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:
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:
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:
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!
For those interested in imaging and image processing with DSLR, CCD, webcam or film.
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