Continuing my questions/theoretical search for getting more bang out of a buck (see the "achro and apo hybrid"-discussion), I've also been wondering about this:
If you look at this topic here; https://www.cloudyni...inodobson-r3071 , you'll see that the twin dobsonian of 18" he's using, is akin/equivalent to one 25" single, using the fact the formula is about 1,42 times the single diameter of the mirror.
Once again, I would think this is cost saving, especially if you go to larger diameters. that's because the expense goes up non-linear the bigger it gets. This is *certainly* true for large apo refractors. I'm wondering though: does the same principle holds up for refractors too, or is it more or lower than 1.42? Also, is it cumulative? (I would think, logically, yes to both questions, but I'm not the expert here).
Meaning, if you have a a double 'binocular' telescope - a quadroscoop, of say 10", would you get 10 x 1.42 = 14,2 x 1.42 = 20,16". Which would be larger than the largest refractor in the world still in use. So, again: why don't we see that? I mean, for smaller apertures, this may not outweigh the extra cost of the tubes and set-up and all that, but that doesn't hold true for big apertures. A 10" refractor might set you back 20000 dollar. 4 times makes it 80000, and add some 20000 extra overhead to it. So, say $100000. However, a 20+" refractror will set you back 3-4 million. So one would think people going for a big aperture refractor would rather go a quadroscope or binoscoop, rather than a single lens.
I'm sure people will say 'collimation' is the big drawback no doubt, but if you read the article, even for the dobsonians that didn't actually present much problem, so it should be even less for refractors. Also, in that article there also a lot of additional advantages described, apart from the bigger 'composed' aparture.
The only thing I could find in regard to big refractor bino's is this one: https://www.cloudyni...saw-firstlight/ . It says it's a bino, but it doesn't give any specifics as to how much more it gathers light in comparison to a single lens. If it's cumulative, and they've got the money, couldn't they as well get rid of the counterweigths one sees there, and put another double apo on it? ;-)
Anyway, why isn't this pursuit more? Yes, I know the market isn't that big for big refractors anyway, but still, where there is, there should be a market for it, but one hardly sees one. As far as I can see, comparing the prices, up to about 8" should be cheaper than the same/similar one big aparture, when it comes to apo's.
Edited by SPastroneby, 20 August 2017 - 11:37 AM.