UW/APO DIMM Seeing Monitor
Note: I'm quite busy in the moment to finish up my degree,
so I won't be able to update this webpage until 2003!
Introduction
The APO Differential Image Motion Monitor (DIMM) was built by Armin
Rest and Chris
Stubbs from UW. This device samples the differential image motion of
a single bright star between two separate optical paths through the atmosphere.
This is accomplished with a 10'' telescope, which has a two-aperture mask
on the front and a prism which displaces the light from one aperture to
produce two well-separated subimages of one and the same star onto a single
CCD camera. Wavefront perturbations caused by turbulence in the atmosphere
(commonly denoted as seeing) cause relative motion of the two subimages,
but common mode motion, such as tracking errors and wind shakes, affect
each image in the same manner and do not introduce relative image motion.
Therefore the variation of the subimage separation is an excellent way
of obtaining a quantitative estimate of the seeing. The software which
runs the DIMM is written by Armin Rest. There are currently two identical
instruments, one of which is currently installed in a movable dome at APO
to monitor Polaris as often as possible. The seeing measurements obtained
are read out remotely via ftp and displayed in real time on a WWW page
using software written by Eric
Deutsch. The idea of the DIMM seeing monitor was adopted from the ESO
Dimm sytem developed by M. Sarazin and F. Roddier (Sarazin, M., Roddier,
F., "The ESO differential image motion monitor", 1990, Astron. Astrophy.227,
294-300).
Current Status
[2001-01-08] The analysis shows that the DIMM performs very well, and can
give an accurate assessment of the seeing (see the trip
report Feb'99 , trip report June'00).
However, the current site of the permanent dimm is not representing very
well the seeing seen by the telescope since the location is close to trees
and other obstacles, thus measuring higher seeing under some weather conditions
(see also Sloan pier results). Plans are under
way to move the Dimm to a more appropriate location. We tested the airmass
dependence, and also the dependence of the seeing measurement on the
exposure
time used in the differential image motion measurements. The current
DIMM dome has permanent power and ethernet supply, and it also has an automatic
search routine, which allows the observing specialist to search for Polaris
and start the seeing measurements remotely from the observing room.
Additional Documentation
Current and Archival DIMM Output
Examples
The pictures below show animated examples of the image acquisition.
The image loop frequency is about 3-7 Hz, depending on the laptop cpu speed.
This high image acquisition frequency with on-the-shelve components is
reached by reading out only subframes, indicated by the white box. Comparing
the relative motion in the picture to the left (about 1" seeing) to the
picture to the right (about 2" seeing), one immediately sees the much higher
relative motion and the higher scintillation for the images corresponding
to the higher seeing.
 |
 |
| image aquisition at about 1" seeing |
image aquisition at about 2" seeing |
. 
. 
Armin Rest: rest@astro.washington.edu
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