Are Longer Focal Length Refractors Making a Comeback??

[Hans Tucker]

TEC recently discontinued their APO200FL F8 but now they have introduced this beauty. Downside is it's a doublet .. not a triplet.


https://www.telescopengineering.com/telescope/apo200fl-f-11/

[glend]

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.

[Camelopardalis]

Optically, doesn’t need to be a triplet for decent colour correction at that focal ratio…

Besides, wouldn’t a 200mm doublet be front-heavy enough :lol:

[Don Pensack]

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.

[anj026]

That chart is a good reference for achromats Don. I don't think it is accurate for fluorite apochromats.

It is possible now to buy a 80mm f10 doublet apo. Also there is a 102mm f11 apo available. There is a niche market for visual observers.

[Don Pensack]

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.

[Hans Tucker]

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.

[m11]

I recently have gotten into longer focal length refractors courtesy of my Broken Hill friend's influence. :rofl:

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. :D

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.

[Bobbyoutback]

Good to hear Mel ' I just love the old long scopes as you know :D

Bobby

[anj026]

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.

[m11]

Haha its all your fault buddy. :P

I think another f/15 scope would be good. :D

[Hans Tucker]

APOMAX 130 f12. One of the finest long focal length APO Refractors. Reportedly only 26 made.

[m11]

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.

[m11]

That is one fine mighty looking refractor. :thumbsup:

[Bobbyoutback]

Or how about that secret big F/17 OTA that's not available :(

[m11]

Haha - thats when you need your massive observatory to house these fine instruments. :thumbsup:

[Bobbyoutback]

In my dreams Mel , but thanks for the fantasy :lol:

Bobby

[m11]

Haha - you never know in life where things take you. :D

[Bobbyoutback]

Yep for sure ,

Both of us have talked about Andy's scopes before ' he knows heaps more about long focus refractors :)

Andy can you advise us ~ Please ?

Best
Bobby :question:

[anj026]

Haha Bobby,

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.

[Bobbyoutback]

Thanks for your input Andy , you sure have had some nice scopes !
Mel & I like you would love to score a classic 8" refractor one day .

I would have trouble lifting such a big scope but Mel would have no problem & could probably handle even a 9" F/15 :lol:

Bobby

[anj026]

Telescope party!

[m11]

That is amazing collection of refractors you have Andy. I think a 8" long focal length refractor would be awesome in you already amazing setup of refractors. :thumbsup:

[m11]

Wow, simply can saw WOW. :eyepop:

I would like to ask, did you make the tubes for the lens?

[dikman]

Mighty colourful collection you've got there!:lol:

[anj026]

They are all gone now. Some links on how the tubes were made;

https://www.iceinspace.com.au/41-340-0-0-1-0.html

https://www.cloudynights.com/topic/325327-5-f12-istar-refractor-project/

[m11]

Thanks for the links - will have a read. :)

[Bobbyoutback]

Wow Andy !
What fantastic looking scopes ''' got me drooling :cool:

[david booth]

barry has 8 inch f30 refractor i have a f20

[m11]

Both special refractors David. :thumbsup:

[Bobbyoutback]

Any chance to get a photo of them ? or just one .
Would make my year to see such wonderful scopes :P

bobby

[gts055]

Long f ratio refractors appeal to me too. Some restoration to this 152mm f15 with lens by Barry Adcock resulted in a beauty. The enamel paint took many weeks to harden before the telescope could be mounted in the rings without marring the finish. Cheers Mark

[m11]

That is a nice looking scope Mark and nice work on the restoration.

I ended up with Barry's 6" triplet which I enjoy using when I went to look at both scopes. :D

[mura_gadi]

for a little steampunk and long F/l refractors is hard not to be impressed by what's on the Moonraker site imo.

https://moonrakertelescopes.co.uk/index.html

[m11]

They look stunning -very bespoke and a modern retro twist. :D

[AG Hybrid]

To answer the original question. If we are honest with ourselves. The answer is no. Astrophotography has gripped the users and manufacturers alike. More and more R&D and products on the market are for shorter focal lengths with more compact and mobile mounts to make setups more - what's the buzz word in the business communities, "agile"?

I don't think large, long focal length refractors have much of a future in the heavy light pollution world we live in. You can at least fit large SCT's and Dobs in the back of a small car to seek those more pristine sky's. The hassle of storing and moving a large long refractor and mount is objectively not worth the effort.