Focusing a Canon EOS and 18-55mm Lens for Wide-Field Astrophotography, e.g., the Milky Way.

There are most likely many techniques for focusing DSLRs for infinity; however, I have found it relatively easy to focus a DSLR having a zoom lens for AP using a Bahtinov mask printed on transparancy film and glued to a cut-out poster board ring. The focusing procedure does not seem to produce a diffraction pattern which is as clearly defined as when used with a telescope but it does seem to give very good results. An outline of the procedure is given below.

1. Go to the Bahtinov generator site app download for Windows at astrojargon.net and use default settings except for the below settings (these are for a Canon 18-55mm zoom lens) which seems to work fine.

• Angle 30 degrees
• f = 100
• Aperture = 42mm
• Edge = 7.5mm
• Bahtinov Factor = 50
• Horizontal Count = 8
• Angle Slot Count = 8

If you have another type of camera/lens, you can experiment with settings that yield good results for you.

2. Cut a ring with outside diameter about 54mm leavng a small amont of transparancy overhang which is good for holding the mask in place. Rubber cement works fairly well to hold the two pieces together.

3. Place mask/ring assembly into the lens opening and focus on a distant street light. Make sure the lens is set in manual mode. I usually leave Steady Shot off.

4. I use the live view feature on the Canon and/or the Canon Digital utility with a laptop. Setting the magnification on the EOS to x10 is important for observing a good diffraction pattern (a magifying glass can be a real help at times). Once focus is achieved, I use BlueStik tacky stuff to keep the focus fixed otherwise it will move if jarred slightly. Usually, the lens can be taken off and carefully transported without changing the focus. Some experimentation with this may be required for your setup. Changing the zoom does require refocusing.

5. When taking unguided wide-angle shots, I prefer to keep exposure times down to 20s or less at ISO 1600 using Tv mode for best results as star motion will then be essentially undectable. Multiple stacked shots will yield a better signal to noise ratio, e.g., 50 or more (the more, the better) 20s subframes can be combined in a freeware program like DeepSky Stacker . Final image processing can be done in Photoshop or other similar programs. One can also experiment with exposues using f/4 in Av mode at ISO 1600. If there is too much pixel noise, you may have to find suitable exposure settings using ISO 800 or lower.