This is a very familiar sight and thousands of pictures have been taken of this little “seahorse” or horsehead in the sky. Taken with the Celestron C11 on an EQ8 mount, Starizona LF corrector for Schmidt-Cassegrains, a Nikon D750 DSLR modified for extra H-Alfa sensitivity. 22 frames of 300 seconds each are combined in this image
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.
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.
This is a famous sight: Messier 51, also known as the Whirlpool Galaxy (not hard to see why!) in the Hounting Dogs constellation (Canes Venatici).
I remember being able first time ever to see this galaxy with 8×40 binoculars when I was 14. That was a major achievement back then. We spent the night under the stars in April with three friends, and as the spring sky is not very rewarding to binocular users, at least this one made our hearts beat faster.
In this picture, images from last year in April are combined with images from February 15th, 2018. All are taken with the C11, Nikon D750 and now in February with the ASI174MM as a guiding camera, replacing the Lacerta MGEN for off-axis guiding work.
The ASI174MM has delivered every time since it’s purchase in January a decent guiding star. With PHD combined with BackYardNikon the guiding goes excellent.