Chris Peterson wrote:
Nitpicker wrote:For the same magnification, a larger aperture telescope makes something easier to see than a smaller aperture telescope.
Not necessarily. At 14X, your eye will only be receiving the photons from four inches of aperture, so it doesn't matter how much larger the aperture gets. We have to consider not just aperture, but magnification as well.
Well, I already mentioned the exit pupil limitation, but you clipped it out:
Nitpicker wrote:Two scopes: 6"-f/10 and 12"-f/5, both have the same focal length of ~1500 mm, so both offer the same magnification and field of view, when used with identical eyepieces. But the exit pupil will be twice the diameter through the larger scope, and so long as the exit pupil is not larger than your dark-adapted pupil, more photons will reach your retina through the larger scope, and so it will appear brighter.
I don't know why you keep mentioning a magnification of 14X. To see the full extent of the Pelican Nebula, you need a field of view of about two moons, or one degree, or 60 arcmin. For sOnIc's 4", f/14 scope, that would require a 28 mm eyepiece with an apparent FOV of 50°, giving 50X magnification and an exit pupil of only 2 mm. But if we increase the aperture to 14" and keep the focal length at 1400 mm, thereby speeding it up to f/4, the same eyepiece will yield the same magnification and actual FOV, but with an exit pupil of 7 mm, which will appear a lot (more than ten times) brighter to the eye. (A healthy, dark-adapted, human pupil is typically said to have a diameter of 7 mm.)
Here is a table I've just whipped up, showing the numbers (and formulae) for a range of telescope apertures, all with a focal length of 1500 mm (like my 6", f/10 SCT), and setup to achieve an actual FOV of 60 arcmin. It shows that only scopes bigger than 14", give an exit pupil bigger than 7 mm. The effect of using a scope too large for this target, is to reduce the effective aperture and slow down the focal ratio, so as to limit the exit pupil to 7 mm.
(It is also in line with Visual_Astronomer's observations comparing the 20" and 12" scopes.)
Ultimately, the point I'm trying to make is that, whilst aperture is the most important factor, other factors are important too. And for the same magnification, bigger apertures do make things brighter to the eye (up to the exit pupil limitation).
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