[Aavso-photometry] AE Aqr

[arne]

There is no unique answer to this question.  The ideal situation is either
two telescopes or a beam splitter, and to take B and V data simultaneously.
Few people can do that, so let's explore the other options.

(1) alternation, such as B V B V B ....
This works fine except you lose time resolution.  Since B is twice as long
as V for Tomas, this means you only get a B frame 66percent of the time
for 1:1 alternation.  If you increase the number of B frames before taking
a V frame, you increase the "duty cycle" but reduce the transformation
accuracy.  Note that you are not really losing data, as the V frames are
also valuable.  Whether alternation is needed, or the duty cycle, depends
on whether the object is changing color within a given interval.  For
AE Aqr, the color should be pretty stable.  What is more likely to occur
is flickering or flaring, so that a given B or V frame may not yield a
very precise (B-V) color.  What I would do is alternate at some duty cycle,
and then use a mean color based on these measurements for the entire night.

(2) single filter, such as B.  This gives the highest time resolution, and
as long as the color is stable night-to-night, you can use a color index
determined on another night (or standardized color) for your transformation.
Or, don't transform.  If the color is stable, this means you just have
a zeropoint offset between you and another observer, easily removed if
there is some observational overlap.

(3) use of a different filter pair, such as V and Ic.  This still gives
you transformation capability, and the Ic exposure will be more like V.
What you then lose is B, where most of the action occurs on a CV, but
with 20-minute exposures, you are missing most flares and other changes
anyway.  By going this route, you have 10-minute exposures instead of
20-minute exposures.  In some cases, the improved time resolution is
the important constraint.

Note that for any of these cases, the comparison stars have known, stable
color and so you can transform your comparison star measures without problem
even with single filter datasets.

I know I'm not helping much!  My choice would be the longer spacing between
V-band observations, such as V B B B V B B B V B B B ...
Arne

Tomas L. Gomez wrote:

>On the night Aug 31/Sep 1, I made a 4 hour time series observation of
>AE Aqr, with B filter (as well as V filter, to be able to transform to
>the standard system). I will send the observations as soon as I reduce
>them.
>
>Tonight (Sep 2/3) I am planning to do another times, but I have a question:
>To get the data transformed to the standard system, I use the usual formula:
>
>B = b + T (b-v)
>
>where b and v are the instrumental magnitudes, and T is the transformation
>coefficient. Therefore, I need to get data also with the V filter, but
>then I will loose time resolution. I guess that the standard way to do
>this is  to take
>frames following the sequence
>
>B V B V B .....
>
>and then calcultate the b-v color index for each B-frame by interpolation.
>To get better time resolution,  last night I took the frames using the following
>sequence:  
>
>V B B B V B B B V B B B  ...
>
>i.e., I took one picture with V for every three pictures with B. Then
>I calculate the
>color index b-v for each B-frame by interpolation. Is this OK?, or is it better
>to use the sequence B V B V B .... to get a more precise interpolation of 
>the color index?.  Or is it better to take frames only with the B filter, and 
>report the instrumental b magnitude?
>
>The aperture of my telescope is only 20 cm, so to get a good SNR I need to give
>exposures of 20 min for B and 10 min for V.
>
>-Tomas L. Gomez (GOT)
>  Madrid, Spain
>
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