[Aavso-photometry] Short-Exposure Photometry

[Arne Henden]

Dan Kaiser wrote:
> This discussion comes at an opportune time.  I have been giving much
> thought to observing a transit of exoplanet HD 209458b.  With my 14" SCT
> it is almost too bright.  I have concluded I will try with a B filter,
> which will cut down a lot of light and hopefully allow exposures longer
> than say 8 seconds.  It will also dim my guide star which could be a
> problem.
> 
I am surprised that your exposures are this short.  At 8th magnitude,
the 1.0m saturates at about 0.5 seconds, which means you should be in
the 5-6 second range at V for HD 209458.  Since the eclipse is hours in
duration, you will be taking many, many images, easily combined if you
want to reduce scintillation.  Certainly going to B will increase your
exposure times by a factor of two-three, which won't hurt.  As I mentioned
earlier, there are many other techniques for reducing the maximum signal
level.

I don't think you need to worry about guide stars.  If the image wanders
a bit on the CCD, that won't hurt, but guiding a 10-second exposure should
not be necessary.

> A heads up to anyone in the eastern  and midwestern americas wanting to
> attempt this challenge.  HD 209458b will be well placed this friday
> night/saturday morning with a transit beginning at 5 hrs UT.  Weather
> permitting I will be there.
> 
This is a good event for the USA.  It is difficult (total depth is 0.02mag),
but if you can't do this one, you need to improve your techniques as all
other potential eclipse systems are predicted to be even shallower.  I am
testing an SXV-H9 camera here and might give the eclipse a try if the
monsoonal skies permit.

> I'm also looking forward to Tim Castellano's Transitsearch talk at the
> meeting next week.
> 
Yeah, I saw Tim's talk at SAS but wanted to hear it again.  Too bad I am
going to miss this meeting.
Arne