A Good Telescope – not a Toy

Now for some details about what makes a good telescope. I have examples below.

Reflector Telescopes

Reflector telescopes use mirrors (a large primary and small secondary) to reflect the image into an eyepiece. The mirrors need to be of good quality to produce a sharp and contrasty image. What constitutes a good mirror?

  • Good glass – a primary mirror made of low expansion glass like borosilicate (Pyrex is one type), vs plate glass is more stable when the temperature changes, keeping the image stable.
  • Good figure – a figure is how the mirror was ground into shape. A parabolic telescope mirror will have a lower level of aberrations than a spherical figure. Look for a parabolic primary mirror.
  • Good focal length – watch that f-number! Make sure the f-number is no lower than 5, and preferably higher, more like 6 to 8. A small f-number indicates a short focal length and good quality short focal length mirrors are much more difficult to produce. They are more prone to figure errors and degraded performance. Short focal length reflector telescopes also need a larger secondary mirror (it directs the image into an eyepiece), and that means a larger obstruction up front. The result – lower contrast, not good for planets, the Moon, or double stars.

Refractor Telescopes

With refractor telescopes it’s all about the objective lens. What makes a good objective lens?

  • Good glass – there are two basic types of good objective lens, Achromat and Apochromat, with the apochromat being superior. They are both designed to reduce common chromatic aberrations inherent in glass lenses. Achromat designs correct for about 85% and apochromat designs correct for 95% or better. Chromatic aberrations cause color fringing around bright images such as the planets, Moon, and stars. These aberrations also reduce contrast, making the image less sharp in appearance.
  • Good focal length – longer focal lengths, such as f10 or f12 benefit achromat designs, reducing chromatic aberrations significantly. This is not a significant factor for apochromat designs. Apochromat optical tubes can be of short focal length and still produce beautifully clean images.
  • The other major difference is cost. Apochromat designs are way more expensive – you pay for the very special glass used in these designs.

Catadioptric/Compound Telescopes

  • These telescopes are closer to reflector designs than refractors even though they include a lens element.
  • Focal length – these telescopes typically come as f10 to f13, depending on the style.
    • Schmidt – Cassegrain: f10. A good all-around telescope.
    • Maksutov – Cassegrain: f12-f13. A bit better for planets, the Moon, and double stars

What’s in the Sky?

December 9; before sunrise; southeast: Early risers can see a waning crescent Moon and Venus share the morning sky.