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Earlier this evening I was looking at the Moon through my cheap toy telescope (x150 magnification) when I noticed a (rather mundane) optical effect I couldn't explain. The Sun had just dipped below the horizon and the sky was still a fairly bright shade of blue, and the Moon itself near zenith. But when I looked at the Moon through the aperture, the backdrop of the sky is notably blacker. My question is what part of the telescope causes this shift in the apparent color of the sky? The best explanation I could come up with is this

Possible Explanation: The objective lens is recessed by a small distance into a hallow cylinder, which reduces the amount of visible light being gathered. So when I look at the sky through the aperture, it's really the same color (wavelength) just dimmer, causing the apparent blackness.

However, while I'm somewhat confident the above is correct, it's disappointingly vague and qualitative in nature. I'm extremely rusty on my basic geometric optics and so I'm curious if there are other factors which could be understood on the basis of simple reflection and refraction, lenses and mirrors, etc. Can anyone help me refine my understanding of this very basic physical phenomenon?

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  • $\begingroup$ This is just a guess off the top of my head that may very well be shot down: because the moon is so much brighter through the telescope, the surrounding sky, due to a combination of physiological and subjective factors, appears darker. This effect might be a analogous to auditory masking $\endgroup$
    – Alfred Centauri
    Commented Nov 20, 2012 at 0:55
  • $\begingroup$ That was actually my first guess too, but easy enough to rule out with a telescope on hand. When I looked away from the moon the sky was the same shade of dark grey. $\endgroup$
    – David H
    Commented Nov 20, 2012 at 1:20
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    $\begingroup$ An interesting link : sky.velp.info/daystars.php . The videos are before sunset, and the sky is bluish. The effect you see must be as you explain, a small portion of the sky is enlarged by the telescope and the color is diluted. $\endgroup$
    – anna v
    Commented Nov 20, 2012 at 6:14
  • $\begingroup$ @anna v: The link you provided is much appreciated. It seems I wasn't so much lacking an understanding of optics, so much as I was lacking an understanding of telescopes. Cheers $\endgroup$
    – David H
    Commented Nov 20, 2012 at 7:49
  • $\begingroup$ @anna, while the sky appears enlarged, the area that light is coming out of (the eye-piece lens) is smaller than the collecting area (the objective). $\endgroup$
    – DilithiumMatrix
    Commented Nov 20, 2012 at 8:30

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Unless you have a filter on the lens, the color shouldn't change noticeably. I think your explanation is the correct one: the telescope simply doesn't transmit anywhere close to 100% of the light it's receiving. Note that with perfect optics, the sky would be brighter by approximately the ratio of the areas of the objective to that of the eye-piece. Because the moon is basically the only astronomical object this type of telescope is really effective for, I wouldn't be surprised if it was designed to dim the source---as the moon can be very bright, and could easily hurt the user's eye. (When observing the moon with larger telescopes, one generally uses a darkening 'moon-filter', for just this purpose).

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    $\begingroup$ A dimmer never would of occurred to me. Like I mentioned, it's an extremely cheap telescope meant for little kids and I was assuming that it wouldn't have parts that weren't necessary for basic functionality. But then, I didn't know the Moon was bright enough to hurt your eyes! In which case I guess a dimmer would be included for basic functionality. And you are correct that my telescope is pretty dismal for observing anything besides the Moon. I'm going to see if I can't track down the original packaging for confirmation. Thanks. $\endgroup$
    – David H
    Commented Nov 20, 2012 at 7:40
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In telescopic (binocular) observation brightness of observed extended surface (like sky, Moon's surface, etc.) strongly depends from applied magnification. Lower magnification makes observed surface brighter. Higher magnification makes surface darker.

Applying 150x on your my cheap toy telescope you must get much more dark image of sky surface than observed with naked eye.

How much darker - depends from size (diameter) of telescope exit pupil. Diameter of exit pupil can be calculated from telescope aperture (diameter of it lens or main mirror) divided on applied magnification.

E.g. in case scope with aperture 100 mm and magnification 50x we have diameter of exit pupil equal 100/50 = 2 mm. Compare to size of observer eye pupil (6-8 mm at night conditions). We see that observation through this telescope reduce useful area of observer pupil in 3 times (9 times in square!) and surface brightness shall be 9 times less then observing with naked eye.

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This observation is true: the brightness in a telescope is reduced whenever the exit pupil of the telescope (i.e. usually the aperture divided by magnification) is less than the aperture of your eye. In such a case, you lose all the rays that would normally come in the outer perimeter of your eye pupil and your retina receives less light than without the instrument - even if there is negligible absorption. This is very usual with high-magnification and/or cheap telescopes.

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