After getting a few fair shots of the recent Lunar Eclipse, I thought I would try taking some more pix of the Moon. One thing I noticed from the previous session was variation in the focus of the stars around the moon. I had refocused with each shot by looking through the viewfinder, but was obviously not getting that very consistent!
Friends in my local astro club recommended using a Bahtinov mask to help with this problem. It’s a very clever focusing aid having a series of grid patterns at different orientations. These create diffraction lines that converge to a common center when your telescope or camera is in focus, and diverge when off focus. This provides a very sensitive indicator of when you are at the very right spot.
My clubbies offered to make a mask for me, but I found some reasonable ones on line and went the lazy way. The mask I got is plastic with some movable plastic screws you can adjust to fit around the end of a telescope or dew shield. It stays on pretty loosely but that seems to work out OK.
I tried playing with this a couple of times and it seemed to work nicely, but I did not set up to take pictures. Then I went out very early in the morning yesterday to catch the Halloween asteroid 2015 TB145. Since a very big, bright moon was in the way, I took a few shots of that while waiting for the main subject to come up above the horizon.
I had setup the scope the evening before and did a rough polar alignment and a single-star alignment. Since there were a number of bright stars visible in the early morning Fall sky, I was able to try and complete a two star alignment of the mount. That was the first time I was able to do so at home as I have so many trees around the house. This turned out to be very helpful in catching the asteroid later on!
Then, I pointed to a bright star and placed the mask over the 6″ Newtonian used previously and the Canon DSLR attached. Looking through the viewfinder, it was pretty easy to focus the scope and get the diffraction lines to cross near the same point. I plugged the camera into my laptop and started Maxim DL to acquire an image. Limiting the image to an area around the star, I was able to tweak the focus a few times and get the pattern pretty close. Here’s what it looked like:
This looked reasonable to go with, so I slewed over to the Moon and tried taking some shots. The Moon was about 4 days past full but still very bright and up near the zenith at 4 AM. I tried taking a number of exposures down to 1/200th. Looking at the brightness histogram in Maxim DL, 1/100 seemed a reasonable exposure so I took a few shots at that time.
Looking at this at first, I thought there was a problem with the capture – perhaps it was still out of focus some or getting blurred through vibration. But maybe the detail is about as good as it can be with this setup. The 6″ Newtonian used has a focal length of 750 mm, which is only several times higher that a decent zoom lens. At this resolution, the Moon spans about 1350 pixels. With a diameter of 3500 km, the resolution is about 2.5 km / pixel at the center. So any feature spanning 5-10 pixels the eye can make something out of would be very big indeed!
One way to increase the resolution is to get a longer focal length, so re-trying this with my C-8 would be worth doing for sure. Another approach is to take a picture of the magnified image through an eye piece – which is what you do when you look through a telescope. I could give that a try as well with the SLR but would also like to try getting a bracket for a phone camera and playing with that.
After getting a few Moon shots I then tried taking some pictures to try to capture the “Halloween asteroid” 2015 TB145. That came out way better than expected, as described in the next post!