Getting out of focus stars into view
Start with You Camera's Live View: Miss-focus still happens to me occasionally, and I continue to see and hear about this disappointment and frustration from my fellow nightscapers, especially those who are just starting out. On page 53 of my Milky Way NightScapes eBook I use three paragraphs on how to manually focus and find your true infinity with the camera's Live View function. I then end with, "Once you’ve found your lens’ true infinity, mark it for future reference, or tape down the focus with a piece of gaffer’s tape. You’ll have one less thing to worry about in the dark."
Temperature Affects Focus: That advice works most of the time, but I've found that you can't always rely on it because the tape can loosen or the focus mark become meaningless due temperature changes. Changes in temperature can cause considerable focus shifts in your lens, so it is best to redo your focus each time you go out shooting at night. And, if temperatures are considerably different than the last time you were shooting, wait several minutes for your lens to adjust to the new outside temperature.
Zoom lenses often have a different infinity focus for each focal length, so no single infinity focus setting will give you accurate results throughout the zoom range of the lens.
Additional magnification to your Live View image: As we get older, it becomes much harder to see if a star is sharply focused on a 3-inch LCD camera monitor, even when the image has been magnified to 10X. For this reason, I carry an inexpensive pair of +3.0 diopter (power) reading glasses from my local drug store. This, or a Hoodman Loupe (with the Hoodman Cinema Strap) allows for hands-free, enlarged views of your little camera monitor.
by Aaron Romano