These galaxies prove to be a real challenge. I gathered data to make some details visible in these fluffy patches in the empty spaces around Leo. We ‘re looking deep into our universe. Picture taken with the C11 and Nikon D750
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 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. To know which collimation screw to handle, it’s best to position the tri-bahtinov in line with the three screws.
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. However, all three axis need to be in line at the same time. And that ‘s not easy when turning 120¨° and seeing one position at the time only.
Do I use the tri-Bahtinov very often? Maybe yes. I believe it’s a great final check for an SCT. It will disclose the slightest error in collimation. For every day use, especially with an SCT that is moved along and mounted for each session, it’s good to check collimation this way. Especially when the scope doesn’t hold it’s collimation very well.
The test with the tri-Bahtinov is rather sensitive, as is shown by turning a collimation screw very very slightly (like 10°): this will become visible in the picture instantly.
Since September the sun has barely appear in our skies and the same is true for stars. So there were only three clear nights without a Moon, all three of which I was so lucky to image the sky. On December 16th I could do this picture with the well-known Pleiades, in a wide field setup.
I used the EQ8 with the old Nikkor 180mm F2.8 ED unguided. An IDAS 2″ filter in front of the objective was used resulting in an effective F/ratio of 3.8. However this seems to have affected the image quality. The star shapes are not ideal.
20 frames of 300″ were stacked and processed by Jean Lammertyn in PI.