RandoziTelescope Eyepiece Comparison
Enter your telescope's focal length and aperture, then list your eyepieces: see the magnification, the exit pupil and the approximate true field of view of each one in a single table.
| Eyepiece | Magnification | Exit pupil | True field (approx.) |
|---|---|---|---|
| 25 mm · 52° | 48× | 4.17 mm | 1.083° (65′) |
| 10 mm · 60° | 120× | 1.67 mm | 0.5° (30′) |
| 6 mm · 82° | 200× | 1 mm | 0.41° (24.6′) |
Nominal values from the focal lengths you typed. The true field is an estimate; the real view depends on your optics and the seeing.
How it works
- Enter the telescope focal length and aperture in millimetres, and a Barlow factor (1 for none).
- List the eyepieces, one per line: focal length in mm, then optionally the apparent field in degrees.
- Read the table: magnification, exit pupil and approximate true field for each eyepiece.
- Copy the table as text or as CSV to keep it with your observing notes.
What people use it for
Choosing the next eyepiece
See the gap in your set: which magnifications and exit pupils you already cover.
Before and after a Barlow
Compare the same eyepieces at 1× and at 2× in two quick runs.
Observing notes
Copy a table of nominal values for a star party or a club handout.
Questions
- What are the formulas?
- Magnification = (telescope focal length ÷ eyepiece focal length) × Barlow. Exit pupil = aperture ÷ magnification. True field ≈ apparent field ÷ magnification, in degrees, times 60 for arcminutes. A 1200 mm telescope with a 25 mm eyepiece gives 48×.
- Why is the true field only approximate?
- Real eyepieces distort the field and have a field stop, so the apparent field is not constant across the view. Dividing it by the magnification is a standard estimate; the manufacturer's figure for your eyepiece and scope is better.
- What does the exit pupil tell me?
- It is the width of the beam of light that leaves the eyepiece, in millimetres. The pupil of a human eye varies with age and light, so no value is guaranteed comfortable or sharp: the number is a comparison, not a recommendation.
- Do the numbers promise a good view?
- No. They are nominal values from the focal lengths you typed. Seeing, optics quality and collimation decide what you can actually resolve.