Quote:

---

Because in order to put the focal point behind the rear end of the OTA, you'd need a flat with a diameter of more than 50% of the primary (this system would be called a folded Newtonian (not a Cassegrain)). Impossible for visual use due to extremely diminished contast (more exactly: MTF-curve); even for phtographical application, a 50%+ obstruction would be problematical.
There are folded Newtonians, but their secondary flat is sized smaller than 50%, so the focal point would lie inbetween the secondary and the primary. To get access to the focal point, a tertiary flat is then used to redirect the beam towards the side of the tube (like the secondary flat in an ordinary Newton does).

Regards,

Ralph

---

Thanks! :-) After drawing some diagrams, I think I understand why. But couldn't the focal point of my arrangement just be left before the rear end of the telescope, after which the image is led out of the tube with a couple of barlow-like lenses? Perhaps you could even use the baffle tube to attach these lenses to.

--------------------
Meade 8" SCT (an older white-tubed version)
Celestron CG5 (non-GOTO)
Teleskop-service 30 mm WA
Teleskop-service 7-21 mm zoom
GSO 2" standard star diagonal

Edited by dddhgg (08/16/08 09:44 AM)

Post Extras