46
Cloud cover statistics could be gleaned from the daily observations performed by the Meteorological Observatory, where we were accommodated.
Data were stored there dating from the beginning of the twentieth century.
The other parameters were more problematic.
The method of determining the transparency of the atmosphere was the
somewhat pedestrian one of estimating by eye the ‘limiting magnitude’ at
zenith. The magnitude limit is determined by the faintest stars that can be
seen with the naked eye. This is obviously dependent, among other things, on
the visual acuity of the observer. The simple determinations of ‘visibility’
made in meteorology, based on estimating the greatest distance in kilometres
that objects can be distinguished, is too crude a method of gauging atmospheric transparency for astronomical purposes.
Nevertheless, in June 1961, using only these primitive tools at the Izaña
Meteorological Observatory, sheer enthusiasm prompted me to write a small
report bearing the title ‘Preliminary study of sky transparency in Tenerife’,
with an introduction, four chapters, a bibliography, and four graphs. The
study served to provide a preliminary idea of the most transparent months.
Still a fledgling physicist, during my holidays in 1961, I managed to acquire
an old Kipp actinometer, property of the Laboratory and Workshop of the
General Staff of the Navy, from the Optics Institute, but without an accompanying measurement system. An actinometer is basically just a thermal sensor located at the bottom of a tube mounted on a tripod. I then had the wild
idea that the apparatus could be used to determine atmospheric transparency
by coupling it to a precise measuring instrument. I set about designing a
Wheatstone bridge capable of measuring milliamperes with precision. As it
could not be built in the Canaries, I persuaded the Nuclear Energy Council
to make it for me. They did it quickly, and by the end of 1961 reliable systematic measurements of atmospheric transparency were being made throughout
the day.
The only method we had of measuring atmospheric turbulence was based
on observing the diffraction rings around stars and then applying a procedure
to deduce the level of turbulence from these observations. In the absence of
an atmosphere, or with a totally stable atmosphere, these rings would be perfect, whereas in a turbulent atmosphere they would become blurred. In the
latter case, star images become jittering luminous speckles.
To get reliable statistics from the data it was necessary to take measurements in a systematic and standardized way, so I drew up a detailed work
schedule, allotting days and hours to specific weekly tasks to be carried out by
each member of staff.
F. Sánchez
Cloud cover statistics could be gleaned from the daily observations performed by the Meteorological Observatory, where we were accommodated.
Data were stored there dating from the beginning of the twentieth century.
The other parameters were more problematic.
The method of determining the transparency of the atmosphere was the
somewhat pedestrian one of estimating by eye the ‘limiting magnitude’ at
zenith. The magnitude limit is determined by the faintest stars that can be
seen with the naked eye. This is obviously dependent, among other things, on
the visual acuity of the observer. The simple determinations of ‘visibility’
made in meteorology, based on estimating the greatest distance in kilometres
that objects can be distinguished, is too crude a method of gauging atmospheric transparency for astronomical purposes.
Nevertheless, in June 1961, using only these primitive tools at the Izaña
Meteorological Observatory, sheer enthusiasm prompted me to write a small
report bearing the title ‘Preliminary study of sky transparency in Tenerife’,
with an introduction, four chapters, a bibliography, and four graphs. The
study served to provide a preliminary idea of the most transparent months.
Still a fledgling physicist, during my holidays in 1961, I managed to acquire
an old Kipp actinometer, property of the Laboratory and Workshop of the
General Staff of the Navy, from the Optics Institute, but without an accompanying measurement system. An actinometer is basically just a thermal sensor located at the bottom of a tube mounted on a tripod. I then had the wild
idea that the apparatus could be used to determine atmospheric transparency
by coupling it to a precise measuring instrument. I set about designing a
Wheatstone bridge capable of measuring milliamperes with precision. As it
could not be built in the Canaries, I persuaded the Nuclear Energy Council
to make it for me. They did it quickly, and by the end of 1961 reliable systematic measurements of atmospheric transparency were being made throughout
the day.
The only method we had of measuring atmospheric turbulence was based
on observing the diffraction rings around stars and then applying a procedure
to deduce the level of turbulence from these observations. In the absence of
an atmosphere, or with a totally stable atmosphere, these rings would be perfect, whereas in a turbulent atmosphere they would become blurred. In the
latter case, star images become jittering luminous speckles.
To get reliable statistics from the data it was necessary to take measurements in a systematic and standardized way, so I drew up a detailed work
schedule, allotting days and hours to specific weekly tasks to be carried out by
each member of staff.
F. Sánchez