Focusing is a hard. Like real hard. Especially with a short focal length camera lens at wide open f-ratio. The slightest touch of the focus ring (no - autofocus doesn't work - too dark even on a bright star) is enough to make your stars look like they have had nothing but Micky Dees super-sized combo meals for a month. That and surrounded by halos of all colors. Bah!
For this trip I mostly just used live view on the Canon back mag'd up to 10x and tried to make the stars as tiny as possible. Worked well enough for the conditions.
Here I took a picture at ISO3200 for 5 sec then reviewed on the camera back (blown up 10x) and by the tiniest touches tried to get that middle spike in between the X. Back and forth. Nudge, shoot, review, nudge, shoot, review (say naughty word), nudge, shoot, review (say naughty word), nudge ... you get the picture. I kept the last picture, said "good enough", resisted the temptation to touch focus again without the mask and carried on with imaging.
Handy tip ... remove Bahtinov mask before imaging run. Yeah. Wouldn't that be something.
Fortunately I had to frame my shot with test exposures and noticed the stars were all dim and funny looking. Yikes.
|Center the Spike! Bahtinov focus on 200mm Canon EF-L at f/2.8|
I only just now thought to take a look at that image under the light of day. Much easier to see on the computer screen than while on your knees craning your neck to see the camera back. Looks pretty good - just a touch off - but it would have taken luck to place that spike any better. The only way would be to use a fine motion focus ring attachment - something I have been working on. More on that another day.
I'm happy with the focus on the Andromeda shot - don't really think I could have gotten it any closer. The Bahtinov mask is cool and a real savior for those imaging with telescopes. I'm not sure it is of much help with 200mm or smaller camera lenses, but it was fun to make while the storms rolled by.