North America and Pelican Nebulae

[NoelC]

How big a telescope would you have to have to see this through the eyepiece?



-Noel

[stargazer_7000]

hi Noe,

this image is terrific! congratulations!
...the answer to your question is: I simply have no clue,...but I guess even the ELT would be too small,...*g*

best regards,
Dietmar

[Chris Peterson]

How big a telescope would you have to have to see this through the eyepiece?

To see it like that, no telescope would do. The eye has no color sensitivity to objects this dim (and big telescopes don't make them any brighter), and at these low light levels our visual acuity is poor.

Nevertheless, the 3.5° field is readily accessible to many ordinary amateur telescopes. This pair is quite visible with a 8" scope under dark skies, and truly impressive with a 16-20" scope.

[NoelC]

big telescopes don't make them any brighter

Chris, I beg to differ. If one could get this field of view in a huge telescope (which may not be possible), the objects would certainly be brighter than when seen in a small telescope.

Are you implying this large field of view - which implies a short focal length - isn't achieved in very large telescopes simply because they have higher focal lengths?

-Noel

[Chris Peterson]

Chris, I beg to differ. If one could get this field of view in a huge telescope (which may not be possible), the objects would certainly be brighter than when seen in a small telescope.

In general, no. A telescope can make an extended object appear dimmer than it would to the naked eye, but never brighter. That's why big telescopes pointed at the Moon don't blind us!

The brightness of an object on our retina is limited by the size of our pupil. Lets consider this wide field area with some specific examples. What magnification do we want? The field is 3.5°; if we view at 20 power, that means an apparent field of 70°, which is a reasonable value for most eyepiece designs. So lets use that.

1. A typical small refractor: 75mm aperture, 600mm focal length. To get 20 power we need a 30mm eyepiece, which is common. This combination will produce an exit pupil (the diameter of the bundle of rays coming out the eyepiece, aperture/power) of 3.75mm. This is actually smaller than a typical dilated pupil (usually given as 7mm, but gets smaller with age), so this scope is providing a dimmer view than you would get naked eye.

2. A typical small Newtonian: 150mm aperture, 1200mm focal length. This requires a 60mm eyepiece, which I've never seen, but which could in principle be made. The exit pupil here is 7.5mm, so this would be just about optimal for our eyes, and would provide the maximum possible brightness: the same as if we used no telescope at all.

3. A nice light bucket: 400mm aperture, 2000mm focal length. For this we need a 100mm eyepiece- even harder to find. The exit pupil is now up to 20mm. Sure, there's a lot of light in there, but we can only squeeze a 7mm zone into our eye. 88% of the light simply misses our retina, and the actual image brightness is just the same as with the small Newtonian. To take full advantage of this larger scope, we would need to increase the magnification until the exit pupil was reduced to 7mm. That would be 57X. That is, if we operated at 57X (using a 35mm eyepiece) we would squeeze all the light into our eye. But at 57X, the apparent size of the field would be 200°, and no eyepiece (or even the human eye) can pull that off.

BTW, this argument works the same for simply getting closer. If we were suspended in space just a few light years from these nebulas, they would appear no brighter than what we see on Earth with our eyes. Visually, we would see a dim gray area of fuzziness, something like the Milky Way.

In this discussion, I'm using "brightness" in its usual sense, meaning intensity per unit area. There is an effect of increased brightness when we enlarge an object on our retina, simply because of the way our eye/brain system interprets things. That's why we find it useful to actually magnify small objects, not because the true brightness increases.