Re: why is it better to look at the SAME fields each night?

[Tom Droege <tdroege@veriomail.com>]

Michael and all,

The question is, if you take 56 frame of a piece of sky on one night, is it 
at all interesting to take 56 frames on the next night?

My plan has been to mix the "follow" and "flat" exposure schemes.

The "flat" run does two things:
1) It collects data to make a flat.
2) It takes one or two measurements of a large area of sky.

The "follow" run takes data on one area of the sky tracking it as long as 
possible

I think this collects data of the kind that Michael wants.  It gets 
measurements at a wide range of time intervals.  OK, it is not 
perfect.  The once a night measurements (done by the flat field data) come 
a month or so ahead of the 56 exposures done by the follow run on a single 
night.

The "flat" run gets data for about 4 x 40 degrees one or two times a night 
(the flat fields overlap).  The "follow" run gets 4 x 4 degrees tracking as 
long as possible for as many fields as fit in for the rest of the 
night.  Eventually the "flat" runs cover the area of each "follow" run many 
times during the nights that are clear.  Note that the "flat" runs get once 
a night data on sky that was covered by "follow" runs in previous 
months.  It might be better to take a second "flat" run at the end of the 
night.  It is a problem to schedule when you are not awake and at the 
telescope.

Tom Droege

At 01:40 PM 10/21/01 -0400, you wrote:

>   Last week, I said:
>
> >   If you can arrange it so that the 2 or 3 fields you follow each
> >   night are _roughly_ the same (that is, there's an overlap of at
> >   least one degree in RA from night to night to night), then your
> >   dataset will be much stronger than if the "follow" fields are
> >   different on each night.
>
>   Tom asked me to clarify.  It's simple: suppose you have 20 clear
>nights over the summer.  If you look at the same fields each night,
>then you have very good sampling of the stars over timescales
>ranging from a few minutes (from one image to the next on a single
>night) to two hours (start to end of run on one night); then a gap,
>and then good sampling again on timescales of 1 day to, say, 60 days.
>If one is trying to find variable stars, this frequent sampling helps
>for three reasons:
>
>       a) it gives one more data on each star, so that one can
>               determine the instrumental scatter more precisely
>               and thus notice variability with small amplitudes
>
>       b) it allows one to determine the period of variability
>               more precisely (for periods of a few to 60 days)
>
>       c) it gives one a decent shot to see rare events, such as
>               the eclipses of binary stars with wide orbits
>
>   I think that it's better to have 20 nights of data on 3 fields
>than to have 3 nights of data on 20 fields.
>
>                                               Michael