We’ve sold a lot of telescopes to beginning astronomers, and one question that gets asked almost every time is: “How far can I see with this telescope?’.
Sadly, this is not the right question to ask. Let me explain:
There are objects in the night sky that are a million light years away, such as the Andromeda Galaxy that are visible in even small telescopes. To put that in perspective, you can see an object that is over 6,750,000,000,000,000,000 miles from us and yet a small 60mm refractor telescope can view it given clear skies and low light pollution. The reason? Well, it is a galaxy, made up of over a trillion stars. It’s big. Real big, and fairly bright too, given how far away it is.
By Comparison, take the dwarf-planet Pluto, which despite its recent demotion is still a resident of our solar system. At its closest, Pluto is 648,000,000 miles from Earth (it varies – a lot) How visible is Pluto? Well look at the image of Pluto taken by one of our most powerful telescopes: the Hubble Space Telescope:
This standard applies in real life. If you ask someone ‘how far can you see’ when viewing on Earth you won’t get a real concrete answer as well. See what? Exactly. You could probably see a skyscraper from miles and miles away. But ask to read a street sign from two blocks away and you will be hard-pressed to even spot the sign, let alone read it.
A perspective on distance can be found in the excellent cartoon XKCD, which has a comic about distances here, using a logarithmic chart. We had this question put to us so often that we ended up buying the poster to explain to customers how the Universe is sized.
So the question that should be asked is not ‘how far can I see’ but rather ‘what can I see’ with the telescope.
In answer to that question we find that at a minimum a telescope, even a small one, should be able to view:
1) The Moon, with sharp details of the craters
2) The phases of Venus,
3) Mars, without much detail
4) Saturn and its rings
5) Jupiter, with a hint of its banding as well as the Galilean satellites
6) Neptune & Uranus, with only minimal details
7) The brighter Deep Sky objects, such as the Orion Nebula, Andromeda Galaxy & so on, assuming conditions
Now keep in mind when we say ‘view’, we mean view with some clarity, and some ability to recognize the object. Any cheap telescope can see Saturn, but if it just an amorphous blob through the cheap lenses what good is it? You should be able to identify that it is in fact a planet with rings around it. It might look like a cartoon of an eye, but it will obviously be Saturn.
Now of course, as you improve the quality of your telescope (namely, increase the aperture of the lens or mirror) the image of these objects will increase as well. Saturn will go from being that carton eye to actually having some more details, and eventually you might make out the Cassini Division (the major separation of Saturn’s rings). Jupiter might have more bands become visible. So on and so on.
So the real question(s) should be “What can I see and how well can I see it?”