F What?

[bkm2304 (Richard Brown)]

Here's a thing. When I get my 6" F5 mirror and focus a light source against the wall, the distance from mirror to focused image on the wall is F14. In the telescope tube the focus is OK at about F5 or 6.

So what's going on? Am I looking at prime focus or what? The focused light source is only clearly focused at F14 - no other distance.

Any explanations?

Richard

[Blue Skies (Jacquie)]

It should looked focused at f/10, twice the focal length. It's to do with optical ray tracing. I can't think how to explain this at the moment (not a good headspace day), perhaps someone else can help out.

[5ash (Philip)]

The distance of the reflected image from the mirror depends on the distance of the object from the mirror . For an image at great distance (approximately infinity) the image distance will be 750 mm from the mirror which is the focal length .In your test the object was relatively close to the mirror giving an image distance of 2100mm. Since focal ratio is focal length divided by aperture your. focal ratio will always be 750/150 = 5. When determining focal length by projection of an image the object must always be effectively at infinity(a very large distance compared to the focal length).
Hope this answers your question regards Philip