Summary telescope & eyepiece magnification?

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Size of objective lens for refractor telescopes

70mm
– the minimum size for good views.
– starts to show more detail on the planets.
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80mm
good size for detailed views of the planets.
– starts to show detail in the detail!
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90mm
excellent detail level on the planets and good views of all objects.
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100mm
bright planets like Jupiter, Saturn, and Mars shine bright in the 100mm reflector.
– even when viewing from urban locations with mediocre seeing conditions.
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110mm
– the 110mm refractor excels in both deep-sky imaging and wide-field observing.
– the 110mm aperture, combined with extra-low dispersion (ED) FPL-51 glass.
– it offers great colour correction, as well as crisp images and fantastic views.
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120mm
all telescopes lose contrast, and refractors are no exception.
– the smaller the telescope, the more contrast is lost.
– however, the increase in contrast transfer between a 100mm and a 120mm scope is only about 20%.
– meaning 100mm refractor telescope which is far less than the gain in light gathering in 120mm scope.

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Formula for calculating the eyepiece

The math is easy division:

  • The magnification is the telescope focal length divided by the eyepiece focal length, in millimetres.
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  • If you use a telescope of 1000mm focal length with a 25mm eyepiece, the magnification would be 33x.

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For example
Here to test all different focal length measurements on the same 30mm eyepiece and 80mm (3 inch) aperture.

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30mm eyepiece

Field of View: 30mm eyepiece
(focal length divided by eyepiece = magnification)

400mm

1000mm

1500mm

2000mm

2500mm

(All above apertures are same at 80mm/3 inch)


25mm eyepiece

Field of View: 25mm eyepiece
(focal length divided by eyepiece = magnification)

400mm

1000mm

1500mm

2000mm

2500mm

(All above apertures are same at 80mm/3 inch)


13mm eyepiece

Field of View: 13mm eyepiece
(focal length divided by eyepiece = magnification)

400mm

1000mm

1500mm

2000mm

2500mm

(All above apertures are same at 80mm/3 inch)


Telescope Eyepieces

Below are rough guides and are dependent on the telescope you use:

  • 2mm-4.9mm Eyepieces:
    These are very high magnification and very difficult to use unless seeing conditions are perfect and the object observed is very bright, like the moon.
  • 5mm – 6.9mm Eyepieces:
    These are good on bright objects such as the moon and bright planets, but are still very high power and work best with steady seeing conditions.
  • 7mm – 9.9mm Eyepieces:
    These are very comfortable high magnification eyepieces and are excellent for observing brighter objects, a must for any eyepiece collection.
  • 10mm – 13.9mm Eyepieces:
    These work well for all objects including brighter nebula and galaxies a good mid/high range magnification.
  • 14mm – 17.9mm Eyepieces:
    These are a great mid range magnification and will help resolve globular clusters, galaxy details and planetary nebulae.
  • 18mm – 24.9mm Eyepieces:
    These will work nicely to show wide field and extended objects, great mid-range magnification for objects like galaxy clusters and large open clusters.
  • 25mm – 30.9mm Eyepieces:
    These are wider field eyepieces for large nebula and open clusters – a good finder eyepiece for locating objects before moving to higher powers.
  • 31mm – 40mm Eyepieces:
    These are excellent for extended views and large star fields and make excellent finder eyepieces before moving to higher powers.
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Eyepieces are the most important part of your observing equipment, choose them and use them well, which will help you enjoy observing through your telescope.

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