What Can You Expect to See in a Telescope?

Buying a telescope is not cheap (as our research shows), so it makes good sense, especially if you’ve never used one before, to question what you’re able to see when you look through a telescope.

There’s a good chance you’ve been brought to the hobby with the lure of phenomenal images from magazines and websites like the one below. But, the reality is, that’s not the color or detail we can see when using a telescope with our eyes.

The Orion Nebula
The Orion Nebula (source)

The question remains then: what can you see when you look through a telescope?

In the rest of this article, we’ll answer that as thoroughly as we can. To get us started, use the summary details in the table below and, if you want more detail, keep reading (use the table of contents below to jump to the section you need).

What You Can See With Different Telescope Sizes

Telescope
Type & Size
What You’ll See Under a Dark Sky
3″ refractor

25x to 125x
Sun/Planets
· Sunspots but no detail (use a filter)
· Lunar craters to a four-mile diameter
· Phases of Venus and Mercury
· Mars as a disc
· Prominent cloud belts and Galilean moons of Jupiter
· Rings of Saturn but not Cassini division
· Uranus and Neptune as colored ‘stars’

Deep Sky Objects
· Double stars separated by 2 arcseconds
· Stars to magnitude 11
· Larger star clusters
· All but dimmest Messier objects
4″ refractor

4″-5″ reflector
or compound


Up to 250x
Sun/Planets
· Detail of sunspot structure
· Lunar rilles and craters to 3-mile detail
· Ice caps of Mars and major features during good oppositions
· Fainter bands on Jupiter with some details
· Galilean moon shadow transits
· Cassini division in Saturn’s rings + bright moons
· Uranus and Neptune seen as discs

Deep Sky Objects
· Double stars to 1.5″ separation
· Stars seen to magnitude 12
· All the Messier objects
· Dozens more NGC nebulae and galaxies
· Some detail is seen in the brightest nebulae
5″ refractor

6″ reflector/
compound

Up to 300x
Sun/Planets
· Lunar features down to two miles
· Spots on the surface of Venus (use blue filter)
· Mars features at poorer oppositions
· More detail in the cloud belts of Jupiter
· Surface shading differences on Saturn and its brightest moon Titan

Deep Sky Objects
· Double stars with one-arcsecond separations
· Stars to magnitude 13
· More stars resolved in globular clusters
· Structural elements of brighter galaxies
· Lots of detail in brighter nebulae
· Many more NGC and IC objects observable
6″ refractor

8″-9″ reflector/
compound

Up to 400x
Sun/Planets
· Features on the Moon just one mile across
· Clouds on Mars at favorable times
· Galilean moons shown as featureless discs
· Up to nine of Saturn’s moons visible
· Pluto observable as a small ‘star’
· Observation limited by seeing and not telescope

Deep Sky Objects
· Double stars less than 1″ apart can be split
· See stars to magnitude 14 or better
· Some globular clusters almost fully resolvable
· Lots of detail in nebulae
· More detail in galaxy structures
>10″
reflector or compound


Over 400x
(very rare due
to air turbulence)
Sun/Planets
· Lunar features less than one mile wide
· Significant surface detail on Mars
· Mars moons Phobos and Demos visible
· Encke division in Saturn’s rings
· Neptune’s moon Triton is visible
· Seeing conditions is limit to visible detail

Deep Sky Objects
· Split double stars as close as half an arcsecond
· See stars to magnitude 14.5
· Almost entire NGC and IC objects available
· Possible color in brighter nebulae

What Determines How Much We Can See?

It’s not just the telescope that you have that determines what you can see through it, although that is a huge factor, of course. There are several other ‘limiting factors’ that curb what’s visible to you on a cloudless night.

The Type and Size of Telescope

There are three basic types of telescope:

  • Refractors, which use lenses to focus light
  • Reflectors, which use mirrors to do the job
  • Compound (or catadioptric) which use a combination of lenses and mirrors

You’ve probably also heard of a Dobsonian telescope. A Dob is actually a kind of reflector, what makes it a Dobsonian is the kind of mount it sits on, not the telescope design.

Whatever type you go for, the simple rule is that more light will help you see fainter objects in more detail. Your first consideration for how much you should expect to see with a telescope is how large its aperture is.

After aperture size, each type of telescope plays to different strengths, but each one also requires a compromise of some sort. Let’s look at their respective strengths and weaknesses.

Refractor Telescopes

Lens-based telescopes are great if clarity and contrast are what you need. Unlike a reflector or compound telescope, there is no secondary mirror to reduce contrast. Whatsmore, because the light passes through just the lens and eyepiece before you see it, more of it makes it to your eye – up to 93% – so, inch for inch, we see brighter images in a refractor.

This makes refractors ideal for lunar, solar, and planetary observation where very small details can be picked out under high magnification. You’ll find that, especially in better quality models, a small refractor will reveal the same detail as a much larger reflector. Refractors are also the scope of choice for splitting double stars because of the clarity of contrast.

Mentioning quality brings us to the downside of refractor telescopes: good ones are expensive and large, good ones are very expensive. Lenses suffer from chromatic aberration, where colors are split and bright objects gain a colored halo. To combat this, manufacturers stick two (doublet) or three (triplet) high-quality lenses together, which bumps up the cost.

That high cost of manufacture also means they aren’t so good if galaxies and nebulae are your thing. These objects are so faint that you’re better off maximizing aperture and gathering as much light as possible. You’ll be hard-pushed to find (or afford) a refractor larger than 6″, but a 14″ Dobsonian reflector is still a relatively affordable proposition.

You can read more detail about telescopes here (opens a new tab), but the headline is cheap refractors are usually poor quality, and great refractors are usually expensive but should be your first choice for solar system detail or splitting double stars.

Reflector Telescopes

The sweet spot for reflectors is in the beginner market and for deep sky objects (DSOs), such as galaxies and nebulae.

They work well for beginners because they are relatively cheap and you get much more aperture for your money. Sure, the clarity is not quite as high as a refractor, but the difference is nowhere near enough to outweigh the cost difference when you’re just getting started.

Deep space work is the other area where reflectors excel because you get a huge aperture for a reasonable cost. When you want to see detail in galactic structures or tendrils within a nebula, what you need more than anything else is light-gathering power. That means a large aperture and a reflector is the cheapest way to get one of those.

As an example, you can pick up a 16″ truss Dobsonian with goto tracking from Orion telescopes for about $4000. Yes, that is a lot of money, but compare it to the 16″ compound telescopes on the same page… they retail at $17,000 to $21,000. You can literally buy four 16″ Dobsonian telescopes for the price of a 16″ compound!

Mars seen approaching the Trifid Nebula
Mars and the Trifid Nebula. Detail that is captured by a camera. (Courtesy Damian Peach/Chilescope)

So, what are the downsides when it comes to what you can expect to see with a reflector telescope?

The two big ones are contrast and coma. As we saw earlier, the secondary mirror in a reflector reduces contrast making finer details harder to observe. Mirrors are also prone to coma, which is where stars towards the edge of a field grow little tails, like the comma in a sentence. This is less of an issue in the center of the field and higher-end scopes.

The other thing to keep in mind with these scopes is bulk. They are larger and heavier than their refractor counterparts. Additional considerations are that their mirrors need collimating (link shows how to do this in a new tab) from time to time to keep them perfectly focused, and bigger models need cooling to get the mirror and tube to match ambient temperature for best seeing.

Catadioptric (Compound) Telescopes

These models take the benefits of both refractors and reflectors and mash them up into a single body. At the end collecting light is a lens (like a refractor) that focuses light on a mirror, like a reflector.

They are set up in such a way that their focal length is much longer than their body, so we achieve high levels of magnification with a larger aperture and a short body. This means that the largest models are still tripod-mounted and much less bulky to store and transport.

Pretty much all compound scopes come with motorized goto and tracking and where they have a double-arm mounting, they tend to be very secure and suffer less vibration.

Compound telescopes experience less coma and chromatic aberration when compared to reflectors and refractors respectively. They deliver large apertures, high contrast (but not as high as a pure refractor), and minimal distortion, making them a great all-rounder scope.

So, what’s the downside?

Well, price is it really. As you saw earlier, they are significantly more expensive than a reflector but you can buy them in sizes matching the largest commercial Dobsonian designs… for four times the cost!

There is no limit to what you can see with a compound scope with a large enough aperture. If you have the money and want the ability to enjoy all kinds of objects, then you should definitely consider one.

Magnification

It’s hard for us to stress this enough: magnification is not the most important factor when working out what you’ll be able to see with your telescope!

Small scopes can deliver high magnification (which is calculated by dividing the focal length of your telescope by that of your eyepiece) but high magnification is completely useless without lots of light.

To get lots of light, you need a big aperture, so large magnifications don’t work at all on small telescopes. A magnified image is ‘spread out’ and, if there’s not much light to begin with, spreading it out makes the image dim, low contrast, and unrecognizable. Read more about how telescopes work (opens a new tab).

Fortunately, most of the time we astronomers rarely need more than 100x magnification to get stunning views. We only need more when we’re looking at bright objects, like the Moon and planets, or we have a bigger telescope that can handle it.

Seeing Quality

When astronomers talk about ‘seeing’ they are referring to the quality of the air they’re looking through.

Turbulent air moves around and the currents are picked up and magnified by a telescope make images more blurry, or move in and out of focus with each passing second. This is more pronounced under higher magnification, i.e. the more detail you want to see, the more turbulent your seeing becomes.

Poor seeing is also more likely in a larger telescope. Big optical tubes contain a lot of air which takes time to cool to ambient temperature, as do large, bulky mirrors in a big reflector. Both of these are warmer than the surrounding air, causing mini thermals (air currents) in the tube which diminishes seeing quality.

When you own a large telescope, the detail it can see is ultimately determined by seeing conditions. Your scope may technically deliver 600x useful magnification, but Earth’s atmosphere will rarely let you push it beyond 400x because the image you see will be too disrupted.

Read more about good seeing and how to achieve it (opens a new tab).

Dark Skies

How dark your sky is plays a massive role in what you’re able to see with a telescope.

The darker your sky, the more objects you will see, and the more detail you’ll observe. Conversely, if, like so many of us, you live in a light-polluted area, there will be dimmer objects unavailable to you, and brighter objects reveal less detail.

As an example, you might be able to see over 7,000 deep space objects under a dark sky, which reduces to below 1,000 under light-polluted skies. Read more about light pollution’s effect on astronomy (opens a new tab).

As we looked at in our article on the number of stars you can see, light pollution also significantly reduces the number visible to just 400 under city lighting conditions, compared to more than 4,000 under a dark sky.

Your Telescope Experience

It may sound odd, but you will get better at seeing objects and details when you practice looking. Inexperienced observers note much less intricacy in a view than a seasoned skywatcher.

The simplest route to improving what you see is to practice averted vision. This is where you look at an object out of the side of your eye, using peripheral vision, rather than directly at it.

This works because the biology of your eye means the central area is rich in ‘cones’, which are our color-sensing cells, whereas the periphery of our view is saturated with ‘rods’ which work best in low light.

Pretty much all eyepiece viewing is ‘low light’ so we’re better at perceiving detail when we let the specialist rod cells do the heavy lifting, hence, averted vision.

The second route to seeing more detail is one of practice and repetition. The more you observe an object, the more detail you’ll see on it, especially if you observe on successive nights.

This works because you get used to seeing certain aspects, so your brain is not working hard to identify them anymore, leaving it free to pick out more nuanced detail that it hadn’t previously noticed.

When you read of a backyard astronomer seeing an object with his 4″ Newtonian that you’ve not seen in your 6″ compound, this is often the reason why. They’ve just learned to perceive more detail than you have.

Surface Brightness

We’ve written extensively about magnitude (see the main article here), which is the measure of how bright an object is. However, this doesn’t tell you the whole story when it comes to what you can see in a telescope because surface brightness is a more important measure for large diffuse objects like nebulae and galaxies.

Galaxies and nebulae are given a magnitude measurement based on their total light output, whereas surface brightness measures that magnitude per unit of surface area.

This means that when light is spread over a larger area we have a lower surface brightness than if the same amount of light was spread over a smaller area.

When it comes to what galaxies you can see with a telescope, you’ll see ones with higher surface brightness before ones with lower surface brightness, even if the two have the same magnitude.

As an example, M31, Andromeda Galaxy (link gives a guide to seeing it in a new tab) has a bright magnitude but covers a huge area, so has a low surface brightness. By comparison, M57, the Ring Nebula, has a lower magnitude but is much smaller, so it has a higher surface brightness making it much easier to see.

Telescope Filters

The final item that impacts what detail you can see with a telescope is filters.

These are special attachments that, as the name suggests, filter out light that we might not want and only let through the light we do.

Different filters do different jobs. Solar and Lunar filters block out light so we’re not dazzled (or blinded) when we look at these bright objects with our telescope.

Light pollution filters block out certain wavelengths of light, such as that produced by sodium street lighting, to make images clearer under light-polluted conditions.

Colored filters are used to bring out contrast on a planet’s surface, such as Jupiter’s Great Red Spot (link shows how to see it), the Martian ice caps, or the hexagon of Saturn.

The final type is a filter for deep-sky objects, particularly nebulae, to emphasize the detail with their gas clouds. These narrowband or line emission filters are very precisely tuned to only let light from the nebula through, delivering superb contrast.

What You Should Expect to See in a Telescope

Now you understand what limits or improves the detail you can see in a telescope, let’s take a look at the different kinds of objects and how they are viewed with different telescope sizes.

The Sun

You should only ever observe the Sun with extreme caution, as we set out in this article (opens a new tab). It is so bright that looking at it with the naked eye will cause damage. If you look at it directly with binoculars or a telescope you will blind yourself, and ruin your equipment.

Use proper solar filters or specially designed solar telescopes to observe our nearest star directly. Filters can be relatively cheap and mounted over the aperture of any telescope. Specially designed, more expensive H-alpha filters produce the best images and details.

The Sun through a hydrogen-alpha filter
Surface features of the Sun, seen through a hydrogen-alpha filter (source)

You will see sunspots in even the smallest scopes or astronomy binoculars (these are our favorite). Larger scopes reveal more finessed features, such as the shape of the sunspots, graduated shading on their surface, as well as spicules and granules caused by convection cells on its surface.

Even though the Sun is the brightest object available to us, you will get the best results observing it with a high-quality refractor, e.g. a doublet (like this SkyWatcher – opens a new tab) or triplet (like this Orion APO model), because you’ll get better resolving power than is available in a large refractor in the daytime air turbulence

The Moon

The biggest and brightest object in the night sky is a perennial favorite for new and experienced astronomers alike.

Even a small kids’ telescope, like these, will reveal more craters and other surface features than you might suspect.

The lunar surface is the gift that keeps on giving. No matter how big your telescope or how much experience you have, there is always more detail to be teased out. The late British astronomer Sir Patrick Moore, was known for his love of studying the Moon.

Waning Gibbous Moon
Waning Gibbous Moon with a typical crater view (source)

However, no matter how fantastic your setup is, you’re not going to see any evidence of the Moon landings. The best, largest telescope on Earth can only get to a resolution of 0.4 arcseconds, which translates to 500 meters on the Moon’s surface, far larger than any equipment or markings left by humans.

It is a very bright object though, so looking at it when it’s in its gibbous phases requires a lunar filter for more comfortable study.

The Planets

This is the most common area of interest amongst new backyard astronomers, which is why we’ve picked our favorite telescopes for seeing them (opens a new tab).

The other seven major bodies in our solar system are a mixed bunch, from fast, featureless, and hard to see Mercury, to huge, steady, and ringed Saturn.

The amount of detail they have to see varies enormously between them, Venus has next to no features, while Jupiter is rich in them. Mars is kind of between the two, it has lots of surface features, but they’re only visible every two years or so when it reaches its closest approach to Earth.

As you’ll see in the table at the top of this page, the more aperture your telescope has, the more detail you’ll be able to see. And, as we noted earlier, if your passion is surface detail, then a quality refractor will reveal the most intricate features.

Stars

These bright objects may be giants (there’s a huge array of sizes) but they are so far away that even the largest backyard telescope can only show them as points of light.

All telescopes perform a kind of magic when it comes to stars. Even the smallest, cheapest telescopes reveal many more stars than you can possibly see with the naked eye. All scopes, but especially larger ones, will pull in enough light to distinguish star colors too. This article explains star colors, in a new tab.

Bigger scopes reveal progressively fainter stars. Unaided, your eyes can see stars to about magnitude 6, of which there are about 9,000. A small telescope will reveal stars to magnitude 11 (under a dark sky), and there are about 1.8 million of those!

But, we’re nowhere near done. If you have a still-modest telescope with six inches of aperture, you can now see over 15 million stars brighter than magnitude 11. Beyond that, the numbers get, well… stellar (haha). A 12″ scope will reveal 45 million stars below magnitude 14 (as the table on this page reveals).

It takes more and more inches to see to dimmer and dimmer magnitudes. While six inches reveals magnitude 13, it takes 12 inches to get to magnitude 14, and 20 inches to get to magnitude 15. However, once you get there, you’ll have 130 million stars in your grasp.

The bigger consideration for telescope choice when it comes to stars is how interested you are in splitting doubles or seeing the details in star clusters. The clarity afforded in a good quality refractor will do a great job of this for much less aperture than a reflector.

M13 Great Globular Cluster
M13 Great Globular Cluster with resolved stars (source)

Smaller telescopes show globular clusters of stars as a typical ‘faint fuzzy’, i.e. a patch of indistinguishable light, whereas a large, ‘light-bucket’ compound scope works wonders on resolving extremely close doubles and the stars inside a globular cluster. Although, you still won’t get views like that above, which comes from the Hubble Space Telescope.

In some cases, small is beautiful. Larger star clusters, like the Pleiades (learn how to see the Pleiades) look their best under low magnification. This is because you get a wider field of view at low magnification, so you can enjoy the beauty of the whole cluster which is otherwise lost at higher magnifications.

There are hundreds of thousands of binary stars above our heads, and a small scope will split the wider ones. A four-inch refractor will reveal two stars as separate when they are more than 1.5 arcseconds apart. This gives us plenty to hunt for and larger scopes will split closer pairs.

Nebulae

Nebulae are clouds of gas, illuminated by stars, that reside in our galaxy. They are usually faint, although there is a handful that are bright enough to be seen with the naked eye under very dark skies.

Despite the pictures you’ve seen, we only ever see nebulae in black and white through a telescope because they don’t produce enough light to stimulate the color receptors in our eyes.

Small telescopes do a good job of revealing the blur of a bright nebula, such as the Ring Nebula or Orion Nebula, but larger scopes will help you see texture such as tendrils and dust lanes, within the structure, especially when coupled with an appropriate filter.

Faint objects often look best at low magnification, even in larger telescopes which gather more light and reveal more detail in their structure. Use the light-gathering power of a large telescope and keep the glory of the image by not over-magnifying it.

Galaxies

Galaxies are unimaginably huge and can contain hundreds of billions of stars, but they are also an inconceivable distance away from us, which means that ultimately they all look small and faint in a backyard telescope.

Smaller scopes reveal the brightest galaxies, like Andromeda, but only as a ghostly smudge against the background of stars

Larger scopes show arms and dust lanes, but no model is large enough to tease out individual stars at this distance. Larger scopes will also show you smaller, fainter galaxies.

If you think galaxies will be your object of choice, then you need to invest in a large-aperture telescope, like one of the models in our best DSO telescope lineup (opens a new tab).

Comets and Asteroids

These are two kinds of space objects that fly through our sky from time to time. We can see them move against the background of stars over hours or days, depending on how far away from us they are.

Comets can reveal some stunning detail as they approach the Sun and develop a long, impressive cloud-like tail as the icy material they’re made of melts and is shed in the opposite direction from the direction of travel.

Asteroids, on the other hand, are lumps of rock and always appear as ‘stars’ in your eyepiece, picked out because they move relative to the real stars.

Summary

What you can see in a telescope is determined by many things. When it comes to the telescope itself, its aperture size is the most important factor, but the type also has an impact on the detail you’ll see.

Outside of the telescope, you need to get dark skies, great seeing, and have lots of practice to maximize the number of objects you can see and the quality of their appearance.


Written by Adam Kirk