The optics in the TEL301 telescope produce an image of the source at a mirrored surface. The user, looking into the eyepiece, sees the image with a black spot corresponding to a hole in the mirrored surface.

The light which passes through the hole is transmitted either directly to the monochromator, in the case of the direct coupled device, or into a flexible light guide. The fact that the source is imaged at the hole ensure that only light from the area covered by the spot seen by the viewer is measured. The use of the fibre bundle allows the telescope to be mounted on a camera tripod or measuring device.

The TEL301 comes with four interchangeable mirrors and a choice of lenses ranging from general purpose, capable of focusing infinity, to microscope which can collect light from a 30m diameter area. Addition of a DH400-VL photometrically corrected photodiode and an ORM400 optometer turns the TEL301 into a versatile luminance measuring spot meter.

Four interchangeable apertures are supplied giving angular fields of view (fov) at 6', 20', 1° and 2°.

Linear coverage at a distance D is: 2d tan (fov/2)

Eyepiece magnification = x10.

Spectroradiometric polarisation error is less than 1% when used with Bentham monochromators.