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Calculating Focal Ratio

The focal ratio or resolving speed of your telescope is measured by dividing the aperture of your primary mirror or objective lens with the focal length of your telescope.

  • For example, a 100mm aperture telescope with a focal length of 600mm would calculate as 120mm / 600mm = 5, represented by convention as f/5
  • Low focal rations or smaller numbers are considered as fast ratios and are good for resolving faint or deep sky objects.
  • Slower focal ratios are considered better for Solar System views.
  • Note: ensure your dimensions for focal length and primary/object components are the same unit of measure, for mm, inches for inches.

Calculating magnification

The magnification currently in use on your telescope is calculated as the Focal length of your telescope divided by the focal length of your eyepiece.

e.g. a scope with focal length of 600mm and using a 10mm eyepiece will be performing at a magnification of 600mm / 10mm = 60 X magnification.

An important thing to remember is that as magnification is increased the view through the telescope becomes dimmer.  In fact, doubling the magnification makes the image four times dimmer!

Tripling the magnification makes the image nine times dimmer, and so on. Therefore, many objects look best at low magnification. If you try to view a dim object at a higher magnification, the object may vanish from view entirely!

In general, the field of view through a telescope decreases as the magnification is increased. Many celestial objects are quite large in apparent size, many being as large or larger than the full moon! This means that many objects will only fit in the field of view at lower magnifications.

  • Simulated views above of the Orion Nebula at magnifications of approximately 50X, 80X, and 120X.
  • Notice that as the magnification increases the field of view shrinks and the image gets dimmer.
  • A quick way to modify this magnification is to introduce focal changers.The most cost effective and common way to do this is with a Barlow Lens. This effectively increases the focal length of your setup thereby increasingmagnification. Barlow Lens typically come in 2X and 3X configurations.

Limiting magnitude is the visual limit or capability of your telescope to see astronomical targets.

Below is a table that details typical levels and the benefit that you achieve as your aperture increases. These measures are dark sky measures so city/suburban lighting will diminish these capabilities.

Telescope Aperture   Faintest magnitude
2 inch (51mm)            10.3
3 inch (76mm)            11.2
4 inch(102mm)           11.8
6 inch (152mm)          12.7
8 inch (203mm)          13.3
10 inch (254mm)        13.8
12.5 inch (318mm)     14.3
14 inch (356mm)       14.5
20 inch (508mm)       15.3
30 inch (762mm)       16.2