The question is simple, but I suspect the answer is not.
If you were building a cost is no object fast astrograph (I don't have the funds to do this, I'm just curious), what would that telescope look like? The goal is the best realizable optical design, and since these are fast scopes, over a very large field/image circle; ideally diffraction limited edge-to-edge. To even ask that question, I have to define a few things and set some rules.
1) Fast = f/4 or faster. However, if you have a good argument for something slightly slower, I may allow it.
2) Targeting a very large image circle; at least 55mm, but probably a nice corrected field closer to 70+mm to accommodate future sensors.
3) This must be a "reasonable" amateur telescope. Hubble, James Webb, etc. don't count. Thus, I'm going to say it must be under 500mm aperture. That's probably still a bit large/heavy, but it's a nice round number. The lightest 500mm fast scopes I can find are a f/2.4 Dreamscopes Hyperbolic Newton, which comes in at just under 90lbs. They do make a 500mm f/8 RC at 55lbs, which can be made much faster if you have the money. Generally, if it's technically possible to make it under 50kg, then it's good to go.
4) Must be ideally suited for todays and tomorrows fast CMOS sensors with small pixels (one of the reasons to go fast). I'm thinking something like the current awesome IMX533/571/461/411 family. i.e. image scales must make sense; CMOS pixels are likely to get even smaller.
5) No external constraints on support equipment such as computer process/storage, etc.
Some options I've seen out there:
1) Astro-Physics 305mm F/3.8 Riccardi-Honders Astrograph - AP claims it has virtually perfect color and all 5 primary aberrations corrected across the entire field. https://www.astro-ph.../producthistory
2) Dreamscopes f/2.4 Hyperbolic Newton Astrographs - Notionally the 16" and 20" meet my requirements when configured as f/2.4 systems optimized for the IMX461/411. Sure are pricey though! Also, while their client list looks impressive, these are so expensive that I don't think I've ever heard of one in use for amateurs. http://dreamscopes.c...-astro-list.htm
3) Dreamscopes f/x.x RC Astrographs - Nominally these are f/7.5 with a field flattener which brings them to f/8 and they would not meet my definition of a fast scope. However, Dreamscopes claims to be able to make these as fast as f/3.375! Given how expensive #2 are, I imagine a super fast RC is going to break the bank, but the RC design is sound. http://dreamscopes.com/pages/DRC.htm
4) ASA H400 (hyperbolic newton) - To my knowledge, ASA dropped the smaller versions and this is the only thing they make that qualifies. I've heard of a few minor quality issues on the smaller scopes. No idea since they sort of dropped the amateur market. Just as pricy as Dreamscopes. https://www.astrosys...scopes/asa-h400
There are a lot of manufacturers with interesting designs that are a far better value and practically good enough, but don't really meet the qualification of absolutely the best. E.g. Planewave CDKs, Officina Stellare RH/RiFAST, etc. I know that may upset some people, but there are optical interferometer tests out there showing these are good, but not the best. Under practical seeing conditions, they're generally good enough and as I said, far better trade of usable quality vs money. It's just not the question I'm asking.
I certainly haven't listed all the possible options. There's some rare designs out there with amazing theoretical performance (bring them on!). There's certainly other custom, high-end manufactures as well. I'll leave you with this link for some practical considerations for a fast astrograph: http://hbastro.com/F...iderations.html
So what's your perfect fast astrograph?
Edit: if you care, reading about the AP 305mm RH got me thinking about this.
Edited by LuxTerra, 05 December 2020 - 12:32 AM.