NavList:

Hi Greg,
If my memory doesn't fail me, the A-7 bubble octant is an A-6 model with averager incorporated.
Like the A-6 the upper and lower glasses of the bubble cell enclosure are concave down ward, the radius of the upper glass curvature is equal to the focal length between the object and eye piece lenses. This allows you close to almost 8.5deg field of movement (the diameter of the field of observation in the bubble chamber is 9 deg), this means that, to be on the safe side, anywhere within one sun diameter from the side of the bubble chamber,the readings are accurate as long as the bubble and body are centered. It then boils down to a size of bubble that allows you sufficient room around the body for collimation without being to small to stick due to surface tension (e.g. the larger the bubble within the limit the better results you get).
Anywhere between 2 dia. to 3 dia. sun if fine as long as you feel comfortable with the aiming or in other words, as long as you have enough room around the body to judge alignment to the best of your ability.
If you can find pure grade Xylene (Xylol) it is one of the top liquid for filling being about 15% less dense than water and very stable to temperature changes.
My two cents.
Paolo
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