Answer key: Telescope Magnification Worksheet
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Goal: Calculate magnification, exit pupil, field of view and resolution, and choose a sensible telescope and eyepiece for a target.
Completed results table
| Setting | Magnification | True field | Exit pupil | Dawes’ limit | Max useful power |
|---|---|---|---|---|---|
| Default (1,000 mm, 25 mm eyepiece) | 40× | 1.5° | 2.5 mm | 1.16″ | 200× |
| Default with a 10 mm eyepiece | 100× | 0.6° | 1 mm | 1.16″ | 200× |
| Wide view | 20× | 3.4° | 4 mm | 1.45″ | 160× |
| Lunar detail | 120× | 0.5° | 1.25 mm | 0.77″ | 300× |
| 8-inch Dobsonian | 48× | 1.08° | 4.17 mm | 0.58″ | 400× |
Answers
How do you calculate magnification? Check your rule with the default setup.
Divide the telescope’s focal length by the eyepiece’s focal length: 1,000 mm ÷ 25 mm = 40×.
When you swapped the 25 mm eyepiece for a 10 mm one, what happened to the magnification, the true field and the exit pupil?
Magnification rose from 40× to 100×. The true field shrank from 1.5° to 0.6° and the exit pupil from 2.5 mm to 1 mm, so the view is closer but dimmer and shows less sky.
The full Moon is about 0.5° across. Which preset shows the whole Moon with the most room to spare? Which preset makes the Moon fill the entire view?
Wide view: its 3.4° true field is almost seven Moon-widths across. Lunar detail: its true field is 0.5°, about the size of the Moon, so the Moon fills the view — good for craters, not for the whole disk.
A box says a small 60 mm telescope (700 mm focal length) gives “175× power” with a 4 mm eyepiece. Enter it in the lab. Is that a useful magnification? Explain.
The lab shows 175×, but the maximum useful power for a 60 mm aperture is about 120×. Beyond that the image just gets bigger and blurrier, with a tiny 0.34 mm exit pupil, so 175× is not useful.
Which preset can split the closest double stars? Use Dawes’ limit to explain.
The 8-inch Dobsonian. Its Dawes’ limit is 0.58″, smaller than Lunar detail’s 0.77″ and Wide view’s 1.45″; a bigger aperture resolves finer detail.
Why must you never look at the Sun through a telescope without a proper solar filter?
A telescope collects and focuses a huge amount of sunlight into your eye and can cause instant, permanent eye damage. Only a purpose-built filter covering the front of the telescope makes solar viewing safe.