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    Re: A12 vs A8-A sextants
    From: Geoffrey Kolbe
    Date: 2020 Apr 25, 11:16 -0700

    I followed this thread, but there did not seem to be any direct comparisons between the A12 and the A8-A.

    I have had A12s for +30 years and so am very familiar with them. I recently obtained an A8-A. So, I thought I would give my initial impressions of a comparison between the two types.

    The first thing to say is that though they are similar in terms of size, principle and operational layout, the cost of production for the A8-A must have been about ten times that of the A12. It is altogether a much more sophisticated instrument.

    The altitude readout on the A12 is directly off the index arc, using a vernier with 2' precision. If you are very careful you can read it to a minute, but you need to stare hard at it. By contrast, on the A8-A, the altitude is read out directly on a drum scale with a precision of 1'.

    Determining the index error on the A12 is not easy. It is best done at local noon when the sun hangs in the sky for some minutes and then taking a series of sights. Or, as I have done, you can create a 'virtual star' using a 2 metre cardboard tube with a 2m focal length lens at one end and a piece of tin foil at the other end. The tin foil has a pinhole in it and a light bulb behind it. This is hung from a couple of pieces of wire at some angle, which is determined with accuracy using a theodolite. Setting this up is a major event! Adjusting the A12 to remove or reduce the index error is hit-and-miss, a real trial, and never totally successful. 

    By contrast, the A8-A has a prism and horizon lens. By using the bubble-size pump, you can remove the bubble completely and use the A8-A like a marine sextant. There is a sun filter on the front of the horizon lens, so you can determine the index error as you would a marine sextant, and then adjusting out the error on the A8-A is a relatively straightforward procedure.

    And talking of the bubble-size pump, the bubble size on the A8-A is adjustable fairly easily and quickly from nothing at all to bigger than the bubble chamber and anything in between. This is not the case on the A12, which (usually) comes with two bubble chambers with bubbles adjusted for sun and star sightings. (I have always found that one bubble size served for both without any problem, so only one bubble chamber is needed, saving the trouble of carrying around a second loose bubble chamber.)

    I have found that the A8-A  does not seem to be as easy to use as the A12. The main reason is that on the A12, the index arm is adjusted using a wheel which rotates on a horizonatal axis. Since the sextant is held fairly solidly so that it is fixed about the horizontal axis, turning the wheel does not upset the sextant and move the bubble off the sun. On the A8-A, the index mirror is adjusted via a small, knurled 'thumbwheel', which rotates about a vertical axis. This tends to upset the sextant and so you have to be careful how you adjust the sextant so as to minimise movement. I guess it is something that is mastered with practice. On the other hand, the superior lenses mean that the bubble image is sharp and clear on the A8-A and there are a couple of vertical tramlines up through the bubble chamber to help you centre the image and bubble in the bubble chamber - despite the fact that the bubble chamber has a suitably curved surface so that it is 'tilt invarient' and it does not actually matter much which part of the bubble chamber you use.

    I have not yet used the A8-A at night for star sightings. The battery chamber(s) on the A12 are internal so that the batteries are inside the instrument. On the A8-A there is an external battery pack which is connected to the instrument via a length of cable, which is not as convenient. The A8-A also has a tiny bulb type that I would imagine would now be impossible to locate, whereas the A12 uses standard flashlight bulbs such that replacements can be sourced very easily.

    The A8-A is slightly smaller than the A12, but not so ergonomic as the A12. I will use the A8-A some more, particularly for star sightings, and report back. My impression so far is that though the A8-A is definitely a superior instrument, it is not so 'handy' as the A12 and will probably not give me more accurate results.

    I see from the manual that it was issued to the RAF. I wonder if David Pike has memories of it?

    Geoffrey Kolbe

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