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Old August 14th 03, 11:07 PM
William Hamblen
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Default Focal Reducers, how do they work?

In article nk.net,
Richard Kenney wrote:

How does a focal reducer work on a SCT? Does it shorten the efective
focal lenght of the scope? I am not an optical theorist so if ypou
choose to respond please do so in terms any bum off the street can follow.


The focal reducer is a positive lens that reduces the effective focal
length of the telescope. "Positive" means the focal length is a positive
number (you can use the lens as a magnifying glass). The amount it
changes the effective focal length of the telescope depends on the
focal length of the focal reducer and the spacing between the focal
reducing lens and the eyepiece or film. Focal reducers have been
around a long time. Lumicon included a focal reducer in its Cassegrain
Easy Guiders for years. Celestron used to sell a "rich field adapter"
that screwed into an eyepiece and was a focal reducer by another name.
Research telescopes have had them for many years.