The Astronomical Unit


This page last modified 1996 December 20th

112mm f/18 Refractor

112mm f/18 refractor

I made this atm 112mm f/18 refractor from scratch. The objective is a Littrow-type lens:


I made it so that R1 = -R2 = -R3; R4 = inf.

One of the things important in lens-making is the control of "wedge", or differential thickness of the lens elements, which causes the lens to behave as a prism, giving false colour. This is identified by measuring different parts of the edge of the lens element with a micrometer and a simple jig.

The objective/dew cap assembly unlatches with 3 suitcase latches (assymetrically placed so that there is only one orientation which fits).

The lens elements are not bloomed. However, instead of spacers between the elements, I used thickened cedar oil (sold as microscope immersion oil) in order to reduce internal reflections. It makes a great deal of difference and gives the objective end of the telescope a smell reminiscent of Mediterranean forests (or, if you're cynical, lavatory disinfectant).

The tube is 6 inch aluminium irrigation pipe, with a stack of internal baffles made of 2mm MDF mounted on 3mm dowel. The baffle stack is quite flimsy, but is very secure inside the tube.


The finder is based on a Ross 300mm camera lens, mounted in a modified down-pipe bracket.

The instrument performs quite well, but the lens does star-test as slightly undercorrected. However, it is far too long -- it won't fit in the family car (Citroen ZX diesel saloon) and it is a real pig to manouever onto the mount. Once up, it catches the slightest breeze and oscillates like a politician's jaw. For this reason, it was rarely used and is now in bits, awaiting a 3" flat, in preparation for being folded. I hope the folded version will be a bit more user-friendly.