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    Re: Improving the Link A-12
    From: Paul Dolkas
    Date: 2012 Aug 24, 22:29 -0700

    That would explain a lot. Unfortunately, there was no diffuser on either of 
    the two bubble cells that came with the A-12, and I don't see one on the 
    parts list. But I'm not surprised that somebody didn't think of this before. 
    And there I was, thinking I was so clever...
    The red background really does help though. I got the idea from reading about 
    the sextant they used on the Apollo spacecraft. The illumination for the 
    reticule was a red lamp, so that it was easy to differentiate between it, and 
    any stars in the background. Also any defects in the glass would also show up 
    red, and the crew wouldn't mistakenly take a sighting on a scratch or a speck 
    of dust thinking it was a star. 
    You are also correct about which bulb goes where, now that I've gone back and 
    taken a second look at the parts drawing. Unfortunately, you are also right 
    about the lack of the correct bulb for the back light. There might have been 
    one when I got it, but (as one would expect for a $100 sextant) it was frozen 
    in place behind a severely  corroded WW11 vintage battery. There wasn't much 
    left of either after I was finished with the Bridgeport mill...
    Lightbulbs aside, I am still rather stoked on my idea of taking a sighting 
    with the star on the top and again on the bottom edge of the bubble. It 
    really does improve accuracy over trying to guess where the middle of the 
    bubble is. I've also used it to take sightings on a crescent moon, which is 
    something else I've had trouble doing accurately before. 
    Thanks - Paul
    >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.

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