[jerry3672 (Jerry Hailey)]

Originally Posted by citivolus

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.



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

I think you will find that the seeing conditions causing the Bahtinov mask to "dance" is more useful to capture the best average focus over a mask that is not a sensitive. My method is to focus on or near your image target. Use the same camera settings to focus as you plan to image, this will yield the best average focus in your image. In my opinion this is the best feature of the Bahtinov mask. Focusing on dim stars.

Collimation will affect the mask, just as it affects the final image. Rotating the mask 90 and 180 degrees will test collimation. However, if your mask is not centered or lines are not a mirror, then this will also magnify the error.

The best method to test various designs is to use a deep sky target image and measure the results with something like CCD inspector. Look for the best FWHM value. After all, this is intended to be used for imaging.

Lots of people have claimed the device works just because they see diffraction spikes that move with the focus knob. Just my 2 cents worth.