[Aavso-photometry] Questions about Z And Photometry

[Jim Jones]

Steve

If I understand what Bruce Gary is doing, he isn't using the artificial
star as a comp star in the normal sense.  He uses the artificial star as
a comp (or reference star in MaxIm's scheme) to measure the same  bright
star in each image (he calls it the "extinction" star).  This gives him
some idea  if clouds have moved in or some other nasty event has
occurred.  It also gives him some idea of extinction of the bright star.

Then he backs that solution out (click on the "Back" button and untags
the ref1 and obj1 stars in MaxIm's photometry display) and measures the
target star in the normal matter choosing normal comp (reference) and
check stars.

Hope this helps.

Jim Jones


Steven Orlando wrote:
> What is the difference between a reference and a check star?
>
> AIP4WIN doesn't use the term reference star. Maxim DL does, and when you do 
> photometry in MaximDL, you get in the csv file that the REF all zeros.
>
> When I submit my exoplanet data to Bruce Gary, he likes to use an artifical 
> star as a reference.
>
> Can someone please sort this out for me? I am confused!
>
> Steve
> ----- Original Message ----- 
> From: "Barbara G. Harris" <barbharris1 at hughes.net>
> To: "'arne'" <arne at aavso.org>
> Cc: "'AAVSO Photometry'" <aavso-photometry at mira.aavso.org>
> Sent: Tuesday, January 01, 2008 11:51 AM
> Subject: Re: [Aavso-photometry] Questions about Z And Photometry
>
>
>   
>> Thanks for your explanation.  This will also teach me to look at the
>> photometry table a little closer.  I did not notice the large error in the
>> 9.2 star.
>>
>> BTW. I am so impressed with AAVSO and mainly because of your close
>> involvement in the discussion groups.  Thanks again.
>>
>> -----Original Message-----
>> From: arne [mailto:arne at aavso.org]
>> Sent: Tuesday, January 01, 2008 10:29 AM
>> To: Barbara G. Harris
>> Cc: AAVSO Photometry
>> Subject: Re: [Aavso-photometry] Questions about Z And Photometry
>>
>> 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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