What are the challenges for making fast APO refracPosted By Vic Maris |
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
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
