[Dennis]

Hi Boxy

Simplistically speaking, when used visually your telescope has two main components - the objective lens or primary mirror that gathers the light and your eyepiece which you then use to view the image produced by the objective. The objective actually magnifies the image so at the prime focus, there will be a magnified image of e.g. the Moon or Jupiter. Your eyepiece then “looks” at this magnified image and magnifies it even further.

This is a bit like taking a magnifying glass and focusing the Sun onto the back of your hand – OUCH – DON’T DO THIS. The magnifying glass is the equivalent of your primary mirror and the burning sensation on the back of your hand is the IR part of the Sun’s spectrum making things hot. If you had the mind of a Zen master and could tolerate the burning, you would see a small visible light image of the Sun on the back of your hand. This is what your webcam will be taking an image of.

I have read in several places that inserting a webcam in place of an eyepiece is like inserting a 6mm focal length eyepiece. So the image of the Moon or Jupiter on your computer will look similar to what you might see visually using a 6mm eyepiece.

In (terrestrial) 35mm film photography, a standard lens of 50mm focal length (fl) was said to have produced a photograph at x1 magnification, similar to what the eye sees. If you replace the 50mm fl lens with a 200mm fl telephoto lens, the 35mm film image will now be magnified 4x (4x50=200).

If you then fit a 1000mm fl telephoto lens to the camera, the image on film will appear 20x closer, or magnified 20x (20x50=1000).

So, when the telescope objective is focused correctly, the webcam will be “seeing” an image of what the primary mirror is pointing at, and won’t really be taking a photo of the secondary mirror that your eye appears to be looking at.


Cheers

Dennis