Prime Focus vs long telephoto lens (Canon, 50mm, adjustment, camera)

[johnrex62]

I read a blog recently in which the author purports his 4" telescope has a 600mm effective focal length and he uses it in Prime Focus mode to photograph deep space and planetary objects. He states the effective magnification is 600mm / 50mm = 12x. How is using a telescope in Prime Focus mode different than using a long telephoto lens?

I have a Canon 7D, a 400mm lens and a 2x teleconverter, which makes an effective 1280mm 35mm focal length. That would seem to me to be 16x without the frame size adjustment and 25.6x with the APC frame equivelency. How much different is this than using a 4" telescope as a prime focus lens?

I don't know much about telescopes, but at this level of simplicity it seems the long lens would be more effective than the 4" telescope once the tracking system is added.

[Fuzz]

Originally Posted by johnrex62

How is using a telescope in Prime Focus mode different than using a long telephoto lens?

Not much. The difference is tracking. Almost all telescopes nowadays come with computerized mounts that can track stellar objects. You'd have to buy a special mount for your camera or create your own "barn door" mount.

For true long-exposure astrophotography (exposures > 30-60s), you really need guiding (either auto or off-axis), which you can only do with a telescope on an equatorial mount.

[TN Tin Man]

Numerous sites on this. http://www.tucsonastronomy.org/barndoor.htm