Comments on: A telescope with 20cm primary mirror has f# of 5. What focal length of the eyepiece to get 100x magnification? http://www.telescopebuyingguide.com/blog/a-telescope-with-20cm-primary-mirror-has-f-of-5-what-focal-length-of-the-eyepiece-to-get-100x-magnification/ Thu, 17 May 2012 19:38:31 +0000 http://wordpress.org/?v=2.7 hourly 1 By: magicninja http://www.telescopebuyingguide.com/blog/a-telescope-with-20cm-primary-mirror-has-f-of-5-what-focal-length-of-the-eyepiece-to-get-100x-magnification/comment-page-1/#comment-1636 magicninja Wed, 03 Jun 2009 00:42:59 +0000 http://www.telescopebuyingguide.com/blog/a-telescope-with-20cm-primary-mirror-has-f-of-5-what-focal-length-of-the-eyepiece-to-get-100x-magnification/#comment-1636 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. 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.

]]> By: Larry http://www.telescopebuyingguide.com/blog/a-telescope-with-20cm-primary-mirror-has-f-of-5-what-focal-length-of-the-eyepiece-to-get-100x-magnification/comment-page-1/#comment-1635 Larry Tue, 02 Jun 2009 05:21:39 +0000 http://www.telescopebuyingguide.com/blog/a-telescope-with-20cm-primary-mirror-has-f-of-5-what-focal-length-of-the-eyepiece-to-get-100x-magnification/#comment-1635 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 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

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