Can an astronomical telescope view objects on Earth?

  • I am new to telescopes and I'm planning to buy an astronomical telescope like the "Celestron AstroMaster 114 EQ Reflector". Just curious to know whether it can be used to view objects on Earth (e.g. traffic and people miles away from tall buildings). Telescopes are used to observe stars; they must have higher power than binoculars. Right?

    The power of a telescope lie not really in its ability to magnify, but rather in its ability to collect much light, enabling you to view very dim objects. Many nebulae and galaxies are actually not very small, they're just very dim. So be prepared that you may be disappointed when trying to view people from miles away. For instance, a person at the distance of two miles spans an angle of 1.8m / (2x1609m) = 2 arcmin = 1/16 the diameter of the full Moon.

    Do you mean that if a telescope that has a power to view the diameter of moon from its straw is turned towards a person on the Earth (2 miles away), the result view will be so much magnified that i will be able to see only 1/16 of that person?

    @user3633267 - no, it simply means an average person at 2 miles away is about 1/16 the size of the Moon - which is a very small size. Sure, a telescope would magnify it and would let you see more details, but see my observations below about astro scopes in general not being easy to use for terrestrial observations. Emphasis on "in general" - I had pretty good results with a small home-built dobsonian when I pointed it at the horizon, but YMMV.

  • Yes, but many (not all) telescopes made for astronomy will invert the image. Presenting the image upright is important for terrestrial instruments, but it doesn't matter for astronomy. Also, it usually requires fewer optical components to make an instrument that inverts the image.

    Also, terrestrial instruments are made to operate in an environment where there's plenty of ambient light (daylight), whereas many telescope designs (such as truss dobsonians) are built to operate in a dark environment and are usually hampered by excess ambient light.

    So, in a nutshell, yes, you could turn a telescope towards objects here on Earth, and it would work, but for best results and easiest operation just get some good binoculars.

    Finally, "power" is a much misunderstood characteristic of optical instruments. It is technically true that a large dobsonian could in theory outperform any pair of binoculars you could care to try, but in practice a large optical stack is difficult to swing around and is fickle with regard to collimation, thermal equilibrium, etc. If you're willing and able to put up with the operating difficulties, with the necessity to flip the image around, etc, then I guess that indeed a large dobsonian could surpass anyone's little binoculars in a terrestrial context.

    But very large magnification always reduces the field of view. Go ahead and crank magnification all the way up, and it's like looking through a straw. It's hard to locate anything. Astronomers have specific methods to deal with this difficulty, but for a terrestrial observation the narrow field of view is a liability, not an advantage.

    There's a reason why various optical instruments have evolved into different shapes and designs to operate in different environments. "Horses for courses", as they say.

  • Yes.
    Most consumer telescopes can be used for terrestrial viewing (or at least the ones I have used) and are often utilized for wildlife viewing or other uses for extreme telephoto requirements.

    The manual doesn't list the minimum focus distance (usually the information provided for photographic lens to describe how near they can focus), but the manual does include a short passage (pg 25) on terrestrial viewing.

    Terrestrial Photography

    Your telescope makes an excellent telephoto lens for terrestrial
    (land) photography. You can take images of various scenic views,
    wildlife, nature, and just about anything. You will have to experiment
    with focusing, speeds, etc. to get the best image desired. You can
    adapt your camera per the instructions at the top of this page.

    You can always make a scope focus on an object situated closer than usual by using drawtube extenders for the focuser - or, sometimes, by simply pulling the eyepiece 1 or 2cm out from its normal position in the focuser. Also, each eyepiece is different in terms of the position of its field stop. So the "minimum focus distance" is not really a characteristic of the telescope.

    What kind of telescope do you have?

  • Yes, there is no hard distinction between a telescope and a camera with a long focal-length lens.

    When it's stuck directly to a camera body, it's often referred to as a "mirror lens" because they still want to call it a "camera lens" but it's made from a mirror:

    But when it's stuck directly to a tripod and the camera body is stuck to it, we call it a telescope. You can point it at terrestrial or extraterrestrial things, it doesn't make a difference.

    Something to remember, no matter what you are looking at, extended objects like wildlife, barns, the Moon, resolved nebula, etc. get dimmer the more you magnify them or spread them out, whereas stars remain at roughly the same brightness, independent of magnification, until you go too high and they go out of focus. So While a large aperture telescope can show you stars that a thousand times dimmer or even ten thousand times dimmer than what you can see with your eye, if you try to use a telescope at high magnification to look at terrestrial objects at night, they can become really dim. This is why for a good line of binoculars, the higher the magnification, the larger the lenses become.

    From What's the largest aperture telescope sent beyond the Earth-Moon system?:

    "Mirror Lens"

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    Sources: top, bottom


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    "Telescope? Camera?"

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    above x2: image of a Dragonfly refractive array telescope [from][]. Image: P. Van Dokkum; R. Abraham; J. Brodie Images borrowed from What (actually) is the “ deprojected half-light radius” of this almost-all-dark-matter Galaxy?

  • An astronomical telescope can view objects on Earth if they’re far enough away to be focused, and if the telescope can be aimed at them. Most often, the targets will appear to be inverted, but you can get prisms to correct that.

    Space satellites cannot place on Earth they are around the Earth but they can see things on the ground less than a foot across.

  • For terrestrial use, avoid Newtonian reflectors. The eyepiece goes directly into the focuser, and you'll get an inverted (upside down) image. Fine for astronomy, a pain for terrestrial use.

    Other scope types - like refractors, Schmidt cassegrain telescopes (SCTs), and Maksutov cassegrain telescopes, use a diagonal mirror or prism between the focuser and the eyepiece. This flips the image vertically, so you end up with an image that's the right way up, but left/right reversed. This can often be OK for wildlife, but if you want correct reading text (with no reversal) you can get specialised alternative diagonals ("Amici prisms") for terrestrial use. They're not quite as good for astronomical use, but do give you a correct image for terrestrial use.

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Content dated before 7/24/2021 11:53 AM

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