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DIMM-1998 and DIMM-2016 have been running in parallel during one full year. While DIMM-1998 was well known for over-estimating bad seeing conditions, DIMM-2016 produces seeing data much better correlated to what is measured inside the UTs.
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It is believed that a DIMM located upwind close to a much higher building will over-estimate low level turbulence due to the backflow of wind near the windward surface.
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DIMM-1998 was mostly affected by nearby 30m high UT3 and 20m high VST buildings situated downwind of the 5m high DIMM tower. Hence the apparent worsening of Paranal observing quality over the years of the development of the Paranal Observatory seen in the historical evolution below was in fact a local artefact not affecting science observations.
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1-NTT First Light (02-1989)
2-NTT Inauguration (02-1990)
3-Choice of Paranal as VLT site (12-1990)
4-Start levelling Paranal mountain, then 2664m asl (07-91)
5-La Silla DIMM destroyed by thunderstorm (05-92)
6-End levelling Paranal mountain, now 2636m asl (12-92)
7-Start planning NTT upgrade (09-1993)
8-Paranal DIMM tower (6m) completed (04-1994)
9-VLT Infrastructure Construction starts (08-1994)
10-NTT 'last light' (07-96)
11-NTT Big Bang (07-97)
12-Paranal DIMM Upgrade (03-98)
13-VLT/UT1 first light (05-98)
14-VLT/UT1 Science Verification (08-98)
15-La Silla DIMM decommissioning (11-98)
16-La Silla DIMM recommissioning (03-99)
17-VLT/UT2 first light (03-99)
18-VLT/UT3 first light (01-00)
19-VLT/UT4 first light (09-00)
20-VLTI first fringe (03-01)
21-NAOS-CONICA first light (12-01)
22-New M1 for Paranal DIMM (03-03)
22a-VST enclosure erection (10-02 to 04-03)
23-MACAO-VLTI first light (05-03)
24-VLT/AT1 first light (01-04)
25-New M1 for La Silla DIMM (06-04)