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    Re: Sextant scopes and an old-bold empiricist
    From: Gary LaPook
    Date: 2006 Apr 14, 02:24 -0700

    >| Now the sticky wicket--determining magnification. Problem at hand, no lab,
    >| no optical bench, no degree in optical engineering.  I need to take sextant
    >| scopes with no specs (except diameter of objective lens, and determine their
    >| respective magnifications.
    >| All I can figure is to focus all the scopes with a given eye, solidly tripod
    >| mount them, put shades (filters) in front of the objective lens to protect
    >| the (glue?) mounting of lenses/prisms from the heat (noting an astronomy
    >| caution that telescopes can take the heat, but binoculars cannot), and point
    >| at a known object (sun?).
    >| Then introduce a target (piece of white board) behind the exit pupil and
    >| move them to bring the sun's projected image into sharp focus (like viewing
    >| the sun through a telescope and projecting it onto a piece of paper/board).
    >| Measure and compare the diameter of the projected circles to the sun's SD.
    >| (Caveat here being that image size will change as the scopes focus--barrel
    >| extension--is changed. Hence common focus. Much easier if the magnification
    >| of one of the scopes is known as a reference.).
    Or, if you can dissassemble the telescope, you can find the focal length
    of the objective and of the eyepiece. Then the magnification is the
    focal length of the objective devided by the focal length of the eyepiece.
    To find the focal length of these lenses simply focus the image of the
    sun onto a piece of paper by adjusting the distance from the lense to
    the paper until the edge of the image of the disk of the sun has sharp
    edges as projected onto the paper. Measure the spacing between the lense
    and the paper at this point and you then know the focal length of the lense.
    Gary LaPook
    >I don't think that's the right way. A telescope is usually adjusted to take 
    in, and put out, parallel light, or nearly so. Correctly
    >adjusted for an observer who doesn't need specs for distant vision, that 
    would be exactly true. Adjusted for someone who requires
    >specs for distant vision, but wanted to use the telescope without wearing 
    those specs, then the light from the telescope eyepiece
    >would be convergent or divergent, as appropriate, but only slightly so.
    >What you need is to measure is the diameter of the pencil of parallel(ish) 
    light, output from the eyepiece, so DON'T try to focus it
    >down to a spot. Do without the shade. Just put a piece of tracing-paper (or 
    similar) CLOSE UP against the eyepiece, measure the size
    >of the resulting spot, and compare it with the size of the objective. The 
    ratio is the magnification. This assumes that there's
    >nothing except the objective that restricts the light path through the 
    instrument: no internal baffles getting in the way, and a big
    >enough eyepiece to let all the light through. That's usually a good assumption.
    >The brightness of the spot on the tracing paper, placed so, can be no 
    brighter than that of the incident sunlight, multiplied by the
    >square of the magnification. So a factor of 36 brighter if the telescope was 
    6x. That's without any dark shade intervening. It's
    >nothing like the brightness you can get with a burning-glass, a big 
    short-focus lens that converges the sunlight down to a tiny
    >spot. Even so, you may feel it's safer if you cover the objective, and just 
    flick the cover away for a fraction of a second. NEVER
    >look at the Sun (not through the telescope or even naked-eye) without a 
    proper filter, black as night. Look at the spot on the
    >tracing-paper from an angle, NOT directly along the axis of the telescope.
    >There's another way to get a rough figure for magnification. Look at 
    something like the strakes along a garden fence, with both eyes
    >open; one through the scope and one not. Estimate how many of one will fit 
    one of the other. I am assured it can be done, though not
    >by me, as since childhood I have a "lazy eye", that I've never bothered to 
    focus. Luckily, it's my left eye, not the sextant eye.
    >contact George Huxtable at george@huxtable.u-net.com
    >or at +44 1865 820222 (from UK, 01865 820222)
    >or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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