[ATM] 20" telescope

["David Williams" <dwilliams1128@earthlink.net>]

Hello ATM list

I am still building the 20" telescope (that's the mirror's external diameter, but the sonotube is 24" outside diameter:)  and I was wondering what kind of secondary mirror to use for that purpose.  I could use a 3, 3.1, 3.5 or even a 4 inch minor dimension axis secondary, but does anyone know what kind of percentage or blockage of the primary each would be, and the difference in contrast?  

Of course the bigger the mirror the bigger the price and so I'm looking for the smallest price and there fore hope to find a happy compromise between the cost and the performance.  


PS 

My tube is 8 feet and 3 inches long and 24 inches wide (1/4 inch thick) and the mirror is 1 inch thick, 20" diameter, focal length 100 inches, and the mirror cell is 4 to 5 inches thick from front to back.  Where should I locate the mirror cell (rear edge) from the back of the tube and where should I put the spider with the secondary mirror and the eyepiece focuser too?

Thank you.
David Williams
Bye
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