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Can someone explain prime focus for me please?

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Hi all,

Im a wee bit confused with prime focus, I building up my equipment to get an imaging set up for the winter. My newest addition is a Canon 300d with the apropriate adapter to pop it in my Orion ED80 refractor.

Been reading Steve's intro to astrophotography he says you need an extrention tube to replace the focal leagnth thats missing when you remove the diagonal. But how do i know what leagnth to get?

Also..being eager to test out the camera and not having the extention tube yet i thought that maybe i could get focus through the diagonal in the daytime, but it was just a blur. As i dont have a lens for the camera or the extention tube, i was just wondering if there was away to get focus through my scope, but maybe im just being impationt and should wait for the extention tube first? :D

Thanks in advance.


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But how do i know what leagnth to get?

If you remove the eyepiece and the diagonal and point the refractor at a lightbulb and then hold up your palm behind the rear opening at various distances you should be able to see where prime focus is as that point the image on your palm wil be a sharp lightbulb. That's where the sensor of your camera should end up (provided the lightbulb is quite far way, that is, much further than the focal length of your refractor).

Edited by themos
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the focal point will be slightly nearer the tube for stars (at infinity) than it will be for something indoors. It might vary by about a cm.The bigger the difference between the distance to the lightbulb and the distance to 'infinity' the bigger discrepancy between the two focal points.

i recommend using a tree that is several hundred metres away. This will be near enough infinity so as to mean that the focuser will only need to be moved by millimetres to get perfect focus on a star.

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