Re: Simultaneous Observations

From: Joe Patterson <*jop_at_astro.columbia.edu*> Date: Wed, 18 Aug 2004 11:53:53 -0400 (EDT) · jop_at_astro.columbia.edu

On Wed, 18 Aug 2004, Michael Koppelman wrote:

> Maybe Joe could confirm or deny, but I have found that simultaneous
> observations of objects, when uncertainties are carefully determined,
> can yield extremely high-quality photometry. You can push the errors
> down to nothing with as few as two observers observing simultaneously.
> It is also helpful in figuring out zeropoint offsets between various
> observatories.
>
> Cheers,
> Michael Koppelman
> http://www.lolife.com/astronomy/
>
> On Aug 16, 2004, at 5:16 PM, Stupendous Man wrote:
>
> >   If anyone else in my longitude zone is working tonight (and
> > it's rare clear night in the Northeast US), perhaps you could
> > go for a different target?

Well, this is worth some discussion. This (some strict simultaneity of
observation) is very critical for our network, because we often observe
unfiltered and with different comparison stars.  Without simultaneity,
there is an unknown offset in the delta-mags between data sets.  We can
still use 'em for many purposes, since many period-analysis procedures use
subtraction of the mean anyway... but measuring this offset allows us to
examine for possible long periods (say 0.3 days and above), which we are
otherwise blind to.

For observers in the same time zone, the improvement is smaller but still
significant - just because everybody's weather/energy is different, so we
typically get lots of nonoverlapping data sets in the same part of the
diurnal cycle.  With even ONE overlapping observation, we can roughly
blend those data sets - but not otherwise.  In practice, a little more
overlap is desirable, because it gives an idea of how accurately the
offset is measured.  (I've noticed variations of as much as 0.03 mag, for
no obvious reason.)

It's not really a matter of "errors" (dispersion of measurement at a given
time).  You can't realistically improve on that much - it's basically
fixed by aperture, star brightness, and sky quality.  With many dozens of
overlapping nights with similar data quality, it would be worth the effort
to squeeze out a little more accuracy by combining the sets.  But we're
never really in that regime.  Meanwhile there are stars popping off
all over the sky, and otherwise taunting us by defying our precious
theories...... following all these delicious mysteries usually beats
calculating a few thousand more square roots!

joe