A differential image motion monitor (DIMM) is in semi-automatic operation on Calar Alto since October 2000. The instrument was provided by L'Heritier/Paris and consists of an 8inch Schmidt-Cassegrain (CELESTRON) telescope, an LH760 ULL pulsed intensified CCD camera (756x581 11 microns pixel size) by Lhesa Electronique using an S20 R photocathode (P20 phosphor). It provides an absolute measurement of the seeing at 20sec intervals. The instrument is similar to two DIMMs which have been installed on La Palma. The software is produced under licence from Vernin and Munoz-Tunon.
The DIMM will provide quantitative measurements of the seeing on Calar Alto, which in turn will help to objectively characterise the site. To the joy of all involved, first measurements have resulted in absolute seeing values down to 0.5-0.6 arcsec. Our first measurements are in clear constrast to annoying comments that the seeing on Calar Alto is poor. Nevertheless, the seeing at the 3.5m telescope, at the same time of the DIMM measurements, was significantly worse. This does suggests, in a strong way, that significant degradations of the seeing at the 3.5m telescope occur by local effects. A number of possible remedies have been identified to improve local seeing. Most importantly, money is secured to modify the 3.5m dome, and to install dome ventilation vents. The vents will consist of a number windows with a total net aperture of 19 square meters. This will secure efficient and natural ventilation of the dome interior during observations. People trust that significant improvements will result, similar to the improvements which were seen at other 4m class telescopes after the later installation of dome vents (UKIRT, KPNO, CTIO, etc).
Unfortunately, our DIMM suffers from a few problems which prevent a robust operation on Calar Alto. We are in contact with L'Heritier to remove the instabilities in the operation of the DIMM. A software engineer from the company will arrive on Calar Alto on March 13 to install a new version of the programme. Promise is made that the remaining problems will disappear once the new software is installed.
