I'm observing the full moon, from a major city, with heavy light pollution and dust.

The objective size is fixed, for this comparison, something between 4 and 6 inches. The question is about the tradeoff between magnification and dimness of the image. I'd like to know what range of exit pupils works best. Again, the goal is to observe the full moon.

To put it differently:

  • What's the smallest exit pupil at which the moon gets so dim that you can't take advantage of the increased magnification? ("Empty magnification").
  • What's the largest exit pupil at which the moon is bright enough, so a larger exit pupil doesn't help?

The only reference I could find is this, which claims that 1-2mm works best for the moon. Is that correct?


Light pollution does not matter for the Moon. Even transparency doesn't matter that much. What does matter is seeing, a.k.a. air turbulence.

It is very rare that an exit pupil smaller than 0.5 mm is useful for anything - perhaps for some tight double stars, but that's about it. So take that as a hard lower limit.

In terms of a "soft" limit, it depends. If seeing is excellent, if the optics are very good, if the instrument is in perfect collimation and at perfect thermal equilibrium, then you can push e.p. below 1 mm, especially for a small aperture like the range you mention.

If seeing is less than perfect, if optics are so-so, if the instrument is miscollimated or too hot, then the minimum usable e.p. goes up. Each one of these factors influences the observation, so it's hard to pin a number.

Bottom line is, use what works for you.

claims that 1-2mm works best for the moon. Is that correct?

2 mm seems too big for a small 4...6" aperture when observing the Moon. It might be okay if you want to capture the whole disk, but it seems large if you're interested in small details.

Again, it's all relative.

I have a 6" reflector with a self-made primary mirror of excellent quality. I always take care to collimate the scope, and to keep it at thermal equilibrium. I live in California, so seeing is often good. It's not uncommon for me to use less than 1 mm exit pupil when observing the Moon. I use 0.84 mm as a starting point, sometimes pushing it down as low as 0.6 mm.

If I use an e.p. of 1.1 mm for the Moon that means seeing is pretty bad - but in those cases I'd rather do something else, instead of being frustrated by bad seeing. But at that e.p. in a wide eyepiece (82 degrees apparent field) you see the whole Moon at once, which is always nice when you do a public demo (sidewalk astronomy).

For much larger apertures, the ideal exit pupils tend to be somewhat larger even when observing the Moon.

To predict seeing, go on the Clear Sky Chart site, find a location near your home, and look at the third row in the chart (labeled Seeing); when it's dark blue, seeing is good.

For the largest practical e.p., don't worry about it. The Moon is so bright, losing light is not something you should pay any attention to at all.

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Since the magnification is determined by focal length of the objective/focal length of eyepiece it is difficult to have an eye-piece with the same f ratio as the objective. Assuming pupil aprature 0.25" then the ideal eye-piece aperature will be 0.25" for most effective light transmission of the eye.A generally acceptable f8 would have a focal length of (0.25x8)=2". A 2" eye piece x 100 magnification would require an objective focal length of 200",for f8 this means 200/8 which is 25" diameter.This would give comparable brilliance at that magnification. Any deviation from the ideal means loss of resolution due to lens errors of manufacture,the combination of these errors result higher arc" resolution. You will find a lower magnification more useful as the lower mag system may effect finer arc",res and additional light to eye/optical system efficiency. There are no easy answers and sadley cost raises the barriers

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    $\begingroup$ That makes very little sense. f-ratio is not a useful parameter for eyepieces. The exit pupil is a characteristic of the whole instrument, it's not the "aperture" of the eyepiece (which, again, is not a thing). $\endgroup$ – Florin Andrei Jan 20 '15 at 19:34

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