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?


3 Answers 3


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.


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").

This is hard to say. It depends on the features being observed, and comes down to the contrast transfer function of your personal visual system. At a certain point, there is so little light that your eye would have a hard time distinguishing contrast between pure black and pure white. But that can depend on the shape of the contrasting feature, and its relative size. There's no hard answer here. Some low contrast features of the Moon will become imperceptible at larger exit pupils than other very high contrast features.

What's the largest exit pupil at which the moon is bright enough, so a larger exit pupil doesn't help?

It will depend on how far your pupil constricts, which itself will depend on the total luminance of the Moon as seen through the eyepiece, which in itself depends on how much magnification is at your disposal and how wide the apparent field of view of the eyepiece is.

To simplify things, let's suppose the view of the Moon completely fills the field of view of a 100 degree apparent field eyepiece, presenting your eye with maximum light. It's as if the Moon spanned 100 degrees across the sky.

In such a situation, suppose your pupils constrict down to 1.5mm. What this means is that any exit pupils of 1.5mm and above will show the Moon at the same brightness, since your pupil is the limiting factor of how bright the Moon will appear.

But things can get complicated. Suppose you have a smaller telescope that cannot supply enough magnification to get the Moon to span a large apparent size on your retina. Suppose it's only 25x magnification, meaning the Moon only spans roughly 12.5 degrees of your retina rather than the full 100 degrees as before. This means there is quite a bit less light hitting your retina. So that might mean your pupil only constricts down to 2.5mm. At that point, any exit pupil 2.5mm and larger will show the same apparent surface brightness of the Moon.


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). $\endgroup$ Commented Jan 20, 2015 at 19:34

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