A common question that new astronomers have about telescopes is how the aperture affects the view, particularly how much brighter it will make faint objects or how different apertures will perform in light pollution.

The typical answer is to describe how much more light gathering power one telescope has over another. For example, the mirror of an 8" telescope has a surface area of approximately 50 square inches. A 12" telescope has a surface area of approximately 113 square inches — 2.25x the surface area to collect photons with, meaning 2.25x the light gathering power. (More simply, you can just square the ratio of the apertures: (12 / 8)² = 2.25). Sure sounds like a lot, right?

While the math is correct, the issue is that it’s not very useful information. What does 2.25x more light really mean? Will objects be 2.25x brighter? Will they be easier to see? It turns out, the answer to that is more complex than it seems.

A Quick Summary

Let’s start off with a quick summary of the effects, and then get into the explanations for them.

The extra light gathering power of a telescope only directly affects the brightness of stars and star clusters. If a 12" telescope collects 2.25x more light than an 8" telescope, stars will be 2.25x brighter, or a little less than 1 magnitude brighter. This means globular clusters like M13 will appear significantly fuller and brighter. For extended objects like nebulae and galaxies, the extra light gathering power does NOT make these objects brighter relative to the sky that surrounds them, thus they don’t have any better contrast. If an object like M1 (Crab Nebula) looks washed out in an 8" telescope, it will be just as washed out in the 12" telescope. The only difference is it will be 1.5x larger in the 12" telescope at a given view brightness, and thus possibly look more detailed. Alternatively, the object will only be 2.25x brighter when viewed at the same magnification as the 8" telescope, but the contrast won’t be improved. The whole view will be brighter — light pollution and all, but the object of interest itself won’t necessarily stand out more.

Let’s look at why this is the case.

A Crash Course in Telescope Optics

Telescopes do two fundamental things: they collect more light than the human eye, and they magnify things. However, in the process of magnifying things, they end up diluting the light they collect by the same amount, so the net gain in perceived brightness for many objects is zero. The exception being stars, which are optical point sources and do not respond to magnification, thus their light does not dilute with magnification. But everything else does — the moon, planets, nebulae, galaxies, and even light pollution.

Consider a 12" telescope (304.8mm) vs a human eye dilated to 7mm. The telescope has a whopping 1,896x more light gathering power than the eye. (304.8 / 7)² = 1,896. However, for all that light to be utilized by the eye, it must fit within the ~7mm diameter of the eye’s pupil, else it is wasted. To do this, you need to use 43.54x magnification (more on this in a bit), which just so happens to spread the light out over 1,896x the area (43.54 / 1)² = 1,896.

This means the view through the eyepiece is not actually 1,896 times brighter. Rather, it is the same brightness as what your unaided eye can gather, it is just 43.54x larger. In fact, because no optical system perfectly reflects or transmits light, the brightest possible view of extended objects through a telescope is actually a bit dimmer than the naked eye.

Again, it’s important to note that this effect only applies to anything that can be magnified. Stars can’t because they are too small and too far away, but nebulae, solar system objects, and galaxies can be.

Exit Pupil Is The Limiting Factor

Let’s look more closely at this relationship between magnification, and getting light to “fit” into a certain pupil size.

It turns out that when you magnify something using an optical system like a telescope, binoculars, or microscope, you produce a virtual aperture known as an exit pupil. The exit pupil is most conveniently thought of as the diameter of the “beam” of light which exits the eyepiece, and enters our eye. It behaves exactly like the iris in a camera lens, and the iris in our eye. The smaller it is, the less light it lets through. The larger it is, the more light it lets through. It’s ultimately the exit pupil (combination of aperture and magnification) which determines how bright the view is, NOT the telescope aperture alone.

One formula for determining the size of the exit pupil is as follows:

Aperture (mm) / magnification = exit pupil (mm)

If we plug in the values from before, we can see how they relate:

304.8 mm / 43.54x = 7 mm exit pupil

From the formula we can see that if we increase aperture, or decrease magnification, we can increase exit pupil size. Let’s try it:

609.6 mm / 43.54x = 14 mm exit pupil

In the above example, we doubled our aperture from 12" to 24" while keeping magnification the same, and thus doubled the size of our exit pupil. Unfortunately, 14mm is far larger than a human pupil, so while the telescope is technically producing a very bright view, your eye can’t make use of all of it. At this magnification, the view of light pollution, nebulae, galaxies, the moon, and even planets in the 24" telescope is identical in brightness to the 12" telescope. Only stars and star clusters will be brighter!

This is a crucial point to understand — the useful pupil size range of our eyes (depending on our age) is from about 0.5mm to 7mm. Thus exit pupils must also fall within this range, regardless of the telescope size you’re using. While you can produce larger and smaller exit pupils in a given telescope with different eyepieces, it doesn’t matter, since our eye is the limiting factor.

When looked at this way, extra aperture of a telescope isn’t for increasing view brightness, it’s actually for increasing magnification at a given view brightness. Let’s compare an 8" telescope, a 12" telescope, and a 36" telescope at the same exit pupil, but different magnifications.

203.2 mm / 50.8x = 4 mm exit pupil 304.8 mm / 76.2x = 4 mm exit pupil 914.4 mm / 228.6x = 4 mm exit pupil

The view brightness between all three telescopes is identical because they each have a 4mm exit pupil. The difference is that the 12" telescope is producing 1.5x the magnification of the 8", and the 36" telescope is producing 4.5x the magnification of the 8". What’s interesting is that a 12" telescope is 1.5x the diameter of an 8" telescope, and the 36" is 4.5x the diameter of the 8". It’s not a coincidence that these aperture differences just so happen to be exactly the same as the magnification differences. Exit pupil, magnification, and aperture are all mathematically related.

Thus to best understand the difference between any two telescopes on any given diffuse object like a nebula or galaxy, you can think of a larger telescope as producing more magnification proportional to its aperture increase. A 10" telescope is 1.67x larger than a 6" telescope, and thus will produce an image 1.67x larger in size, at the same brightness level. A 100" telescope is 10x larger than a 10" telescope, and thus produces 10x the magnification at the same brightness level. Diffuse objects in that 100" telescope aren’t any brighter or more contrasty than the 10" telescope, they are simply larger. This increased size has important qualities which I will get into later.

Brightness

Let’s talk a little bit more about brightness and exit pupil.

Using the formula for exit pupil and magnification above, what if we kept the magnification constant between those telescopes to change the exit pupils?

203.2 mm / 150x = ~1.35 mm exit pupil 304.8 mm / 150x = ~2 mm exit pupil 914.4 mm / 150x = ~6 mm exit pupil

From those results, we can see that the 8" telescope produces a somewhat dim 1.35mm exit pupil at 150x, while the 36" telescope produces a really bright 6mm exit pupil at 150x. So does that brighter exit pupil mean faint diffuse objects will be easier to see? Not necessarily — it depends.

By increasing the view brightness with a larger exit pupil, we increased the brightness of both the light pollution, and the object, equally. This means the contrast of the object has not changed. However, the view is brighter, which means the eye-brain system has more signal to work with, and that extra signal may help it identify the faint diffuse object more easily.

There is also such a thing as too little exit pupil for a given object. Let’s use different apertures to illustrate this:

60 mm / 150x = ~0.4 mm exit pupil 203.2 mm / 150x = ~1.35 mm exit pupil 914.4 mm / 150x = ~6 mm exit pupil

From the above, we can see that dropping down to a 60mm aperture while maintaining 150x magnification gives us a super tiny 0.4mm exit pupil. Depending the size and contrast of the object, this may render the object invisible. Some objects like small, high-surface brightness planetary nebulae remain nicely visible at such small exit pupils. But low surface brightness / low contrast galaxies may become too dim or spread out for the eye to register them. Surface brightness is another concept I’ll go into more detail about in the next section.

In general though, the eye still works remarkably well on a wide variety of objects, even at small exit pupils. Larger exit pupils (brighter images), may help improve the signal to noise ratio of some objects and features, but it’s ultimately contrast that governs how well they stand out.

Contrast

Contrast — not brightness — is ultimately what makes us go “hmm, that looks faint and washed out” or “wow, that really stands out!”, and contrast is defined as the ratio of light from the object, to total amount of light being gathered by the telescope.

Remember, light pollution sits between us and the object we’re looking at, so any light we see is the combination of light pollution plus the object’s own light. This means that technically speaking, light pollution can never fully obscure an extended object. The place where the object is located will always be a tiny bit brighter than then surrounding light pollution.

However, the more light pollution there is, the lower the ratio of light coming from the object vs total light being gathered. Once this ratio gets too low, we can no longer perceive the object.

What’s important to note is that all extended objects have a measurement known as surface brightness. This is an inherent aspect of the object, and it cannot change with aperture. Even the brightness of the sky due to light pollution or moon light or day light has a surface brightness measurement.

The surface brightness is usually measured in magnitudes per square arc second (mpsas). The darkest skies on earth have a mpsas rating of about 22.0. A heavily light polluted area in the middle of a city might be as bright as 13.0 (in comparison, the surface brightness of the moon is around 4.0 mpsas, Jupiter is about 5.4 mpsas, and a clear blue daytime sky is about 3.0 mpsas).

A galaxy like Messier 51 has a mpsas rating of 21.7. This is almost as dim as the darkest skies on Earth, so how is it that we can still see it even from moderately light polluted skies? Again, it’s because the light is additive. It’s actually possible for an object to have a lower surface brightness than the light pollution in you area, and still be visible. Conventionally speaking, it can be about up to 5 magnitudes dimmer and still be detectable. So if you live in an area where light pollution clocks in at 18.0 mpsas, you can still see objects as dim as 23.0 mpsas (though this strongly depends on the nature of the object itself. Fuzzy objects like galaxies that tend to fade away and have no well defined edges are much harder to see when contrast is low, than well defined objects like many planetary nebulae). This is because there will still be a sufficient ratio of object light to total light — that is, there is still sufficient contrast.

Another important thing to understand is that a telescope cannot add contrast. It cannot selectively gather light from the object while ignoring light pollution. It gathers light from the object AND light pollution equally. This means that increasing aperture cannot increase contrast. However, if the object is a star, then increasing magnification can increase contrast. Magnification reduces exit pupil, which makes sky glow/light pollution dimmer. But because stars are point sources, and do not spread their light out with magnification, they stay the same brightness regardless of exit pupil.

Magnification

Earlier I had mentioned I would talk a little bit more about the effect that magnification has on dim objects. Magnification is the third concept to understanding the visibility of faint objects.

Here’s an experiment. If you live in an area where you are able to see Messier 51 through a telescope, try to see it with the naked eye. Chances are, you can’t.

But why not?

Is it too dim? From what we learned above, we know that if it’s bright and contrasty enough to be seen with a telescope, it must also be bright and contrasty enough to be seen with the naked eye.

Is it too small? Nope. It’s 7x11 arc minutes in size, making it about the size of some of the seas on the Moon. We can easily see those seas with the naked eye, so why not M51?

It turns out it’s a combination of both factors. The human eye can see detail as small as 1 arc minute, if the contrast is very high and it’s sufficiently bright. The dimmer it is, and the lower contrast it is, the larger it must be in order for our eyes to detect it. (Here is some technical reading on this subject if you’re interested. There is also visual detection calculator by Mel Bartels that might help interactively demonstrate the relationship between all of these concepts).

In practice, what this means is that if an object is seemingly too faint to see, but you think it’s still within the threshold of detectability with your given light pollution levels, try throwing high magnification at it. Make it big enough to detect it.

This also applies for smaller details inside of already large objects. If you can fairly easily see M51, but can’t see detail in the spiral arms, just try more magnification. Even though you’re making it dimmer by making it larger, your eyes can still work fairly effectively even at small exit pupils. The benefits of magnification more quickly outweigh the detriments of a smaller exit pupil.

There’s a limit to this though, at some point it does become counter-productive to keep using higher magnification against the object, and this is where larger aperture starts to make a difference. Larger aperture affords higher magnification at reasonable exit pupils.

Color

There’s another misconception that more aperture is needed in order to see color in objects, but the reality is that our individual genetics are the limiting factor.

Eyes have two basic types of photoreceptors — rods, and cones. Rods are what our eyes use to see in low levels of light — known as scotopic vision. Rods are achromatic, meaning they do not detect color. Cones are active in moderate to bright levels of light — known as photopic vision. Cones are chromatic, meaning they are what allows us to see color.

Mesopic vision is when both rods and cones are activated, such as during late twilight.

Thus in order to see color, the area of the retina illuminated by the object must enter at least mesopic vision, but ideally photopic vision. The threshold at which receptors in your eye move from scotopic (black and white) to mesopic or photopic (color) vision, depends on the individual.

However, to give your eyes the best chance of seeing color, you need to flood them with as much light as possible, and that usually means the larger the exit pupil, the better. Since we know that exit pupil is a function of both aperture and magnification (not just aperture) we can achieve large exit pupils in smaller telescopes, by using lower magnifications.

To achieve say, a 7mm exit pupil in a 6" telescope, you’d want to use 21.77x:

152.4 mm / 21.77x = 7mm mm exit pupil

That will produce the brightest possible view in that telescope, giving your eye the best chance of triggering mesopic vision. While 21.77x is not very high, and may not magnify a lot of smaller diffuse objects enough, it could be enough to show you color in larger objects like M42 (The Orion Nebula). As you go up in aperture, you will be able to add more magnification while maintaining that exit pupil, thus letting you potentially see color in broader range of object sizes.

However, light pollution is once again the real downer here. Any color you might be able to see will be quite subtle, and poor contrast as well as false color added by the light pollution will likely destroy any chance of seeing it. Getting out under really dark skies will give you the best chance of seeing color, regardless of your telescope’s aperture.

It should also be noted that there are plenty of bright planetary nebulae in the sky that show obvious bright blue and green color even in small apertures, and even at small exit pupils. Some examples are the Cat’s Eye Nebula (NGC 6543) and the Blue Snowball Nebula (NGC 7662).

Resolution

I’ve spent an exhaustive amount of time covering the effects of aperture on DSOs like nebulae, galaxies, and star clusters, but what about other objects like the planets, the moon, and double stars?

This is arguably where a telescope’s aperture really struts its stuff.

Larger aperture increases the telescope’s resolving power, which is what allows you to see finer and finer details in planets, and the moon. It also lets you split double stars that are very close together.

Even when magnification isn’t being pushed too hard, a larger aperture will appear to show more detail than a smaller aperture, even at equal magnification. The difference in detail that can be seen on Jupiter between say, a 4.5" telescope and an 8" telescope is astonishing. In the 8", the Moon explodes with fine textures that otherwise look flat in the smaller instrument.

However, just as light pollution is the enemy of contrast, atmospheric turbulence (“seeing conditions”) is the enemy of resolution. A larger aperture, even with perfectly figured optics, won’t reveal its potential if you live under turbulent skies. In some ways, a larger aperture becomes detrimental when the seeing conditions are not steady.

The reason for this is because bad turbulence means you have to drop down to lower magnifications to maintain sharpness. This ends up increasing the exit pupil size. But our eyes don’t work very well when our pupils are wide open. We seem to have the best visual acuity at between 1.5mm and 3mm pupils (this varies from individual to individual). So if the exit pupil becomes larger than this, we are not seeing the object as clearly as possible.

As an anecdote, there are some nights where I am limited to about 100x when viewing Jupiter. 100x in my 12" F/5 telescope means about a 3mm exit pupil. But 100x in my 8" F/10 telescope means about a 2mm exit pupil. Despite the smaller aperture, I find the view of Jupiter to be cleaner in the smaller telescope. The reason is because my eye tends to focus light from a 2mm exit pupil better than a 3mm exit pupil. Jupiter is so bright at 3mm that some of the more subtle cloud textures get washed out, but are still visible in the 8" at 2mm. The 100x magnification means I’m not really taking advantage of the higher resolving power of the larger aperture anyway.

If I’m looking at the Moon in poor seeing at 100x in both, then the exit pupil size becomes irrelevant because the Moon is so bright, my own pupil is constricted smaller than the exit pupils of either telescope. Meanwhile the low magnification and atmospheric turbulence is erasing the difference in resolving power between the two telescopes, so the view is basically the same in each despite a significant difference in aperture. It’s only on nights with exceptionally steady seeing that the advantages of the larger aperture really start to become apparent.

What this means is that while aperture is king when it comes to seeing fine details in the planets and the Moon, the atmosphere may be the limiting factor for you depending on where you live. If you’re considering a big telescope primarily to see as much detail as possible in the planets, you may want to start off with a smaller, cheaper telescope and evaluate the Pickering seeing conditions in your area for a few months to be sure your skies warrant a larger instrument.

Double Stars

Double star observing is greatly influenced by aperture since it depends on resolving power. The tighter the double stars are together, the more easily they are separated by larger apertures. However, sometimes more aperture isn’t always the answer. Larger aperture also increases star brightness, and due to various optical aberrations in most peoples’ eyes, that extra brightness can cause stars to look spikey and bloated unless you have very well corrected vision. The extra aperture also shrinks the Airy disk (the central diffraction peak of a star), which can, ironically, make the star appear less distinct to the eye. A smaller aperture producing larger Airy disks for stars, and also being slightly less affected by poor seeing, can actually produce a cleaner split of double stars that are within its resolving power.

For example, Epsilon Lyrae (the Double Double), is almost always easily split into four nice Airy disk diffraction patterns in my 90mm Maksutov Cassegrain. They are not too bright and don’t overwhelm my vision. Conversely, the look messy in my 12" and 15" scopes because they are so much brighter. While they are easy to split, the added brightness and messiness of the larger aperture requires more magnification to split them further apart, and atmospheric seeing conditions might not support such magnification, so they look blurry.

Double star splitting is very much a game of matching the aperture to the magnitude and angular separation of the stars. Different pairs of stars have different sweet spots for different instruments.

But it also depends on what your preference for the presentation is. Albireo in Cygnus looks glorious in big apertures — like two colored diamonds sparkling on a velvet background. In smaller apertures though, they look kind of bland by comparison. They are far enough apart that any aperture will resolve them, but the bigger the aperture, the more “jewel-like” they appear.

Recap

From the above, we have learned a few truths: