2021 January 14
If you have a DSLR, mirror-less or any other type of digital camera including a smartphone camera, have you thought about trying your hand at Wide Field Astrophotography? You may be surprised at how capable your camera is. Your device will most likely have a large range of settings accessed by push buttons, touch screen menu options or even smartphone or tablet control capabilities. It will be worth your time exploring the control options in the comfort of your home during the daytime with the camera manual and a cup of tea to hand. It is best to maximise your time under the dark sky instead of trying to sort out the settings and buttons that now appear to be hidden or have mysteriously moved in the dark! Some smartphones have dedicated night modes, and certain models can be used in an “Astrophotography” mode, please experiment. You do not need a telescope, your digital camera with a standard or a short telephoto lens and a tripod is an excellent start.
This can be really easy, as all you need is some object to hold your camera steady during the longer than normal exposure times you will need to use. You can balance your camera on many items, but a tripod of any size will allow for a quicker setup and a capability to point more accurately at your chosen target. If you take an average daylight shot your camera may calculate the exposure to be 1/1000th of a second at F5.6 using ISO 200, you can handhold your camera and standard lens with ease at these settings. Unfortunately, these typical daylight settings will not work very well for night time astrophotography. You will need to set the ISO to be at least 800, 1600 or higher to minimise the exposure time. Exposures in the range of ¼ second to tens of seconds will be required depending on the target chosen. You may use your lens at its maximum aperture, keeping an eye on the images’ quality, especially if you have recorded many stars in the corners of the images. Suppose there is a bright star, planet or the moon in the sky you are imaging. In that case, your cameras’ autofocus system may work. If not, you will have to set your camera to manual focus and adjust the lens as appropriate, starting with the lens set to the physical infinity mark or the equivalent in software. If your camera can display the scene in real time, often called live view this makes focussing much easier. If you have a 3x, 5x or 10x live view magnification option, experiment with it. You are looking to get the moon to appear as sharp as possible if it is in your scene. If the scene only contains stars, they have to appear as the smallest dots of light as possible on your viewing screen or device. When you start focussing on stars it is usually advantageous to turn the cameras ISO up very high then you should see many stars to focus with. Remember to turn the ISO down as appropriate for your image sequence. For your initial steps, you are looking to take a series of single frames, so you can start by using the JPEG image recording image option. Your camera should let you adjust the cameras JPEG image formatting to get usable colours and sharpness. If you have the relevant software, the opportunity to use RAW files to post-process later is a better option but not mandatory at this stage. If you go onto more advanced astrophotography techniques, the RAW format is a must. If you use a smartphone, it may automate many settings, including image stabilisation and stacking. Experimentation is the key. .
Framing the scene
You may be able to capture a pleasing image by including some foreground interest. In figure 1, the target was the Moon and Venus. From my location, they were very low in the north-west evening sky. However, several foreground trees were partially illuminated, so the task was to get a good image of the Moon and Venus with the trees adding some local interest. The camera settings were:- ISO 1600 with a 100mm lens set at F3.5, it was manually focussed on the moon, and a 1-second exposure was used. The camera was mounted on a small tripod. You can also see two stars in the top left of the image. These are Mu (closest to the corner) and Eta Geminorum. Exposures from 4 seconds down to ¼ of a second were taken to capture the scene accurately.
The 1-second exposure gave the best overall exposure of the scene. This exposure kept the colour of the twilight sky, and the earthshine on the moon visible. You have to work quickly as the sky brightness diminishes soon after the sun sets and you may lose the scene you wanted to capture. The image was not post-processed; it is the cameras standard JPEG file; it has only been cropped..
The goal for this imaging session was to image the Milky way rising vertically out of the sea. There are several computer programmes or smartphone apps that you can use to show you what is visible at your location at any time you require. Search online for “astronomy session planning app”.
The camera settings for figure 2 were ISO 6400 with an exposure time of 30 seconds. A zoom lens setting was 17mm focal length and the aperture used was F2.8. The camera was on a sturdy, fixed tripod. ISO 6400 was used to keep the shutter speed as low as possible to prevent blurring due to the stars’ movement. Once again, several exposures were taken and the best selected for display. This scene was taken at 01:13. The foreground was illuminated with a small LED torch. This image was post-processed from a RAW file to bring out more of the faint milky way that was just visible to the naked eye. The camera was manually focussed on the stars using its live view capabilities extended onto a tablet computer via a good quality USB cable. This allows for a comfortable viewing angle, irrespective of the camera’s orientation and extends the cameras battery life, as the camera’s display, was turned off. The tablet also triggered the camera’s shutter. Many cameras can be controlled over Wi-Fi, be careful of the battery life in this mode of use.
With simple software, it is possible to create very pleasing images. Search online for “Star Trails software”.
For this project, the camera was mounted on a sturdy tripod and pointed towards the pole star. A zoom lens set to 16mm at F2.8 and 199 images at 800 ISO were taken with no gap between each exposure. The exposure time of 20 seconds per exposure was determined by taking various images and checking that the sky brightness caused by light pollution did not swamp the stars or the scene. The camera was programmed and controlled remotely. The lens was manually focussed on the stars. Some cameras or smartphones allow the number of exposures and the gap between exposures to be directly programmed in, without extra software. This is usually called the interval timer or intervalometer shooting mode. It is advisable not to touch the camera during the sequence as any slight re-alignment will spoil the resultant image. Ensure you have plenty of storage available on your memory card and your camera battery is fully charged.
Figure3 shows a small sample of 4 images out of the 199 RAW frames that were taken. The images look disappointing when individually viewed, with the stars only showing as very faint dots.
Once the image sequence is run through the star trails software, the scene produced is shown in figure 4. The stars’ apparent rotation around the north celestial pole is clearly shown, and the colours of the stars can easily be seen. Polaris can be seen just above the centre as a point. The image shown is just over 1 hours worth of exposure or around 15 degrees of the earth’s rotation.
Suppose your camera has a dark frame subtraction routine that is used for reducing noise in longer exposures. You will have to disable it as the images have to be taken in sequence as quickly as possible, so no gaps are apparent in the final image. Any extra noise in the images will be lost in the final result. Some software will also allow the star trails to be shown as a moving video image. On the more advanced smartphones, some have a dedicated night-time mode. Please experiment with the exposure settings to get the best possible images for your location and sky conditions
The next steps
If you would like to extend your imaging capabilities and are new to astrophotography, then some tracking mount is advantageous. There are several small but capable tracking mounts on the market. These are well suited for DSLR’s and their lenses and are also very portable. They are capable of carrying a small telescope when the need arises. Figure 5 shows a small tracking mount on a sturdy tripod carrying a DSLR and a telephoto lens. This type of set up is easy to carry and use. Often portability is overlooked but becomes a must if you are travelling on foot to get the scene you want to record. There are also techniques using a fixed camera and taking many hundreds of short duration images to accomplish something similar. Still, the downside is the large computational time required and the loss of resolution compared to a tracked image. Shown in figure 6 is an image of the Pleiades open star cluster, also known as the Seven Sisters. It is in the constellation of Taurus. A 70-200mm zoom lens set to 200mm at F3.5 and manually focussed. A lightweight tracking mount was used. The cameras ISO was set to 1600. The image is cropped to remove the star image distortions and field illumination errors at the image’s edge. The image has also been post-processed to remove the effects of the local light pollution. Depending on the type of zoom lens you have to make sure the lens does not change its focal length as some lenses zoom barrels will move as the camera is pointed skyward. For this image, 53 exposures, each of 30 seconds have been stacked. More exposures were actually taken, but only the best were selected for use. The stacking process allows many short exposures to be averaged together to form an image that is equivalent to 1 long exposure. For this you will need some software, there are some excellent free programmes available, search online for “star stacking software”. As shown in the image, some of the blue reflection nebulae associated with the Pleiades have been captured.
As shown in this article, a wide range of ISO settings and exposure times have been used depending on the outcome required. There are no hard and fast rules regarding your cameras ISO, shutter speed and the number of images needed for a particular project only guidelines. Try using a range of ISO settings from 800 and above and shutter speeds from 1 to 30 seconds and critically examine the resultant images. Local conditions such as sky quality and light pollution play a significant role in what you can achieve. Make notes of your successes and use those settings as a starting point for your next project. Above all, enjoy yourself.
Tony Morris, Imaging Advisor, Equipment and Techniques