Tuesday, December 6, 2011


In addition to the spreadsheet discussed in my original post (below) here are some eyepiece links friends on the Stargazers Lounge and Cloudy Nights forums have suggested:

Original post: November 28, 2011
I created a simple Excel spreadsheet to calculate power, exit pupil, and true field of view for my eyepieces. I use the table when I'm observing to pick the best eyepiece for whatever I'm viewing. I also use the spreadsheet when I'm considering a new eyepiece.

I have set the spreadsheet up for my telescope and eyepieces, but you can input your own equipment and the spreadsheet will give you results for your setup. I'd be happy to share this spreadsheet with you. you can contact me through my profile page. (I have not figured out how to attach the spreadsheet to this blog.)

The table below shows information calculated for my eyepieces and telescope. My telescope has a focal length of 2,500mm and an aperture of 254mm. See below for the formulas to calculate the power, exit pupil, and true field of view for your eyepieces and telescope.

Eyepiece focal length (mm) Power Exit pupil (mm) Apparent field of view (degrees) True field of view (degrees) True field
of view (arcminutes)
40 63 4.1 60 0.96 58'
26 96 2.6 52 0.54 32'
23 109 2.3 68 0.63 38'
20 125 2.0 60 0.48 29' 40mm + 2x barlow
13 192 1.3 68 0.35 21'
9 278 0.9 52 0.19 11' illum. reticle
6.7 373 0.7 82 0.22 13'

Formula: Power = telescope focal length (mm) / eyepiece focal length (mm).
Comment: There appears to be a useful maximum power of 50 times telescope aperture (in inches). However, in the case of a 10" aperture, that results in a maximum of 500 power, while most literature indicates maximum useful power is closer to 300x due to atmospheric interference.

Exit pupil
Formula: Exit pupil = telescope aperture / telescope power
Comment: The practical maximum exit pupil size is around 5mm and a 0.7mm exit pupil size is pushing the lower limits.

True field of view
Formula: True field of view = apparent field of view / power.
Comment: For comparison, the moon is approximately one-half degree in diameter, or 30 arcminutes.

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