What size secondary mirror for 6" Reflector with 2" EP??

What size secondary mirror do you think would be optimum for a 6" f/8 Newt reflector for use with a 2" EP in a Starlight focuser?
Steven Elliott

Steven, forgive me if I misunderstood, but diagonals are not used normally on a newtonian... the eyepieces go straight into the focuser itself.

Hello Steven
You can google search Woden Optical Diagonal Calculator. Another program is NEWT which is a newton design program where you can load in your scope specs and play with different aspects and in your case the best secondary diagonal size.
Hope this helps
Buck

Steven,
A firm answer is impossible without knowing your scope's geometry, that's why you need to do the ray tracing yourself. But my experience may shed a little light.

I converted my 6" f/8 to 2" eyepieces a few years ago. I wanted to keep the diagonal as small as possible, so I used the lowest-profile focuser I could find, which turned out to be a helical model, and inched the focuser as far from the primary as possible so the eyepieces would just reach focus at the lowest setting. Just for kicks, I tried my original 1.25" diagonal, and obviously no way it was big enough, since the field with a 46mm field stop 2" eyepiece was basically plumb dark.

I then followed the recommendation that 70% illumination at the edge of your widest field was a good thing to shoot for, used web software, "Newt" I believe, which came up with a 1.6" diagonal. I ordered and installed it and am mostly satisfied.

Under a dark sky, vignetting isn't noticeable. You're looking in the middle, and wouldn't know it if a star on the edge was dimmed slightly. But, under a bright sky, with the 5mm exit pupil provided by this 40mm eyepiece, there is no escaping seeing that the skyglow is dimmer at the edge of the field. This is a little bit disturbing.

So 70% is fine for dark sky, but you might want to aim for something like 85 or 90% if your sky is always bright and you think this effect would bother you.
Ron

Steve:

As diagonals and holders are relatively cheap, get two or three very high quality ones. I suggest a 1.25" or 1.33" minor axis and a 1.00" and just try them out. That's roughly 21-22% and 16% respectively central obstruction by diameter. The 1.00" diameter diagonal can give great, high contrast high power results and I've found that light drop off at the edge is gradual and the eye is not that sensitive to it. But if you truly insist on a big fully illuminated field, also try a 1.5".

"We learn by doing and learn best by screwing up in the process. Well, I've learned a lot. "

Does anybody remember the booklet Newtonian Notes? That was a great source for such information back in the pre-computer days.