Refractors: How big can you go ?

[reezeh]

17 hours ago, Demonperformer said:

I believe the largest refractor ever constructed was the Yerkes 40" (although there is a lens in the 48" schmidt camera). Beyond this deformation becomes an (insurmountable?) problem.

Well, if the clouds you get are proportional to the quality/usefulness of the purchase, I would guess I would get a clear sky just about at the end of the mortgage I would need to pay for it.

I dunno, although there's definitely an upper limit, maybe a permanently spinning lens might extend on what has been built. I do know spinning liquid mirrors have been done, but they're not tiltable.

It'd definitely be an interesting project for some research department. Or just build it in space or on the moon.

[Demonperformer]

But I think it would only be of academic interest. Suppose someone managed to engineer a 2m apo refractor (at huge expense) ... is that really going to out-perform the modern linked 10m reflectors, which can almost certainly be made at a fraction of the cost. The phrase white elephant springs to mind!

As for a spinning lens, I'm not sure how that could work. Spinning mirrors are (basically) liquid in a container, but the light has to pass through a lens, so it cannot be in a container. And, even if you had spinning liquid glass that didn't fly apart, how would you keep the different types of glass from mingling?

[Tommohawk]

I think in principle it should be possible to fabricate a segmented objective lens, in much the same way as segmented reflectors are constructed. This has a huge advantage in that the overall weight would be much reduced of course. Suspending the array of individual elements would be a nightmare and would probably pose an insurmountable practical barrier. Also the diffractive effects at the junctions would be greater I think than those of the equivalent reflector because of the steps in one or other surface. Chromatic aberration would be a third order nightmare - each component would need its own achromatising segment.

A Fresnel lens achieves a similar result and its easier to see how the optics work, but of course this suffers the same issues.

I'm guessing NASA will probably stick to mirrors!

[Demonperformer]

Yes, but I am still stuck on why would anyone want to (apart from to prove that they can)? Surely the aberrations produced by light passing through a segmented lens would be much greater than those produced by bouncing light off a segmented mirror. And, even if everything was perfect, would even the most ardent frac supporter maintain that a 2m frac would outperform a 10m or even an 8m mirror? Consider the comparison between a 4" frac with a 20" or even a 16" dob.

[Tommohawk]

1 minute ago, Demonperformer said:

Yes, but I am still stuck on why would anyone want to (apart from to prove that they can)? Surely the aberrations produced by light passing through a segmented lens would be much greater than those produced by bouncing light off a segmented mirror. And, even if everything was perfect, would even the most ardent frac supporter maintain that a 2m frac would outperform a 10m or even an 8m mirror? Consider the comparison between a 4" frac with a 20" or even a 16" dob.

Absolutely - but the OP did ask how big can you go!