Should I focus more on Aperture or Focal Length for a telescope?
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?
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.