A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2017 Aug 26, 16:59 -0700
This afternoon I decide to try and get my A12 working. I cleaned the index glass and the shades and generally dusted it off. After a struggle I was eventually able to remove the scale illumination battery holder and the bubble illumination battery holder. I got the scale illumination working well, but bubble illumination remained intermittent and difficult to control. I got the bubble chamber out, but both the upper and lower glasses were pretty opaque, and there was no bubble, probably because the chamber is empty. Without the proper Bristol key to turn the filler grub screw, this was as far as I could go.
Tonight was about 4/8 Cu, so I was getting intermittent glimpses of the stars. Using direct viewing was no problem. Basically, if you can see a star through a single glass window, you’ll see it through an A12. As Paul Hirose has said, the main problem is matching bubble chamber illumination to the stars intensity. My bubble chamber appears as an amber disc. The degree of illumination is controlled by turning the Bakelite knob, but after 70 plus years, my rheostat unit is decidedly iffy, despite a squirt of WD40. The brighter stars like Deneb will burn through all but the brightest amber disc, but to allow dimmer ones to burn through, the bubble chamber disc needs to be barely visible. With a recently renovated dimmer unit, better light control ought to be possible. I can’t say how easy it would have been to see the bubble at this low lighting intensity, because, as I said earlier, I don’t currently have one. I tried to test the A12 using reflected view, but without a bubble it was almost impossible, and I would expect that even with a bubble it would require a lot of practice at night.
One other thing I noticed using direct viewing was for star altitudes greater than about 55 degrees, you really need to be in a deck chair or something, because otherwise you end up with a very stiff neck. DaveP