The optical system of the INT is a conventional Cassegrain configuration with a
paraboloid primary mirror and a hyperboloid secondary. The
primary has a diameter of 2.54 m and a focal length of 7.475 m, giving
a focal ratio of f/2.94 at the uncorrected primary focus.
The mirror is made of Zerodur and has a negligible coefficient of expansion:
On axis, 80 percent of the light from a stellar image lies within a circle of
0.3 arcsec diameter, as shown in Figure 
at the end of this
chapter. Table and table summarize
the optical characteristics of the INT.
Table: Summary of mirror characteristics for the INT
Table: Optical characteristics of INT foci
A focal corrector of the type described
by Wynne (1974), but with a flat rear surface and increased back focal
distance, is used at the Prime focus in order to enlarge the useful field,
where the images should be 0.5 arcsec over the unvignetted field. The
corrected focal ratio is f/3.29.
The three elements of the Prime-focus corrector are made
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 below.
Table: Coating options for rear element of INT prime focus corrector
Single-layer coatings produce reflectivities smaller than 2 percent per refracting surface over the appropriate wavelength range. Three-layer coatings give reflectivities of less than 0.5 percent per surface, but the performance deteriorates very rapidly (to worse than for uncoated glass !) outside the useful wavelength range. Most observations are done using the broad band/blue corrector. Note that changing from one rear element to another is a major operation and a day-time job. Frequent changes are strongly discouraged.
The Cassegrain secondary mirror gives a focal ratio of f/15. Spot diagrams
for this focus are shown in Figure at the end of this chapter.