A focussing tool for Astro-photography. The Bahtinov Mask is a very elegant solution to one of the biggest problems that besets the deep sky astro-photographer - focus! It is common practice to focus your camera DSLR or astro-CCD on a bright star as this star and the object you want to image will both be at infinity so getting the star in focus will ensure that the deep sky object will also be in focus. However, trying to do this with a DSLR is extremely difficult as the pentaprism viewfinder is designed for daytime use and even a 'bright' star appears dim in the viewfinder.
|Published (Last):||27 October 2004|
|PDF File Size:||11.74 Mb|
|ePub File Size:||14.1 Mb|
|Price:||Free* [*Free Regsitration Required]|
The Bahtinov mask is a device used to focus small astronomical telescopes accurately. The telescope is pointed at a bright star , and a mask is placed in front of the telescope's objective e. The mask consists of three separate grids, positioned in such a way that the grids produce three angled diffraction spikes at the focal plane of the instrument for each bright image element.
As the instrument's focus is changed, the central spike appears to move from one side of the star to the other. In reality, all three spikes move, but the central spike moves in the opposite direction to the two spikes forming the "X".
Optimal focus is achieved when the middle spike is centered between the other two spikes. Small deviations from optimal focus are easily visible.
For astrophotography, a digital image can be analyzed by software to locate the alignment of the spikes to sub-pixel resolution. The direction of this displacement indicates the direction of the necessary focus correction. The mask must be removed after accurate focus is achieved.
The mask works by replacing the aperture stop of the optical system normally the circular shape of the objective itself with a stop which is asymmetric and periodic. Viewing a point source such as a star yields a diffraction pattern at the focal plane representing the Fraunhofer diffraction transform of the aperture shape. This pattern normally would be an Airy disk resulting from a circular aperture, but with the mask in place, the pattern exhibits asymmetric spikes representing the transform of the mask pattern's spatial frequency and orientation.
A very bright star and very dark sky are required to produce highly contrasted spikes that are clearly visible. The diffraction effect is similar to producing sunstar patterns in landscape photography with ordinary camera lenses, where the mechanical iris of the lens is adjusted to a small polygonal shape with sharp corners. In the example below, the central pattern shows good focus. The central spike is noticeably displaced from the central position in the left and right images.
Designing a mask using drawing, design, or CAD software can be challenging, as the specifics of the mask change for each telescope based on aperture, focal length, and how you plan to attach the mask to the scope. This generator makes generating masks significantly easier - simply enter some parameter values specific to your telescope on the next page, press the Generate! Note: While using the browser to preview the SVG is useful in understanding the effect of changing parameters, I no longer recommend printing these directly from the browser. Testing larger aperture designs yielded printed masks that were increasingly "off" - also sometimes known as "wrong. All credit for the mask goes to Pavel Bahtinov for creating the design, and to Dennis Sakva for championing it on Cloudy Nights. Thanks, guys!
Make Your Own Bahtinov Focusing Mask
Did you use this instructable in your classroom? Add a Teacher Note to share how you incorporated it into your lesson. This is a quick and easy solution to make your own Bahtinov mask for a telescope. This design should be adaptable for different sized telescopes as well as for camera lenses. Recently I decided to join those 2 activities. I shopped around, did my research on getting started in Astrophotography and made my purchases. Now I was all set.
Home Made Bahtinov Focusing Mask & Templates
The Bahtinov mask is a device used to focus small astronomical telescopes accurately. The telescope is pointed at a bright star , and a mask is placed in front of the telescope's objective e. The mask consists of three separate grids, positioned in such a way that the grids produce three angled diffraction spikes at the focal plane of the instrument for each bright image element. As the instrument's focus is changed, the central spike appears to move from one side of the star to the other.