No, actually, I didn't get it right. Moving the primary alters the
distance between primary and secondary but so does moving the secondary.
Same difference. Either one is altering the focal length and
magnification when focusing by moving either mirror. Only moving the
camera to meet the position of the focal plane will keep the focal
length constant. Just like most of our older lenses (especially primes)
focus by moving the entire set of optical elements toward or away from
the focal plane.
I'll have to change my diagnosis to a difference between the lenses in
the degree of magnification created by the secondary. Both would still
change their focal length upon moving either mirror but the one with the
strongest secondary magnification would change by the greatest
percentage of its base focal length.
Chuck Norcutt
Jan Steinman wrote:
>> From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
>>
>> As to the apparent non-difference seen in the image sizes between the
>> Bausch & Lomb and Celestron I speculate it may be due to the focusing
>> mechanisms. Some cat's focus by moving the primary mirror back and
>> forth and this has the effect of varying the focal length and
>> magnification for close objects vs. the specification for infinity.
>
> Good call! The B&L moves the primary mirror, while the C90 moves the
> secondary mirror.
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