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Tables gif and gif summarise the optical characteristics of the INT. Spot diagrams for the various foci are shown in Figures gif and gif.

The three elements of the prime focus corrector are of UBK7 glass, coated to minimize reflections and thus improve efficiency and reduce ghosts. The first two elements are not interchangeable and have a broad-band, single layer MgF coating. The rear element of the corrector, however, is available in several coating options, summarized in Table gif. Single layer coatings produce reflectivities smaller than 2% per surface over the appropriate wavelength range given above. Three-layer coatings give reflectivities of less than 0.5% per surface, but the performance deteriorates very rapidly (to higher reflectivities than uncoated glass) outside the useful wavelength range.

Most observations use the broad band/blue corrector. Note that changing from one rear element to another is a major operation and is a day-time job. Frequent changes are strongly discouraged.

Table: Coating options for rear element of INT prime focus corrector

Tue Aug 15 16:42:46 BST 1995