Reflector with interchangeable secondary?

[Tommohawk]

This might sound a bit bonkers, but has anyone made a reflector with an interchangeable secondary, ie maybe an Ritchey Chretien type for long focus, or conventional secondary for short length low F work with side focusser?

I suspect the primary design would be the issue ie paraboidal vs hyperbolic.

I'm still pretty new to all this so maybe a rather naive question - any thoughts though?

[Laurie61]

It is possible to combine a Cassegrain and Newtonian optical design. As a Cass it is a long 'f' scope and as a newt it is much shorter 'f'. You would have two observing positions, on the side of tube for newt and behind primary for Cass type.

http://www.takahashiamerica.com/includes/link.htm?target=http://www.takahashiamerica.com/CN212.htm&title=Ta

[Peter Drew]

I used to manufacture a dual focus 10" F5 - F10 Newtonian. There were two focuser apertures on a common adjustable plate one fed by a standard secondary size at F5, the other minimum size secondary fed a buit in 2X  Barlow to give F10. The secondarys were mounted on shafts and preset for collimation and could be interchanged by sliding the shafts into a holder set in the spider, stops and indents assuring lateral and rotational  position and collimation. 

[niallk]

Here's an interesting patent for a variable focal length Newt that I happened across a while ago:

http://www.google.com/patents/US6061175

Rotatable double sided primary !

[Tommohawk]

It is possible to combine a Cassegrain and Newtonian optical design. As a Cass it is a long 'f' scope and as a newt it is much shorter 'f'. You would have two observing positions, on the side of tube for newt and behind primary for Cass type.

http://www.takahashiamerica.com/includes/link.htm?target=http://www.takahashiamerica.com/CN212.htm&title=Ta

Yes! Thats what I had in mind. So, not an original idea then (again!) This looks to be discontinued, and I dont see anythng else comparable out there - wonder why?

I used to manufacture a dual focus 10" F5 - F10 Newtonian. There were two focuser apertures on a common adjustable plate one fed by a standard secondary size at F5, the other minimum size secondary fed a buit in 2X  Barlow to give F10. The secondarys were mounted on shafts and preset for collimation and could be interchanged by sliding the shafts into a holder set in the spider, stops and indents assuring lateral and rotational  position and collimation. 

That sounds interesting. I cant quite get the configuration from the description - any pics around? Did it work well in practice I wonder?

Here's an interesting patent for a variable focal length Newt that I happened across a while ago:

http://www.google.com/patents/US6061175

Rotatable double sided primary !

Looked at some of the drawings - I think he got a bit carried away! Two mirror surfaces sort of defeats the object to my mind. Did this design ever see the light of day - or dark of night more to the point?!

Thanks all, any further thoughts welcome.

[ddtwelve12]

years ago at wyas they had a 18" Newtonian with a 3 mirror arrangement light hit the main then came back up to the hyperbolic secondary which in turn shot the light down to a flat mirror to the outside the top hyperbolic mirror could be removed to make a f5 Newtonian against f15

[ddtwelve12]

here we go parks optical telescopes HITs f3.5 Newtonians and f15 cassigrains all in one tube

[ddtwelve12]

www.parksoptical.com

[Tommohawk]

www.parksoptical.com

Hey thanks - thats the one for me! So technically, it obviously can be done.

I think the dollar would have to fall a long way before I splashed out an that though.  $31,999 !! Could I persuade Synta to make a budget version??

Maybe I could just get the OTA and mount it on my HEQ5. Its only 118kg. Just for visual, obviously.

SPECIFICATIONS

Primary Mirror Diameter: 16"/406mm

Focal Length, Newtonian Mode: 64"/1626mm

Focal Length, Cassegrain Mode: 288"/7315mm

Focal ratio: f/4 Newtonian; f/18 Cassegrain

Focuser: 2" with 1.25" Adapter

Drive System: Dual axis with declination motor and hand controller

Oculars: One each, 15mm & 25mm focal length, 1.25" Parks Optical Gold Series

Finderscope: 8x50 Polar Finderscope with Illuminated Reticle

RA/Dec Drive-shafts: 2.5" Stainless Steel

Shipping Weight

Optical Tube Assembly: 260 lb/118 kg

Mount and Rest of System: 657 lb/298 kg