[Aavso-photometry] Questions about Z And Photometry

[Walt Cooney]

Arne,

Since Z And is getting attention, would you mind releasing the SRO BVRI 
photometry of the "92" star in an email to the list?  I'm shooting BVRI 
of Z And tonight.  I'd like to use the "92" star as my comp star since 
it is similar in brightness to Z And at the moment.  Otherwise, I'll 
wait until the improved photometry is released later this month before 
submitting my measurements.

Thanks,
Walt
(CWT)

arne wrote:
> 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
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