Getting perfect focus with the telescope when making astrophotographs can be very difficult to do, and serious astrophotographers have long used various tools such as Hartmann Masks, or knife-edge focusers. In 2005, Russian astrophotographer Pavel Bahtinov created a mask that fits over the front aperture of a telescope to solve this problem with high accuracy.
The Bahtinov 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 flipåÊfrom one side of the star to the other. Actually all three spikes move but the central spike moves in the opposite direction to the two spikes forming the 'X'. Best focus is reached when the middle spike is centered between the other two spikes.
In digital astrophotography it is relatively easy to examine the stars for tiny deviations in focus with images made with theåÊBahtinov Mask with image processing software. Once you gain experience with this device you can even understand which way to turn the focus knob to achieve sharpest focus.åÊThe Bahtinov Mask is truly a must have device for almost any astrophotographer.åÊ
The image below shows a simulation of the Bahtinov mask diffraction pattern as the image is brought from out of focus to focused to out of focus again. Image souce: Wikipedia.
We carry the full range of Bahtinov Mask devices from Astrozap. They are made from lightweight aluminum, finished in a durable textured black powder coat finish. This series have three subapertures that open for focusing, and can close to serve as a focusing device and a front dust cover cap that attaches to the front of the telescope, giving double duty for focusing and protection. Of course the mask is removed when making astrophotographs or for visual observations when the most perfect focus is desired.
Astrozap makes these Focusing Caps with the Bahtinov Mask to fit telescopes with apertures ranging from 90mm to 405mm. They are held in place with three thumbscrews to to outer edge of telescopes's front cell or dew shield.