1. Bahtinov Mask 77mm

Introduction

Focusing mask which was invented by Pavel Bahtinov is considered to be the easiest, the fastest and an accurate tool for manual focusing. During the focusing procedure the mask is placed in front of telescope's aperture and then the telescope should be pointed to a bright star. Bahtinov mask produces 3 diffraction spikes: Two of them form an 'X' shape, and another spike crosses it, forming a 'Ж' shape. The central spike moves left or right (relative to the diagonal 'X' spikes) according to focus position. When precize focus is achieved - the pattern becomes symmetrical.

Farpoint Bahtinov Mask for 3.5' - 6.5' OD Telescopes and Dew Shields. The Bahtinov Mask makes it easy to focus your DSLR camera, CCD camera, or webcam when imaging the night sky. The Farpoint Astro Medium Bahtinov Mask works with any telescope with an outside diameter of between 3.5' and 6.5'. Astro Photography Tool (APT) We are happy to provide a link to an excellent astrophotography tool called APT (Astro Photography Tool)www.astroplace.net. APT has a function called Bahtinov Aid based on Bahtinov Grabber technology and it provides an excellent visual and digital reference when using a Bahtinov mask to come to focus. AT72ED Bahtinov mask by tacticaltaco is licensed under the Creative Commons - Attribution - Non-Commercial - Share Alike license. Give a Shout Out If you print this Thing and display it in public proudly give attribution by printing and displaying this tag.

This method is very sensitive, since the displacement of diffraction spikes is apparent even with the slightest focus shift, and takes guessing out of the equation. It can also be used while focusing 'regular' camera lenses for astrophotography. When focusing manually - in my opinion Bahtinov method is superior to reading FWHM value, because the human brain interprets geometrical shape much faster than a numerical value (especially while it constantly changes due to atmospheric turbulence). Bahtinov mask is also superior to 'Hartmann' focusing mask, its predecessor, however it is more complicated to make.

Printable PDF Templates

I have generated printable PDF templates for the most popular telescope apertures using the original bahtinov mask generator from 'Astrojargon' website. You should use Adobe PDF or a similar software for printing. In printer settings make sure to select the correct page size (A3 or A4), 600dpi resolution and set the scale to 100% (or select 'actual size'). Note that for larger apertures you'll need to print a 'mosaic' of several pages, and then connect them together. Each page has 1 inch overlap and markings which indicate how to connect each piece.

Making the Mask

The most elegant ways of building the mask are laser cutting, CNC or even 3D printing. I've also read reports on printing the mask on paper with a laser printer and then transfering it to a glass using a hot iron - method which is suitable for making a small mask for DSLR or a refractor.
I've made the mask for my 8' reflector using materials commonly avaliable in an office supply store. This is the same method I used to build an aperture solar filter as well.

Required tools and materials:

• Binder made from hard, thin plastic (must be non transparent)
• Stanley or a hobby knife and scissors
• Contact glue
• Ruler and a marker pen
• Printer (A3 if possible for larger scopes)

First step was to print the mask - you can use the provided templates, or use the original bahtinov mask generator, and then print it (any modern browser can open the generated .svg file, however chrome works better). In my case I had to print the mask on two pages.

Then I cut one half of the plastic binder, and attached to it my printed mask . I used a masking tape, making sure it's smooth and held firmly in place. It is important to keep the lines straight and parallel (to ensure that diffraction spikes will be bright and thin)

Then the annoying task - cutting the plastic. I did it by placing a metal ruler over each slot, and slicing several times over each edge, without applying too much pressure.

after the slots were cut out and the paper removed - the mask looks like this:

Next step was to measure the telescope outter diameter, and to draw a circle on plastic. Then I cut the mask out using scissors, while leaving 12 small rectangular sections around the circle. These sections should be bent 90° inside after cutting:

Then I cut a few strips of plastic, to fit these 12 cutouts (2.5cm wide in my case). I glued the strips to the bent sections using a strong contact glue. You should follow the instructions written on the glue tube, which usually tell you to spread the glue on both surfaces and then let it dry for 10-15 minutes before attaching them together:

In a similar way - a daylight solar filter can be made, by glueing a sheet of 'Astrosolar' film (or a similar product) between two circles of plastic.

Another tip is to make the inner diameter of the mask slightly larger than required (3-4mm) and add a thick soft tape inside - this way the mask will be held in place much firmer.

The Bahtinov mask is a device used to focus small astronomical telescopes accurately.Although masks have long been used as focusing aids, the distinctive pattern was invented by Russian amateur astrophotographer Pavel Bahtinov (Russian: Павел Бахтинов) in 2005.^[1][2] Precise focusing of telescopes and astrographs is critical to performing astrophotography.

The telescope is pointed at a bright star, and a mask is placed in front of the telescope's objective (or in front of the aperture).

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. Rotating the mask through 180° will reverse the direction of spike movement, so it is easier to use if placed on the telescope with consistent orientation. 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.

• Example diffraction patterns produced by a Bahtinov mask
  (middle: in focus, left and right: slightly out of focus)

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• Simulation of the Bahtinov mask diffraction pattern, using 'Maskulator'. Each frame shows a focus difference of 140.6 μm.

See also[edit]

References[edit]

1. ^Pavel Bahtinov (22 September 2005). 'Optional Focusing Mask'. Retrieved 17 July 2017.
2. ^Harris, Lawrence (2010). So You Want a Meade LX Telescope!: How to Select and Use the LX200 and Other High-End Models. Springer Science & Business Media. ISBN978-1-4419-1775-1.

External links[edit]

Bahtinov Mask 77mm