Making a APO with normal glasses

[Jim Fusco]

Hi Andrey,

I was wondering if an apo was ever made of normal glasses. I'm sure you know something about this.What I understand on of the glasses need to be higher with the abbe number , but this can be done with noraml glasses.

Jim

[astreya]

Hi Jim,

as my work is design of APO lenses and other optics, I surely should know all in this field. And I try to.
Of course, nowdays most APOs are made on ED-glasses. This way is simplest in APO design. But there are some problems with ED-glasses which limit production of big APOs for example. So designers are searching for other solutions, including making APO on glasses with normal dispersion.
And I'm interested in this problem too and also have my own calculations.
I know several projects, but only one realisation by now. Such APO was made this year in Novosibirsk, Russia. It was shown at some star parties here. This lens has long air spaces design (LAS Lens).

It's not a simple thing to design such APOs and by now they are rather complicated, have more elements than ED APOs and are longer because of long air spaces. But I think that it's necessary to research this way so as to find some better solution because problem with ED glass for big apertures became obvious. We're working in this way.

[Jim Fusco]

Interesting, however there must be a abnormal element 1 or 2 in the optical layout or not. A wide air gap design would allow a rather balance scope. I guess air pockets could develope, however the cell supports could be open to allow movement of air.
Do you know the size of the image circle offered by this design? Now this is not a Petzval design, but a wide air gap, what is the difference? For a petzval to offer APO performance it will need a ED element, where the wide air gap doesn't. I'm not sure.

Jim

[astreya]

Hi Jim,

First of all, I'd like to say, that it is only one reason to speak about LAS design - this design uses only normal glasses.
This is the main and principle difference between this design and all others. If we use ubnormal glasses, in particular ED-glasses, there is no reason to make LAS design, because with ubnormal glasses it's possible to make compact doublet or triplet which have no disadvantages of LAS lens design.
Then, Petzval is a design for imaging. It consists of two components, placed rather far from each other. Each component usually consists of two elements stuck together among themselves or shared by thin air spaces. In this LAS design there are three components. Front component is a singlet with very small optical power. Second component (multielement) placed on a distance from front element which is significantly more than in Petzval design. And rear component is also multielement and placed not far from the second one. The diameter of second and third components are 1.5 times smaller than front singlet diameter. Total length of the design is very big. About image quality. I have my own calculation of such design and can say that it is possible to make rather good lens for visual and imaging with real APO quality, like ED-triplets have. It needs 6-7 elements of normal glasses.

[Jim Fusco]

Andrey,
This seem like a interesting approach and thanks for sharing this info. Being that high quailty optical glasses are use and no ED, large APO's could be more afforable. Is there any plans on size? Will the LAS system offer a fast design F/7- F/6. As far a color correction, how tight Chromatic Shift might be between the 700 - 400.

Jim

[astreya]

As for optical characteristics of LAS lens, see diagrams for 180 F/7 design.
You see that color correction is rather good from 479nm to 656nm. Of course, it's not super apo but real apo. Faster design can be also made.
Total length of the design with back focus is approximately 1700mm.
I think some time is necessary to realise all pro and contra so as to make a decision about making LAS lenses.