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:

lens


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

wedge
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.

baffle

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.