Quote:
Originally Posted by Kirkus
Thank you all for your answers.
So I assume the same principle would apply with aperture (f ratio?). Using the same focal lenth of 750mm, let's say it was an f/5. If I were to use a 2X barlow that would double the FL to 1500, but would that also reduce the aperture by half?
In other words would a 750mm-f/5 scope using a 2X barlow be the same as using a 1500mm-f/10 scope without a barlow?
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No. aperture remains unchanged but yes, your last statement is essentially correct. Note though the distance from the Barlow or Powermate to the focal plane can change the amplification factor. See
http://www.televue.com/engine/page.asp?ID=42
In a given camera lens the iris can control the aperture and therefore f ratio but a given telescope has no iris and only a change in focal length can control f ratio unless you manually mask the front of your telescope with a smaller aperture mask (basically cardboard with a hole in it).
Note a smaller aperture will lower the resolving power of the telescope.