8" F/5 question...

Hi,

I'm planning to build a 8" F/5 Newt,
and wanted to get feedback from any
experienced Newt builders.

I've been crunching numbers trying to
go for the smallest reasonably-sized
diagonal. And I'm coming up with 1.52".

Tube Diameter is approx. 10 in.
Planned Focuser is JMI NGC-DX3.

My usage will be about 75% ccd
imaging, 25% visual use.

Using the program, Newt, everyting seems
fine.

But I'm worried about illumination drop off.
Should I worry about this? (CCD chip is
about 7 mm wide)

Or should I just jump up to a 1.83" diagonal?

Appreciate your insights/feedback.

Kwang

I would imagine with a 7mm chip you would not have a problem that would show up on the ccd images, and that being the case, I seriously doubt you could visually detect the difference. However, from personal experience with an 8" f/5 years ago with film, I did get noticible falloff with a 1.83 diagonal and had to go to a 2.14 to get full even illumination on 35mm film.

Walt

For 1.52" diagonal and 7mm 100% illumination you'd need no more than 160mm diagonal-to-focus separation. For field of full illumination diameter in a Newtonian you can use
(A-s/F)/(1-s/f), with
A=diagonal minor axis, s=diagonal-to-focus separation, F=f# and f=focal length. It has to be a positive number; negative number would indicate too small a diagonal (i.e. stopping down the aperture).