[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.
> 
If you look at the photometry table for Z And, you will notice
that the 92 star (AUID 000-BCR-684) has very large errors, while
the 112 star errors are ten times smaller.  You ran into one of the
problems that happened with automatic population of the photometry table
in October.  These stars are from Henden and Munari, as Z And is
a symbiotic star and we calibrated the fields of about 100 such
stars.  In this case, the 92 star is saturated, which is why it has
large errors.  Your 9.49 value for that star is probably much closer
to the truth.  As you indicate, using a check star is always a good
idea, and with CCD images, be sure you save the images as well so that
you can go back and find other stars if necessary at a later time.

Thank you for pointing this out - I'll revise the 92 star's photometry
for the January release of the comparison star database.
Arne