Collimating the C14 with a tri-Bahtinov.

Finally clear skies with a half Moon yesterday! so perfect for fine-collimating the C14.

Last week, astro-friend Erik (Hemiksem) could lasercut two tri-Bahtinovs, one for F11 native and one for the F7.5 configuration using the Starizona reducer.

This was the intial image before collimation:

Tri-Bahtinov image of the C14 before collimation

As can be seen on the image above, the collimation is pretty good, actually near-perfect. The principle of a Bahtinov is that the middle line is perfect in between the two outer lines. A Bahtinov does that with one axis (on set of lines resembling a star), and a tri-Bahtinov does that on three axis. By aligning the Bob’s knobs with the axis of the mask, you can easily adjust each knob untill the coïnciding middle line is perfectly centered.

Closely looking at the starting image, it’s clear that the 10 o’clock-4 o’clock axis line is a bit off. After some small tweaks with the corresponding knob it looks as follows:

A possible pitfall is that the collimation already needs to be near-perfect for this procedure to work. A SCT can be misaligned but still show an aligned tri-bahtinov. So take care to collimate as perfect as possible visually on a star first.

Collimating the C11 & ASI290MM

It’s been a while that the C11 was put to use, today I could clean the corrector, and also I found the very very small engraved numbers close to the edge, that line up with the line (and the same number next written next to it) found on the back of the primary. With the tri-bahtinov I could focus and collimate the optics. See also the links here and here.

make sure the three area’s of the mask line up with the three collimations screws of the secondary. The procedure is to focus like a normal bahtinov (ate least one set of lines), and then to look for the area (one of three) where the central line is not nicely central between the spikes. That becomes easily visible when covering an area: when that dims or obscures that is the guilty area, and the corresponding collimating screw needs to the turned (very very slightly). You see the effect on the central line. When centralized, repeat for the third area when necessary.

This is really easy, choose a star high in the sky. On the accuracy of this method I have little data or calculations unfortunately. What I do see is that (with a star in-focus) a very slight turn on the collimation screw (Bob Knob’s in my case), like 1/8 of a turn, gives an immediate effect on the Bahtinov image. The lines across the three sections will not be symmetric anymore. The area that corresponds to the collimation screw that was changed, will display a Bahtinov cross that is not centralised. From this observation I do believe that this method is accurate enough for collimating a SCT.

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Tri-Bahtinov Mask

We all know the classic Bahtinov mask: it allows to focus a bit more objective compared to visual star focusing.

The Bahtinov will produce Cross-shaped stars, with one clear line that moves along the cross as you focus. When the line is centered in  the cross, you should have achieved perfect focus.

So what is the tri-Bahtinov?

It essentially three Bahtinov masks in one, rotated over 120 degrees. It gives you three lines and three crosses to check collimation, along three axis simulteaneously.

In addition to focussing, the tri-Bahtinov so also indicates collimation status along three axis.

This is interesting for e.g. Schmidt-Cassegrain telescopes, where the collimation is done by push-pull screws of the secondary mirror along three axis.

When you position the tri-Bahtinov alongside these axis, it will give you insight not only per axis of the focus, but also how different these focus points are for each axis.

 

 

 

 

 

 

 

 

A picture of a star using the tri-Bahtinov and the ASI174 Webcam.  Very slight disalignement is visible.

You could basically do the same by turning your classic Bahtinov at 120° angles and check focus each time per individual axis.

Do i use the tri-Bahtinov very often? Not really. I believe it’s a great final check for a fixed SCT on a permanent mount. It will disclose the slightest error in collimation. But for every day use, with an SCT that is moved along and mounted for each session, it might be too cumbersome.

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