Incidentally, what lens are you using?
The stars are indeed too small for the auto-focus system to work and it does require manual focus. But there are focusing aids you can use.
There are two things you can do to help without using any additional aids, and then there's an option to use focus aids (more on that in a moment.)
First...
When focusing on the night sky, know that if "anything" in the sky is focused, then "everything" in the sky is focused. This means that if you can see the moon or any particular bright star -- even if that's NOT the part of the sky you want in your image -- you can use that to focus. Switch to manual focus, focus the lens on those easier objects, then point the camera lens back to your intended target (knowing that as long as you don't bump the focus ring, you'll get a tack sharp image.)
Second...
Don't just use live-view... use 10x live-view. When you switch to live-view, the normal image will show a preview of what the whole sensor can see, but at this 1x magnification it's difficult to tell when focus is optimized. In the upper-right corner on your camera body back you'll see two buttons.. one is magnifying loupe with a "+" in it, the other has a "-" in it (zoom in or zoom out). You can press the + button a few times to take the live-view up to 10x size (you get a small box in the live-view screen when you are NOT zoomed in which shows the area that will get magnified when you do zoom in. Use the directional navigator buttons to move that box to frame the section of sky that contains a bright object in the sky.) This makes it MUCH easier to refine focus.
Third...
Also, Canon cameras have "exposure simulation" when using live-view. This means if you crank the ISO to max, the f-stop to the minimum (that your lens allows) and also crank the exposure time to the longest time possible (even though this is _not_ what you plan to use for the shot) it will brighten up those dim stars and make them easier to focus. After focus, return your exposure to more sane values.
These three things help reduce the difficulty of focusing on the night sky and anyone can do them becuase they don't require anything that you don't already have.
But I mentioned that there are some focusing aids you can employ (this requires making or buying more "stuff").
I use a Bahtinov focusing mask for my night sky focusing. There are some variations on this, but basically it covers your lens and has slots cut into it. The slots are cut in such a way that they cause any point of light to throw diffraction spikes roughly in an "X" like pattern but with a vertical bar "|" through the X. As you focus, you'll see that vertical bar move to the left or right side of the "X". You have achieved perfect focus when the vertical bar is in the very center of the "X".
You can make your own mask by visiting a website such as astrojargon.net and using their Bahtinov mask generator tool. It will ask you questions about the lens you are using (if you don't know what values to enter, tell me the exactly model of your lens (and if it's a zoom I need to know the focal length) and i can provide you some sane values to use.) This gives you a template file to download and print and you can trace the template onto something a bit more durable (since printer-paper will quickly turn to mush in the night time dew) and cut it out with a hobby knife.
You can also buy pre-made masks.
Farpoint Astro makes masks. Kendrick makes masks. Gerd Neumann makes masks (a bit pricey). These are pre-made and come in various sizes which either thread onto your lens (using the filter threads on the front of your lens) -OR- there are some that snap into the front threads of an existing filter (e.g. if you have a clear or UV filter you can attach the mask to an existing filter.)
But there is one down-side to these masks... given that they are basically a piece of solid black material with slots cut into them, you'll notice that roughly 50% of the surface are is blocked and about 50% are clear slots. This means you are literally losing about HALF your light that makes dim stars appear even dimmer. Dim stars don't create very big diffraction spikes. You want a bright star (e.g. something like Sirius, Arcturus, Altair, Vega, etc.) and actually while planets tend to appear brighter than even the brightest stars, planets are not "point" sources of light and this means they don't produce clean diffraction spikes. Don't use a planet when using a Bahtinov mask... use a bright star (it's much easiser).
Anwyay... Lonely Speck makes a device called the "SharpStar2". This is a square slide-in filter (requires that you own a square filter holder such as a Cokin holder or a Lee filter holder) and the slide-in focus aid is completely clear (rather than being 50% blocked). To create the diffraction spikes, they laser-etch in the same lines as you would have seen cut as slots on a normal Bahtinov mask. This means they work "like" a Bahtinov focusing mask but they don't block so much light (which means they are more effective at night.)
Again, all the same guides apply... use live-view, crank to 10x size, crank up the exposure values, etc.
If you still cannot find a bright enough star even after doing all of this, then just take a test exposure (the longer the exposure, the bigger the diffraction spikes in the test image).
Also (very important), if you do use a focusing mask, don't forget to remove the mask before you start imaging. That may sound "obvious" but you are working in the dark so it's an easy mistake to make. ...not that I have ever made such a mistake (whistles innocently).
Tim Campbell
5D III, 5D IV, 60Da
