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    Some Link A12 Sextant Questions
    From: David Pike
    Date: 2018 Jan 21, 08:11 -0800

    Tracy you wrote:

     I've found that only the brightest night objects allow indirect viewing, the Moon, Venus and Jupiter. Mostly due to getting the bubble illumination low enough to see the reflected celestial object in the combined image. With some paitence, stars can be found in the backyard, but when trying to get a quick series of shots in the plane, it's been hard to find stars indirectly.  I had Enif dead ahead and low last week and even had a somewhat dificult time with a direct viewing sight on it, again, getting the bubble illumnation low enougth to see a fainter star, but then maybe it's my older eyes... 

    It was a clear last night for once, so I thought I’d try out the A12 on stars.  I could see Sirius easily directly and indirectly, and I could even see Polaris to the north, my most light polluted direction, but only using direct view.  Mine’s still got its WW2 filament bulbs, which I’ve been trying to replace, because they appeared so dim.  I realise now that that was exactly what was required.  You only need the slightest redness in the filament to view the dimmest stars, so apologies Mr Link.  The A12 is growing on me.  The little attaché case is so transportable, and the light path is so direct that you don’t have the attenuation of the greater numbers of 75 year old glasses and mirrors of the RAF MkIX series.  I have three questions of the experts however.

    The battery in bubble illumination chamber soon runs down even when not in use.  I was wondering if this has anything to do with the fact that modern alkaline batteries have all metal tops while the tops of WW2 carbon batteries were mainly pitch with only a brass cap at the very centre. Could the metal top of the battery be coming into contact with the alloy top of the container?  I was thinking of slipping a paper washer into the top of the container.

    I’ve been using Zippo lighter fluid in the bubble chamber, but my bubble only stops small for less than 24 hours (no, I’ve not lost the little ball).  Before starting a complete refit, I thought it might be worth using white spirit to fill the chamber instead.  This is considerably less volatile.  Comments please?

    The sextant appears to be over reading by about five degrees, which is a lot to carry as an instrument error.  Before I attack the index glass with a Bristol wrench, I wonder if there is any significance between this and the fact that one turn of the drum is also equal to five degrees.  It’s difficult in the dark to tell when you’re approaching the end of the scale.  Could the index glass have been accidentally driven against a clutch or over a tooth?  DaveP

       
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