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Old 15-12-2011, 07:13 AM
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Merlin66 (Ken)
Spectroscopy Wizard

Merlin66 is offline
Join Date: Oct 2005
Location: St Leonards, Vic
Posts: 7,223
Like any lens the reducer has a focal length, this is the distance from the lens to the point of focus. i.e. if you used it to image the Sun onto a piece of paper, when the image on the paper is smallest, it's at the focal point.
This distance for a x0.63 reducer is usually 220mm.
Why is this important?
When fitted to the telescope, the actual reduction (ie x0.63) is achieved with a fixed distance between the lens and the CCD chip in the camera, for the standard reducer this distance is 105mm.
If the focal length of the reducer is say only 145mm, then this distance would be close to 70mm - a big difference from the anticipated 105mm.
Hope this helps.
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