What are the challenges for making fast APO refrac

There are few who make fast F/5 widefield APO refractors Takahashi with the FSQ106 and the Televue 140 (past tense) and when you get into bigger APO's such 160 and especially in the 180 and up the fastest scopes are F/9. I see Nikon can make an F/2 in a camera lens what are the technical obstacles when the aperture increases for APO telescopes? Is it there are just fewer and substantially more costly pieces of flourite as the size goes up and no matter how sophisticated the grinding equipment you always end up with pieces in the junk pile? When the term hand figured is used what exactly does this mean? A person is hand removing the material from the lens being ground? If an F/5 can be made in the 4" aperture range is it possible to even make a faster such as an F/4 in the same 4" aperture and would you want it if it was possible? It seems f/4 is commonplace in dobs I would just think in all the advances in technology at some point there would be cost effective ways to make faster APO's.

When I hear of people talking about the difficulties in making fast scopes is the amount of scrap (and it associated amortized cost the biggest factor) is the ratio something like for every good lens that is made three end up on the junk pile? Is their also a higher degree of difficulty in the manufacture and assembly of these fast APO even if the lenses can be ground accurately?

Could a really large company with greater resources and maybe technical knowhow such as Zeiss, Nikon or Pentax make these large fast APO telescopes especially as Nikon can make an F/2 lens?

I know achromats can be made faster vs. APOs and why is this the case?

Thanks in advance for the replies.

A nice 600mm F2 lens would be a foot in diameter and two feet long. It would weigh about as much as your riding lawn mower. Well, maybe not that much but you'll need a good tripod.
Harry

I not positive but I think spherochromatism puts a limit on how fast you can make a lens.

Mark Nevitt

I consider a wide field three degrees or better. If you play with the numbers, you'll find you can not get much aperture with such a wide field and get all the available light into your eye. A 150mm f/4 can provide a 4.4 degree field with a 2" 55mm Plossl or a 41mm Panoptic, but either case your eye will act as an aperture stop.

A 27mm Pan would provide a more useable 6.75mm exit pupil, which some folks could fit through their pupil, and would give a field of 2.9 degrees. Most of us older folks would not use the entire aperture.

Clear skies, Alan

We live in a wonderful time for people who love high end apo refractors. Some prefer longer apos and some faster systems. There are distinct advantages to both. But making a fast apo triplet that has the same degree of correction as a longer system is a big challenge.

We wanted to make the shortest, fastest apo triplets we could. The market already has some excellent Petzval designed fast apos, like the Televue, so we saw no reason to make another one of these. Televue fills this niche nicely. Instead, we opted for an ultra fast apo triplet, with all lens elements in front and a shorter main tube. This offered better portability length wise but resulted in a heavier optic (since the glass is thick and steeply curved). So some will prefer one over the other. Doing something different has always appealed to me.

We worked with our team including our extremely talented lens designer and optical fabrication people in Russia for three years in developing this system. We wanted to make apos up to 135mm at around f-5 but we wanted to meet the following rigorous criteria:

1. Euqal or better spherochromatic correction than conventional telescopes that are slightly longer - the biggest challenge
2. Reduced reflections
3. A more corrosion resistant, thermally stable cell
4. Since these would be coveted by Ap'ers, a large aperture apo field flattener would be needed for each.

All of this was accomplished by using not one but two fluoro-phosphate elements in our triplet design. A four part lens cell was designed with centering rings that are literally machined around each fragile glass element. This allows mechanical centering to better than 3 microns! Fortunately, the Russians perfected this method years ago as part of their space/missle program. So the final solution resulted in an extremely fast, accurately aligned apo triplet that is literally rugged enough to "launch".

But it does make for a much more expensive optic. Even when we reduced our introductory sale price down to cost (more or less) the dual fluoros are expensive to make. For example, our 5" f-7 is about 5K whereas our 135 f-5.44 is about 9K at the intro pricing level. We also have two others dual fluoro apo triplets, the 111 f-4.95 (!) which is $5990 and the 127 f-5.7 which is $7900. Again, one should not compare these to normal apo triplets. These telescopes are the fastest triplets using very short tubes and providing exceptional performance despite their fast speed. The ray fans and spot diagrams are extremely impressive.

We can only make about 12 of these objectives per year since they are very difficult to make. But as owner of Sv135 #1, I can attest to their incredible performance.

Vic Maris