A good description of what a Bahtinov mask is and how it works can be found at its Wikipedia entry. It was invented by Pavel Bahtinov to get rid of the tiring procedure of traditional focusing where you try to find the minimum spot size. This mask can help you with focusing on stars easily when it is placed in front of your telescope.
The working principle is that the light of the stars is diffracted in a special way and creates a characteristic pattern which can be easily judged to recognize a perfect focus position of your telescope.
The product offered here is an optimized version for the 135mm f/2.0 telephoto lens by Samyang, but it can be used with any lens or refractor with a front diameter less than 94mm! (It fits perfectly to all versions of the Samyang lens, both the manual focus version and the Cine/VDSLR version. The two versions share the same sunshield where the mask can be placed.)
There are even some software tools that can assist you with the stripe alignment during focusing.
As a member of the OSHWA I'm totally convinced of the idea to offer products as Open Source Hardware instead of protecting them by patents. The OSHWA (Open Source Hardware Association) is one of the most well-known organizations that support the open-source-hardware ideas.
This Bahtinov mask is also based on an Open Source Hardware project hosted by me at Thingiverse:
You can find there additional informations about the creation history of this variant. It was derived from the great work by Thingiverse users brentb and Ryoko.
The source code (OpenSCAD format) can be found at GitHub:
You are invited to work on this mask, if you have any ideas. I'm always pleased to get some input, whatever to criticise is (I'm open to both positive and negative critics!). GitHub contains a pretty good issue tracker, you may enter your suggestions or issues there directly!
Copyright Notice: SAMYANG is a trademark of SAMYANG OPTICS CO., LTD.
Disclaimer: I am not licensed, approved or endorsed by Samyang