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>
>
>What I found, was that the unit has some serious 
>limitations when used at night. For one, the 
>lighting system on the index vernier scale 
>seriously sucks, at least as far as my 55 year 
>old eyes are concerned. So I replaced the old 
>filament bulb with a red LED drop in 
>replacement*, and ended up with something I 
>could actually read in the dark. But by far my 
>biggest gripe was that the backlight for the 
>bubble cell really doesn’t work at all. What 
>you want to see is a uniform luminous 
>background, preferably red, where the bubble 
>stands out as a nice black circle and the star 
>is clearly visible as a small white dot. What 
>Link provides is a far cry from that.
>
>Because I’m an engineer by trade, I am 
>genetically disinclined to leave well enough 
>alone � especially when “well enough” 
>doesn’tt really work in the first place. 
>Luckily, the lighting system in the A-12 is a 
>small flashlight-like affair that can easily be 
>removed. So I designed a small cap that fits 
>over the light bulb which does the trick. (See 
>the attached shot.) It backlights the bubble 
>cell with a uniform red glow. I’m having a 
>really hard time figuring out why the good folks 
>at Link didn’t think of this 70 years ago. Works like a charm.

Nicely done Paul, but there are a couple of 
points. First, there should a diffuser on the 
bubble unit which diffuses the light. Another 
light diffuser should not be necessary to achieve 
a uniform background light for the bubble - at 
least, this has never struck me as a problem. 
Second, the correct bulb for the unmodified 
bubble illuminator is the round type with a 
single uncoiled filament wire. That is important 
as you get a much more linear variation in light 
intensity as you vary the rheostat with a 
straight filament lamp than you will with a 
coiled filament lamp. That lensed bulb you have 
in your modified bubble illuminator unit is the 
bulb for the index scale illuminator. The round 
one with the straight filament may not have come 
with your octant, which may have contributed to 
your problems. They are probably impossible to 
find now, but that is the type of bulb that should be used.

Converting the scale and bubble illumination to 
red LED is a good idea in principle, though 
personally I have not found that the bubble 
illumination ruins my night sight and I don't use 
the scale illuminator. I use a red LED head lamp 
to read the scale, read my watch and write down 
the time and altitude. I made a variable red LED 
background illumination device for my theodolite 
and to make sure the illumination varied in a 
linear fashion with potentiometer rotation, I 
powered the LED from a two transistor 
multivibrator so that the time the LED was on for 
varied in a linear way with potentiometer 
rotation. A similar unit could be made to fit 
into the battery holder of the bubble illuminator on the A-12.

Geoffrey

   

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