It does NOT fit with the Hyperstar or Fastar camera installed. But if you look carefully, you will see a slit that goes almost all the way across one side. Just complete that slit and you can flex (twist) the mask to open it up to get around the camera. The OD is 10.25" and the ID is 3.375".

Tri-Bahtinov Focus and Collimation Mask for Celestron 9.25" SCT with Hyperstar or Fastar
The Tri-Bahtinov mask can also be used for collimation of your telescope.

About Bahtinov Masks

Pavel Bahtinov, a Russian amateur telescope maker and astronomer, created this simple, very intuitive method and the device has universally been named after him: The Bahtinov Mask.

Attach a camera to the telescope, center a fairly bright star and achieve rough focus. Then place the mask over the front aperture of your telescope by “hanging” it on the plastic screws from the front dew shield (Custom fit masks for specific telescopes are also available). Take a short exposure. The image of the star will show diffraction spikes. The “center” spike will most likely be shifted to one side or the other. This is showing the defocus.

Adjust the focus of the telescope and re-expose the star, once again showing the diffraction spike pattern. Iterate the process until the central diffraction spike is centered against the star and the other spikes. When a symmetrical pattern is achieved, the best possible focus has been obtained.  Simply remove the mask, center the target object and take amazing, sharp images!

Farpoint manufactures this "lifesaving" astrophotography tool out of virtually indestructible ABS plastic, which comes in three major types and in a range of sizes. In addition to Bahtinov masks for telescopes, Farpoint also makes Bahtinov masks for DSLR camera lenses.

Tri-Bahtinov mask can be used for collimation of your telescope.

_{Star in focus via a Bahtinov mask}

Software Assist tool

Recently, free software tools have become available to further analyze the images created by the Bahtinov and Tri-Bahtinov mask and provide assistance in adjusting focus and collimation.

The Open source software package can be downloaded here: https://github.com/insertnamehere1/Bahtinov-Collimator/blob/master/published/setup.exe

The software is simple to use and works with your existing image capture software.

In addition, Bahtinov mask analysis has been integrated into APT image capture software.

The diffraction spike pattern from a bright star using a Bahtinov mask

As you adjust the focuser of your telescope, the central spike will move across tthe X-pattern created by the mask.  Adjust the focus of the telescope so the center spike becomes centered and symmetrical with the other diffraction spikes.  This is very intuitive and quick. 

Bahtinov masks work equally well with DSLR cameras.