Determination Of The Suitability Of Mt Gamsberg In Namibia For Millimetre Wave Astronomy By Measurements Of The Precipitable Water Vapour

Abstract

Precipitable Water Vapour (PWV) is the amount of water vapour in the atmospheric

column above a location equivalent to the amount of liquid precipitation that would

result if all the water vapour in the column was condensed. Water vapour is the main

source of opacity in the Earth’s atmosphere at infrared and millimetre to sub-millimetre

wavelengths. The Event Horizon Telescope (EHT) is a large network of millimetre to

sub-millimetre telescopes across the globe that is used to image supermassive black

holes. The Africa Millimetre Telescope (AMT) is planned to be built on Mt Gamsberg

in Namibia and aims to complement the EHT. In this study, PWV at Mt Gamsberg

and at the H.E.S.S. site was determined in order to asses the two sites regarding their

suitability to conduct millimetre wave astronomy and for the AMT to be built on Mt

Gamsberg. PWV at the H.E.S.S. site was indirectly determined from sky temperature

data which was taken from 2004 to 2019 by radiometers on four of the H.E.S.S.

Cherenkov Telescopes (CT). The PWV at H.E.S.S. as a function of sky temperature

as given by the CT radiometers was determined by relationships given by data from

two other equipment present at the H.E.S.S. site, namely the NASA AERONET station

and an ATMOSCOPE with the equipment recording PWV and sky temperature,

respectively. The PWV at Mt Gamsberg was determined by scaling the PWV from the

H.E.S.S. site to what it would be at the height of Mt Gamsberg. The relative frequency

distribution of PWV, monthly mean PWV along with its seasonal variations were comi

puted for both sites. The presented results indicate Mt Gamsberg is a suitable site for

millimetre wave astronomy and to host the AMT. However it was recommended that

direct PWV measurements should be taken at Mt Gamsberg to confirm the result of

this study, as these were not based on direct in-situ measurements.