[vsnet-chat 7636] Z Cam sequences

[gibbon butt]

Z Cam is a relatively crowded field, but not too crowded to cause
serious blending.

Using the AAVSO VSP engine the AAVSO recommended sequence stars for 12
and a half Johnson V magnitude and less were examined and their V and
B magnitudes as derived from Henden photometry that was taken with the
1.0m USNO NOFS telescope noted.

These were compared to the APASS data release 1 B and V values for the
same stars, these values being based on _three_ photometric nights, as
far as the cryptic and undocumented data of APASS data release 1 can
be assessed to be (pretty much just a beefed up ascii file of the ilk
the old Henden photometry from NOFS used to be disseminated in.
Linkage was via coordinates from NOFS photometry and APASS dr1
photometry plotted up in Guide 8 graphically for certainty of
matching).

V ranges from around 0.08 to 0.15 ish fainter in APASS dr 1, B about
0.1 to 0.2 fainter, for around half a dozen stars down to fifteenth
and a bit mag.  Pretty much consistently across the board.

Probably a negligible difference in terms of ensemble techniques, no doubt.

But the question is, both the Henden 1.0m photometry and the APASS dr1
data are deemed correct.  They both can't be, obviously.

The difference isn't large, but it's larger than the typically 0.03 to
0.05 error range quoted in maillists.

It's about the scatter range quoted in the APASS online documentation,
albeit B tends to be twice as worse off as V.

And it's a range of scatter around a systematic offset that can be
seen elsewhere around the sky when old sequences are checked against
APASS dr1.  That is, old V versus APASS dr1 V will give spread
(standard deviation upon their differences if preferred) of greater
than 0.05, whilst offsets are always more than 0.05 away, nearer 0.1
or so.

And B is not as good.

This V isn't much better than can be gotten globally via V from UCAC3
using the fmag and 2MASS J and Ks, whether significantly so in terms
of utilisation of ensemble techniques I haven't figured out yet.

APASS is going to need to be shown to be good, down the road, via
referencing against allsky cases, with Global homogeneity, and not
just via some passing comparison clean and equatorial Landolt
references.  Not just said to be so, not just shown to be so bottom up
because "it must be" given that a CCD was used and a computer software
route was taken.

It'll need rigorous testing against the sky, or at least some of the
level of quoting of precision loosened.

It will likely be very useful, especially in areas where there are no
alternatives.

But if some of the current claims are not watched, not checked, and
just taken on faith by users, some colour epoch photometry conclusions
based on data reduced via it could well come a cropper.

caveat utilitor

John