A telescope with 20cm primary mirror has f# of 5. What focal length of the eyepiece to get 100x magnification?
Sunday, April 25th, 2010 at
1:27 am
A telescope with a 20 cm primary mirror has an f# of 5. What would be the focal length of the eyepiece required to give a magnification of 100? I know there's an equation, but I don't know what it is. Thanks in advance.
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Assumption ; by a “20 cm primary mirror” you are talking about a mirror 20 cm in diameter.
The magnification is equal to the primary focal length divided by the eyepiece focal length.
To find the primary focal length, multiply the primary diameter by the f number.
Primary fl = 200 mm * 5 =1000mm
( 20cm = 200mm)
Mag = Primary fl / Eyepiece fl
rearranging:
Eyepiece fl = Primary fl / Mag
Eyepiece fl = 1000mm / 100 = 10mm
You have to vonvert to mm since that is what you would be working with. 200mm mirror / focal length of eyepiece = magnification or 200mm/2mm=100x
Ohhhh yeah. Skipped a step. Good show Larry.