How is the focal length, and therefore also the focal ratio, of a MAC-Newt measured? It appears to me that a SCT's focal length must start from the corrector plate, but evidently the focal length and ratio of a MacNewt are measured from the primary mirror?
SCT and MacNewt F.L. ???
Started by pnbhgwt, 02/11/2009 09:18PM
Posted 02/11/2009 09:18PM
Opening Post
Posted 02/11/2009 09:52PM
#1
Paul Bailey said:
How is the focal length, and therefore also the focal ratio, of a MAC-Newt measured? It appears to me that a SCT's focal length must start from the corrector plate, but evidently the focal length and ratio of a MacNewt are measured from the primary mirror?
Paul: With compound scopes one cannot simply measure the distance from the mirror or the corrector to the focal plane, often there are elements which change the focal length by magnification.
In the case of the SCT, the primary mirror is generally around F/2 and the secondary is convex and provides approximately 5x magnification which results in the F/10 focal ratio. I believe the corrector plate provides no magnification, ie, it is unity.
In the case of the MAK-Newt, I believe one merely measures the focal length of the primary mirror as the corrector is essentially 1 power and the secondary is flat.
That's my two cents and I am sticking to it even if I am wrong...
(Not actually, more like: Live and Learn)Jon
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