night (I did not take into
account stars with less than 10 observations a night).  I have plotted
the difference between the TASS V and I magnitudes and the
corresponding V and I values from Brian's mail against Brian's V-I in
the accompanying gif file (zipped, because otherwise it apparently
doesn't get through).  Error bars denote the standard deviation of the
TASS data for the night.  

There is good agreement between the two nights JD380 and 412, but the V
data and especially the I data for JD455 are fainter, probably because
another set of stars was used in the calibration, leading to a
different zero point.

It is clear also that the TASS V data are in good agreement with
Brian's V, but that the difference in I is a function of V-I.  Rough
formulae give:
I = TASS I - 0.33 * TASS(V-I) + 0.2 (for the nights JD380 and 412)
I = TASS I - 0.2 * TASS(V-I) (for JD455).

(Just another illustration of what has already been discussed before
here: the I data is in need of better calibration).

Patrick







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IC4665.GIZ