Magnification October 5, 2020
Understanding Equipment

The light captures through a telescope needs to go somewhere.  Astrophotographers capture light using a camera and astronomers observe the night sky through an eyepiece.  Both a camera and an eyepiece have their own focal length.  It is only by combining the focal length of the telescope and the focal length of the eyepiece or camera that the magnification of a setup can be calculated.

Magnification

The magnification of a telescope is the ratio between the effective focal length of the telescope and the focal length of the eyepiece or camera.  In order to calculate the magnification when using barlows or focal reducers, their multiplication factor needs to be applied to the telescope focal length to obtain the telescope’s effective focal length.

As the telescope focal length is fixed, the only way to get some flexibility in the magnification of a setup is to combine a telescope with different eyepieces, barlows, and focal reduceFocal Length:

  • A 1000mm telescope and a 10mm eyepiece provide a 10x times magnification
  • The same telescope with an 18mm eyepiece will result in a 55x times magnification
  • The first setup with a x2 barlow shows a times 20x magnification

Manipulating the magnification does not change the amount of light that is captured by the telescope though.  Increasing the magnification, zooming in, does make an image dimmer.  Brightness and resolution only depend on the size of the aperture, not on focal length.

Power

Magnification is also talked about in terms of power. High power refers to high magnification and low power to low magnification

The maximum power you can get out of a telescope is about 2 power per mm aperture, so a 200mm aperture telescope has about a maximum magnification of 400 before you exceed the resolution limit of your telescope.  Earth’s atmosphere will also limit the maximum amount of magnification you can get out of a telescope to about 300 power. Over-magnification just makes images blurry as you keep zooming in without adding more details.

Observing Planets vs Deep Sky Objects

Planets are located in astronomical terms very close to Earth.  They require at least a magnification of 100 to appreciate them:

  • Venus phases can then be seen using 7 power binoculars
  • The 4 biggest moons of Jupiter are already visible with 10 power binoculars
  • Jupiter’s cloud bands need at least 75x magnification to appreciate them
  • Saturn’s rings at 100 magnification or 100 power are clearly visible
  • Even when Mars is closest, you need at least 200x magnification and good conditions to distinguish its polar caps
  • Uranus will start showing as a tiny blue-green disc upwards from 200x magnification
  • Neptune should also show as a tiny blue disc upwards from 200x magnification

Deep-sky objects, on the other hand, although faintly visible with the naked eye, can be very big and do not require much magnification.  M31, the Andromeda galaxy actually has the size of 3 full moons in the night sky!