Set Up Before You Go – Considering your eyes will require time to adjust to the darkness in the field it is easier to adjust your camera settings before you head out or while you are still in your vehicle.
Shoot RAW – Most amateur and many enthusiast photographers are quite content shooting jpegs. For astrophotography you definitely need to make the jump to shooting RAW. This allows the greatest amount of flexibility when post-processing your shots. Think of RAW files as your old film negatives and jpegs as the prints from the photo kiosk. Having good negatives allow you to print photos with subtle changes in processing.
White Balance – Every light source produces a colour cast. Your WB setting helps correct for this. I usually set a custom white balance (CWB) of 3600 - 3800 Kelvin. White balance, or colour temperature as it is also called, is based on the range of colours that a strip of platinum goes through as you heat it. It is measured using the Kelvin temperature scale. If you prefer to use one of the programmed WB settings I recommend Tungsten (or Incandescent). They will give your night sky a pleasant blue tone. Of course, if you are shooting RAW you can adjust this on your computer during post-processing.
Focusing – Go Manual. Imagine focusing your lens with your eyes closed. That's what you are asking your camera to do in the dark. The stars are too faint for the auto-focusing to work effectively. Switch to manual focusing and set your lens to infinity, but don’t trust the infinity symbol on your lens. Instead use the manual focus assist features (magnification and focus peaking). If you are using the E-M1 Mark III then engage Starry Sky AF and it will do the focusing for you.
Aperture – "My gosh those stars are far away. I better use the smallest aperture possible to get the greatest depth of field." Nope! Only half of that statement is correct. The stars really are far away, but to your lens they are simply small points of light all on the same plane. Selecting a large aperture (f/2.0, f/2.8, or f/3.5) will ensure that your lens will gather as much of that light as possible in the shortest amount of time.
Shutter Speed – During the day we are usually measuring shutter speeds in very small fractions of a second. But at night you are fighting against two challenges: the very small amount of available light coming from those distance stars and the spin of the earth. It was only when I started experimenting with astrophotography that I realized just how fast the stars move through the heavens. Set a shutter speed that is too long and you end up with blurry stars, instead of nice sharp pinpoints of light. For your lens and camera combination use the 400 Rule to determine the maximum shutter speed you can use. The 400 rule is fairly simple; divide 400 by the true focal length of the lens and this will give you the maximum shutter speed before star trails will become noticeable. The TRUE focal length refers to the full frame equivalent of the lens (or 35mm SLR equivalent from the film days). This will depend on the crop factor of the camera sensor. Olympus sensors have a 2x crop factor. For my 12mm lens, the calculation works as follows: 400 / (12mm x 2) = 16.6 seconds. I would round this down to the closest available shutter speed. Therefore, an exposure of 15 seconds should result in sharp stars. If you have no intentions of enlarging your images you can get away with a longer shutter speed before you would notice blurring of the stars.
ISO – The final part of the exposure triangle is ISO. The higher the ISO, the more sensitive to light the sensor is. Unfortunately, a higher ISO also results in more noise or graininess. Improved sensor technology has dramatically reduced the amount of noise when shooting at higher ISOs. I typically shoot between 3200 and 6400 ISO. If noise becomes too big an issue in your images try shooting at 1600. It's amazing how much detail can be pulled out of an image that appears underexposed.
Noise Reduction – Dive into your camera menu and ensure that noise reduction is turned on. All of that graininess that I just mentioned will be reduced significantly. You will notice however that your exposures take twice as long. This is because your camera is taking a second photo with the shutter closed (a dark frame), seeing where all the noise is and then subtracting it from the original shot. It slows the photographic process down, but the results are worth it. There will still be some noise left on your image but that can be reduced using software.
Program Your Settings – Since your main settings are not likely to change much from one situation to the next, program them into one of your custom settings (C1, C2, etc.). This will save time on your next outing.
Test Shots – I will finish off by starting again - with composition. Once everything is set I take a few test shots. This is mainly to see whether my horizon is level and if my foreground subject is framed properly. Use the Live View Boost to improve the visibility of your dark surroundings.
Lens Condensation – As the front element on your lens cools during those long nights of shooting it is not uncommon to get some condensation forming. This can ruin your images. Rechargeable lens heaters can be purchased to help prevent this. Another option is to use a chemical hand warmer and an elastic band to attach it to your lens. This has worked for me on a number of occasions.