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    Re: MK2A periscope sextant
    From: David Pike
    Date: 2020 Dec 25, 13:45 -0800

    Thanks very much Dave. An interesting bit of background. I knew about the light as it's similar to the MKIX. Everything's working now. Got a couple of moon sights last night which worked out nicely. Both inter epts were short by 3 miles I presume thats the expected accuracy. Certainly very easy on a window ledge. Thanks for that tip. Took a while for my slow brain to work out the light is coupled to the averager not a shutter like the IX. The circuit diagram helped with that. I'd be really interested to see the rest of the diagram showing the pendulous mechanism. I usually take my sextants apart and have a look inside but the 2A seems to be impregnable. My Hercules flights were as an air cadet in the far east in the late 60s. Spent many happy hours on the bench seat at the back of the cockpit. They were brand new at the time. Amazing that they are still flying. Thanks again for the help.

    Yes, you must wind a periscopic sextant Mk2 up for the lighting to work.  That doesn’t mean you have to run the averager. The averager is only there to minimise acceleration errors, which are not present when you’re stationary.  Once wound up, you can take instantaneous observations.

    Periscopic aircraft sextants are complex beasts, that probably cost many £1,000s at todays prices when new.  They required sending to a specialised company to periodically check and service them.  In the UK, it was ‘Fenns of Farnborough’.  I might be tempted to take a spare MkIX apart, but there’s no way I’d touch a moderately serviceable periscopic Mk2.  (I do have a seized Electro-magnetic periscopic Mk1 on my bucket list to have a go at.)  

    You’ve also got to remember that that these devices were never expected to be more accurate than 2’ or so of arc.  If you look at the diagram of the linkage up the tube to the index mirror, you’ll realise why.  2’ of arc is also small beer when compared to the possible acceleration errors at 480kts.  Basically, in this day and age, they’re ‘fun’ nostalgia toys for granddads, so enjoy.

    The best Air Publication section is a bit long for ‘NavList’ photography.  Paul Brewer will provide you with a very nice properly produced version via  eBay for £11.50 inc p&p. Periscopic Sextant RAF No. 2 Technical Handbook. | eBay DaveP

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