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    Re: Can anyone tell me why my sextant has two sunshades?
    From: Bill B
    Date: 2006 Mar 10, 16:27 -0500

    > Can anyone tell me why my sextant has two sunshades?
    > I have a MAC sextant with four shades:
    > 1 'redish' , 1 'greenish' , and 2 that are so dark as to only
    > be for the sun - why two? , also what is the reason for the red
    > & green ones?
    Instead of "sunshades" think of them only as shades with different
    transmission indexes.  Most observations are sun shots, but not all.  There
    also are times you will need a shade for bright reflections on the horizon.
    Our goal, especially with the sun (for safety sake), is to have the body no
    brighter than it needs to be.  If you have been doing sun observations, you
    might have noticed that it is possible to observe through some cloud cover
    or haze, but the dark filter transmits too little light.
    If you tried observations with the moon or stars, or doing lunars (star,
    planet, or sun's distance from the moon) you may find it helpful to dim down
    the moon and/or a star/planet.
    As to red and green, I can only guess.  Note the colors are approx. 120d
    apart on the color wheel.  The affect of combining them is that the whole is
    greater that the parts. (If exactly opposite and equal in transmission they
    would create gray to black when combined.) Let's say the the red transmits
    1/4 of the light, and the green transmits 1/4 of the light. Together you
    might expect that they would transmit 1/16 of the light, while in fact they
    might transmit 1/64 or less. It can also be handy when observing two bodies
    (star-to-star or lunars) or the same body (IE with sun) to have the images
    different colors. When they overlap they form a third color, in this case
    Also note that the color of the filter can increase or reduce contrast
    depending on the color(s) of the objects being viewed.
    Regarding the darkest shades, my Astra has very dark index and horizon
    shades the I refer to as "thermonuclear."  At 40N and above I have never had
    the occasion to use them.  I surmise they may be useful in tropical regions.
    Hope that helps

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