Photographing the stars or the Milky Way at night isn’t all that difficult. However, unlike most other areas of photography, whether it be landscapes or portraits, the equipment makes all the difference in the world, and it can get pricey. The reason being, there isn’t a whole light to work with, so you need to bring in as much of it as possible.
You accomplish this in two ways. The first being the camera body. A digital camera’s sensor is comprised of millions of tiny photo-sensors, which gather light and record an image. A full-frame sensor has larger photo-sensors that absorb more light. This increase in light means the camera can operate better than a crop-sensor in low light conditions and yields better image quality. It also means you can increase the ISO setting without increasing the noise, which is very important when photographing the Milky Way. Usually, a crop-sensor camera can take just as nice of photos as a full-frame camera, but this isn’t the case when photographing at night, or in low light conditions. When shooting the Milky Way, you need to operate around ISO 1600-3200, and a crop-sensor camera is more than likely going to have a ton a noise in that range. Even the full-frame has noise, but it’s manageable in post-processing.
Secondly, a full-frame camera won’t help you take better pictures of the Milky Way and the stars unless you also have a “fast” wide-angle lens. By fast, we mean a lens that can let in a lot of light. An example of this would be the Canon EF 16-35mm f/2.8L III USM, a Sigma Art 20mm f/1.4 or even a cheaper Rokinon 12mm f/2.8. You really want a lens that can open up to f/2.8 or f/1.4, as this will let in a lot more light as compared to an f/4.
I don’t want to debate the quality of the lenses, as each has its advantages and disadvantages. The bottom line is, you want a lens that’s 24mm or wider with an aperture of f/2.8 or larger. You can still take some nice pictures at f/3.2, but once you stop down the aperture more than that, you lose too much light.
This goes without saying, but you’ll need a nice sturdy tripod and a shutter release cable or remote trigger capability. It’s impossible to shoo the milky way hand-held. Exposure times will vary, but most of the time they are going to be around 20 seconds. Anything longer than about 25 seconds and you’re going to capture the Earth’s rotation and the stars will start to appear oblong rather than dots. You could always get some really facing star tracking hardware, but it’s really not needed.
Focusing can be tricky. Simply adjusting your lens to focus “infinity” won’t work. The reason being, the lens can actually focus beyond infinity such that nothing is in focus. You have a couple options.
The first is, go into Live View if your camera supports it, point the camera at a bright star, and rotate the manual focus to get the star to look as sharp as possible. If you have a Canon camera, while in Live View mode you can press the magnifying glass button twice to zoom 10x. This makes focusing easier.
The second option is to take a picture, view it, zoom in as far as you can and see if the stars are in focus. Make an adjustment either clockwise or counter-clockwise with the manual focus, take another shot, and compare it to the first shot. This involves a lot of trial and error.
Even with a full-frame camera, there is going to be noise at ISO 3200, so post-processing is important. You can minimize the noise, but you’ll never get rid of it without losing a lot of the detail. Cranking up noise reduction in Adobe Lightroom will remove many of the smaller stars and take away from the photo. There are other techniques in which you can take several photos and stack them, but this requires a lot of knowledge on how to use the applications that can process the photos.