by David J. Watkins
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Astrophotography Aquiring Images - Focus
Focusing to infinity:
Focusing is the probably most important thing to master first. In astrophotography, perfect focus an aboslute necessity. Forget autofocus with a DSLR or point and shoot camera, it just won't work on a star. You may get away with it on the moon but that is where autofocus and astrophotography end. You will have to manual focus. You will want to focus at infinity to get sharp pin point stars. In the old days with manual focus lenses, you could just rack the focus to the infinity setting. But with the lenses of today, simply moving the lens indicator to infinity will NOT achive a suitable infinity focus. Autofocus lenses will focus past infinity (doesn't sound possible does it?). They do that so the autofocus motors do not slam against a stop, but can gently overshoot and move back to focus when near infinity. If you use a telescope you will have to manually focus, though some software packages will control your electronic focuser and use the image from your camera to focus. But you probably won't be starting out with equipment like that. Photographing galaxies, planets, star clusters, nebulas, etc is not like photographing terrestrial objects or landscapes. In the sky, everything is at infinity. So if you focus on the moon which is only a couple hundred thousand miles away, you will still be in focus to image Saturn which is more than 750 million miles away, or the Sunflower galaxy (Messier 63) which is over 35 million light years away. The only thing that will force you to refocus, is a change in temperature or mirror shift in a Catadioptric scope. Focusing on stars is like learning a new skill.
Focusing with Live View Mode (DSLR):
Live View mode is the best way to achieve focus on stars. It is the only way that I have ever focused for astrophotography. If your camera does not have live view mode, you can probably find an alternative method by searching the web, there are also focusing aids available.
There is a trick to live view mode focusing on stars. You will need to find a bright star to begin. Bright stars tend to be the only ones that will show up in live view mode. Set the ISO as high as you can on your camera. Many cameras today can be set to ISO 25600 or ISO 12800, if not, ISO 6400 will work. Next set the camera to Bulb mode usually the "B" setting on the camera. Turn on Live View mode on your camera and zoom in to 5X or 10X if your camera will zoom in that far. As you focus the star will swell or shrink depending on how close to focus you are. When the star has shrunk to its smallest size, you are in focus. Cranking up the ISO helps brighten the stars. Take your time, it will take more than just a few minutes to get the best focus. Focusing on a star can take a very long time, so don't be surprised if it takes you 15 minutes or more to achive focus! Once you achive focus you should be good for a while unless the temperature changes or if you encounter mirror shift like in a Catadioptric scope. If you change eyepieces, add extenders, change lenses, etc., you will have to re-focus. After focusing in Live View Mode, don't forget to turn the ISO back down before you start shooting. Extreme ISO's will produce extreme noise.
Focusing Aids:
If you don't have live view mode on your camera, or you don't like using it, there are other focusing aids available. Most of them are for telescopes. So where do you get something like that? AstroZap (http://www.astrozap.com/) sells various focusing aids for different telescope models. I have never used one, so I am not sure how they will work on a camera lens. There are many tutorials on the web to explain how to use them.
Focusing For CCD/CMOS Astrophotography cameras:
Focusing with a dedicated CCD or CMOS astrophotography camera is a lot like using live view mode on a DSLR, but easier! You typically have software controlling the camera and the software usually has one or more features for focusing. The principle is the same, you watch the star swell and shrink, and when the star is the smallest size that you can get it, you are in focus. Focusing on planets, like Mars, Saturn, and Jupiter, you can use surface features to judge when they are sharpest.