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    Re: Eyesight dangers using telescopes
    From: Douglas Denny
    Date: 2009 Jun 29, 08:11 -0700

    Mr. Huxtable,  you do not convince me at all.
    Your pedantry astonishes me.
    You wrote:
    "There is confusion throughout Douglas' posting, between
    aperture-as-diameter, and aperture-as-area."
    I say : no there isn't.
    I have been careful to mention area as the the important parameter in this 
    discussion about light flux energy and its importance with viewing the Sun 
    through a telescope. I think it is quite clear throughout the debate that 
    whenever apertures are being discussed the property which is important is 
    It is you who has been concerned with the issue of exit pupil diameters of the 
    telscope and entry pupil diameter of the eye. For the sake of brevity I have 
    resisted challenging your 3mm criterion for the eye pupil which is a 
    complicated issue in its own right. 
    You wrote: 
    "Increasing magnification decreases luminance squared." I don't understand
    that statement, which appears to be written backwards. If Douglas had
    written instead "luminance decreases as the square of the increase in
    magnification", I would go along with that."
    You are "going along" with what I already wrote.  It is clear enough. (To me anyway).
    I wrote:
    "What happens with aperture changes with constant magnification?
    If a telescope is used of unity magnification, the maths says increase the
    aperture area and the luminance of the image increases. If aperture is
    doubled then L doubles.  If aperture is quadrupled, then L is quadrupled.
    Use a telescope of unity magnification but aperture area double that of the
    pupil diameter of say 3mm and the luminance must double to 2L."
    You wrote:
    "No. That's where Douglas has it wrong. He has assumed the impossible."
    I still believe the maths states the case clearly.  Luminance of the image is 
    directly proportional to aperture of the objective, (and inversely 
    proportional to square of magnification). I have already stated this a number 
    of times.  You seem to have had to have the aperture argument forced upon 
    you. It should be obvious in my opinion.  The magnifying lens burning paper 
    provides experimental proof that anyone can try.
    You wrote:
    "I think in a previous posting, Douglas eventually agreed with me (and with
    Johnson's book) that the ratio of entrance pupil diameter to exit pupil
    diameter of a telescope was equal to its magnification."
    I do not agree with this necessarly,  or Johnson's book.
    I agreed I made an error in quoting what was in fact the radius of the exit 
    pupil - a half-angle result from a ray diagram, forgetting the diameter is 
    twice this.
    I query your result from Johnson's book Re. magnification which I think might 
    be a simplification. Without the text before me from this book I cannot make 
    head nor tail of the quoted result you made.  Nor frankly  do I wish to, as I 
    ma satisfied with the result I calculated at 4.4 mm from a ray diagram based 
    on Longhurst's definition of exit pupil.
    You say 5 mm from Johnson. Immaterial anyway at 0.6 mm difference, and a 
    red-herring as I mentioned, as the angular subtense of the Sun (half a 
    degree) at the exit pupil in my X 10 binocs with seven and a half degrees 
    full field of view, is still much smaller than the 4.4 (or 5) mm diameter of 
    that exit pupil. 
    I deduced the exit pupil diameter from the definition in Longhurst's statement 
    that the exit pupil in an astronomical telescope is the image of the 
    objuective formed by the eyepiece... which indeed it is, if you draw the ray 
    you wrote:
    "But double that diameter, and the diameter of its exit pupil is also
    doubled. However, now most of the light in that larger 6mm exit diameter
    pencil of  now misses the hole in the iris, which remains at 3mm diameter."
    And this is where I think you are wrong.
    The exit pupil will, I agree, increase with increasing diameter of objective 
    as you say (and I am aware of that and have made it clear in my assertions 
    too) - but the limiting factor in this discussion about the Sun is the 
    angluar subtense of the Sun which is the limiting factor with whatever the 
    combination of magnification and aperture is, which in turn, gives half a 
    degree at the exit pupil of the telescope,  to give a diameter (if you 
    insist) of say 3 mm to allow full entry into the entry pupil of the eye 
    ......and this only if you want to work out the conbination of 
    aperture/magnification that gives a _full_ solar disc from objective aperture 
    to eyepiece exit pupil entering the eye.  You are IGNORING the danger still 
    exists for only a _fraction_ of the solar disc appearing focussed on the 
    fovea to cause serious damage.  (I have made that point before too).
    I wrote:
    "So it is clear in my mind, for brightness of image in a telescope there are
    two parameters working in opposition: aperture increasing directly and
    magnification decreasing and by a square function.""
    You wrote:
    "No, the possible objective area and the total input energy increase by the
    magnification squared, as explained"
    I say no to you again..........
    The "total input energy" presented to the telescope as a light wavefront per 
    square metre is constant.  The amount of that energy acccepted into the 
    telescope is a function of objective aperture area.  The amount appearing at 
    the exit pupil is an inverse square function of magnification.
    The amount of solar disc entering the eye depends then on the maximum angular 
    subtense allowable at the telescope exit pupil/eye pupil entry pupil due to 
    the _combination_ of aperture/magnification effects.
    I had indicated I was not wishing to get sucked into a never ending diatribe 
    about this issue.  This is likely to be my last submission. I think it is 
    quite clear now having been discussed in some detail.
    Finally though: As you are quite asserive to challenge this issue of Solar 
    energy entering the eye causing damage, I ask you directly: are you 
    suggesting that in any way it is accceptable to view the Sun through a 
    telescope?  Yes or no?
    Douglas Denny.
    Chichester. England.
    Navigation List archive: www.fer3.com/arc
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