Having been very impressed with the Bahtinov mask I did some experiments and have come up with something that may be an improvement on Bahtinov's idea.
I found that there was still a slight uncertainty in judging whether the central spike was truly central and tried to get an arrangement that would have smaller gaps between spikes and afford more critical comparison.
This is what I ended up with:
http://www.btinternet.com/~gfcarey/m...rey_mask_a.jpg
This gives two 'X' shapes, with slightly different angles. An explanation of what the diffraction pattern looks like is here:
http://www.btinternet.com/~gfcarey/careymask.htm
I am eager for someone else to try it out and let me know if they think it is an improvement.
Regards,
George