Jerry wrote:

For those interested in using a mask to aid in focusing should check out the Bahtinov mask. Read all of the links and try generating one for your scope. http://astrojargon.net/MaskGen.aspx Jerry Foote

Yes, that is is a nice design.  Much more difficult to fabricate in terms of the number of cuts than a Hartmann mask but the Bahtinov mask addresses the drawback of the Hartmann.  The Hartmann mask greatly reduces the amount of light passes the aperture and can result in the object that you are trying to focus on simply disappearing from visibility when you install the H-mask.  - Kurt