Eyepieces October 5, 2020
Understanding Equipment

Eyepieces are small lenses that need to be attached to telescopes in order to see the generated image.

They have two important characteristics that determine directly the performance of a telescope:

These two aspects of an eyepiece are fixed.  The only exceptions are the zoom eyepieces.  These still have a static apparent field of view, but a variable focal length and hence variable levels of magnification.

Without zoom eyepieces, astronomers need to have multiple eyepieces available to obtain different magnifications or different true fields of view.

Eye Relief

Eye relief is the distance eyes need to be from the eyepiece in order to enjoy the entire field of view of a telescope.  Any other distance will result in only observing a reduced field of view.  The higher the magnification and the larger the apparent field of view, the shorter the eye relief.  This is especially important for people wearing glasses as glasses prevent astronomers from getting too close to an eyepiece.

Barlows

Barlows change the effective focal length and hence the magnification of a telescope.  But they also indirectly have an effect on eyepieces:

  • Instead of choosing a 5mm and a 10mm focal length eyepiece, the same result can be obtained with a 10mm focal length eyepiece and a x2 barlow that halves the focal length to 5mm.
  • You can get creative combining eyepieces and barlows:
    • 2 eyepieces and a barlow replace 4 eyepieces.
    • 2 eyepieces and 2 barlows replace 8 eyepieces.
  • Barlows allow you also to enjoy the eye relief of longer focal length eyepieces and still benefit from larger magnifications.

Conclusion

Although they are tiny compared to a telescope, choosing the right eyepieces, combines with barlows, will determine a lot the performance of a telescope.  Choose carefully!