Thanks Dennis, I'll get back to you some time after this weekend as I'm on call through Tuesday so I can't stray far right now.
I have two designs I'd like to compare to this mask. One of them, upon inspection, functions using a very similar mechanism to Bahtinov's design, however his will likely work on dimmer stars due to it only masking about 50% of the light.
One difference that I am speculating on is behaviour during poor seeing. I find that the design that is similar to Bahtinov's dances around a lot due to seeing, while my other design is much more stable with the trade-off being slightly less precision. I'm not sure how his design handles seeing, given I have not tried it out yet.
I'll PM you the designs and a focus movie to look at.
Suggested prototyping materials:
A4 card stock, two sheets wide with some overlap, joined using spray adhesive. You can print to the cards and then cut with a sharp knife after joining them.
3mm or 6mm Depron foam (or the card stock backed foam mentioned elsewhere in this thread) can be used in conjunction with the above to add some rigidity to the design without adding significant weight. The spray adhesive works well here. Sharp corners can be a bit harder in Depron, however if backed with the card stock, precision cuts in the Depron become less critical as the card stock will mask the light anyway.
An observation/speculation on focus being slightly out after using the mask: Collimation can dramatically impact the performance of these masks, as could slight imperfections in the curvature of your optics. Based on information in a few Astrophotography books that I have, the Bahtinov design is similar to mirror testing Hartmann masks which are designed to highlight the imperfections in curvature of lenses and mirrors. Having open regions on such a large portion of the mask could possibly have this negative side effect, as a trade-off for brightness and contrast.
Try rotating the mask 90 or 120 degrees on the OTA and see if it still shows focus as perfect. If it does, then my speculation above is likely incorrect
Regards,
Eric