[ATM] The camera obscura

["Lock,Tony [Dartmouth]" <Tony.Lock@EC.GC.CA>]

When I first read Horace Dall's whimsical article on the camera obscura
in ATM II some years ago I was seized with the desire to build such a
device.  But like so many enthusiasms it weakened as years passed.  But
last year while on holiday in the UK I managed to visit the camera in
Edinburgh. This was the first CO I had seen and immediately the old
enthusiasm was renewed. The image, projected on a slightly concave table
several feet in diameter, was amazingly bright (once one had
dark-adapted) and while the images were notably less sharp at the edge
than at the centre, the useful field of view was much larger than I had
expected. 

So I began to consider the practicalities of making myself such a
device.

	1,  I have a house in an elevated part of town with good
roof-level views in all directions

	2.  I have a tower room 8 feet square at the top of the house -
ideal for a small camera obscura

	3.  I would need a lens of 4" to 6" aperture and perhaps 10 feet
focal length

	4.  I would need an 8" flat of reasonable quality

The lens is the real problem. I have done a fairly complete internet
search, but there is no useful information on the design of lenses used
in existing cameras obscura (camera obscuras?). I would need a long lens
but the longest commercially available lens seems to be a 6 inch f15,
and this is not quite long enough. So I must consider the possibility of
making my own. I have only made one mirror (an 8", f6 many years ago)
but the possibility of making a large flat and a doublet seems possible,
and quite exciting, given that I have just retired and will have time to
develop my optical skills.

But some questions immediately arise:  

Which of the many doublet designs gives the widest useful field?  I
would hope for a table screen about two feet in diameter. 

A six inch f20 lens should allow for excellent colour correction even
with a cemented lens, but is it reasonable to hope for both a wide field
and a relatively flat focal plane?  

Has any of this list's subscribers made a camera obscura? Or does any
member knows where I can buy a suitable long-focus lens.  But if I find
it necessary to make my own (which in my heart of hearts I would really
like to do) can anyone offer me any advice on the preferred design?

Thanks for considering this problem.....and if you haven't read Dall's
camera obscura article recently go an look at it again. It is hard to
read that article and not want a camera obscura of ones own.

Tony Lock
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