Correct placement of a diagonal

[Tom Summers (Tom)]

Can some one help out with this: I have a project on at the moment to build an 8" Newtonian and am having a bit of trouble with the secondary mirror placement. I have a formula which requires a value (h) which is defined as, The distance of the desired location of the focal plane from the optical axis, My question is, is this distance measured with the focuser fully extented with an eypiece in it or in the home position and no eyepiece. I have a grip of all the other bits but this one puzzles me, thanks in advance.
Tom

[bojan]

I would say, home position (just a bit more, to allow for errors).
Also, you may want to attach the camera (with focal reducer - field flattener) one day.. so you should allow for this as well.

[adman (Adam)]

this will depend what you are primarily going to use it for. If the scope is for imaging the focal plane will need to be further from the optical axis than for visual. Often imagers need to fit a low profile focuser so they can get the cameras sensor to the focal plane, other times it is necessary to move the primary mirror up the tube to get focus with a camera.

There really is no best compromise, but for me I set it up for imaging and then use extension tubes with my eyepieces to get focus for the rare times I actually look through it.

Adam

[Blue Skies (Jacquie)]

Originally Posted by Tom Summers

My question is, is this distance measured with the focuser fully extented with an eypiece in it or in the home position and no eyepiece.

No, neither of those - you want the focuser half extended so you have some play up and down in it and no eyepiece. You then measure to the top of the focuser tube, without an eyepiece in it.

[Merlin66 (Ken)]

Tom,
consider putting the prime focus at at leat 55mm out from the focuser edge....this will allow you to add a DSLR without to mauch trouble.