Telescope Reviews: Why aren't eyepieces lighter?

In part it's because there are also some other competing objectives that many people find very desirable in eyepiece designs, such as width of AFOV and creating a sense of "immersiveness, the achievement of which also as a corollary tends to increase the difficulty of fulfilling another important design objective - minimizing aberrations. In short, fulfilling any of these design factors (especially all together) tends to require wider, sometimes thicker lens elements, and more of them. It's much harder to minimize weight while achieving these design objectives than it is with simpler three-element (narrower AFOV) designs like Orthos or even Plossls (many of which aren't necessarily simple three-element designs anymore).

If Al Nagler couldn't figure out a way to make a 31T5 Nagler lighter at the time it came out, it was probably extraordinarily difficult to get a sharp, minimally aberrated 82-degree widefield in a package much lighter and smaller. But nonetheless, TeleVue must consider weight/bulk reduction to be an extremely important objective - the Ethos line is much less dense in weight/size ratio compared to The T4 or T5 Naglers. ALL five members of the Ethos family released so far weigh in at between one and one and a quarter pounds. My guess is that what's slowed TeleVue from releasing the final member of the Ethos line (somewhere between twenty and twenty-five mm focal length) isn't the feasibility of producing an optically excellent eyepiece at a suitable point within that range, but rather doing so within as desirable a limit on bulk and weight as possible.

As to cost of materials used - it isn't the bulk raw silicate glass that's the major component of cost, but rather:
a) the exotic elements added to suitably alter/improve the refractive/transmissive properties of the glass
b) the processing of the glass to obtain suitably high quality for lens-making (rather than e.g. ordinary window-pane) glass
c) the amount of grinding, polishing, coating, etc needed to make a high-performance lens element

So - I'm not sure how much cost advantage could be feasibly gained if was possible for telescope lens maker to switch to some form of polycarbonate rather than glass as the base material - its use in eyeglass lenses suggests there might be some, but OTOH telescope lenses involve additional considerations than do eyeglasses to achieve the necessary performance. The bottom line, however is that a huge component of the cost isn't simply the primary bulk of raw materials involved, but the exotic elements needed plus the processing. Maybe technological advances will change this in the future, but for now, there's good reason TeleVue and Pentax are sticking mainly to (exotic, more expensive) glasses for telescope lenses.

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Chris M., aka "First Sight"
Orion XT12i Dob with Moonlite CR-2 focuser
WO Megrez 90 refractor on UniStar Light mount
Nikon 10x50 Binoculars