A home-made focal-reducer for a webcam

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A home-made focal-reducer for a webcam

Post by rwilkinson » Thu Nov 15, 2012 11:41 am

I got a great tip from Gerald the other day - the object-lens from low-cost miniature binoculars can be used to make a focal-reducer for a webcam.

But why would you need a focal reducer? Well the webcams only have tiny CCD sensors, and so yield a very small field of view when attached to a long focal-length telescope (e.g. 2000mm for a C8). This makes finding and framing the target very difficult, or when used in an off-axis device can mean that there aren't any guide-stars in the frame!
And a halving of the operating focal length also means that the image is four times brighter, thus shortening exposure times.

Now I already had an old 8x21 mini-binocular which I'd split into a pair of monoculars (one of which I use as a finder-scope for my DSLR), and it was easy enough to unscrew the object-lens from the spare optical tube:
Removing the object-lens from an 8x21 DCF binocular
reducer1.jpg (204.65 KiB) Viewed 4015 times
This lens is of cemented doublet construction, and has a focal-length of around 75mm.
I found that if I built up its outer diameter with a couple of layers of black PVC tape it became a push-fit inside the webcam-to-eyepiece adaptor:
Adding PVC tape around the edge of the doublet so that it fits the inside of the adaptor
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The completed focal-reducer assembly with the lens wedged into one of the "steps" on the inside of the adaptor
reducer3.jpg (199.47 KiB) Viewed 4015 times
Then this handy on-line calculator from Wilmslow Astro
predicted that with my 75mm focal-length lens fitted at about 35mm from the CCD, I should expect a reduction of 50%, i.e from f/10 to f/5.

Finally, here's a comparison of the field-of-view of the camera installed in the off-axis unit on my C8 SCT before and after the focal-reducer was fitted:
Part of the structure of a nearby gasholder imaged with and without the focal reducer
comparison.JPG (17.94 KiB) Viewed 4015 times
Incidentally, this picture illustrates another important point - do all your experimental testing on daylight targets, so that you don't waste precious starlight on a clear night!

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