Re: [ATM] lensmaker for a very long focus objective

["Richard F.L.R. Snashall" <rflrs@rcn.com>]

David Harbour wrote:
> This is the easiest thing in the world to accomplish. One may purchase a
> lens of extremely long focal length, made in glass, of about three inches'
> aperture, from any vending optician. The longest focal length lens one can
> buy is .25 diopters; this will make an extremely long aerial telescope. I
> have already made some that perform wonderfully well. He would have to stop
> down the four or five inch lens in any case; he would not have to stop down
> a standard glass ophthalmic blank. Extra bonus is that it will be corrected
> for spherical aberration. With a color filter, it could perform stunningly.
> A .25 diopter lens yields a focal length of four meters; longer than he
> wants; next option is to ask for a .50 diopter ophthalmic blank. I have very
> smoothly fitting, "telescoping" tubes that these ophthalmic blanks (about
> $35 for glass, which he wants) will fit. I know the perfect brand name to
> buy, they are almost taylor made for an telescope with ophthalmic blanks
> (though I note that he wants to dispense with a tube altogether)

But doesn't it really need to be on the order of one-eighth of that or
about 1/32 diopter to give a 100' focal length.

BTW -- For something like this, would it be better for the focal length
to actually be a little shorter (say 90') in order that the infinity
focus, assuming the object to be looked at is stars, is actually
in front of above a person viewing with it?

> 
> David
> Great Plains Instruments
> http://www.atmsite.org/contrib/Harbour/index.html
> scarab@2cox.net
> 



-- 

Rick S.

http://users.rcn.com/rflrs

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