Hall refraction

The hall refraction is a minor anomaly of the astronomical refraction . It arises at the observation slit of an observatory dome when the inner air temperature still shows noticeable differences to the outside temperature.

The cause is a small horizontal or vertical temperature gradient , which deflects the light rays by a few 0.1 " . Physically, the effect corresponds to a geodetic side refraction , which can be ten times that along sunlit walls or in tunnels.

One can keep the hall refraction (English also dome seeing ) low by

1. the domes are given a light coat of paint to protect against sunlight during the day,
2. the dome is opened immediately after sunset,
3. through artificial ventilation (possibly at the beginning of the night).

It cannot be completely avoided because on clear nights there is almost always a noticeable cooling of the ambient air at night .

By actively cooling the telescope room during the day to the temperature that can be expected outside at night, the cause of the hall refraction is eliminated except for the heat sources humans and electrical devices. However, such cooling is energy-intensive. The largest observatories are located far away on mountains and can usually only be supplied with electricity to a limited extent (by diesel generator sets). The energy requirement for cooling increases in parallel with the intensity of the solar radiation, which is why it is attractive to use solar cooling as an energy source, which has recently been planned.

In some modern large telescopes , the hall refraction is reduced by replacing the classic observatory dome with a cube-shaped protective structure that is open at the top or is partially pushed apart at night. This construction is also more stable and less expensive.