Primary Mirror Position and Focus Point

[Sam]

Hi All,

Does anybody know how much the focus point moves when the primary mirror of a reflector is moved up the tube - is it 1:1 or some other ratio?

cheers

Sam

[blinky]

I assume it is 1:1, cannot think why it would be diffrent.

[John]

I think it must be 1:1 as well.

The only other issue to look out for I guess would be if the light cone becomes to big for your secondary mirror as you move the primary mirror towards it.

John

[Sam]

Thanks Guys,

Yeah logically thinking about it the secondary is flat so it must be 1:1. I suppose moving the mirror further up effectively begins to reduce the aperture as the reflections from the edge of the primary will miss the secondary.

thanks for your help

cheers

Sam

[barkis]

Everyone's right. If you move the mirror towards the diagonal a distance of two inches, the focal point will move two inches towards the end of the drawtube. A one to one result. But beware of vignetting if you move it too far.

Ron.