Focal length help please!

[gooseholla]

Hello,

My telescope came without any indication to the focal length. How do I work it out. It is a newtonian. Is it the length from primary to secondary, or is the portion from secondary to focuser also included?

I will either have a 1200mm or 1500mm depending on the answer! Or some new unknown!!

Thanks

John

[Laurie61]

Hi, the focal length is the distance from the primary mirror to the focal plain which is roughly were the eyepiece is when the scope is in focus. Is the scope the OO in your equipment list, if so they come in f4 and f5.3 flavours, the tubes tend to be a bit smaller than their focal length. So an f 4 would be up to 1200 mm and the f 5.3 1600 mm long.

[gooseholla]

Hi. I am quite sure it has OO mirrors in, the guy and the sticker on the primary says so. I am, however, not sure about the tube itself! 

If I measure from the primary to the focuser, it is roughly 1500mm, so I guess, if it were a complete OO original setup, it would be the f 5.3 1600mm (although the mirror was moved up the tube, so would probably be nearer 1500mm now!).

Thanks for the help.

John

[ronin]

It should be Primary to focuser as in Primary to Secondary + Secondary to Focuser.

The positioin on/in the focuser will be approximatly to the tend of the tube you drop the eyepiece in when it is about mid point of the travel. It is not to where the focuser attaches to the scope.