Barlow Lens Page updated

By request, I have updated my Barlow Lens tutorial:
http://www.astunit.com/tutorials/barlow.htm

In addition to some stylistic improvement, there is a new section on
finding the focal length of a Barlow and the addition of a worked
example for calculating amplification by adding extension tubes or
putting the Barlow on the OTA side of the star diagonal.

I hope this makes it more useful. As always, constructive criticism and
suggestions for improvement are welcomed (but may not be acted on
immediately! g).

Best,
Stephen

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