Focusing for visual observing seems so easy compared to astrophotography. Achieving good focus is always important no matter what type of observing you're doing, but the eye has a way of adapting to slight imbalances in focus and dealing with unsteady "seeing." Unfortunately, the camera is not as forgiving.
I recently started planetary imaging using an Orion StarShoot 5MP and deep-sky imaging with an SBIG STF-8300M. I feel like I'm getting pretty good with stacking, processing, and post-processing, but focusing is still a challenge for me. To compound the problem, sometimes you don't realize that the focus was ever-so-slightly off until AFTER you start processing the images, at which point there isn't as much that can be done to correct for it.
Since I own a long focal length (3910mm) Schmidt-Cassegrain Telescope (SCT), precise focus is even more critical. Mirror shift is always a variable with an SCT, but mind handles that pretty well with the built-in mirror locks. Still, even being off focus by a hundredth of a millimeter can make a huge difference in image quality. To try and tackle this problem, I added a JMI EV-1 Crayford-style focuser. This has certainly helped make fine focusing easier, but it's still tough to know when you're spot-on. My imaging package offers a reasonably good fine-focus routine that analyzes the quality of a single star as you make slight changes to focus. Again, this helps tremendously, but I find fluctuations in the "seeing" quality during the focus routine still make this quite a challenge and the quality meter "bounces around" quite a bit.
After much reading and research, I decided to purchase a Bahtinov Mask to improve my fine focusing routine. The mask creates three artificial spikes in the image of a single star. The idea is that when you adjust the focus to converge the three spikes together in the center of the star, you're in perfect focus. Most folks say this speeds up the process and eliminates a lot of the frustrations of focusing. I have yet to try it as the weather has been less than cooperative so far this spring, but I'm hoping that this will simplify one of the key variables in getting good images.
What about you? What focusing methods do you use to tweak your fine focus for imaging?
I recently started planetary imaging using an Orion StarShoot 5MP and deep-sky imaging with an SBIG STF-8300M. I feel like I'm getting pretty good with stacking, processing, and post-processing, but focusing is still a challenge for me. To compound the problem, sometimes you don't realize that the focus was ever-so-slightly off until AFTER you start processing the images, at which point there isn't as much that can be done to correct for it.
Since I own a long focal length (3910mm) Schmidt-Cassegrain Telescope (SCT), precise focus is even more critical. Mirror shift is always a variable with an SCT, but mind handles that pretty well with the built-in mirror locks. Still, even being off focus by a hundredth of a millimeter can make a huge difference in image quality. To try and tackle this problem, I added a JMI EV-1 Crayford-style focuser. This has certainly helped make fine focusing easier, but it's still tough to know when you're spot-on. My imaging package offers a reasonably good fine-focus routine that analyzes the quality of a single star as you make slight changes to focus. Again, this helps tremendously, but I find fluctuations in the "seeing" quality during the focus routine still make this quite a challenge and the quality meter "bounces around" quite a bit.
After much reading and research, I decided to purchase a Bahtinov Mask to improve my fine focusing routine. The mask creates three artificial spikes in the image of a single star. The idea is that when you adjust the focus to converge the three spikes together in the center of the star, you're in perfect focus. Most folks say this speeds up the process and eliminates a lot of the frustrations of focusing. I have yet to try it as the weather has been less than cooperative so far this spring, but I'm hoping that this will simplify one of the key variables in getting good images.
What about you? What focusing methods do you use to tweak your fine focus for imaging?
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