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Old 16-03-2005, 07:19 PM
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iceman (Mike)
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Join Date: Sep 2004
Location: Gosford, NSW, Australia
Posts: 36,709
Hey Nort..

That's true, prime focus means the webcam (or commercial CCD camera) is directly in your focuser barrel. You don't use an EP with this method.

The light is captured directly on the CCD chip, and the telescope is acting like a huge telephoto lens. The magnification comes from the focal length of the telescope mirror. Your 8" dob is like 1000mm FL isn't it? Most people claim that a ToUcam capture acts like a 4-6mm EP, so what I do is put in my shortest FL eyepiece and check the seeing.. if it looks pretty good in the eyepiece, it might be ok with the ToUcam. If it looks bad in the EP, it's not going to be good in the ToUcam.

To get extra magnification with the ToUcam, just like with an eyepiece, you use a barlow lens. A 2x barlow effectively doubles the focal length of your scope, so your 1000mm becomes 2000mm, and your image scale will be twice as big as a result.

Having said all that, you take the lens out of the webcam to do prime focus. If you're using a digicam with a fixed lens, you have to use an eyepiece to focus the light into the lens of the camera. If you've got an SLR or DSLR, then you take the lens off the camera and use the adapters to attach it to your focuser.

Hope that helps.
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