Updated: Mar 5, 2021
Astro photography may be the opposite of Street photography. You might chance a lucky shot on the street, but to get a good shot of the cosmos you need to plan.....
Before I get into the process of astrophotography, I need to preface this blog with the admission that I love technical things. I like figuring out a new computer program, a new device, and different camera techniques . Astrophotography can be technical. Much like macrophotography, the shots need to be planned, calculated and timed perfectly. This contrasts a lot with my candid street photography which is more about finding scenes that are setup with photographic opportunities. I think you don’t have the same pot luck with astro-photography. At best you will get a clear night, but then the rest of the process is up to you. If you can plan properly then you will definitely get some good shots. The fact that it s a more technical process is what makes astrophotography fun!
The workflow documented below should be a good start if you are new to the process.
Often to get the right shot you will need to wait for the right conditions. Typically, this includes a dark, clear sky with little or no visible moon. Even with these conditions you may want to plan the position of the stars. Below is a breakdown of this planning.
A Dark Sky This means getting away from populated areas that have high levels of light pollution. You don’t realise it but lot of the light from buildings and streetlights is reflected in our atmosphere. This effectively obscures the dark sky and the stars. Typically, you need to travel to remote locations to avoid light pollution (or you find yourself in a large blackout!). Check out the following for where to find a dark sky https://www.lightpollutionmap.info/
A Clear Sky
A clear night is also a must. This is the one you will have the least control over. A well planned trip can be easily ruined by bad weather. Some parts of the world, however, are more or less prone to cloud cover (e.g. deserts are great). If your in Australia then the BOM is the best website. http://www.bom.gov.au/
A Moonless Night or a New Moon. The moon is a strong light source. It will obscure stars and is typically not as interesting as the Milky Way. Planning for a night without the moon can be achieved with a quick check on the net. There are also a bunch of mobile apps that help you plan for this. A good website is http://www.moontracks.com/
The position of the Milky Way (for stills)
When taking sill shots the position of the Milky Way will be most important (as this is typically the background of your shot). A lot of photographers tend to blend the milky \way with the foreground of the shot. In terms of positioning, our galaxy will appear at different positions in the sky depending on the time of night, the time of year, and your relative position. Again, there are a bunch of mobile apps that can help plan for when the milky way will appear just on the horizon, or full in the sky. A good resource is http://stellarium.org/
The position of the Celestial Poles (for long exposures). When taking a long exposure of the sky, you will get star trails. The stars will curve around the Celestial Poles (north or south). A tighter arc will appear around stars closure to the pole, and less curved around equator. These are my favourite shots, and also easier to take with my own camera setup. I use the Google Sky app which uses my phone’s gyroscope to show me the relative position of the pole.
An amazing picture of the milky way is still only a background. You will need an engaging foreground to take a great shot. If you google astrophotography, what you will actually find is a variety of foreground subjects against a star-scape. These are typically silhouettes but can be foreground that is illuminated (artificially). This is where you get to be creative…
In terms of equipment, you will need a sturdy tripod, a head-torch, a camera (preferably with good light sensitivity), and a fast wide angle lens. If you want to illuminate the foreground you may also what a light source of the right colour. For long exposures, I recommend a deckchair and some kind of portable heater that can prevent condensation from forming on the camera lens.
Taking a Still Shot
A nice clear view of the cosmos with some interesting foreground is the goal of a still shot. Here the emphasis is on getting as much detail from the sky as possible without the stars moving, and without too much unwanted noise or over-exposed elements.
Mode, White Balance and Focus
Set your camera to manual. Whitebalance between 4000K and 5500K, and focus to infinity. The balance of your shot can be corrected in post so don't stress too much about getting the colour perfect.
Set it to RAW. This will capture the maximum amount of information in the image file so you can pull out more detail in post processing. I’ve forgotten this step a few times and have ended up with shots that have limited options for processing in Lightroom or Photoshop.
Generally you will want to shoot with the aperature as wide as possible. This means the lowest F-stop. However, the problem is that a lot of lenses do not perform well when opened wide up. If you have a non-professional lens then you may want to set it at one stop more than the widest setting. E.g. my wide angle lense can support F1.8 so I may want to shoot at f2.4 to keep it sharp. This is especially important for the stars on the edge of the shot.
ISO Set your ISO high (but not too high). You will need to set the ISO as high as it can go without producing too much noise or blowing out the detail in the brighter starts. Better cameras will be able to achieve higher ISO values without these shortcomings, however, a somewhat lower ISO can still be used if you are utilising the star stacking method (see below). Unfortunately, the right value will change camera to camera so you may need to experiment. A good starting value might be 25,600 for full frame, and 12,800 for cropped frame (or lower with star stacking).
The ideal shutter speed for still shots of the night sky can be calculated from the “500 Rule”. This rule will give you a limit for how long you can set the shutter before your camera will record a perceptible trail for each star (when you really want a point for each star). Basically, it is 500 / Crop-Factor x Focal Length. For my setup I use a 7mm Lens with a Micro Four Thirds Setup. This equates to 500/(2x14) = 35. This means that at 7mm I can get a still shot at 35 seconds or less. Setting the camera to the slowest shutter will allow you to capture more light (stars) or less noise (if your camera needs a lower ISO).
Remote or Timed Shutter. A good way to reduce camera blur is to use a remote shutter so your finger does not move the camera when you press the shutter release. This can be a wired cable, a wireless dongle, or even a wireless app that controls the camera. A simple alternative, however, is a 2 second timed release.
If you want the most possible detail, or your camera is not particularly light sensitive then a clear image of the Milky Way can still be achieved through Star Stacking. Using this technique you will take a few shots in quick succession (using the above settings). Once captured, you will have the raw data needed to perform Star Stacking in post-production. The stacking method will add the data from all the images to achieve clear, full image of the sky whilst removing unnecessary noise from the final shot. This is a good idea if your camera is not particularly light sensitive but you still want to see all the dim stars. The key points to note when star stacking are as follows
Remove noise reduction. The noise reduction will slow down your successive shots and you will want to make the shots as quickly as possible to minimise the movement of the sky. The noise reduction will be performed in post.
Dark Frame. A Dark Frame is simply a photo with the lens cap on which will allow you to remove image noise in post-production. The Dark Frame *should* only contain noise from the sensor so will subtract the noise from this frame from your other images. When taking a dark frame it is important that you take this shot immediately after your other shots.
Composite Software. Most photographers use Photoshop to combine composite shots (and there are many tutorials on this process). However, there is a freeware application called Starstax which can achieve the same outcome without an expensive Adobe subscription. The software will assist with any movement of the sky against the foreground.
Taking a Long Exposure
If you want to show star trails then you will need to perform some form of long exposure. This shot will show how the stars move through the sky and produce an image that looks like a whirlpool of stars with the Celestial Pole at the center. The key points to note with long exposure is that there are two methods. A single long shot or a series of shots that you can join as a composite image in post production. My preference is the latter.
A Single Long Shot
A single long shot is a single exposure of over 60+ minutes. Its not typically the preferred option as the camera sensor will heat up after about 3 minutes and introduce a lot of noise. It is for this reason that I typically prefer the composite approach.
Slow down the shutter. Setting the camera to a slower shutter will mean you will need to forget the “500 rule” and set the camera into “bulb” mode. This will allow you to take a 60+ minute shot which will show the stars moving in arc across the camera sensor until you finish the exposure.
Set ISO Low. A long shot will tend to over expose and produce a large amount of noise if the ISO is set too high. The ISO will need to be set low enough to reduce this but not too low as to remove the stars altogether. A good starting value is 400.
Take test shots. If you’re going to wait over an hour then you will want to take a few test shots to make sure you have everything setup right. I would allow around 10 mins for each test shot until you are confident that you have the right balance with ISO and shutter speed.
A Series of Composite Shots
Composite shots will require 100+ exposures of around 1-2 mins which will be combined into a single image in post processing. There is some extra effort with setup and with processing, however, you will reduce the noise associated with each image which can make a huge difference. The steps are below:
Shutter Speed. The shutter speed will be quicker than the long exposure (to reduce noise), but will also be longer than a still shot (to allow for lower ISO). Finding the right shutter will depend on your form factor. Full frame will be around 30-60 seconds. Whilst cropped sensors 60-120 seconds. The high exposure of cropped sensors will be required because these sensors will need a lower ISO to reduce noise.
ISO. The goal here is to keep ISO as low as possible, whilst getting enough light sensitivity to expose the night sky within the selected shutter speed. Generally, Full Frame cameras can tolerate a higher ISO so between 300-800 is good. For cropped sensors, between 160-500 will work well. To figure out the exact value you will need to experiment. Note, the preference will be to set a longer shutter speed than to increase ISO.
As per the single shot, you will need to take record 60+ minutes, however, this will be achieved over around 60+ images. The time between each shot should not exceed 1 second. If your camera supports continuous automated shooting then this is preferred.
Test Shots. Unfortunately, you will need many test shots to find the perfect balance of Shutter and ISO for your camera. You may waste an hour taking test shots so rug up!
Composite Software. The star stacking process is similar to the still shot stack. You can use Photoshop. However, there is a freeware application called Starstax which can achieve the same outcome without an expensive Adobe subscription.
Overall, shooting long exposures is more challenging than still shots because of the time required. In a single night, I could only one or two decent shots. The best advice I can give is to cosy up with some drinks and a good book. I think the star trails are more rewarding though…
Challenges in Practice
Given the above process you can be forgiven for making some compromises in your pursuit of the perfect astro photograph. Even with compromise, however, you can still produce some very rewarding shots. One of these challenges is illustrated below:
Shooting with Too Much light.
The image below shows a group of sheds over a body of water. The challenge here is the existing light source that blows out the exposure with all shots with high ISO or long exposure. Unfortunately, high ISO or long exposure is required to expose the star scape properly. A compromise is required to expose both the bridge and the stars. In this case, I have slightly overexposed the bridge lights and underexposed the night sky. What we get is some minor blowout of the bridge lights, and fewer stars exposed in the sky. Even with this compromise, however, I think the shot is still engaging. Let me know what you think....