Stuart St. Clair said:
NEWT recommends placing the focal plane at 1/4" below the top of the focuser when fully racked in.
I know it's really late but this sure sounds wrong to me.
Are you sure it didn't say "when fully racked out"?
In theory the field stop of an eyepiece is located at the focal plane of that eyepiece. So, when the field stop hits the focal plane of the scope it should be in focus. Most Televue eyepieces have the field stop 1/4" below the shoulder (where the barrel meets the body).If you put the focal plane of the scope 1/4" below the top of the focuser when fully racked IN these eyepieces will barely (if at all) come to focus only when the focuser is fully racked in. Some eyepiece makers try to put the EP focal plane at the shoulder. These EPs will not reach focus.
This is how I've done it with good results. I take my eyepiece that has the field stop furthest below the EP shoulder and design the scope to focus that eyepiece 1/4" below the fully racked OUT position.
It might be easier to say: [SIZE="Large"]put the scope focal plane where all your EP field stops can meet it[/SIZE].
Of course there are all kinds of variables and considerations. Like if you plan to attach a 35mm camera then you need to put the focal plane a good 2" above the fully racked in position. Or, if all you own are TV plössls and Radians you can put the focal plane at the top of the fully racked in focuser. If you own a TV 55mm plössl and a 31mm Nagler you will need over 1-1/2" of travel between the 2 extremes.
Sorry, its really late and I'm rambling. Maybe someone else will chime in and put it more simply than I have.
Bob