Nice Bahitnov Mask. Beware - it is brittle so it will snap if bent or twisted or overtightened.

I bought the Bahtinov mask for refining focus when imaging. However I use it in almost all sessions including visual. The Williams Optics version is a clear plastic engraved version that gives a clear and bright diffraction pattern. I was actually surprised at first use when setting focus with Canopus how very bright the pattern was and focusing was a snap. More expensive that the cheap versions of course but worth it and a highly recommend.

I'd been using a solid-type mask for a while and using it in a video loop at high gain to try to focus live. that worked fine on super-bright starts but had been marginal on others, so I thought I would try this WO clear version to improve sensitivity. I was a bit fed up at first as it turned what I had before into a mushy mess with no spikes at all. On my third go I decided to try for longer exposures (20s) at much lower gain and hey presto very clear large thin spikes making it very easy detect the even angles of the Bahtinov pattern to refine focus, but a bit more slowly than before. This was on a C8 SCT running at f/7.1 with the Starizona corrector Mark III. So it's great for getting the focus nailed in shoot and refine mode, working well on stars of moderate brightness. So after a meltdown over it I am now very happy. It's very adjustable so I'll be able to fit it on my larger refractor as well. Just measure your dew shield end to get the right size.

Good Product, Good Packing, Fast Shipping, A++++++