Determining When To Use Eyeglasses Advice Article List > Determining When To Use Eyeglasses
Albert Nagler
CEO, Tele Vue Optics

Many peoples' eyes exhibit enough astigmatism to require correction with eyeglasses. When using a telescope, near-sightedness and far-sightedness (but not astigmatism) can be compensated by focusing. So, how can we determine when eyeglass correction is necessary?

First, note that with any eyepiece, the exit pupil can be simply calculated as follows:

eyepiece focal length telescope f/# = exit pupil

Example 32mm eyepiece with f/10 telescope: 32mm f/10 = 3.2mm

  • Using your lowest-power eyepiece to get the largest exit pupil, view a bright star at the center of the eyepiece's field (eyepieces never have astigmatism in the center of the field, but often exhibit some astigmatism at the edge, particularly for simple eyepiece designs).
  • Rotate your head about the eyepiece and notice if any irregularity in the image tracks your changing orientation. If it does, you have astigmatism or some other eyesight irregularity. When some people see image problems with "low power" eyepieces, they mistakenly believe that their eyepiece or telescope is at fault, because there's no problem with medium or high power eyepieces (where the smaller exit pupils don't reveal eyesight astigmatism).
  • If you see a problem at low power, it's helpful to determine the exit pupil size at which the problem is ameliorated. Why? For two reasons:
  1. If you don't need long eye relief model eyepieces to accommodate wearing eyeglasses, you can use less expensive eyepieces that are also usually lighter and smaller, or wider field eyepieces with shorter eye relief. However, please note that some people prefer observing with their glasses on so they can instantly switch between "naked eye sky viewing" and "telescope viewing".
  2. By not wearing eyeglasses, you can eliminate the extra light loss, scattering and possible reflections they cause.
  • There are two simple ways to determine the exit pupil size at which you can observe "eyeglass-free", with no eyesight degradation:
  1. Use higher power eyepieces (or use Barlow lenses to achieve higher powers) until you notice the point where your exit pupil provides ideal imaging. One problem with this approach is that higher powers may introduce telescope and atmospheric limitations to the process. In any case, always use the above exit pupil formula.
  2. Perhaps a better test is to use the same eyepiece that gives you the largest exit pupil, and just tape paper circular masks in front of the telescope objective to "stop-it-down" to change the telescope's f/#. For example, if you have an f/6 telescope and use a 42mm eyepiece, you have a 7mm exit pupil to start with. Making a mask with an opening ½ the telescope's aperture yields a 3.5mm exit pupil; a mask with an opening ¼ the scope's aperture yields 1.7mm, etc. For refractors, it's best to use concentric circular masks. For reflectors, however, use de-centered (also called off-aperture) masks to avoid the obstruction caused by the diagonal or secondary mirror.
  • When you determine the largest exit pupil that does not reveal astigmatism, just multiply that exit pupil by the telescope's normal f/# to determine the longest focal length eyepiece you can use without wearing eyeglasses. For example, if a 2mm exit pupil is okay, and you have an f/5 telescope, a 10mm eyepiece is your upper focal length limit.
Further Reading

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