APOD: The Sombrero Galaxy from Hale (2013 Jul 15)

[Chris Peterson]

Why have a 5mm exit pupil on binoculars
if 5mm is the maximum eye dilation at night?

5-6 mm is pretty typical for adults. Of course, binoculars are largely intended for daytime use, and the makers don't want the objectives to be any larger than necessary (because of added weight and cost), so many binoculars are designed with a 5mm exit pupil. Astronomical binoculars are usually designed for a 7mm exit pupil, optimized for somewhat younger users with fully dilated pupils.

Twice the retina area means twice the number of illuminated rods & cones for each eye.

Indeed, and this does provide some benefit in sensitivity, even though the brightness is unchanged. It's one of the main reasons for using higher magnifications with dim objects.

4 air-glass surfaces => Transmission = (0.96)⁴ = 85%

Which is, in fact, barely noticeable.

Galileo had telescopes with effective apertures larger than an inch?

Not that I know of. A one inch aperture (especially at 5X) makes seeing the Sombrero very easy. While a larger aperture supports more magnification, which means more of the retina illuminated, that isn't necessary at all for simple detection of the object.

[neufer]

[b][color=#0000FF]Flamsteed's Royal Greenwich Observatory mural arc with which he was first human to observe a magnitude 5.32 Uranus.[/color][/b]

---

A one inch aperture (especially at 5X) makes seeing the Sombrero very easy. While a larger aperture supports more magnification, which means more of the retina illuminated, that isn't necessary at all for simple detection of the object.

Given a 85% lens transmittance M104 would have been a fuzzy magnitude 6.8 object in Galileo's 1" telescope.

I have never heard of a magnitude 6.8 object being visible (easily or otherwise) with two good eyes much less one.

Mankind had thousands of years to observe a magnitude 5.32 Uranus with two good eyes but (apparently) they never did before the invention of the telescope.

<<John Flamsteed FRS (19 August 1646 – 31 December 1719) was an English astronomer and the first Astronomer Royal. He was responsible for several of the earliest recorded sightings of the planet Uranus, which he mistook for a star and catalogued as '34 Tauri'. The first of these was in December 1690, which remains the earliest known sighting of Uranus by an astronomer.>>

[Chris Peterson]

Given a 85% lens transmittance M104 would have been a fuzzy magnitude 6.8 object in Galileo's 1" telescope.

No, it wouldn't. You can't manipulate or interpret the magnitude of extended objects the same way you do that of stars. I've observed M51 (mag 8.4) with my naked eye. M101 (mag 7.8) is difficult even in binoculars. M104 (mag 9) is easy in binoculars- I've even seen it in my wife's 8x22s. The apparent magnitude of galaxies isn't very useful at all in determining how visible they will be to the eye or through a telescope, since the magnitude is integrated over the extent of the objects, and fails to consider the intensity gradient. Galaxies like M104, which have bright cores, are much easier to see.

Mankind had thousands of years to observe a magnitude 5.32 Uranus with two good eyes but (apparently) they never did before the invention of the telescope.

I think it is very unlikely that Uranus went unobserved visually. It simply wasn't recognized as a planet- not surprising given its dimness and very slow apparent motion against the background stars.

[Beyond]

All that inventing just to see a far away Uranus. Heck, they could have just used a mirror to see one close up.

[emc]

If you live here, U R A NUS

[Beyond]

[neufer]

Given a 85% lens transmittance M104 would have been a fuzzy magnitude 6.8 object in Galileo's 1" telescope.

No, it wouldn't. You can't manipulate or interpret the magnitude of extended objects the same way you do that of stars. I've observed M51 (mag 8.4) with my naked eye. M101 (mag 7.8) is difficult even in binoculars. M104 (mag 9) is easy in binoculars- I've even seen it in my wife's 8x22s. The apparent magnitude of galaxies isn't very useful at all in determining how visible they will be to the eye or through a telescope, since the magnitude is integrated over the extent of the objects, and fails to consider the intensity gradient. Galaxies like M104, which have bright cores, are much easier to see.

• You have exceptional eyesight and very dark clear skies

<<Theoretically, in a typical dark sky, the dark adapted human eye would see the about 5,600 stars brighter than +6m while in perfect dark sky conditions about 45,000 stars brighter than +8m might be visible.

The visibility of diffuse objects such as star clusters and galaxies is much more strongly affected by light pollution than the visibility of planets and stars. Under typical dark conditions only a few such objects are visible. These include the Pleiades, h/χ Persei, the Andromeda galaxy, the Carina Nebula, the Orion Nebula, Omega Centauri (magnitude 3.7), 47 Tucanae (magnitude 4.91) and the globular cluster M13 in Hercules (magnitude 5.2). The Triangulum Galaxy (M33) (magnitude 5.72) is a difficult averted vision object and only visible at all if it is higher than 50° in the sky. The globular clusters M 3 in Canes Venatici (magnitude 6.2) and M 92 in Hercules (magnitude 6.3) are also visble with the naked eye under such conditions.

Under really dark sky conditions, however, M33 (magnitude 5.72) is easy to see, even in direct vision. Many other Messier objects are also visible under such conditions. The most distant objects that have been seen by the naked eye are nearby bright galaxies such as Centaurus A (magnitude 6.84), Bode's Galaxy (magnitude 6.94), Sculptor Galaxy (magnitude 8), and Messier 83 (magnitude 7.54).

Five planets can be recognized as planets from earth with the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn. Under typical dark sky conditions Uranus (magnitude +5.8) can be seen as well with averted vision. During daylight only the Moon and Sun are obvious naked eye objects, but in many cases Venus can be spotted in daylight and in rarer cases Jupiter. Close to sunset and sunrise bright stars like Sirius or even Canopus can be spotted with the naked eye as long as one knows the exact position in which to look.>>

[Chris Peterson]

• You have exceptional eyesight and very dark clear skies :!:

I have pretty good corrected eyesight (although I know people with better astronomical acuity). And I do, indeed, have very dark skies, about as dark as they get.

I think the list of diffuse objects listed in the Wikipedia article is much too short. In fact, I've sat around at star parties with people trying for naked eye fuzzies, and with good knowledge of position and the use of averted vision, it's surprising how many objects can just be detected. There is no way that most of these would be observed casually or accidentally, however.