Any number of combinations of scope and eyepiece will produce a 4" diameter circle. Your effective focal lenght is about 480 inches so for an 8" scope your effective f-ratio is f:60

The focal length of the scope will be much shorter. You need to be able to use 1/2° field so for a scope and eyepiece combination that will work, you need to know what size field you can illuminate and what eyepiece will work.

An 8" scope with 80" of focal length will produce a prime focus image of the sun that is about 2/3" in diameter. A 25mm eyepiece with a 40° AFOV should project a 4" diameter image about 6" away.
A 6" f:7 scope with the same eyepiece will do the same at about 11.5". Shorter focal length scopes can use shorter focal length eyepieces. Always be sure the eyepiece has a AFOV large enough to include 1/2° true Field Of View (angular diameter of Sun).

Finally remember that the effective focal length required for a 4" diameter image of the sun is 480" Which is f:60 for an 8", f:80 for a 6" or f:40 for a 12". Dividing these numbers by the scopes actual f:# will tell you approximately how many times you add the focal length of the eyepiece to the projection distance.

Good Luck,
-Rick

• Re: Re: sun projector - alejandro - December 10, 2000 - 17:16 UTC