360º VISION: CREATING A NIGHT SKY PANORAMA
How to capture the entire majesty of the night sky using multiple shots stitched together.
Creating a panorama can be a bit of an art. Many aspects need to be considered, and in this article we’re going to explain how to capture the images to form a 360° picture, as well as the equipment needed to do it. You’ll find it a very rewarding experience once you start to see the image come together.
So what is a panorama? Put simply, it’s a 360° picture made up of multiple images taken from one location. They can’t be any old images, though: to get the best results you need to take non-parallax images, where there’s no perspective change as your camera is moved. It’s also best not to spend more than 20 minutes taking the images that will be combined: any longer than that and the Earth’s rotation will cause the stars to shift positions. To minimise this star shift, it’s also better to shoot in rows first, then columns so you can photograph the sky faster. You also want the images to overlap so that they can be easily combined later.
Lens choice plays an important role in the capture technique. I typically like to work at f/2.8; it lets plenty of light into the image but also allows for a
pleasing depth of field. The other advantage of using a wide aperture is that as the camera moves, the edge of one image can be the centre of the next. Wide apertures will generally produce more vignetting, however, and while this can be corrected after capture, there’s a risk of noise entering the image and creating a ‘blotchy’ final picture. I’ve shot at f/2.0 but wouldn’t recommend going wider than that, as coma (blurring) and other lens aberrations can become evident at the edges of an image.
Exposure time is crucial too and the ‘500 Rule’ is invaluable here. According to this, you divide 500 by the camera’s focal length to get the number of seconds you can expose for before star trailing becomes an issue. So with a 24mm lens, 500÷24mm = 20.8 seconds, which means the stars will start to trail with any exposures longer than that. Most cameras have a long-exposure noise-reduction option; this makes the camera take a second image immediately after the first to record the noise of the sensor. The camera will then automatically subtract this noise from the photo, effectively doubling the exposure time. If exposure time is an issue or your camera doesn’t have this function, it’s possible to take a single dark frame image with the lens cap on and the eyepiece covered, which can then be subtracted from the photo manually. Doing this can be very time consuming, though and in reality, it doesn’t save as much time as moving the camera to the next position while the dark frame is exposing.
The theory
As previously mentioned, the images need to overlap. Generally I have a 30 per cent overlap, which makes stitching the frames together relatively easy. It also means the highest resolution part of the lens is used to form the majority of the completed image and if there’s a bad frame you can almost completely remove it by masking in the neighbouring frames.
Armed with the focal length and overlap factor, you can calculate how many degrees to turn the camera. The first step to doing that is to calculate the field of view using the following formula: In this, _ is the angle of the field of view, d is the sensor size (remember to calculate horizontal and vertical separately) and f is the focal length of the lens. The table (above right) shows the field of view that different focal lengths provide with a 35mm sensor, calculated using the formula. Finally, to calculate the angle to turn the camera between shots, you’ll need to take the amount of each image that isn’t an overlap and multiply that by the field of view. For a 24mm lens with a 30 per cent overlap,
the amount of each image that isn’t overlapped is 70 per cent, so (1 – 0.3) x 74 = 51.8° for the horizontal, and that (1 – 0.3) x 53 = 37.1° for the vertical.
To make sure that you can capture all your images within 20 minutes, you now need to calculate the number of images in rows and columns. If you have an automated panoramic head, your camera will be in landscape mode, so: 360÷51.8 = 7 frames for the horizontal. Then for the vertical, it’s: 180÷37.1 = 4.8 frames, which you can round up to five. That’s a total of 7 x 5 frames, which is 35 photographs. Using the 500 Rule you can work out you want a 20-second exposure from the 24mm