How far you can see through a telescope largely depends on the telescope’s specifications. For example, the model, lens, and quality of the telescope.
Surprisingly, the naked eye can see the Andromeda Galaxy, which is 2.5 million light-years away. In reality, this Galaxy has a diameter of approximately 220,000 light-years (67 kpc), however, to the naked eye it would appear pretty tiny.
The further away an object is, the less light that reaches your eye. This means the image takes up a smaller space in your retina - the light-sensitive tissue at the back of your eye. Therefore, details are harder to see, and the image appears smaller.
A telescope allows you to get a better view of stars, planets, and entities, but how sharp an image you get depends on the type of telescope you’re using.
A standard 8-inch telescope allows you to see two billion light-years away, meaning you’d be able to view the brightest quasars present in the sky. The addition of an advanced gravitational lens allows you to view galaxies even further away.Different objects have different optical requirements. For the planets, double stars, the sun, and the moon, a smaller but higher quality 80-mm refractor is preferred to the larger 150-mm reflector. However, a 150-mm reflector allows you to get the best images of the deep sky objects.
How do telescopes work?
Most telescopes - and all large telescopes - work through the use of curved mirrors that gather and focus light from the night sky. Early telescopes used glass instead, but mirrors are lighter and smoother.
The mirrors or lenses in a telescope are called the “optics.” To see very dim objects in the night sky or entities that are very far away, the mirror or lens of the telescope has to be really big.
The bigger the mirror or lens, the more light the telescope can attract, and the light is then concentrated by the shape of the optics. So basically, the light is what we see when we look into a telescope.
The size of the telescope you’re using also impacts how far you’ll be able to see:
Small telescopes usually use a 70-80mm Newtonian reflector or a 50-80mm refractor. With these, you can expect a lower optical quality, which is reflected in the lower price of such a telescope.
A small-sized telescope will allow you to see planets such as Mercury, Mars, Jupiter, Saturn (but not its rings), Venus in certain phases, and the Moon.
As you’d expect, this telescope will provide you with moderate-quality images. They usually use a 150-mm Newtonian reflector, 80-100mm refractor, or 90-130mm Maksutov Cassegrain.
They usually have apertures of 140mm or more especially when deep sky objects are considered.
With a medium-sized telescope, you’ll be able to get a much better view of the moon, the major features of Mars, some details of Jupiter, and the rings of Saturn. You may be able to just about make out Neptune or Uranus but they’ll only appear as dots, and viewing conditions need to be optimal for these two.
These high-quality telescopes usually have a 200-300mm reflector, 150-mm refractor, or 200-250mm Schmidt Cassegrain. The aperture of these telescopes can reach up to about 200mm or more.
This size telescope will give you a good view of Mercury and the distinguishable phases of Venus, a perfect view of the moon and Mars, the major features and moons of Jupiter, as well as the occasional moons and visible rings of Saturn.
Uranus and Neptune however will still look more like dots, and Pluto won’t be visible.
In order to collect more light effectively, a bigger “eye” is needed to magnify distant objects. This increase in light will create a brighter image which can be magnified to take up more space on your retina.
The telescope’s bigger lens, known as the objective lens, collects more light from a distant object compared to your eye.
A focal point inside the telescope draws the light towards it, and from there, the smaller eyepiece lens collects the bright light from the focal point. This will be magnified to ensure more usage of your retina.
How much light the telescope collects depends on how big the objective lens is, as this plays a significant role in gathering and focusing light from the narrow sky region.
The eyepiece works similarly to how a magnifying glass works: it will magnify the light collected by the objective lens. However, the performance of the telescope will vary depending on the aperture or the size of the objective lens.
Location/ Light Pollution
The location you use your telescope in will significantly influence your view of the night sky.
Underneath a very dark sky with minimal light pollution, you’ll have a good view of the Milky Way and will see plenty of stars dotted across the sky.
With the aid of a 250-mm telescope, you’ll be able to view 5-7 thousand sky objects. This should provide you with a good level of detail, too. If you are in a dark area but can barely see the Milky Way, then it’s likely you’ll only be able to see small and less detailed images.
In an urban area with some light pollution, you should still be able to get a glimpse of some features of the night sky, but you should bear in mind that you won’t get as detailed a view as in darker areas.
In these areas, it’s best to focus on objects such as double stars, open clusters, and some bright planetary nebulae.
Ultimately, there is no one answer to how far a telescope can see, as this largely depends on the telescope you’re using, the diameter of the lens or mirror, and the location you’re viewing the night sky from.
All of these factors will influence the quality of the images, and how far your telescope can see.