## Re: Light collecting power and aperture.

This may be different for DSOs. Stars are always focused at a point but DSOs are surfaces so we need to consider the light flux. That flux is proportional to the square of the aperture D but inversely proportional to the square of the magnification M. That magnification is not always the same for telescopes of increasing aperture because eyepieces are usually maximally 30 mm in focal length.

So if the light flux of the human eye for a surface area is normalized to 1 and the binoculars are 7x50 for instance, then the light flux relative to the human eye will be (D/(7*M))**2 = (50/(7*7))**2 (**2 meaning squared, my Android has no other character for it). This is approximately 1.04.

If we assume a 30 mm eyepiece and a focal ratio of 5 for all telescopes of different aperture then all of them will have the same light flux. Namely, because for a fixed focal ratio the magnification is proportional to the aperture, (D/(7*M))**2 is the same for all so there will be no difference in light flux between them. If we take D=100 as an example then F=500 and the magnification is M=500/30=16.6. So (D/(7*M))**2 is about 0.74.

For all these scenarios the light flux on the retina is about the same. The difference is the magnification on the retina. From seeing M51 through a 24" Dob once I know that this still significant, I believe I saw color if I remember correctly. Obviously the math is completely different here.

I hope this makes sense. I don't have this from a book so who knows if I overlooked something please let me know.

PS At first when I wanted to post this I canceled because I thought this seems to contradict the experience of most observers. So I searched for surface brightness and found a CN discussion of 3 pages with exactly the same reasoning. I did not read the end of it but keep in mind the key assumption here is that we use a fixed 30 mm eyepiece with the telescope so the magnification is proportional to the aperture. It has to be the sensitivity of the human eye to larger objects that makes the difference.

... Henk. **Telescopes:** 6" Mak-Newt (Comet Hunter), ES ED127CF, ES ED80, Zhumell Z12, Coulter Odyssey 10, AT6RC, Venture RX-7, Celestron Skymaster 20x80, **Mounts and tripod:** Losmandy G11S, AVX, LXD55, Tiltall, **Cameras:** Fuji X-a1, Canon SX40, Xt, XSi, T6, ELPH 100HS, **DIY:** Dob and camera barndoor trackers,

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adapter, Dob with foldable base and Az/Alt setting circles, **Accessories:** SSAG, Plossls, Barlows, Telrad, laser collimators (Seben LK1, Z12, Howie Glatter), Cheshire, 2 Orion RACIs 8x50,

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