(cba:news) Re: observing techniques

Joe Patterson jop at astro.columbia.edu
Fri Jul 2 14:56:01 EDT 2004

> Joe, I get it but would wonder -- I thought it would be much easier to
> combine observations if they were made filtered. I know you've combined
> tens of zillions of observations so this must not be the case?
> As a side note, I usually observe filtered because I don't have a clear
> filter and don't want to refocus. (read: lazy).

Yes, that's a point.  If everyone used the same filter and the same
comparison star, it would be quite a significant point; the splicing of
light curves from various observers would be very easy, and that might
even be enough to justify the factor of 8 loss in photons.  But we're a
ragtag army, with every soldier possessing different weapons - and
fighting the war really on their own terms.  So I've been used to
studying data obtained with different cameras, different filters, no
filters, different comparison stars.  By being that omnivorous, we usually
manage to get a LOT of data.

So how do you splice it?  Well, from many hundreds of nights of photometry
simultaneous between multiple observers, a simple answer emerges: SIMPLE
ADDITIVE CONSTANTS (in delta magnitude).  You can reduce the systematic
observer-to-observer difference to about 0.03 mag that way.  You can't do
much better; differences of 0.01-0.02 mag exist from night to night even
when nothing (perceptible) changes, and there are during-the-night drifts
of 0.02-0.03 mag (presumably due to differential extinction).  These
annoying little effects can be measured and partially removed, but by then
you're spending a lot of time on calibration - time that is better spent
observing our fascinating stars!

Often it happens that there is little or no observer overlap, in which
case there's nothing to splice.  In that case I just subtract the mean
delta mag, and trend if there is one.  That is THE most powerful way to
prepare a time series for period search (as long as you remember that you
have blinded yourself to the lowest frequencies by this subtraction!).
In practice the scientific usefulness of the time series is mostly set by
the QUANTITY and STATISTICAL QUALITY (not calibration quality) of the
delta magnitudes.

I imagine some observers are worried about what Saint Peter is going to
say about all this when you meet him at the Pearly Gates.  On the off
chance that he happens to be a photometrist, I grant you that he's more
likely to be a classical than a high-speed photometrist.  In which case
you may well be in trouble, because the classical art thrives on
calibration.  I'm sorry, you'll just have to do some quick talking.
(Personally I am not worried, since I'll be rejected on numerous other

BY THE WAY...  Jonathan tells me that we now have a "cba-chat" exploder
enabled.  Send messages to cba-chat at cbastro.org whenever the
item is more like chat than like news.  We'll see if it works.  Thank you,
Michael (Richmond), for prodding us to actually implement this idea.


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