Re: [ATM] The camera obscura

[Ken Hunter <atm_ken_hunter@yahoo.com>]

OR...

If you already have a lens of too short focal length,
you can use another lens to lengthen the EFL.

A barlow lens (negative achromat) can be placed into
the light path and adjusted so the EFL is right for
your main-to-image separation. The main restriction is
that the Barlow needs to be "non-vignetting" at the
desired position.

Ken Hunter

ps... Have you tried a pinhole camera obscura? They
are kinda neat and no lenses required.

--- Michael Lindner <mikell@optonline.net> wrote:

> On Wednesday 25 August 2004 11:13 am, Lock,Tony
> [Dartmouth] wrote:
> > 	3.  I would need a lens of 4" to 6" aperture and
> perhaps 10 feet
> > focal length
> 
> D&G Optical advertises 5" through 12" achromats at
> f/12 to f/30 at 
> http://www.dgoptical.com/objective.htm
> 
> > 	4.  I would need an 8" flat of reasonable quality
> 
> Diagonals of 6" and more are available commercially
> at Newport 
> (http://www.newportglass.com/appxdiag.htm) and other
> places. If your lens is 
> 6", the diagonal probably doesn't have to be much
> bigger (in fact, if you put 
> the lens before the diagonal, the diagonal can be
> smaller than 6").
> 
> Either way, I'd recommend an optical window in front
> of the first element to 
> protect it from the elements (pun intended). You
> could either grind regular 
> window glass for that or get a thin water white
> plate glass blank from 
> Newport (http://www.newportglass.com/acorretr.htm)
> or other vendors...BTW 
> Newport's catalog is at
> http://www.newportglass.com/angwcat.htm
> 
> This isn't intended to discourage you from making a
> lens or flat, but if you 
> choose to buy, the parts are out there.
> 
> BTW, I'm not recommending these as the vendors you
> should use or not use, 
> these were the first Google came up with.
> 
> Clear skies...
> -- 
> Michael Lindner
> http://www.starastronomy.org ***
> http://home.att.net/~mikel
> http://www.atmsite.org *** http://www.atmlist.net
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 



		
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