Tri-Bahtinov mask for the Skywatcher Explorer 200PDS f/5 Newton

The Bahtinov mask is one of the most useful tools for astrophotography. It is very easy to focus with.

The Tri-Bahtinov mask has an additional function compared to the classic Bahtinov mask: with it, the telescope can not only be focused but also collimated.



What are the advantages of the Tri-Bahtinov mask compared to the classic collimation using the Airy disk method?
the mapping of the Tri-Bahtinov Makse is independent of atmospheric conditions. Often the Airy disk cannot be seen clearly enough to achieve optimal collimation.
the spikes generated by the Tri-Bahtinov mask are easier for the human eye to interpret than the center of a diffraction disc. Thus, the brain can process this information better and allow a more objective assessment of the collimation.


How does a Tri-Bahtinov mask work?
In a perfectly collimated reflecting telescope, the path lengths of all light rays from the primary to the secondary mirror are the same. However, if the collimation is not optimal, the light rays have different path lengths and thus different foci. This allows focus to be achieved at one point in the image plane, but not at other points.

For the three adjusting screws of the primary mirror, three Bahtinov masks have been integrated into one. Two additional spikes are added to each of the three main directions of the original Bahtinov mask, which can be used for collimation.

[Source: https://github.com/cytan299/tribahtinov]





Using the Tri Bahtinov Mask:
With the Airy disc, first bring the shadow of the obstruction as centrally as possible. It is easier to do this with Airy disks than with the Tri-Bahtinov mask directly. In this step, however, the obstruction does not have to be perfectly centered, a rough centering is sufficient
The Tri-Bahtinov mask is put on, aligning the markings with the adjustment screws.
An area of ​​the Tri-Bahtinov mask is covered with a sheet of paper, so it can be found out which collimation screw corresponds to which direction of the Tri-Bahtinov-Masek.
Now the three collimation screws are set using the spikes.
It may be necessary to refocus in between to bring the spikes back into focus.



In the picture you can clearly see that the collimation is not correct (3 o'clock and 7 o'clock positions)


Software:
K. Evans wrote software that supports focusing and collimation:

https://github.com/1CM69/Tri-Bahtinov_Grabber


Hints:
The Tribahtinovmasekn are calculated individually for optimal performance for each telescope.

The TriBahtinov masks were calculated, designed and printed by us. We chose white so you can easily find them in the dark. They are intentionally a little looser so as not to damage the delicate surfaces in the dew shield.