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Dave

All will be revealed on the blog in the next week or so, including a view of the bubbles. There are no fiducial marks on the longitudinal vial and once the object is in coincidence with the bubble, it stays there within the range of the bubble, just as in most bubble sextants, in which the radius of curvature of the vial is matched to the focal length of the collimating lens. The fourth photo shows the objective lens of the telescope to which has been added a vertical slice of a positive lens to bring the bubbles into focus. I haven't measure the focal length of the combination or the sensitivity of the level, but I will. The equivalent of a pinhole is also provided, in the form of a narrow slit which, just like a pinhole, gives an increased depth of focus.

Bill Morris

Pukenui

New Zealand