THE NEW SKIN OF THE 4M DOME
Up to a few years ago, there were traditionally 2 kind of paints used to cover telescope domes: 'Lomit' and titanium oxide.
Lomit is the grey-looking (silver- or aluminium-like) paint. It typically overheats by up to 20degC during the day in the sunshine and overcools by about 2degC at night after reaching equilibrium with ambient air (delay due to thermal inertia of structure).Its emissivity is 0.21.
Titanium oxide is the white paint which had, for example, always been in use at CTIO. It typically maintain thermal equilibrium with ambient air during the day (no overheat) but overcools by about 5degC during the night. Thus, although titanium oxide is far better than Lomit during the day, it is slightly worse at night, and the air cooling around the dome tends to fall/roll down off the walls and eventually enter the dome through the shutter or the lateral doors, eventually causing dome seeing. It is frequent in summer time on Cerro Tololo to see the domes wet while the RH is only 75%: this is because the outside skin of the domes overcools several degrees below ambient and can drop below the dew point.
In the early nineties, our colleagues at Las Campanas Observatory have used an aluminium sticky foil to cover their domes and got very positive results (more recently they used it for the Magellan twin telescope domes). Our measurements have shown that, with such a coating, the dome typically overheats by 5degC during the day and by about 2degC during the night. It has therefore better characteristics than the 2 older techniques.
* white titanium oxide paint: day=ambient ; night=ambient-5deg
* grey Lomit paint: day=ambient+20deg ; night=ambient-2deg
* aluminium foil: day=ambient+5deg ; night=ambient+2deg
The difference with the Lomit paint (both are greyish and reflectivish) is mainly that the foil is made out of an insulating layer of adhesive about 0.12mm thick (5/1000") coated with aluminium. Consequently there is no direct thermal contact between the ambient air and the metallic structure of the dome (thus avoiding the diurnal overheating). The product is called Compaq #804 Aluminium foil tape, it comes in rolls of 30"x60yds and it is sold by Bron tapes Inc. Adhesion is increased by application of a primer on the substrate, and is good enough that no maintenance is required over periods as long as 7 years. The foil can also be washed without deterioration.
Our hope is therefore to improve the thermal performance of the 4m dome and decrease dome seeing. New temperature sensors with RF transmitters and receivers will be mounted on the dome (which is a rotating part!) to attempt quantifying these modifications.
M.B., 14th November 2000