The Three-Mirror Telescope
at the Institute of Astronomy

by Steve Jones, from technical notes provided by Dr R Willstrop

The CAA has been offered the use of the Institute of Astronomy's unique Three Mirror Telescope. If you have attended any of the viewing evenings at the IoA you may already have seen this fine instrument.

The Three Mirror Telescope at the Institute of Astronomy, with its designer, Dr Roderick Willstrop.

The Three-Mirror Telescope (or 3MT) was developed by Dr R Willstrop at the Institute of Astronomy. Its design is unique, being the only telescope to combine a wide field of view, very sharp images, and all-reflection optics (no lenses). In other designs of reflecting telescope, the usual narrow field can be widened to only a degree or two, by correcting lenses placed in the converging lightpath. In the case of the Schmidt camera, a field of 7° or 8° is possible, but the corrector lens here comes first, far in front of the mirror, and limits the aperture to about 1.3 metres. The mirror then needs to be larger than the lens to minimise the loss of light towards the edge of the field.

The 3MT has a field of view of 5°, with a theoretical image size of 0.33 arc seconds everywhere, improving to 0.1 arc seconds over the central degree. Because no lenses are needed the images are perfectly achromatic, and in principle such a telescope could be built with a larger aperture than any Schmidt telescope.

It has a very compact design which allows it to be housed in a small, economical building (here the roof rolls off on rails, as can be seen in the photograph). About half the height of 2.5 metres is taken up by the equatorial mount. An altazimuth mounting would be much lower.

The 3MT has a focal ratio of f/1.6 - an aperture of 500 mm and a focal length of 800 mm. It is currently fitted with a CCD for digital image recording. Unfortunately this reduces the field to 30 x 40 minutes of arc. The picture elements, or pixels, are 6.8 microns square, which is equivalent to 1.75 arc seconds, closely matching the diffraction limit of this relatively small prototype. The practical limit of resolution is affected by seeing, small tracking, focus and alignment errors and imperfections of the mirrors, and is close to 3.5 arc seconds or 2 pixels (full width at half maximum intensity).

The radical design of the 3MT compares very favourably with the more conventional Schmidt. For example, the tube length of the Three-Mirror Telescope is just 1.2 metres - much more compact than the tube of the nearby Schmidt, which also has a field of 5°, gathers only slightly more light, but has a tube length of over 4 metres. The CAA is very privileged to be offered the use of this fine instrument.

The telescope is currently fitted with a CCD, but this can be replaced with a film holder.

Any members wishing to be trained in the use of this instrument should refer to the Contact List for details.