DIMM is considered to be the best
method to measure the astronomical seeing of an area. The atmospheric
turbulence is responsible for the known effect of the stars´
´twinkling´ which can seriously affect the quality of data
gathered. In plain terms the value of the astronomical seeing gives us an idea
of the atmospheric distortion (in other words how much a star twinkles) in the
area.
DIMM
observations use a special mask with two (or more) holes placed in front of the
telescope, a CCD camera and an appropriate code for the reduction. Because of
the mask, each star gives two images in the CCD frame, under the condition that
the star is slightly defocused (when in the prime focus the star will give one
image despite the mask... that's one of best ways to focus your
telescope!). What the code does is to calculate the distance in pixels between
the images of the star in a single frame and then compare these distances from
all the frames. Using some algorithms the code turns these distances to the
seeing value in arcsec.
We
started the observations on the Mt. Holomon Astronomical Station with the
Celestron GN-8 telescope 2 years ago and continued eversince. The results so
far clearly indicate that this region is one of the best in Greece for
astronomical observations, as we have a seeing average of 0.82"! The
observations continue and the full analysis is being conducted in the
Observatory of Thessaloniki using a DIMM analysis code, written for this purpose.
A
sample of DIMM graphs can be foundhere. You
can see a poster about seeing atMt.Holomonand National Observatory of
Athens. You can also see a relevant paper here.