Bird-Jones Newtonian Modifications?

[Gutross]

Is it possible to remove the lens at the bottom of the focus tube and make a 1000mm that is useless for prime focus into a 500mm that prime can be acheved?

I am just curious if it is possible at all. I would love to be able to use the modified webcam i have and can not use because of the focal point.

the scope is  a celestron 114gt

[Cornelius Varley]

Removing the corrector lens is a bad because it serves two purposes. The first is to give the focal length (1000mm) and the second is to correct the spherical aberrations of the primary mirror.

Jones-Bird telescopes discuss here https://en.wikipedia.org/wiki/Newtonian_telescope

If you are having problems achieving focus (insufficient inwards focuser adjustment) try using a barlow lens to move the focus point outwards.

[kalasinman]

I wonder if the mirror can be adjusted up the tube or the tube shortened., as I did with my GSO newt.

[Cornelius Varley]

I wonder if the mirror can be adjusted up the tube or the tube shortened., as I did with my GSO newt.

The primary mirror has a focal length of 1000mm and the corrector is used to allow the use of a shorter tube. The primary is also spherical so probably not a nice view without the corrector anyway. Recollimating the primary farther up the tube would have the same affect as will your GSO, moving the point of focus outwards.

[Gutross]

thought as much, was hoping someone had done this. oh well will just ahve to wait till i get somthing proper

[kalasinman]

In the OP he says he cannot achieve focus. Why not just leave the lens and move the focal point out? Shortening the tube without removing the lens could accomplish that.

[Cornelius Varley]

It would be simpler to recollimate the primary using longer bolts and reversible. Also the use of a barlow is desirable when imaging with a webcam.

[Peter Drew]

The focal length of the primary mirror will be 500mm, it is the 2x Barlow type corrector that gives the effective 1000mm focal length. As mentioned, the "Barlow" is part of the correction system, without it the 500mm focal length spherical primary would give unacceptable spherical aberration. Because of the "Barlow" characteristic a small amount of primary mirror upwards in the tube would give a substantial increase in back focus and should be enough for the webcam.