Re: About secondary mirrors

I'm building a planetary Newtonian for the Mars opposition around an 8" f 10 full-thickness mirror. If I use a 9" or 10" tube with a 1.25" low profile focuser, what would be the optimum size for the diagonal mirror? I have a 3 curved vane spider and 1.25" M.A. 1/20th wave diagonal, but would it be advantageous to use a smaller one? This is going to be a dedicated planetary scope.

James,

Your 1.25" secondary is a 16 percent obstruction. Using a .75" secondary will be a 9 percent obstruction. Your focuser will have to be at least 2.5 inches and you need a 9 inch tube. Your secondary holder needs to be the same size as the secondary or smaller. You alraedy have a 1/20 wave secondary. You would have to consider that in your decision. But its, as far as I can tell, do-able.