Measuring refractor focal length...

While I'm waiting for my big telescope to come, I'm keeping busy with little projects. As you know, there is nothing more dangerous than a beginner with a box of objective lenses from the Surplus Shed. Here's my really basic question.

If a refractor objective has a focal length of say 350mm, does that mean that the length from the objective to the mirror in the diagonal us 350mm? Or is the total length from the objective to the eyepiece supposed to be 350mm?
And how does this measurement relate to the focal lengths of my eyepieces? I've had troubles where I get something set up just fine for 25mm eyepiece, but I have to totally readjust for a 10mm eyepiece. Thanks!

[QUOTE]Lee Spain said:

While I'm waiting for my big telescope to come, I'm keeping busy with little projects. As you know, there is nothing more dangerous than a beginner with a box of objective lenses from the Surplus Shed. Here's my really basic question.

If a refractor objective has a focal length of say 350mm, does that mean that the length from the objective to the mirror in the diagonal us 350mm? Or is the total length from the objective to the eyepiece supposed to be 350mm?
And how does this measurement relate to the focal lengths of my eyepieces? I've had troubles where I get something set up just fine for 25mm eyepiece, but I have to totally readjust for a 10mm eyepiece. Thanks!


The focal length is measured from the objective lens to the eyepiece. Eyepieces have their own focal point depending on design, hence your having to refocus when using different eyepieces. I you find that you often use one eypiece that is quite different from your other, you may be able to solve the problem with a "parfocal ring" which simply keeps the eyepiece from inserting fully into the diagon alley.

If a refractor objective has a focal length of say 350mm, does that mean that the length from the objective to the mirror in the diagonal us 350mm? Or is the total length from the objective to the eyepiece supposed to be 350mm?
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It means the distance from the objective to the focal plane/point of the objective is 350mm.. If you image a target that is at infinity, it will focus 350mm from the objective.

The eyepiece has it's own focal plane...

Jon

Thanks guys! You've helped me a get a little more scientific than my "snip of some more tube" approach!