[Aavso-photometry] Questions about Z And Photometry

[arne]

Barbara G. Harris wrote:
> I submitted a CCDV observation of Z And taken on 12/30/2007 @ 01:52 UT.  I
> use MaximDL for photometry.  I used the 9.2 star as my reference star (9.165
> from field photometry).  Using that as a reference, I go a Mag of 9.25 for Z
> And.  I submitted this to AAVSO.   Later after looking at other observations
> around that time I noticed that the 9.25 was not consistent with the other
> observations around that time (9.7226 CCDV, 9.6 Vis).  I then went back to
> my image and used the 11.2 star as a reference (11.194 field photometry).
> With that star as a reference I got a mag of 9.56 for Z And which is more in
> line with the other observations at that time.  When I selected the 11.2
> star as my reference and then selected the 9.2 star as a check star, the mag
> that I got as a check for the 9.2  star was 9.49.   If I use them both as
> reference then I get a mag of 9.40 for Z And.  I have gone back and modified
> my observation in AAVSO.
> 
>  
> 
> So that teaches me that I should always use a check star.  My question is:
> Why was there such a disparity in my 9.2 star?  I consider myself still new
> at CCD photometry so any other advice or recommendations would be
> appreciated.
> 
I've done a little checking.  The 92 star is given by Tycho2 as
000-BCR-684  V = 9.030 0.034  (B-V) = +0.429 0.038
and by TASS (patches2 catalog) as:
000-BCR-684  V = 9.051 0.060  (V-Ic) = 0.444  0.073

Significantly brighter than my saturated photometry, but actually
within the quoted errors in the photometry table.
So these are the values you should use for the 9.2 star.  These disagree
with your estimate of 9.49 for the star, so you may be calculating things
wrong (or, the star may be variable, though the TASS results
look pretty constant). My guess, without knowing your CCD system,
is that you may be saturated at the V=9 level, which is why your
9.2 star disagrees so radically.

By the way, the TASS system gives for the 112 star:
000-BCR-679  V = 11.224 0.067  (V-Ic) = 0.623 0.093
and the VSP photometry table, using Henden unsaturated photometry, gives
000-BCR-679  V = 11.194 0.013  (V-Ic) = 0.567 0.020
so very similar between these two stars.
Arne