Focal length of Vixen 120mm f/8.3 *objective only*

Hello all,

The subject title pretty much says it all. I am using the objective lens from a Vixen 120mm f/8.3 scope as a collimating lens in an optical setup. The scope itself of course has a focal length of 996 mm, but the scope also has some additional (corrective?) optics towards the back of the OTA. So what I'm wondering is...does anybody know what the focal length of the *objective lens* itself is?

I have estimated it to be somewhere in the neighborhood of 46.5 +/- 1.0 inches. What I'm wondering is if anyone knows what the exact number is for sure (back focal length would be great).

Thanks!
Jason

Don't know the exact answer and it might vary a *little* from one sample to another but, point it to the moon and place a piece of paper about 46" behind it and try to get a sharp image, that will be your focal plane. You can do the same with the sun but of course (as I'm sure you know...) the heat will burn your paper and might start a fire. Hope it helps.

oren.