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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Andromeda with the old Nikkor 180mm ED ’70 tele

and the D750, 141x60s on ISO400.  WIth a 50% Moon low in the South. This image was taken before the Moon set, and with the public lighting still switched on.  From midnight onwards the Pleiades were imaged.

M31 Andromeda 141×60 ISO400 Nikkor 180mm ED tracked on a EQ8 with Lacerta MGEN II and SW8x50mm Moon 50% in the sky

 

Pleiades continued

During the night of Monday October 15th, I could add 21 times 5 minute shots to the Pleiades project. Unfortunately the coverage of fields is slightly off, so I have an artefact on the lower right corner, where the images did not overlap. Well, there will always be something… 🙂

M45 Nikon 180mm ED D750 6u 400ISO SQM2015 Meldert Oct2018 NoFilter 66% JPG

And my friend Marc Verhoeven has processed the image above a different way:

M45 same picture, but processed by Marc Verhoeven in PixInsight. Different flavor!