Date: Sat, 9 Oct 1999 09:32:16 -0700 (PDT)
From: Chris Stubbs
Subject: so, you thought that was a little terse?
hi.
I feel obliged to address some of the questions that have been raised
about the DIMM. It's important to realize that it's only making
one measurement- namely the differential motion of a star as seen through two
columns of atmosphere that are a few inches apart. It then uses
a *model* to determine what a large aperture telescope would see.
This model (Kolmogorov turbulence) is not guaranteed to work
under all conditinos. For example, the DIMM is totally insensitive
to teh (usually dominant) tip-tilt term. If the seeing is dominated by a
low-lying air bubble then the DIMM will predict seeing better
than what is achieved, as tip-tilt would in that case be a larger
term than it's expecting.
ALso, there is overall "fudge factor" (multiplicative) that
we have yet to fully calibrate.
Finally, there is an airmass dependence that needs to be accounted for.
Our plan is to make a web page that interrogates teh 3.5m and
2.5m TCCs, and predicts the seeing at the telescope's
zenith angle.
In our experience the DIMM does provide useful information, but you
need to keep its limitations in mind.
Cheers,
Chris