Secondary size for 14" f/4.5 primary mirror

[Doogs38 (Alex)]

Howdy. I have skywatcher 14" conical mirror with 2.6" secondary in a large DIY dob. I'm in the planning stages for rebuilding the dob (more lightweight) using "The Dobsonian Telescope" book by Kriege & Berry - it's an excellent book. Like many before me, I'm paining over what size my secondary mirror should be.

The 66mm (2.6") secondary I currently have gives a fully illuminate field for 12mm (0.5") diameter field size; the calcs from the book for a 20mm diameter field size result in the requirement for a 75mm (3") secondary. My question is: for purely visual observing using up to a 30mm eyepiece (68° AFOV), is there any significant practical benefit in using a 3" secondary vice a 2.6" secondary?? I've read plenty on the web and there are arguments either way & it can be quite confusing ... I'm interested in your experience(s). Thanks Alex.

[glend (Glen)]

I have the book as well. The secondary on my 16" is 88mm. What's your focuser height? The reason being you can make the secondary smaller by using a short focuser. Have you read page 96, the last three paragraphs seem to present the options for you. I think the telling advice is to resist buying a secondary that is too small.

[Peter.M]

http://stellafane.org/tm/newt-web/newt-help-scope.html

That site seems to recomend that for visual use your 100% diameter should be half of the field stop that of an eyepiece that you intend to use. So for your 30mm it would be 15mm. The site also suggests half an inch of illumination would be a good amount. I have used their ray tracing to design my scope and I would say that it is very reliable but I have no experience with visual observation.

[anj026]

For general visual use 0.5" fully illuminated should be fine. This table by Mike Lockwood is a useful reference.

http://www.loptics.com/ATM/diagonals.html

[Doogs38 (Alex)]

Hi guys and thank you for your helpful replies Glen, I agree - don't skimp on the size & quality of the secondary. I've read page 96 several times and also a very help article by Gary Seronik (http://www.garyseronik.com/?q=node/8). I've also found a great little DOS program by Alan Adler called 'Sec' that plots 1-1/4" & 2" eyepiece illumination profiles for various combos of primary mirror size and focal ratio. In essence, a 2.6" secondary mirror does a great job and a 3" does an excellent job for my primary 14" mirror:

a. 2.6" secondary - for a 1-1/4 inch eyepiece the edge illumination is ~85%, and for a 2" eyepiece it's ~70%.

b. 3.1" secondary - for a 1-1/4 inch eyepiece the edge illumination is ~98%, and for a 2" eyepiece it's ~85%.

Alex