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    Re: Observator Mark 4 Sextant
    From: R B Emerson
    Date: 2008 Aug 15, 12:47 -0400


    engineer wrote:
    [...]
    And Rick wrote:
    "Even if the filter is, in effect, fail-proof, the instrument itself
    will certainly heat up (after all, the Sun's busy emitting in the IR,
    too) and that poses a number problems.  Should the filters either burn
    through or the control inadvertently be flipped to "clear", the
    observer's eye is at risk."
    
    This may be so with astronomers' telescope with their large objectives
    and high magnifications and I certainly do not advocate deliberately
    looking at the sun through a telescope, but most of us at various
    times must have received an accidental blast of sun light over the top
    of the horizon mirror when taking a sun sight(some Heath and Tamaya
    sextants have blank shades to prevent this). I certainly have on a
    number of occasions and my optometrist has never said that I have
    retinal scars. The danger is there, but we blink or look away too
    rapidly for damage to occur.
      
    To clarify, I'm speaking of backyard observers (the ones more likely to misuse a Herschel wedge or get one with a very low-cost scope, whose objective might only be 70 mm).  As an aside, light-gathering isn't much of an issue with solar observing.  ;-)  Actually, even lots of magnification is usually a problem, as it exacerbates problems with atmospheric aberration.  Lower magnification doesn't make it go away, of course, but does make it less intrusive.  Further, my 10" Schmidt-Cass Meade is actually rather poor for solar observing because of the location of the secondary mirror in the middle of the correcting plate at the objective end of the scope.  Imagery from 70 and 90 mm simple scopes is often better. 
    As to heating effects within the optical system of a telescope I would
    expect most of the infra red to be absorbed by the glass of the
    mirrors and lenses and in any case, the light is not brought to a
    focus within the lenses or in the case of the Observator at the
    filters. If this were not so, I don't think Mr Wild would provide two
    eyepiece shades with his theodolites, nor sextant makers with their
    sextants, as there might be a risk that they would heat up and
    shatter. The telescope may well heat up but not, I suspect, very much.
    Bill Morris
    Pukenui
    New Zealand
      
    Compared to a telescope staring directly at the Sun, a sextant's scope certainly receives less energy, in part because observations don't take that long and because of the mirrors.  Nonetheless, filtering out IR before it hits the scope strikes me as more sensible than doing it in the scope and hoping the enclosure is an adequate heat sink/dissipater. 

    All in all, I suspect this explains why the Observator design wasn't attempted more often.  It like, for example, the C. Plath Navistar Professional, answers questions that aren't often asked. 

    Rick Emerson
    S/V One With The Wind
    On Aug 15, 9:06 am, "Richard B. Emerson" <pavil...@pinefields.com>
    wrote:
      
    Forget all that.  You're quite right in raising the the issues of allowing "raw" sunlight into the scope and questioning the light gathering for star shots.  While dimmer stars might still be visible, the horizon may pose a problem in that reduced light gathering will mean the horizon can become unusable sooner than with a brighter image.  But this is minor compared to the following...
    As to allowing unfiltered sunlight into the scope, in astronomical circles this generally a major no-no for solar observing.  While Herschel wedges can be used as light attenuators, if they fail, the observer will get a blast of concentrated sunlight (resulting in anything from corneal burns to retinal burns to permanent blindness).  Even if the filter is, in effect, fail-proof, the instrument itself will certainly heat up (after all, the Sun's busy emitting in the IR, too) and that poses a number problems.  Should the filters either burn through or the control inadvertently be flipped to "clear", the observer's eye is at risk.  Given this issue alone, concern over the filters' surfaces being parallel or not is an exercise in "rearranging deck chairs on the Titanic".  (For the record, I also question the reasoning that supports "the filters don't have to have parallel surfaces")
    Rick Emerson
    S/V One With The Wind
    All in all, this sextant strikes me as an interesting, but ultimately futile, attempt at a better sextant mouse trap. 
    George Huxtable wrote:A further thought about this "Observator" sextant has struck me. Bill Morris wrote, in [6123]- Members can read the original patent document athttp://v3.espacenet.com/origdoc?DB=EPODOC&IDX=EP0082556&F=0&QPN=EP0082556. Its claim to originality are that the filters are contained safely within the viewing means and that they can be made of cheap material like photographic film, as they do not have to have flat parallel faces, lying as they do behind the objective lens of a Galilean telescope. ============= Which brought this comment from me in [6129]- It's true that the two filters do indeed "lie behind the objective", but so far behind it that they are closely in front of the Galilean eyepiece. There, they sit side by side, the horizon shade to the left and the reflected-light shade to the right, controlled by separate adjusting knobs. ============= But is Bill's comment correct, that therefore "they do not have to have flat parallel faces"? I
    magine a thin wedge prism, being deliberately interposed into the optics, in just one side of the split viewline, just before the light enters the eyepiece lens. Wouldn't that displace the apparent direction of one of those images, by the deflection-angle of the prism, as seen through the eyepiece, and not the other? If that's correct, then the requirement for optical quality in the shade, in its new position, is no less than it is in a traditional sextant design. And if so, the suggestion that the accuracy of the instrument would not be degraded by the use of photographic film, instead of optically flat glass, is at least questionable. George. contact George Huxtable atgeorge@huxtable.u-net.comor at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
        
    
    
    
      

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