If you put a ccd or similar at prime focus.say even a toucam.The field of view would be 31'x23' with a focal length of 444mm at F 1.2!
As you increased sensor size the field curvature of the prime image could be a problem.A meniscus lens would flatten the field.
The other problem would be getting an accurate focus and centering of the sensor.But the real time image would make it straightforward.
You can get around the field curvature problem ( or at least lessen it) by using an aperture mask, if you masked it down to say 14" or maybe 13" bring the F ratio up to say f2 or f3 ,much less curvature.
I am currently building a 8" Wright/Schmidt using a F3.75 mirror, match this with a large format camera and you get one hell of a lens :-)) super fast with a large field of view.
You shouldn't have a problem getting focus, just allow the for the height of the focuser in your calculations.
I have no problem getting focus with a 8" F3 Wright/Schmidt, although I haven't tried with any form of CCD on this scope, Hmm! something to try next time the scope is out maybe
David.
You can get around the field curvature problem ( or at least lessen it) by using an aperture mask, if you masked it down to say 14" or maybe 13" bring the F ratio up to say f2 or f3 ,much less curvature.
I am currently building a 8" Wright/Schmidt using a F3.75 mirror, match this with a large format camera and you get one hell of a lens :-)) super fast with a large field of view.
You shouldn't have a problem getting focus, just allow the for the height of the focuser in your calculations.
I have no problem getting focus with a 8" F3 Wright/Schmidt, although I haven't tried with any form of CCD on this scope, Hmm! something to try next time the scope is out maybe
David.
With a mirror 16" in diameter I had a mental picture of my 20D at prime focus.Hows that for a central obstruction?Maybe a 300D is better,it is smaller.Then as you said you could introduce an aperture at 2F to limit spherical abberation and field curvature.It was purely a gedanken experiment,but one worth trying if you could obtain a mirror this big relatively cheaply.
I only threw this in to see if anybody had similar evil thoughts.
I think if you can get this mirror cheap enough it would be well worth the effort to construct a superfast large lens, I have a friend who has a 18' F2.8 scope, he uses the modified 350d the FOV is huge and his images are nothing short of spectacular.
I have seen a few of these mirrors on Ebay over the last 12mths and they all seem to sell for around $800.00 US, abit too high a price for me, I will stick to finishing the 8"
it only cost me about $75.00 AU for the mirror.
I just did a quick calculation and found the central obstruction of a 20D
on a 16" mirror is about 15%!Assume the camera is behind a 160mm dia circular disc.It would be even less behind an ellipse.US $800 mmmm....
I think we have our telescope design's crossed :-(
The wright/schmidt use's a normal secondary the same as any newt, the focuser is in the same place as a newt at the focal point the only difference is the tube is twice the length,so the central obstruction is the same as a normal newt around the 15 to 17% with the 16" mirror you would still need a secondary around the 3" mark.
using a mask the effective focal length changes from F1.2 to F2 or more depending on the size of mask.
I think this would be a much simpler design to build.
Your'e thinking of this type the C18 (which happens to be my friends business.) http://www.astroworks.com/
You are correct.I was only thinking of a quick and dirty experiment.I have a Canon 300mm F2.8L and this keeps me busy enough.But we are all after more aperture and lower focal ratios.that could explain the (dare I say it) dobs.
That is a superb design at a correspondingly superb price.Unfortunately that is the first rule of the universe.You dont get anything for nothing.
If we all had the money we all could own a C18 but alas it's not to be, so I will make do with my cheaper scope.
For those that don't know the Wright/Schmidt design heres a piccy, all self explanatery I think.
mine will be slighty longer the mirror is exactly F3.75
