[Aavso-photometry] Re: [AAVSO-DIS] CCD- methods used

[Richard Huziak]

Jean,

I use differential photometry, one star prefered, since I understand a 
future transformation to a standard system is straightforward.  This 
seems to work well if the field is decent and you have confidence that 
the reference star is not variable.  I will always use a series of about 
5 frames shot sequentually on the same night, then to make sure of 
non-variability on a non-calibrated visual sequence, I first do ensemble 
photometry with at least 3 stars, but generally check out all stars that 
have been assigned magnitudes (ie. off of AAVSO visual charts).  From 
this I then look at the errors and attempt to choose a single star again 
that I can use for reductions.  I often find that no single star works 
well, and end up doing 2- or 3-star ensemble.  Using frames from 
additional nights to reduce errors is better, but they are not always 
available.

It depends a lot on what my end goal is for the data, and if it is that 
important that my results are to be on the standard system or not.  For 
differential, the obvious problems is what star to use as the reference. 
 It shouldn't matter too much if the field  has been calibrated and you 
have access to the accurate sequence table. (Head office is apparently 
working on a database of easier accessible sequences, so I'm told). 
 However, if working off of a non-calibrated visual sequence, everyone 
will choose their own best reference star, and so offsets between 
observers will be common (all future-correctible to standard, though). 
 Generally, I assume v = V, then I test the field before completing 
reductions, then choose the one star that makes data fit best to the 
visual sequence, and has check stars make sense to the sequence.   
(Because visual charts are increasingly being used as references for CCD 
work, I suppose it makes sense to pre-assign a reference and check star 
so we are all using the same ones, but that opens up a whole new can of 
works over who would do this and keep track of problems as they come up. 
 Would simply be best just to calibrate the field).  It should be noted 
that there are a lot of 'bad' visual fields out there (for both visual 
and CCD observers!).

Right now, I'm staying away from all-sky, because a) the suspects 
project I'm working on doesn't depend on absolute standards, and b) I 
mooch off of University lab photometers whose exact set-ups I cannot 
control night to night.  All-sky is a lot of work, which I will take on 
one day soon, I'm sure.

Rick (HUZ) Huziak

Jean.Gunther wrote:

>A question to all observers using CCD
>
>Two methods seem to be used:
>
>-relative photometry using comparisons stars
>
>-"all-sky" methods using standard stars, transformation coefficients and absolute determination of the magnitude of studied star
>
>Which method do you use?
>
>What do you think about advantages and drawbacks of both methods?
>
>Thank you for your answers
>
>J. GUNTHER
>St TRINIT  FRANCE
>
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> 
>  
>

-- 

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Richard Huziak
Manufacturing Engineering
SED Systems, Saskatoon
tel. (306) 933-1676
<huziak at SEDSystems.ca>
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