33 replies to this topic

[RichA]

GOTO made six in the 80mm f/15 configuration and three at f/12.5

Amazing, I've never seen any of these telescopes in the wild, they must have produced very few of these things.

[photoracer18]

Pentax briefly offered the standard achromat and ED doublet apochromat versions of their 100 f/12 at the same time. For a while it was possible to buy the ED objective separately as an upgrade. I think they were clearing out the last of the standard objectives, and testing the waters to see if the market would pay for the apochromat before discontinuing the achromat. The f/12 ED design wasn't around for long, as they then switched over to the shorter EDHF designs. 

 

Chip W. 

Not sure about that. At the time the 100/12 was out so was the 85/12 but the ED version of the 85 was F9 not F12.

[ccwemyss]

That's why I didn't mention the 85. My recollection was that only the 100 had the option of an ED upgrade. The non-ED 85 was often mistaken for an apo, so a switch to ED wouldn't have made a big difference for it. The 100 had noticeable CA, and ED tamed that at f12. ED allowed the 85 to become shorter. 

 

Chip W. 

[Jon Isaacs]

True! More glass is always worse other things equal.

 

And the APO will be figured to eliminate spherochromaticism, which will affect the spherical correction.

 

-drl

 

Are you using singlets?

 

All other things are never equal, particularly when comparing a 2 element design with a 3 element design.

 

Jon

[deSitter]

Are you using singlets?

 

All other things are never equal, particularly when comparing a 2 element design with a 3 element design.

 

Jon

 

A Tolles solid eyepiece is said the be the last word in detail on planets. But I don't have one.

 

The problem with a singlet is that without a cooperating 2nd element, the only way to get short focal length is with extreme curvature, leading to aberrations too close to the axis. Nevertheless people have used tiny spherical singlets. The first good microscopes were actually tiny spherical singlets.

 

-drl

Edited by deSitter, 09 April 2025 - 12:39 PM.

[ccwemyss]

Well, it appears that Pentax also had an ED 85/1000.

 

https://zenmarket.jp...ode=h1180979517

 

That's the first one I've ever seen. I was still working in the scope shop in '82 and don't recall ever seeing an announcement about an ED model. 

 

Chip W. 

[Lagrange]

GOTO made six in the 80mm f/15 configuration and three at f/12.5

That triplet fluorite super APO must have been staggeringly expensive!

 

Does it use a positive-negative-positive design with two fluorite elements, or is it one fluorite and one ED glass?

[RichA]

A Tolles solid eyepiece is said the be the last word in detail on planets. But I don't have one.

 

The problem with a singlet is that without a cooperating 2nd element, the only way to get short focal length is with extreme curvature, leading to aberrations too close to the axis. Nevertheless people have used tiny spherical singlets. The first good microscopes were actually tiny spherical singlets.

 

-drl

Not the first microscopes, but the best at the time.  People may have these restricted field eyepieces, but likely don't use them very much.  A Tolles is no better than a singlet, perhaps worse because of its glass thickness.  One comparison suggested it, a monocentric and a regular Plossl produced roughly the same central definition. It's essentially a glass rod with two convex faces.  I got excited once, thinking I found one, but it turned out to be an optical plug with flat faces.  But, as you'll see, even in something as forgiving as a small f15 refractor, it's not so great.

 

https://www.cloudyni...d-construction/

[PawPaw]

That triplet fluorite super APO must have been staggeringly expensive!

 

Does it use a positive-negative-positive design with two fluorite elements, or is it one fluorite and one ED glass?

Here is an article on Goto I authored a couple of years ago.  

https://www.cloudyni...-for-1976-r3393