The concept of Exit pupil and eye relief are essential for general astronomy. There relevance increases in context with the apparent and true field of view.
in this section, we will discuss about the robust concepts of exit pupil and eye relief. The knowledge of these two considerations makes it easier to better your observations.
What is an Exit Pupil?
In older optical literature, Ramsden Disk is called the Exit Pupil. The phrase is used to describe the light beam that leaves from an optical outlet.
The exit pupil is expressed in terms of the light beam’s diameter. It is calibrated in millimetres or mm.
This value equated the amount of light that enters the observer’s eye. This is true for when you are making an observation through the microscope, binocular, and eyepiece.
Depending on the amount of light, your pupil might dilate or constrict. For astronomy, the exit pupil is an important concept.
Reasons that make the exit pupil necessary in astronomy
Your eye can dilate up to a maximum of 5mm to 7mm. The extent of dilation largely depends on your genetics and age.
The more dilation, the larger amount of light touches your eye’s retina. This allows for much better visibility, even in low light conditions.
As an astronomer, this gives you quite an advantage. It would help if you chose a proportional eyepiece of a perfect focal length.
Choosing the right eyepiece will allow you to image as bright as possible. Following the rule, you need to use a low-powered scope with an aperture measuring up to 3.5 inches.
When you up the power scale on your scope, the size of the exit pupil decreases. This, as a direct effect, increases the contrast, while the background will appear significantly darker.
For most astronomers, it is more convenient to follow the 60x rule. This rule dictates using a 60x inch aperture.
Many variables come into play regarding exit pupil. Some of these include the optical quality, stillness of the air around the telescope.
Depending on these factors, you can use an aperture larger than 60x. But using, higher magnifications are not practical for reflector type telescopes.
Calculating the Exit pupil
You will need to the aperture of the objective lens of your telescope for calculating the exit pupil. Convert the aperture calibration from inches to millimetres.
Divide the aperture calibration with the eyepiece observatory magnification. We will give you an example for more clarity.
For instance, if your telescope’s aperture size is 127mm with a focal length of 952mm. With these units, the magnification will be around 203x.
When you divide the magnification with the aperture size, the total value comes to be 0.62mm. This is the value of the exit pupil.
What is the best Exit Pupil for me?
The brightness of the celestial object and the atmospheric conditions, the exit pupil varies. You need to experiment with different focal lengths to find the right unit.
For larger stars and lunar astronomy is best to use an exit pupil of 3mm to 5mm. For relatively smaller objects in the deep sky, 2mm to 4mm of the exit pupil is sufficient.
For detailed Astro imaging, and celestial planets, you need 0.5 to 2 mm of the exit pupil. This is true when the atmospheric conditions are perfect.
What other consideration should you make for eyepieces?
There are different eyepieces in terms of magnification and the diameter of the pupil. You can also procure eyepieces with designs for diverse fields of view, eye relief, and exit pupil features.
Most scientific eyepieces have 52, 62,68,82,92,100, and 120 degrees of apparent field of view. These lenses, with different eye relief calibration, decide the actual field of view of your telescope.
They also classify the amount of distance you need to maintain while observing. This feature acts as a comfort factor while making astronomical observations.
The sport optic’s exit pupil is proportional to the amount of dilation of your eye. This determines how well your vision is adapted to darker lighting conditions.
What is eye relief for sports optics?
Eye relief is a safe distance between your eye and the eyepiece pf your telescope. This distance dictates the full field of view be observed comfortably.
The distance for eye relief is measured from the end of the eyepiece to the plane behind it. The backplane of the eyepiece is the region where all the light rays are focused.
The eye relief and exit pupil are closely interlinked with each other. Your eye should be positioned at an accurate eye relief position for a full field of view.
While 15mm of distance is the best eye relief measure, you can begin with 10mm. If you use specs or glasses, you might need a more extensive eye relief distance.
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