Diagonal focuses, eyepiece doesn't

[LinusVanPelt]

Hi folks,
after many years of trying I have finally got the school I work at to buy a telescope. I have had to teach astronomy and astrophysics purely from a theoretical approach and am really looking forward to developing my and my students skills in practical observing.
We are testing the water with a Skywatcher 102mm refractor which may hopefully lead on to bigger and better things if I can garner enough interest with the students.

I was out trying it out tonight and I am puzzled.
I can get a nice sharp image if I use the 45 diagonal prism, but I can't get focus if I put the eyepiece in directly.
I need to focus out to get it in focus, but the focuser won't travel far enough.
I don't understand, why does the diagonal work? Surely the diagonal is simply a plane mirror? I'm a bit stumped.
Thanks heaps!

[DavidU (Dave)]

This is quite common. Refractors are usually produced with a some what short focus tube length so a diagonal with it's inherent extra focal length will focus well. We can also run into trouble when using a camera without a diagonal also. A diagonal can add 20-30mm extra light path travel.

[OneOfOne (Trevor)]

If you don't want to use the diagonal, you can also try putting in an extension tube. This will give you the extra travel you need. They don't cost much, $20-30. You can get one from BinTel, York, AstroOptical or any of the other scope retailers, have a look at their sites. Probably not worth trying to get one cheaper interstate as the postage is a killer on something this cheap.

[LinusVanPelt]

Thanks for the replies. What I don't get is that there is heaps of extension left on the focuser - far more than extra distance the 90 diag gives (not 45 as prev stated - long day).
I can crank out the tube far more than the distance added by the diagonal I can't see why I need even more length. Hmmmm

[Dennis]

If the diagonal contains a prism rather than a mirror, then there will be a longer light path through the prism than a front surface mirror?

Cheers

Dennis

[Robh (Rob)]

Originally Posted by LinusVanPelt

Thanks for the replies. What I don't get is that there is heaps of extension left on the focuser - far more than extra distance the 90 diag gives (not 45 as prev stated - long day).
I can crank out the tube far more than the distance added by the diagonal I can't see why I need even more length. Hmmmm

Yes, but how far out is the focuser with the diagonal in it? The diagonal will add maybe another 8cms or more to the light path.
See picture. Red path is the extra distance light travel.

Regards, Rob.