Obstucted Designs: Newt V's Cassegrain Approach

[John]

Another current thread has already discussed the imacts of central obstructions on light grasp, resoloution etc. I've a related, but slightly different question so I've started it under a new thread.

With a newtonian design (including the mak-newtonian) the primary mirror is intact but it's central region is masked by the secondary mirror. In a cassegrain design the primary mirror has a hole in the centre to let the light pass through from the secondary to the eyepiece.

What I'm intrested in is whether, with a newtonian, the central region of the mirror (ie: an area equal to the area of the secondary obstruction) plays makes any contribution at all to the light grasp, resolution etc of the scope - or might it just as well not be there as in the cassegrain design ?.

The obvious answer is that it makes no contribution at all as it is masked by the secondary - but sometimes the obvious is not in fact what happens - so I thought I'd ask the question !

John

[Kaptain Klevtsov]

John, I seem to recall a Tak. that could be a Newt. or a Cass. design by changing the bit at the top for the other one that comes with it. It worked as a Newt. so the shadowed bit can't do a lot. This one had a hole drilled through the mirror. Best paint the centre of my Newt's mirror black to stop reflections and stuff.

Captain Chaos

[John]

Best paint the centre of my Newt's mirror black to stop reflections and stuff.

Thanks CC - with all the care that people go to in flocking their tubes, blackening focusser tubes etc I would have thought that blackening the central part of a mirror would be a good idea too - if you've got the nerve to do it that is !.

John

[Astroman]

Blackening the obstructed part of a Newt primary wouldn't gain much, if anything, since there are no collimated light rays landing on that section. The hole in a Cassegraine illustrates this. When Pro scopes place a tertiary mirror, as I've mentioned before, it goes just above the center obstruction to deflect the light sideways. You can Google the Keck scopes for an illustration.

[Kaptain Klevtsov]

Just did thanks. I couldn't guess where you meant, so I had a look, it's in front of the hole in the big mirror, underneath the small one. Now thats got mo wondering about a lens in front of the Newtonian secondary mirror, with another same sized or smaller secondary mirror back to back with the normal one. Built in guide 'scope anybody?

Captain Chaos