: How big of a secondary mirror do I need for an : 8" f/6 Newtonian?

Hi there. To answer your question you need to decide on a few determining factors. How far away from your primary mirror's focal point do you wish your diagonal to be placed. l*----------------f1-------------*l l l l ----* "P" *----- l l l l---------------------------* l l * * l l *D D2 * "+" l * * Focal ----------------------------* Point

Dimension "P" determines the size due to its set- back from the focal point. Here is a formula I picked out of Texereau's book. D1" x P" D2"= -------- f1" D1 = diameter of your Primary mirror, 8"inches p = the setback shown in the sketch "?" f1 = your focal length, eg 48"inches.

8" x 10" eg 1.66" --------- minor axis 48"

Check to see what's available at your local supply house, and let me know how you make out. Good luck