Fitting a prismatic telescope to a Davis Mk25 sextant

The whole Davis 25 with the prismatic monocular fitted

The Davis Mark 25 is good value for a back up sextant, and while not as accurate as a metal one it is lighter and cheaper. Its main failing is you need to calibrate index and side error every time you pick it up.

During the Covid 19 epidemic I am on lockdown and can't go to Tui  but I have the Davis 25 at home and I am practising sun sights with a baking tray of water or oil as  an artificial horizon.

It has a little x3 plastic telescope. It us actually  better than it looks and x3 does help. But I don't have great visual acuity and more magnification would help at least on land 

I bought a cheap 8x20 miniature monocular. The objective lens is in a conical tube that unscrews. This allows it to mount in the front scope bracket and tightening it centres the monocular.  The frame that holds the prism and eyepiece fits flush against the rear scop bracket and while the whole is to big it still holds in firmly.

I get a clear sharp view of the sun and its reflection and the field of view is about 5 degrees wide which is plenty. Of course the sun appears bigger and for me easier to judge when superimposed or "kissing".

I paid only £2.69 for this on Ebay, they have gone up to abut £5 now. Just search for 8x20 miniature monocular.

The new prismatic monocular fitted, and the standard "x3" (in practice more like 2x)  telescope underneath for comparison

Showing how the mounting works

A dimensioned photo from another ebay listing

 

More ebay listing photos.  The  optics are coated and  so  the objective appears orange from outside. In practice the magnification is more like x5 thanx8. Don't trust the description!

If you want to get the best from your plastic sextant I recommend the book How To Use Plastic Sextants by David Burch. If you want to know more about the mechanism of sextants, including telescopes I recommend The Nautical Sextant by W J Morris. Both are available in print and electronic format