OASI: 110 mm reflector

A new telescope has appeared at Orwell Park Observatory - let me explain its origin. In the 1980s, I constructed a 110 mm f/5 Newtonian as a finder and guidescope for my astrocamera. I found it to be extremely useful. With a low power eyepiece, e.g. a 40 mm Kellner, it gave a field of view of almost 3°, ideal for finding objects, but with a 2x Barlow and 6 mm eyepiece it gave a high magnification suitable for guiding for astrophotography.

OASI already owns a small equatorial mounting on a tripod but has no suitable telescope to mount on it. Therefore I decided to construct a small Newtonian, similar to my own, which could be used on the balconies at Orwell Park Observatory or taken on field trips e.g. for grazing lunar occultations.

Good quality mirror blanks are available at reasonable cost from H V Skan in Birmingham [1]. I bought three 110 mm Duran 50 (Pyrex) blanks with the idea of constructing two identical f/5 mirrors with a spare blank for a Cassegrain secondary mirror. I started grinding the two mirror discs alternately on the same grinding tool so that they would end up with the same focal length. I used 80 grade carborundum (C.80 [2]) for initial rough grinding, taking one-and-a-half hours for each disc to give a focal length of 64 cm. I then changed to C.180 and reduced the focal length to 60 cm and made the surfaces smoother. Grinding is usually carried out with the mirror on top of the grinding tool but for part of the C.180 stage I used the tool on top of the mirror. This ensured that the edge of the mirror was fully ground and also prevented the focal length from becoming too short. The remaining grades used were C.280, C.400, A.600 and A.1000 and, at the end of grinding, the focal length was 57 cm. After the A.1000 grade the mirror was very smooth and ready for polishing. I made a polisher as described elsewhere [3] and spent about three hours on each disc completing initial polishing. After initial polishing I Foucault tested the mirrors [4]: unfortunately this showed that the centre of the mirrors had too long a focus and the edge too short a focus, i.e. the mirrors were oblate spheroids, the exact opposite of what was required! However, this was not too serious as mirrors are rarely close to the correct shape after initial polishing and after another hour or so of intermittent figuring and testing I achieved a close approximation to the desired parabolic shape, see image below. Final analysis of the Foucault measurements showed that the mirrors were accurate, apart from the extreme edge, to ~1/10 wavelength.

And how much did the reflector cost? Well, the mirror blank cost £12, the aluminium coating £10 [5], the focussing mount £39 [6] and the diagonal mirror £33 [6]. All the other components (tube, diagonal mount, mirror cell, etc.) were made from scrap bits and pieces.

I am loaning the telescope for an indefinite period to OASI for use on the balconies at Orwell Park Observatory and also on field trips away from the Observatory with my permission. The only useful telescope is one that is used, so please do use it as often as possible!

Note: in early 2008 Mike terminated his generous loan of the telescope and it is therefore no longer available for use by members of OASI.

Foucault test.

The telescope on an equatorial mount.

Footnotes

[1]

H V Skan Ltd, 425-433 Stratford Road, Shirley, Solihull, West Midlands, B90 4AE.

[2]

Grinding powders are referred to for example as C.80, C.180, A.1000, etc. C stands for carborundum and A for aluminium oxide. The latter is softer than the former and yields a finer glass surface. The number refers to the size of particles: the bigger the number the finer the abrasive.

[3]

Sky & Telescope, November 1983, p 454.

[4]

How to Make a Telescope, J Texereau, Willmann-Bell, 1984.

[5]

Image Optics Ltd, Image House, Harvey Road, Burnt Mills Industrial Estate, Basildon, Essex, SS13 1ES.

[6]

Astro Promotions, 89 Lalleford Road, Luton, Bedfordshire, LU2 9JH.