[raymo]

Taping the lens in place doesn't guarantee accurate focus because it is almost impossible to hold it absolutely still whilst taping it, and also during an imaging session you can need to refocus to compensate for
expansion/contraction of the lens as the ambient temp changes.
Trevor, the three circle method is a variation on the Hartmann mask
which has two circles. They make the object look doubled or tripled, and
focus is achieved when the object appears single. The Bahtinov mask has
become more popular than the Hartmann because it is easier to judge
when lines cross each other, than when an object becomes single.
Bahtinov masks are simply scaled up or down in diameter to suit the
diam. of the scope or camera lens. My mask has sliding pegs that allow
it to be used with 6-7-8" scopes. You can get my size one for about $40
from Bintel. I imagine smaller ones would be a bit cheaper, and vice versa. Alternatively, you can make your own by downloading a template from the net. You could tape a lens whilst watching the mask effect
in Live View, to make sure you don't disturb focus while doing it, and do
the same again if you need to refocus during the session.
raymo