Secondary Size

Hi Group:

I currently own a Guan Sheng (GS) manufactured 10"/F5 scope (247mm mirror face, 1255mm FL = F5.1). The standard secondary is 63mm which is a 26% CO, I am hoping to be able to drop the 1.85" (47mm) secondary supplied as standard on the 8"/F6 GS scope straight in without modification for a 19% CO. I have plugged all the numbers into NEWT but I am not 100% sure on what all the numbers mean. The scope will be used for visual work only. My widest FOV eyepiece is a 2" 32MM (63 deg AFOV) plossl for 40X and TFOV of 1.6 degrees. I don't use this eyepiece all that often and normally my Maximum FOV that I use is 1 deg in a 25mm ortho. I have listed all the necessary paramaters for NEWT below, could someone please check it out for me and tell me if it works ok. I think it will work fine.

Primary Diameter = 247mm (illuminised surface)
Focal Ratio = F5.1
Diagonal Minor Axis = 47mm or 1.85"
Tube Inside Diameter = 295mm
Tube Thickness = 1mm
Focuser to Front of Tube = 140mm
Mirror Face to Back of Tube = 70mm
Focuser Height = 57mm
Spare Focuser in Travel = 23mm
Additional Camera Height = 0
Focuser Inside Diameter = 50mm

Sorry to give you guys the measurements in metric but thats primarily what we use in Australia and I didn't have an imperial ruler or calipers handy.

Thanks in Advance
John Bambury

John, with diagonal-to-focus distance of 228.5, your min diagonal minor axis with an f/5 scope is 228.5/5=45.7mm.
This means that a 47mm diagonal edge would overlap the axial cone converging from the primary by as little as 0.65mm. That is not a desirable situation. Keeping the diagonal edge at least a couple of mm away from the light forming diffraction limited field guards you from having likely imperfections around the edge erea impair your high-quality field. With diffraction limited field diameter for an f/5 of ~2.6mm (from F^3/47), you'd have that minimum achieved with 52.6mm or, say, 2 inch minor axis. That would give an 8.4mm (0.39 degrees) fully illuminated field diameter, and 62% illumination 17.5mm off-axis (the very edge of your widest eyepiece).

Most people agree that pushing the diagonal size to the very minimum is likely to be counterproductive. In this case, difference between the two sizes in terms of influence on image quality is similar to that between 1/6.6 wave and 1/7.2 wave of spherical aberration. Bringing possible diagonal edge imperfections into
diffraction-limited field can sure have more damaging consequences.