T3D custom lenses for viewers

[Peter Abrahams <telscope@xxxxxxxxxx>]

John, the conference is only a few more days.  Don't let it breeze by while
answering this:

>the guy designs oculars as a specialty for
>a living and has a company of twenty people and he's

Well that takes the air out of my  objections....but of course I still have
things to say.

>> As far as I know, you need to view on-axis in any normal 
>> eyepiece.
>The Star D has no IPD adjustment and it works fine with 
>a simple lens albeit non-ortho at 50 mm.  The V-M is also
>fixed at 65 mm.

Give me a break on the VM, except as an example of what not to imitate.  I
haven't seen a Star D, but it doesn't sound like a quality optic.

>To lay numbers on it, the 5th percentile is 58.5 mm and the
>95th percentile is 68.5 mm so the standard stereo IPD of 65
>should cover quite a few people with +/- 5 mm. 

Ask your contact about viewing through an ocular lens of (x) dia., 5mm off
axis.  It would be fantastic if he said you could still see a flat field
across a 7p chip, at (~40mm) f.l., while viewing offset from the axis by
5mm.  More likely, he'd have to trace it out before opining.

>I don't believe
>the other requirements of a loupe are 100% compatible with the
>other requirements of a stereo ocular.  Needs such as entrance
>pupil size/location or field coverage would be different in 
>degree if nothing else.

I need convincing on that...field coverage for 7p as opposed to a full 35mm
neg. is how much, and entrance pupil is different in what way?
Offhand, it seems to me that a loupe is a pretty close model.

>> The price for these lenses almost certainly wouldn't include a cell
>Then I've been misled.

No, I'm probably wrong, but I believe most lens fabricators don't routinely
thrown in a cell.

>> Fabricating a viewer is a very difficult project, if you're 
>> going to include IPD and focus adjustments.
>I agree and would suggest these need to be mated to existing chassis.

So the project is to take an existing viewer and fit a lens to it.  I
doubt, but without conviction, that the small diameters used in most
viewers would accomodate a flat field view of a 7p chip.  They could be
machined out to a larger dia. without too much trouble.  I wouldn't want to
use the red button for this, it is already the best (or among the best)
optics (why cannibalize the best?), so pick one that has good mechanical
parts, is available, and has mediocre optics.
_______________________________________
Peter Abrahams   telscope@xxxxxxxxxx
the history of the telescope, the microscope,
    and the prism binocular


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