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How Do I Measure Exit Pupil?


What is the Exit Pupil?
The exit pupil, also called the “Ramsden Disk” as described in older literature on optics, is the beam of light that exits from an optic. Usually expressed as the diameter of the beam in millimeters, the exit pupil will determine how much light will enter your eye through a telescope, binocular, or spotting scope eyepiece.  The human eye’s entrance pupil constricts and dilates depending on the lighting conditions of the environment. To visual astronomers, the exit pupil diameter of a given telescope/eyepiece combination is an important value for various reasons, including:

  • The human eye can dilate to its maximum from about 5mm to over 7mm depending on your individual genetics and age. Of course, the larger your eye can dilate, the more light can contact your retina, making it possible to have greater ability to see in low light level environments, which is a plus in observing deep sky objects.
  • Matching the exit pupil diameter by choosing the right focal length eyepiece for your telescope to your fully dilated eye entrance pupil will allow you to see the brightest images possible from your telescope. This usually means using the lowest practical power of your telescope with an average of 3.5X per inch of aperture of your telescope.
  • Increasing power reduces the size of the exit pupil, and often has the effect of increasing contrast, by making the background sky look darker. Most astronomers use a rule of thumb of a maximum of 60X per inch of aperture of their telescope. However, depending on the quality of the optics and especially the stillness of the atmosphere (called seeing quality), it may be possible to exceed the 60X per inch rule.
NOTE: Using extreme magnifications. If your telescope is a reflector, such as a Newtonian or a Cassegrain type then the secondary mirror blocks some of the light to your eye. If your eye is not dilated enough to match the exit pupil coming from the eyepiece, then you can see the shadow of the secondary mirror. This often happens when using reflector telescopes at low power during the daytime. Using too much magnification produces images that are too dim with reduced contrast. High power also creates a very small exit pupil which first-time beginners often have a hard time centering their eye over the tiny beam of light exiting the eyepiece. 
How Do I Calculate Exit Pupil?
To calculate the exit pupil diameter with a given eyepiece on your telescope you take the aperture of the objective lens of the telescope in millimeters and you divide it by the magnification produced by the eyepiece being used. For instance, the Explore Scientific ED127 Air-Spaced Triplet has an aperture of 127mm and a focal length of 952mm. If you use a 4.7mm eyepiece on this telescope it will yield a magnification of about 203x (to calculate magnification divide the focal length of the telescope by the focal length of the eyepiece). 127mm divided by the value of the magnification of 203 produces a value of about 0.62mm, a very small exit pupil indeed. If you use a 40mm eyepiece then the magnification is only 23.8x, but will yield a large exit pupil of about 5.3mm, which will not only produce a bright image, but will be more easy for first-time beginners to center their eye over the lens to observe through your telescope.
What Exit Pupil is Best for the Object that I am Looking at?
Depending on sky conditions, the brightness of an object, and the observer, the right exit pupil is often found by trying different focal length eyepieces until the object is best seen. However, in an article from Astronomy Magazine they make the following recommendations as a starting point:
Target                                                                                        Exit pupil (mm)
Large star clusters, complete lunar disk                                     3mm to 5mm
Small deep sky objects (especially planetary
nebulae and smaller galaxies), double stars,
lunar detail, and planets on nights of poor seeing                     2mm to 4mm
Double stars, lunar detail, and planets on
exceptional nights                                                                       0.5mm to 2mm
What Else Do I Need to Consider When I Choose Eyepieces?
Aside from calculating magnification and exit pupil diameter, you can purchase different designs of eyepieces that have different apparent fields and different eye relief distance characteristics. For instance, Explore Scientific has eyepieces with 52°, 62°, 68°, 82°,92°, 100°, and 120° apparent fields. These different eyepieces with their different eye relief distances will determine how much true field of view that you see through the telescope. It will also determine how close your eye will have to be to the eyepiece, which affects eye comfort when viewing.
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