[Aavso-photometry] GJ 436 Campaign Web Page

[Wolfgang Renz]

Hi Arne

So you recommend using 2 filters to be able to transform.
But how high should be the "image ratio" between the two filters ?
50:50 ?
Then one will get effectivly a B and a V lightcurve. But as the color
of the target should not change, one will loos data in one filter that
might be required to reach the needed precisition by appying a
box-car average to the data of one filter.
In real photometric nights, wouldn't it be better to image primarily
in B and take a deep V band image just every 4th, every 6th or
every 8th image and us a smoothed, interpolated V band mag
for the transform ?

In non-photometric nights (e.g. with slight cirrus, changing extinction,
..., or even worse) it might be better to use 50:50 to be able to reject
outliers. But under non ideal conditions it might not be possible to
reach the required precision at all.
How good must the night be to make sense to start with GJ 436 ?

How many bias/dark/flat frames do you recommend to take to not
introduce to high errors by the calibration frames and to be able to
reach the required precision in the few mmag range at all ?

Clear skies
 Wolfgang

-- 
Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO



----- Original Message ----- 
From: "arne" <arne at aavso.org>
To: "Robin Leadbeater" <robin at leadbeaterhome.fsnet.co.uk>
Cc: <aavso-photometry at aavso.org>
Sent: Saturday, June 09, 2007 2:45 AM
Subject: Re: [Aavso-photometry] GJ 436 Campaign Web Page

> Robin Leadbeater wrote:
>> If  the purpose of the exercise is to determine the timing of the transits 
>> to the highest precision, what  advantage does multiple measurements at 
>> different wavelengths offer?
>> 
>> Concerning scintillation, I assume there must be some  break point dependent 
>> on read out time and read noise that makes it more advantageous to filter to 
>> increase exposure time rather than summing multiple short exposures post 
>> event. Can anyone point me to an example of this calculation?
> 
> The purpose is to not only determine the timing of the transits of GJ436b,
> but also to look for other transiting exoplanets around this system.
> Since the transit depths are likely to be small, it is easier to
> combine people's datasets if they use filters, and if they transform
> their data.  In order to transform, you need color information.
> Since the colors of the comp stars and of the target star are known,
> you can use those colors plus your transformation coefficients to
> transform your single-filter measures, or you can take 2-filter
> sets and transform with "live" data.  Either method works.  We
> may use other stars in the field to obtain rough transformation
> coefficients for those who have never done this before.  As mentioned
> in the Alert Notice, this is a tough project!
> 
> It is highly recommended that you use a filter for these observations.
> This is *very* different than CV observations.  You are dealing
> with millimagnitude variations of a red star.  You have plenty of
> light, so using a filter also gains you with longer exposure times
> to beat down scintillation.  It is easier to work with a single
> longer exposure than to have to sum groups of shorter exposures.
> Why make more work for yourself?  However, to answer your question,
> there is no difference between taking a single long exposure and taking
> multiple short exposures that add up to the same length when it comes
> to scintillation noise.  The multiple shorter exposures will have
> more read noise, which may be important in this case.
> Arne