Tri-Bahtinov mask for the Skywatcher Quattro 150P Newton

The Bahtinov mask is one of the most useful tools for astrophotography. It allows for very easy focusing.

The Tri-Bahtinov mask has an additional function compared to the classic Bahtinov mask: it can not only focus the telescope but also collimate it.

What advantages does the Tri-Bahtinov mask offer over classic collimation using the Airy disk method?

The image produced by the Tri-Bahtinov mask is independent of atmospheric conditions. Often, the Airy disk cannot be seen clearly enough to achieve optimal collimation.

The spikes produced by the Tri-Bahtinov mask are easier for the human eye to interpret than the center of a diffraction disk. This allows the brain to process this information better and enable a more objective assessment of 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 means that focus can be achieved at one point in the image plane, but not at others.

For the three adjustment screws of the primary mirror, three Bahtinov masks are essentially integrated into a single one. This adds two additional spikes 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 disk, first center the shadow of the obstruction as much as possible. It is easier to create this using the Airy disk than directly with the Tri-Bahtinov mask. The obstruction doesn't need to be perfectly centered in this step; a rough centering is sufficient.
The Tri-Bahtinov mask is placed on the lens, aligning the markings with the adjustment screws.
A sheet of paper is used to cover a section of the Tri-Bahtinov mask, allowing you to determine which collimation screw corresponds to which direction of the Tri-Bahtinov mask.
Now, the three collimation screws are adjusted using the spikes.
It may be necessary to refocus occasionally to bring the spikes back into focus.

In the image, you can clearly see that the collimation is incorrect (3 o'clock and 7 o'clock positions).

Software:

K. Evans has written software that supports focusing and collimation:

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

Notes:

The Bahtinov masks are individually calculated for each telescope for optimal performance.

The Bahtinov masks were calculated, designed, and printed by us. We print the masks in black or white, depending on availability. They are intentionally slightly looser to avoid damaging the delicate surfaces in the dew cap.