For visual use alone a small telescope only needs a well illuminated
image circle of about 1" (25mm) diameter. For 35mm size images one
needs an image circle larger than that which requires a larger secondary
mirror. The larger secondary blocks light transmission and reduces
contrast through additional diffraction. One needs to make a good
compromise in the design.
Chuck Norcutt
Moose wrote:
> Chuck Norcutt wrote:
>> ... For telescopes of 90mm diameter or less the overwhelming design choice
>> is a Maksutov which uses a spherical primary and a very thick meniscus
>> corrector lens. This is your typical camera mirror lens design which (if
>> it's done correctly) can have the secondary mirror simply be an aluminized
>> spot on the back side of the Maksutov corrector lens.
> That is the design of my Meade 90mm diameter, 1000mm fl, F11 telescope.
>> .....................
>> .... They are true telescopes with secondary mirrors sized for visual
>> use... meaning a small image circle. Probably OK on an E-body but with some
>> evident vignetting on 35mm size.
> The Meade I have does not vignette. With focal length and aperture
> marked prominently on the front ring, it is probably designed for camera
> use as well as telescope use.
>> But it will be equipped with a finder and means for sturdy mounting and have
>> options for removal of the visual
>> back and replacment with a larger focusing tube and T-mount for a camera.
>>
> Yup. Both brands require an adapter that screws onto the body with
> eyepiece attachment removed and has the back end threaded for T mount
> adapter for the particular camera.
>
> Moose
>
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