Measuring points to gain focal length?

[trick]

I understand that for a F5 primary mirror this equals 50 inch focal length?
Yes No

I know I measure from the top of primary the distance up and thru the secondary for a total of 50 inches.

Yes No

If I am correct to this point where? or what point in or around my focuser is the end point I am measuring to. See attached diagram...........

Thanks in advance
Trick

[Ian Robinson]

Play with his handy program http://www.catseyecollimation.com/designie5.html

it'll give you a handle on this and help you calc the point there the focal point is in your scope.

[trick]

Thanks Ian

[Barrykgerdes]

Originally Posted by trick

I understand that for a F5 primary mirror this equals 50 inch focal length?
Yes No

I know I measure from the top of primary the distance up and thru the secondary for a total of 50 inches.

Yes No

If I am correct to this point where? or what point in or around my focuser is the end point I am measuring to. See attached diagram...........

Thanks in advance
Trick

No
F5 is the ratio between the focal length and the aperture
the approximate focal length will be 5 times the aperture in aperture units of measurement for that lens/mirror.
If the mirror is 10" at F5 yes the approximate focal length will be 50"

This distance will be the sum of the distance to the centre of the diagonal plus the distance from there to the focal plane of your eyepiece.

Barry

[trick]

Yep sorry I should have stated its a 10" primary...