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Let's assume there are three types of telescopes with the following specifications:

    Telescope                  Aperture              Focal Length
    --------------------------------------------------------------
    Example 1                     70 mm                    400 mm
    Example 2                     60 mm                    700 mm
    Example 3                     60 mm                    900 mm

As a beginner, should I look for a higher value in the aperture or the focal length?

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3 Answers 3

up vote 8 down vote accepted

There is one rule that is generally true for all deep sky objects (nebulae, stars, galaxies,...): Aperture matters!

For solar system objects, aperture is not that important.

The second most important thing is: What size are the objects you want to look at: Small objects need long focal lengths and high magnifications, large objects need short aperture for low magnifications.

With 400mm you could watch objects like:

  • Andromeda galaxy core
  • Orion nebula, other large emission or reflective nebulae (e.g. Pleiades)
  • large star clusters
  • low magnification lunar observations

With 900mm you could watch objects like

  • Planets (Saturn, Jupiter, Mars, ...)
  • high magnification lunar observations
  • planetary nebulae (e.g. ring nebula)

Note that 60 and 70mm aperture are still very small for telescopes! The aperture influences two things:

  • Light sensitvity: The larger the aperture, the more light you can collect. Very important if you live in a city!
  • Maximum resolution: Rule of thumb is that you can do aperture in mm times two as maximum magnification. I.e. for 60mm a 120x magnification is the absolute maximum which still is feasible.

The magnification is created by the eyepiece. E.g. when you have a 400mm focal length telescope and use a 10mm eyepiece, you get 400mm/10mm = 40x magnification.

Note: the shorter the eyepiece focal length, the more difficult it is to build. Good 5mm eyepieces can cost 100 USD and up. I personally started with a 750mm Newtonian with 150mm aperture and 25mm and 10mm eyepieces. That's a good allrounder, even though planets will appear rather small with the 10mm eyepiece. But you can later invest more money in good eyepieces, which you can re-use on better telescopes which you may buy later on.

Edit: One more thing -- the telescope mount is equally important as are the eyepieces and the telescope itself. A mount that fits the telescope easily is as expensive as the optical tube assembly itself. Hence many beginners start out with a Dobson telescope, which uses a very, very simple yet sturdy mount.

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It depends on what you plan on using it for. For dimmer, deep sky objects you should be concerned with the balance of aperture and focal length (with a focus on aperture for light gathering power). For brighter objects, like the Moon or the solar system planets you probably don't need much light gathering power (aperture) so a long focal length for imaging finer details should be ok. Just a reminder though: never look at the Sun without proper filters, you'll go blind instantly, and looking at the Moon without filters will wreck your night vision and may hurt at first.

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+1 on the Moon tip. I once accidentally pointed my scope at part of the moon and had a shadowy streak on my vision for some days. Good neutrally grey polarization filters that can be tuned from 50%-95% absorption can be had for around 50 EUR (for 1.25" eyepieces). –  Arne Oct 10 '13 at 7:00

Right or wrong, myself as a beginner I started my decision based on aperture to get the appropriate light gathering ability for my area. At that point focal length became pretty much a non-issue. My choices were down to 2, and 1 of them would have been long enough that getting it outside easily would have been difficult.

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Could you add some more detail which might be more helpful to future people to your answer? As it stands, this reads more like a personal story than an answer. –  Undo Oct 9 '13 at 0:47
    
Point being that focal length probably is not going to be an issue at all. Once you satisfy your other more important parameters, you probably won't have a choice. –  Brian Knoblauch Oct 9 '13 at 12:31
    
-1 No justification given, very little explanation. –  called2voyage Oct 16 '13 at 14:40
    
Yeah, I'm sorry, I really have no idea what you guys are looking for. A question was asked on what one should do, I answered it, I thought that's what this site was for? –  Brian Knoblauch Oct 16 '13 at 19:37

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