Focal Plane Placement

I'm building a 16" f/4.5 dob. NEWT recommends placing the focal plane at 1/4" below the top of the focuser when fully racked in. I'm looking for confirmation of this recommendation, or alternate recommendations. Thanks!

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

This is how University Optics recomend focal plane placement:

http://www.universityoptics.com/hints-tips.html

Find the recomendation in Hint & Tips the 3rd Question & Answer that begins with "I have my tube, focuser, mirror cell and main mirror. Where do I put the holes for all the parts?"

If using a low profile focuser, I'm not certain that there will be enough back focus to switch from using eyepieces to camera without making a forward adjustment of the primary mirror. My old 8" f/6 Meade Research Series Reflector has two sets of three screw holes each at the bottom of the OTA to secure the primary mirror mount. The set of holes nearest the bottom of the OTA is used when using eyepices, and 1.5" from that set of holes - toward the front of the tube - is the second set of holes to be used to move the primary mirror mount forward when using a camera, and thats with a high profile focuser with a long draw tube.

Jim

If I'm not mistaken, NEWT asks YOU how much in-focus you want from the focal plane. With your entry, NEWT will determine the location/measurements of the rest of the scope from there.