Davis 25 sextant telescope upgrade

[Bi111ion]

Land locked and locked down I am practicing sun sights with a baking tray of water as an artificial horizon in my garden. The Davis Mark 25 plastic sextant has a times 3 plastic telescope. It is better than it looks but I could do with higher magnification, especially on land. I found a very cheap but amazingly good value 8x20 minature prismatic monocular that fits neatly. More pics and details on my blog post.

Maybe there is a whole web site somewhere called "pimp my plastic sextant", but I have not seen anyone else try this before.

 

[neil_s]

You're not alone! I have made a zero mag 'sight tube' for my Ebbco - I find that good for Sun sights - it cuts down the glare. I would appreciate more magnification for stars, though, my old eyes aren't so good anymore!

 

[Norman_E]

Neat conversion, but I think I would struggle with 8X magnification at sea. I was given my friend's Mark 15 after he died and I find its 3X scope good enough. So far I have only taken sun sights at home and will have to wait until I get out to the boat to try it properly, but it seems to be a very good instrument. Next plan is to take it to a beach where I can use a sea horizon.

 

[Bi111ion]

You're not alone! I have made a zero mag 'sight tube' for my Ebbco - I find that good for Sun sights - it cuts down the glare. I would appreciate more magnification for stars, though, my old eyes aren't so good anymore!

Now I thought the Ebbco had a sight tube? Maybe yours was missing. In these days of 3D printing I expect we could make all sorts of handy sight tubes and telescopes. People are even making their own simple plastic sextants. The beauty of this monocular is it just works without any modification - as well as being incredibly cheap!

Also the Davis 25 is a pretty good sextant. On mine I have to correct index and side error every time I pick it up though so I wonder if that could be improved.

 

[neil_s]

My Ebbco sextant has a X2.5 telescope - looks very similar to that fitted to the Davis. I've never used a metal sextant so have no idea how stable they might be. The Ebbco index error does vary between 1 and 3 on, but it cost me £17 needing some repairs so I can't be too critical!

 

[neil_s]

A thought came to me - one of the things that needed fixing on my sextant was the engagement of the worm screw on the quadrant. It was a bit slack, leading to a slight backlash when turning the micrometer drum. An improved spring fixed it for me. - Possibly why you have an unstable index error?

 

[Bi111ion]

I wonder if another option might be to use optical rifle sights. They come in low magnifications including 1x (fancy sight tube) 3x 8x etc and small objective lenses. The fittings are somewhat standardized to perhaps making them interchangeable. I wonder if anyone tried this. Especially if you had a Davis or similar with broken scope mounts.

 

[Bi111ion]

Thanks I will check that! Also the mirror adjustment screws and the mirror spring spring clips fall under suspicion.

A thought came to me - one of the things that needed fixing on my sextant was the engagement of the worm screw on the quadrant. It was a bit slack, leading to a slight backlash when turning the micrometer drum. An improved spring fixed it for me. - Possibly why you have an unstable index error?

There is a set of spares for springs and screw availiable Replacement Hardware Kit for Mark 15 or Mark 25 Sextants | Davis Instruments