[ATM] ATM: My re to a fellow interested in very long focusobjectives made with opthalmic blanks

["David Harbour" <scarab2@cox.net>]

Abelardo,

I am very enthusiastic to hear from you.

I remember an article in Sky and Telescope many years ago where the author
showed how to use two opthalmic blanks, spaced about three or four inches
apart with a little cardboard tube- these opthalmic blanks (about $25 each,
from any dispensing optician (eyeglass seller, eyeglass lensmaker) could be
mounted in different combinations of diopters to produce ENORMOUS focal
lengths; he cobbled one up to project the image of a total eclipse of the
sun on a wall for some children- and since the aperture of these blanks is
nearly three inches, and the e.f.l. was ENORMOUS (60 feet or so, I think) he
obtained a very bright and surprisingly sharp (and very large) image.

The author gave a formula for calculating which two blanks to use to get
whatever effective focal length one wanted; of course, I have slept since
then, and I do not remember the formula. I will poll the atm list members to
see if any of them remember this article; one could probably find it by
checking out old copies of Sky and Scope published near the date of solar
eclipses. I sure wish I could remember that formula; because it would be
much easier to string two inexpensive lenses together for a 60 foot long
telescope than trying to grind such a shallow single objective. He had a
formula to figure which two lenses (negative and positive, I should imagine)
together to give enormous focal lengths. Pretty easy to make: cut some
tubing, tape in the blanks.

Perhaps some of the other memebers of the list know about this author, and
his ingenious two element objective made with a short cardboard tube, tape,
and about $50 worth of eyeglass blanks, and his formula for picking the
right blanks. I will get back with you.

Are you a member of the atm list? Let me know if you would like to join; all
the atms who are worrth their salt are on the list. If you would like to
join, I will ask Michael to send you an invitation to join. Betrween all of
us together, THERE IS NOTHING WE DO NOT KNOW ABOUT TELESCOPE MAKING!!!!!
(Ha, ha,- a little humor, occasionally, too!)

I am going to post a copy of this note to the list, so we can get help
finding that article, and then I am going to visit your link to see these
camera obscura photographs. We'll see if we can find out something useful
for you, Abelardo-

Dave

----- Original Message ----- 
From: "Abelardo Morell" <abe@abelardomorell.net>
To: <scarab2@cox.net>
Sent: Thursday, May 05, 2005 9:37 AM
Subject: lensmaker for a very long focus objective


> Dear David Habour:
>
> I read your very interesting note regarding long focal length lenses.
>
> I have been making camera obscura pictures inside large dark rooms with
> just poking a hole and letting an image of the outside come in. Lately
> I have been using diopters to let more light in and have the image more
> in focus. I have used  a + .25 which focuses on the opposite wall at 12
> feet and it's great. Now I want to get a diopter that focuses longer
> than that to 18, 20 and even 30 feet. It seems hard to get diopters
> ground accurately to below .25 to say  +.18 , +.10 etc.  you can see my
> camera obscura photographs at:
>
> www.abelardomorell.net
>
> Do you have any ideas. Thank you.
>
> All best,
>
> Abelardo Morell
>


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