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I'm looking to buy a telescope, and every mainstream website says that aperture is the most important factor, and buy the largest telescope your budget can afford. However, a website I found offering more practical advice stated that there were all too many large telescopes being sold by amateurs, listed as seldom used because of the difficulty in using them, and larger telescopes take on more light pollution anyways. But now that I value portability more,

I haven't been able to find a decent guide on how much the quality of view increases per aperture increase. Can anyone describe what I can expect to see through telescopes of different apertures? (For example, object X will become detailed at aperture Z.)

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    $\begingroup$ Where will you be observing from? My 250mm reflector shows me a lot more from the city than my 80mm refractor, but the refractor's easier to travel with and from a dark location it'll show me more than my reflector does in the city. Of course the best of both worlds is the large aperture telescope from a dark location, but those opportunities are fewer. $\endgroup$
    – Aaron F
    Jul 12 at 23:12
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    $\begingroup$ And a common saying is "the best telescope is the one you use" :-) $\endgroup$
    – Aaron F
    Jul 12 at 23:13
  • $\begingroup$ @AaronF Your 80mm refractor shows more than a 250mm reflector in the dark? $\endgroup$
    – Rafael
    Jul 13 at 2:25
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    $\begingroup$ @Rafael No, I think Aaron was saying the 80mm in the dark shows more than the 250mm in the city (and the 80mm is the only one that's practical to get into the dark). $\endgroup$
    – nanoman
    Jul 13 at 7:07
  • $\begingroup$ nanoman is correct. I wasn't very clear in what I wrote, so have tried to expand it into an answer, below $\endgroup$
    – Aaron F
    Jul 13 at 16:34
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In Observing Handbook and Catalogue of Deep-Sky Objects, Luginbuhl and Skiff describe how hundreds of deep-sky objects look in different apertures. For example, for galaxy M63 (NGC 5055) in Canes Venatici:

Messier 63 is an easy object for 6 cm, located 3'.5 E and a bit S of a mag. 8.5 star. It is elongated E-W, passing just S of the star. The broadly concentrated core has a stellar nucleus. 15 cm shows the halo 3' × 1'.5 in pa 110°, an oval galaxy with a stellar nucleus. In 25 cm the S side appears flattened. 30 cm reveals a granular core surrounding a stellar nucleus. The halo is distinctly more abrupt on the S flank with an overall size of 3'.5 × 1'.3.

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    $\begingroup$ Worth noting, as the authors of the book do, that most of their observations were conducted a location with a limiting magnitude of 6.2. What they don't say is this is better than most typical locations. If you live in a typical urban location, the brightness of the sky background will result in many of the objects listed as visible in a 60mm telescope may not be visible even with a 200mm scope. $\endgroup$
    – Dr Chuck
    Jul 13 at 10:12
  • $\begingroup$ Good answer! Sadly according to them it would seem that most objects aren't visible without the use of a 10 inch (25 cm) scope, and I can't really get more than 100mm (even used). Although I'm not in such a light-polluted area. $\endgroup$
    – Rafael
    Jul 13 at 16:21
  • $\begingroup$ @Rafael A 100 mm scope can show all the Messier objects! $\endgroup$
    – Mike G
    Jul 13 at 20:04
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Both statements are correct: aperture is the most important factor, and many large aperture telescopes are resold due to lack of use.

The brightness of any object you observe is determined by the aperture of the telescope ("light grasp"). With larger objects, such as planets, the aperture also determines the resolution: the amount of detail you're able to see. As you increase the magnification (by changing eyepieces) you'll notice the image will become dimmer, and after a certain point it will also become blurred and will lose detail (atmospheric disturbance, which changes from night to night and from minute to minute, also affects this).

The trick is to get the largest aperture telescope that you will actually use, and this is very personal:

  • How heavy a telescope can you physically lift and carry outside?
    For many people, as they get older they can't carry heavy things as easily as they once could. For this reason you see many amateur astronomers downsizing when they get into their 70s.

  • Where do you live?
    There's a big difference between opening a door and wheeling out a large telescope, and carrying a 35kg metal tube up and down several flights of stairs.

  • What are the skies like where you live?
    If I put my 250mm aperture telescope alongside my 80mm aperture telescope then the views in the larger aperture are a lot brighter, (and bigger, due to the longer focal length). But I can easily carry the 80mm up a mountain and away from light pollution, and in those conditions I'll see a lot more with it than my city-bound 250mm will show me.

  • Do you have a car?
    My 250mm reflector fits perfectly on the back seat of a car, and in two hours I can be far away from the light pollution; but I personally have to hire a car if I want to do that.

You'll need to weigh up all of these considerations, as well as your budget, and when you've done so the perfect telescope for you should become apparent.
When I performed this exercise before buying my first telescope it was clear to me that a 10-inch reflector on a Dobsonian mount was the right one for me: I live in a city, I have a wide balcony, and a limited budget.

And if you come to the conclusion that you have to get a telescope with a relatively small aperture, but you live under heavy light pollution, then don't be disheartened: instead look into "electronically assisted astronomy" (EAA) - using a small telescope, a sensitive astro-camera, and a computer, you'll be able to detect objects that your eyes alone would never be able to see.

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