The Secondary Mirror


Making a 4.25 Inch Dobsonian Reflector Telescope




          Secondary mirrors for Newtonian reflectors are elliptically shaped.    From a 45 degree angle they look like a circle.   These can be obtained at various sites on the web.   The prices are strangely exponentially related to size.    There are a few free calculator programs on the web for figuring out how big your secondary should be.


          Since the mirror was small, I opted for a purchased mirror.   The common wisdom is that it is harder to figure a flat mirror than a parabolic one.   I'm not convinced however testing this will need to wait for a future project.