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[MattJ]

I was looking at the moon last night using my ST80.  I used 40x and 80x, and I decided to try using the aperture mask, which consists of the lens cap but with a small opening in it.  The aperture is reduced to, I think, 42mm.  As expected, this reduced the false color on the moon quite a bit.  What surprised me was that the image seemed to me only slightly darker than the image with the full 80mm aperture.  Since the 42mm has only slightly more than a quarter of the light gathering area of the 80mm, I would have expected a much darker image.  

 

I was wondering if anyone had more insight on this.  Thanks!  

[triplemon]

That is as expected. The moon is really bright (about as bright as granite or other grey rocks in direct sunlight on earth) and your eyes do have plenty of ability to adapt to dimmer views than that. You're nowhere near even using your scotopic (night) vision.

 

Try the same with a much dimmer object - like a globular cluster or a double, like Alcor-Mizar, Albireo, Double-Double. Then the light loss becomes much more apparent.

Edited by triplemon, 12 September 2024 - 12:55 PM.