Large chopping secondary mirror for the 15-m submillimeter James Clerk Maxwell telescope.

A 75-cm diam chopping secondary mirror has been developed for the 15-m diam James Clerk Maxwell telescope. The large focal ratio (D/F = 1:0.36) requires a highly convex secondary mirror, which was manufactured out of a solid slab of high-grade aluminum, consisting of a 3-mm thick shell with a support ring along the edge. The mirror surface approaches a predescribed hyperboloid with a rms precision better than 9 microm, which is adequate for use in the submillimeter wavelength region. The design aspects, the performance of a two-axis chopping mechanism, and the construction of the mirror are described.

[The institutional population: trends]. / De institutionele bevolking: omvang en ontwikkeling.

"The institutional population includes the population in homes for the aged, nursing homes, psychiatric hospitals, prisons and educational and religious institutions. On 1 January 1990 there were 278,000 people in this population group in the Netherlands, which amounts to 2% of the total population. During the period 1960-1990 the institutional population showed a smaller increase than the total population (6% vs. 30%); during the eighties the number has decreased. In this population the share of people of 65 or older has increased from 29% in 1960 to 67% in 1987." (SUMMARY IN ENG)

[A new measure for degree of urbanization: the address density of the surrounding area]. / Een nieuwe maatstaf voor stedelijkheid: de omgevingsadressendichtheid.

A new method of measuring the degree of urbanization of a given area, developed by the Dutch Central Bureau of Statistics, is introduced. "The measure classifies areas according to what is called address density of the surroundings: the average address density within a radius of 1 km of an address in the area." The advantages are discussed, and the measure is used to classify the 647 municipalities in the Netherlands into five groups according to urbanization level. (SUMMARY IN ENG)

High speed frequency modulation of far infrared lasers using the Stark effect.

Electronic frequency tuning of an optically pumped far infrared waveguide laser has been achieved by using the Stark effect. Frequency modulation with a 50-kHz modulation frequency and an index greater than 1 has been observed as well as a maximum modulation frequency of 300 kHz.