The RoboDIMM measurement represents the best atmospheric seeing obtainable
on a large telescope over the preceding minutes, observing at zenith and
unaffected by real-world effects
such as dome seeing, telescope tracking or optical aberrations. It is calculated for
an observing wavelength of 500nm. For other wavelengths (and airmasses),
please use the seeing miniview
(a) the measurement takes about 2.5 minutes, so that you can expect to obtain better (or worse) seeing in shorter
exposures in the interim (b) the value published on the Met system and Weather page is updated only every 5 minutes
(the latest measurement is available here) (c)
two nearby telescopes
will not necessarily measure the same seeing, even when observing the same star at the same time, because they
are observing through different parts of the atmosphere and
(d) the more variable the seeing the more difficult it is to compare two measurements.
From experience, comparing seeing measurements from nearby telescopes is only possible by first averaging measurements
over a 15-20 minute period, even in relatively stable conditions.
The seeing estimates produced by RoboDIMM are calculated from image motion,
using the approximation from Sarazin and Roddier's DIMM paper (PASP 1990), are based on the
Kolmogorov theory of atmospheric turbulence in the free atmosphere. The calculated values are
essentially the expected image width (FWHM) in a "long" exposure
(meaning, more than a few seconds), without correction for diffraction, which
is insignificant in large telescopes anyway.
By measuring image motion from the 4 images formed on the
detector, RoboDIMM simultaneously produces four independent estimates of the
image FWHM. Since all 4 should be in agreement, the data thus allows a check
that the instrument is working as designed. Telescope optical parameters such as
the image scale at the focal plane, the entrance subarperture size and separation distance,
are the only input values in this calculation, besides the image motion measured
by the monitor.
The DIMM method assumes that all of the image motion is caused by Kolmogorov type
turbulence. Since it measures differential motion between image spot, telescope
movement does not affect the seeing estimate (and trailed images are rejected by RoboDIMM).
Exhaustive turbulence analysis using the JOSE camera (1999 MNRAS 309, p379) has shown that the Kolmogorov theory
accurately describes the seeing obtained with the WHT for the great majority of observing time. Non-Kolmogorov
turbulence was a negligible component and rarely detected in the samples taken (25 half-nights plus
short samples from more than 45 other nights between 1995-99).