Take the 2-minute tour ×
Astronomy Stack Exchange is a question and answer site for astronomers and astrophysicists. It's 100% free, no registration required.

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?

share|improve this question
add comment

migrated from space.stackexchange.com Oct 8 '13 at 21:36

This question came from our site for spacecraft operators, scientists, engineers, and enthusiasts.

3 Answers

up vote 7 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.

share|improve this answer
add comment

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.

share|improve this answer
+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
add comment

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.

share|improve this answer
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
add comment

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.