Introduction

The standard Bahtinov mask is one of the most useful tools for focusing used in astrophotography. However, I think it can be made even more useful by making it three-fold symmetric, i.e. to add two more Bahtinov masks. The result is what I call a "Tri-Bahtinov" mask. With the Tri-Bahtinov mask, I have added the ability to check whether the SCT is also well collimated. Below are my instructions for generating any arbitrary sized Tri-Bahtinov mask and for using it for collimation.

Generating the Tri-Bahtinov mask using Satoru Takagi's webapp

Thanks to Satoru Takagi, you can use his webapp to generate the mask rather than downloading and installing my program to do this. Here's the link to his Tri-Bahtinov mask generator:

Satoru Takagi's Tri-Bahtinov mask generator

Generating the Tri-Bahtinov mask with downloadable program

The tribahtinov mask can be generated by using the executables in the releases directory. Please follow the instructions there for installing generate.

The required input parameters

The required parameters are

• focal length of the telescope in millimetres. If you want more or less slots, you should change the focal length.
• outer radius of the mask in inches. This parameter allows the mask to rest inside the telescope aperture.
• inner radius of the mask in inches. This parameter allows the mask to clear the secondary mirror obstruction so that the collimation adjustment screws are accessible.

The mask that is generated comes in 3 graphics formats:

• tribahtinov.pdf the tribahtinov mask in pdf format
• tribahtinov.png the tribahtinov mask in png format
• tribahtinov.svg the tribahtinov mask in svg format

The above mask is an example that was generated by generate with the following parameters

• focal length is 1000 mm.
• outer radius is 4 inches.
• inner radius is 1.5 inches.

Note: there are some artifacts that generate creates in the mask. This can be easily erased by hand.

Maskulator movie of the diffraction pattern

I can check that the generated mask works with Maskulator. The program allows me to check how the diffraction pattern changes as the focus changes. For example, the mask that I made and the diffraction pattern calculated by Maskulator is shown below

Maskulator can also generate a movie that shows how the diffraction pattern changes during focusing:

Makulator movie

Using the Tri-Bahtinov mask for SCTs and RCs

• I check that the shadow of the obstruction is roughly centred before I use the mask. The doughnut can be far off like I show below

If this is the case, it is more efficient to centre it using the Airy disk method than by using the mask to get it close.

If the shadow of the obstruction is already roughly centred or if this is just tuning up,

• I put on the mask, align the notches of the mask, or the white thumb screws to the collimation screws.

• I use a sheet of paper to block out one set of the Bathtinov masks to figure out which collimation screw corresponds to which direction by looking at the shadow it casts on the diffraction spikes.
• I focus the SCT so that one of the Bahtinov diffraction patterns indicates that the SCT is in focus. I'll call this the reference Bahtinov diffraction pattern. When this direction is in focus, I will probably find that the other two are not.
• I use the collimation screws that correspond to the other two Bahtinov diffraction directions to get them in focus. The collimation screws that I use was determined by the paper shadow test in the earlier step.
• If necessary, I refocus the SCT again so that the reference Bahtinov diffraction pattern gets back in focus. I repeat the previous step with the other two Bahtinov diffraction patterns until I am happy with the result.

Using the Tri-Bahtinov mask for Newtonians (S. Takagi)

Method is from S. Takagi's entry in Cloudy Nights.

• Do an initial collimation with another existing collimation tool.
  • For example, if it is a laser collimator, adjust the secondary mirror so that the optical axis passes through the center of the primary mirror. Next, roughly adjust the primary mirror so that the laser light reflected from the primary mirror returns to the center of the collimator.
• Remove that collimation tool and set up Tri-Bahtinov mask. At this time, match the orientation of the marks of the mask with the adjustment screws of the primary mirror. It is not those of the secondary mirror.
• Set a test star in the center of the field of view and focus. Be sure to evaluate the collimation with the test star always in the center of the field of view. This is the same for SCT.
• The collimation using the Tri-Bahtinov mask after that is done only by adjusting the adjustment screws of the primary mirror.
• The relationship between the adjustment screws of the primary mirror and the diffraction image looks the same as that of the SCT, so adjust it just like SCT.

Bugs and Suggestions

You can submit bugs and suggestions via the issues tab.

There is a known bug that I'm not quite sure how to fix at this time:

• There is a faint dashed line that marks the transition between the upper angled slots and the lower angled slots. The workaround is to either to ignore it or to erase it with a graphics program.

Copyright

All the software, documentation, hardware that I have written is copyright 2016 C.Y. Tan.

All software is released under GPLv3

All documentation is released under Creative Commons Attribution-ShareAlike 3.0 Unported License.