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Capital Adventures
November 28th, 2007
07:32 pm
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A personal history of digital photography

Yesterday, I was thinking about my earliest digital images.

I was thinking back to a previous life in which I was training to become an astronomer, trying to remember the specs of the CCD camera I used to take spectra at Mt John, when it occurred to me that those weren't actually the first images I ever made.
These are.






It was about 1994 or 95, and I was looking for a relatively clean and undisturbed space in which to finish figuring this mirror. Such a space was not available in typical student accommodation so I asked around the physics department. This came to the attention of Dr William Tobin who was writing a book on Leon Foucault and was interested in seeing a Foucault test performed. We came to an arrangement under the guise of a lab-course project which allowed me to co-opt a darkroom for a few weeks and these pictures were the result.

The camera was an Electrim camera, more usually used in an undergraduate astronomy lab for CCD photometry. I seem to recall that it was 640x480 pixels - that would be 0.03 megapixels, but I could be confusing it with some other piece of kit. Basically a CCD on a stick (for mounting on optical benches) it was controlled externally from a PC.

The mirror is actually 35cm in diameter and at f / 7.5 is not far off spherical so does not actually show much in the Foucault test except for that ghastly turned down edge. William and I have different ideas about what constitutes almost perfect and it's his book. I would have gone with perfectly adequate for visual astronomy once that edge has been masked off. That is my reflection in the left hand photo and is probably the closest we'll ever get to a self portrait in this blog.

I still have the mirror - you can see it in the photo, an appropriate secondary and eyepiece, and haven't yet made it into a telescope.
One day...



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From:wcg
Date:November 28th, 2007 01:29 pm (UTC)
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You could mount the mirror in a cardboard tube and make a dobsonian. It'd give you a chance to star test the whole rig.

Looking at the images, I'd guess you only need to mask off a few millimeters around the edge. I've seen commercially produced mirrors sold as 'diffraction limited optics' that were worse.
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From:dianavilliers
Date:November 28th, 2007 06:11 pm (UTC)
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A dobsonian was the plan, although I personally prefer a square plywood tube. As for the edge, a black permanent marker line gets rid of it nicely.
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