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HARTMAN MASK - Possum Observatory
This handy focusing aid (16" on left, 8" on right) is cheap and easy to make and works fairly well. The mask is inserted over the telescope end and the two-star image visible through the SLR camera view finder is focused until they form one star - with spikes (if the star is fairly bright). If a 2x magnifyer is used to look through the camera FOV it accentuates this process... I initially tried four circles, but after reseach found that two triangles (one pointing at the middle of the other) worked better. Example photos can be seen below...
Happy imaging - John Drummond, Saturday 23 April 2004