Max Planck Institute for Astronomy - Annual Report 2007

Differential Image Motion Monitor
Unambiguous knowledge of the current seeing conditions
is essential to the success of large ground-based
telescopes. Such knowledge permits reactive observing,
in which the queuing of ongoing programs can be matched
to current atmospheric conditions. Over the longer
term, correlation of seeing measurements with other
seasonal and meteorological indicators can help in predicting
the image quality on any given night. This al-
IV.1 Instruments for the LBT 101
lows more effective planning and better use of scarce
observational resources.
During 2007, a team of MPIA scientists constructed
and delivered a Differential Image Motion Monitor
(DiMM) to the Large Binocular Telescope (LBT). This
device measures the seeing by recording the differential
motion of two images of a single star, which pass
through two slightly different paths in the Earth’s atmosphere.
Although conventional in most ways, the MPIA
Di MM faced a peculiar and difficult challenge arising
from restrictions on construction at the LBT site. A typical
Di MM installation includes a tall tower (ca 10 m) located
sufficiently far from other structures to allow sampling
of the free air turbulence. However, the LBT sits in
a scientific natural refugium and new construction permits
are essentially unavailable. As a result, the MPIA
team developed a novel mounting and guiding system
which allows the DiMM to point at and track a star while
mounted to the LBT telescope, which is itself pointing
at and tracking a different target.
Johannes Schmidt, Ralf-Rainer Rohloff, Armin Böhm
Fig. IV.1.14: Test camera #1 (above) mounted on the adaptive
optics test tower in Florence. Test camera #2 (below) attached
to the Gregorian focal port, which Lucifer will occupy in Fall
2008.