When you say 24mm eyepiece are you referring to field stop diameter or focal length? If the latter you need to work out the FSD from the manufacturers data or from calculation using the apparent field of view.
eg. A 24mm Panoptic has a field stop diameter of 27mm (as does a 32mm Plossl as does a 40mm plossl). A 24mm Plossl would have a field stop diameter of approx 20mm.
Now, have a look at Mel Bartels online diagonal sizing program at
http://www.bbastrodesigns.com/diagonal.htm
Either use inches or mm, but note the eyepiece off axis radius will be given in the same units so convert to mm to make it easier.
10.5" = 267mm
fr=6
f.l. = 1600mm
Option 1 diag minor axis of 1.83" = 46.5mm
Option 2 diag minor axis of 2.14" = 54.4mm
Distance from diag to focal plane (subject to tube diameter and focuser height variation (Use NEWT for further refining). Say 9.5" = 241mm. This figure is critical since it cannot be easily changed once you have the tube diameter set and focuser chosen.
The aim is to get no less than 70% illumination at the edge of field so as not to drop too much magnitude. When you plug in the above figures you get the following:
Option 1 (46.5mm diag): 69.92% (say 70%) at the 16mm off axis radius. ie. 32mm field stop diameter.
Look at
http://www.televue.com/engine/page.asp?ID=144 for field stop diameters of the Tele Vue eyepieces and you will see a few eyepieces just under and just over the 32mm FSD. eg. 27mm Panoiptic is 30.5mm, 22mm Nagler is 31.1mm, 17mm Ethos is 29.6mm. If you had you heart set on the 21mm Ethos you could still get 65% illumination at the 18mm off axis radius (36mm FSD) position and it would be quite useable with less than 0.5mag drop.
Option 2 (54.4mm diag): 68.29% (say 70%) at the 22mm off axis radius. ie. 44mm field stop diameter.
Again look at
http://www.televue.com/engine/page.asp?ID=144 for field stop diameters of the Tele Vue eyepieces and you will see a only the 55mm Plossl and 41mm Panoptic have in excess of 44mm FSD. The 55mm would give you to large an exit pupil and is not really suitable. Therefore if you enjoy low power viewing and you intend on getting a 41mm Panoptic then the 54.5mm diagonal is made for that combination but is an overkill for anything else. A secondary mirror of around 50mm (if available would be ideal) or use the 46.5mm mirror you may be able to get a lower profile focus but make sure you have enough back focus to accommodate a range of eyepieces. Note I am referring to visual only not photo as an extra low focuser is needed for photo further complication the design.
The program is also telling you that if you want maximum exit pupil of 7mm (42mm eyepiece in your case) and 68 deg apparent field (41mm Panoptic is closest to this) you need a 70% illuminated field at the 24.89mm off axis radius position to use a 41mm Panoptic. To get this you need a larger diagonal or move the focuser closer. If you go with a 2.6" diagonal you will then achieve the illumination required at the expense of contrast loss since the diagonal is now 25% of the primary diameter. Unless you are a variable star observer you want to limit your diagonal to about 20% or better if possible but dont undersize too much. For planetary only observing you could easliy use a 1.83 or 1.52" diagonal. Variable stars observing requires even illumination across the field to ensure accurate magnitude estimates.
Another important note too is that your secondary holder will obscure part of the secondary mirror and will give you extra contrast obscruction in the process, (unless you glue the seconday without a holder around it). You have to take this into account and if you do have a holder you will need to measure the non-obstructed minor axis and use that in the calculations for illumination. Also do not be too concerned about secondary offset now as you can adjust the position of the mirror later if at all. The offset is to give you a circular illumination ratrher than a slightly elongated one. The effect is very negligible at f6.
The Mel Bartels site
http://www.bbastrodesigns.com/ has a lot more ATM goodies and is very informative. I hope all this has helped to clear up and not cloud up any design issues you are having. Enjoy!
