There are still plenty of long focal length refractors around, and they do not have to be triplets. Up above f12 a good Doublet is well corrected, and much lighter than a triplet. My iStar Doublet is a very nice scope, but living with a long refractor can be problematic due to the swing room they require, not to mention the storage space.
You might find this useful:
The normal rule for a doublet is that the f/ratio needs to be 3-5x the aperture in inches for the f/ratio.
I.e. an 8" refractor doublet needs to be f/24-f/40 to be without chromatic aberration.
With some of the exotic glasses available today, as short as 2X the aperture for the f/ratio, i.e. f/16.
An f/8 doublet 8" scope is not a scope I'd want to own, no matter what glasses were used in the lenses.
It might be usable for widefield scanning with a strong minus violet filtration.
Correct. As I said, a Chromatic Ratio of 2 is acceptable for FLP-53/FCD100/CaF2 lenses in doublets.
But you can't work miracles--a 2-element solution only crosses the zero point at 2 places, and it's usually the C & F lines.
That makes an 80mm f/6 OK, a 100mm f/8, or a 6" f/12.
Below those f/ratios, though, color will still appear, especially at high powers.
Celestron marketed a 4" CaF2 fluorite refractor back in the '80s, but it was f/9. I asked them about that and they said they had tried f/7 but "it had too much color".
I think that chromatic aberration seems to be less important to astrophotographers (they can tune/filter it out), but a lot of people are buying astrophotographic refractors and using them for visual use in pursuit of shorter f/ratios.
But no glass is going to make an 8" f/8 acceptable in a doublet.
Correct. As I said, a Chromatic Ratio of 2 is acceptable for FLP-53/FCD100/CaF2 lenses in doublets.
But you can't work miracles--a 2-element solution only crosses the zero point at 2 places, and it's usually the C & F lines.
That makes an 80mm f/6 OK, a 100mm f/8, or a 6" f/12.
Below those f/ratios, though, color will still appear, especially at high powers.
Celestron marketed a 4" CaF2 fluorite refractor back in the '80s, but it was f/9. I asked them about that and they said they had tried f/7 but "it had too much color".
I think that chromatic aberration seems to be less important to astrophotographers (they can tune/filter it out), but a lot of people are buying astrophotographic refractors and using them for visual use in pursuit of shorter f/ratios.
But no glass is going to make an 8" f/8 acceptable in a doublet.
The subject scope, APO200FL, is an F11 not F8. TEC have replaced their APO200FL F8 (Triplet) with a APO200FL F11 (Doublet). No doubt we will hear about the performance when the first scopes ship and reviews come in.
I recently have gotten into longer focal length refractors courtesy of my Broken Hill friend's influence.
I have a new refractor alt az mount coming soon for two of my long focal length refractors - one is a 6" f/8.3 triplet and the other one is a 6" f/12 Istar(pictured).
I find them somewhat drawn to them and a joy a use when use for the particular objects I like to use them on. I am plan to keep all my long focal length refractors and add to the collection where possible.
I am aware that most people are going to shorter focal lengths for refractors and as the years go back will see less people wanting to own and use one. I hope there will be more people that become aware of them and still happy to do some visual viewing with these scopes.
There is a lot of nostalgic charm to long refractors. I've had several in the past and still miss them at times. That is a great looking mount Mel.
Hi Andy,
Yah I agree, there is something about them that makes them a joy to use and something special that some them are quite old as well and still being used.
Haha thanks, not my work - Peter Read was nice enough to make a heavy duty mount for me which I will get later this month.
I just have a big soft spot for these things and a bit of experience. The attraction started for me when I became frustrated with the reflectors I had at the time. I used to browse Sky and Telescope magazines at the local libraries. Then I ordered a D&G 6" f15 lens from the USA and was amazed by the high contrast and flat field. The telescope was big and bulky but I made it work by having a permanent pier always ready to go with a simple alt az mount. The telescope was less then 15kg and easy to bring out of the shed and set up quickly. I never had to deal with collimation or tube currents or mirror recoating. Later I also had an Istar 5" f12, Antares 90mm f16.7, and a D&G 5" f15. All with lightweight thin ply tubes. Still think I would like to try a 6" or 8" f12 one day.
