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From: Bill (no email)
Date: Fri Mar 10 2006 - 16:27:15 EST
> 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?
Greg
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
yellow.
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
Bill
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