Measuring Newtonian OTA focal length

[mhc]

Hi Folks,

Is the focal length of a Newtonian OTA measured simply from the primary to the secondary, or does the path include the distance from the secondary to the eyepiece too?

My ancient SX150 seems to be only 700mm from primary to secondary. Does that sound low?

Thanks,

Mark

[barkis]

Hi. Mark,

The focal length of a Newtonian mirror, is half it's radius of curvature. But basically, the focus from an object at infinity, ie a star,

must fall somewhere inside your focuser. So the distance from the surface of your mirror to the centre of your diagonal mirror plus the distance from the centre of your diaganal mirror, to a midway point of your drawtube, should give you a rough measurement of the scopes focal length.

If you shine a pen torch down the tube in a horizontal position, get a reflection of the torch in your mirror, slowly walk backwards and you will see the light from the torch start to flood the mirror with light. At a point where moving the torch gently up and down makes no difference to the light, that is roughly the radius of curvature of your mirror. If you can measure that distance, then divide it by two, that will be the focal length too. Bit boring, but it works

[mhc]

Not boring at all!

Many thanks, will give it a go tomorrow.