The eclipse enthusiasts can't wait until the day of the Total Solar Eclipse of America-2017. While solar glasses are one way to watch the eclipse, there are other indirect methods to look at the sun. One can use lens combinations to make image projectors or 'Safe Solar Viewers' (SSV) as termed by Astronomy professor Richardson of College of Charleston. I am using the idea provided by Prof. Richardson in his website . He had done a length study using various lens combinations to get the ideal combinations. Reference to Prof. Richardson's work will be provided at the end of this document.
I used the following values for focal lengths of the object lens, barlow lens and the distance between the lenses d.
fo = 500 mm = 19.7 inches
fe = -27 mm = - 1.1 inches
d = 21 inches
You may have to adjust the position of the barlow until you get a satisfactory image on the screen.
Reference: Prof. Richardson