Monday 2 March 2015
Above is a photo of the Sun, taken on 4th February, 2015, at Oulanka Research Station. Unfortunately thin clouds made observation and focusing difficult. The photo is a test shot to see how the equipment will perform during Sodankylä Geophysical Observatory's live webcasting of the solar eclipse on 20th March, 2015. See SGO's blog for more information. Here, I want to discuss a few technicalities, which will make a live web cast possible.
The setup for the image above as well as the web cast will be as follows: a full-frame Canon EOS 6D will be tethered to a MacBook, and fitted with a 2x converter as well as a Tamron f/5.6 200-400 mm zoom lens giving an image of the Sun of a bit over 1000x1000 pixels. Most importantly, the lens opening facing the Sun will be covered with "AstroSolar" folio, which in Finland you can buy, e.g., here.
First, a word of warning: Do not look at the Sun without this folio! If you intend to use welding goggles, make sure they are at least factor 13 or darker! The sunlight can destroy your eyes, and you won't notice it at the time of observation, but only afterwards.
As discussed recently with respect to photographing the Northern Lights, in order to prevent stars to appear elongated due to the Earth's rotation, the exposure time, in seconds, must be chosen to be less than 500 divided by focal length. The above setup yields a total focal length of 800 mm: 500/800 = 0.625 seconds. Thus an exposure time of half a second or faster is sufficient.
The aperture will be set at the widest possible, which means f/5.6 here. But the 2x converter reduces the aperture by two stops, thus the combined f-stop above is f/11. It turns out that at f/11 with the AstroSolar folio in place, the exposure time needed for the above image was 1/100 s, thus no problem with blur due to Earth's rotation.
However, the field of view of an 800 mm lens is only about 3.1° (diameter), and therefore I will have to readjust the pointing direction constantly in order to keep the Sun in the field of view. This means that in the camera image frame, the Sun will appear in different locations due to my manual pointing.
In order to get neat, full resolution, and fairly similar images throughout the event, I wrote a software, which centres on the Sun and then crops the image around the Sun at 1500x1500 pixels. How does that work?
First, I decode the RAW file (.CR2) using a utility called dcraw, which will develop it into a .ppm file. This file is then further processed using the fantastic ImageMagick tools. First, it is converted into pure black and white, meaning there are only two colours left (use -threshold). Then, using the -fuzz option, the crop geometry is obtained. However, this is a tight crop, which is not very pleasing, and thus it is easily converted into something a bit bigger than the Sun, with the Sun in the centre. Some sharpening is then applied. Finally, the time stamp and copyright texts are placed under the image using the label: option. The last step is to use exiftool to copy the EXIF data from the RAW file to the final JPG. The last step will be to copy the image to a web server, but the address has not yet been agreed upon and thus this step is still missing.
The photo above does not have any other modifications done to it, no lens correction, no contrast or brightness adjustments.
Text and photo: Thomas Ulich.
PS: You can download the source code for that software from our server here.
Tuesday 3 February 2015
Often I am asked how to take photos of the Northern Lights. How does one need to set up a camera to be successful? There are, of course, other considerations as well, but here I want to concentrate on the main parameters that determine how sensitive the camera is to light. If your camera can operate using the values given in the chart below, you will be able to photograph the aurora.
Focal length is the least important factor discussed here. The Northern Lights often span the whole sky and thus you want to get as much sky as possible into the image. Therefore, zoom out as much as possible or choose a lens with a wide field-of-view. Focal length is a measure of field-of-view, and the smaller your focal length, the more sky you can see. Typical wide-angle focal lengths are 14mm to 35mm, but also 50mm might work for some scenes (for a full-frame camera). For a crop-sensor (APS-C) camera or a micro-four-thirds camera you need to multiply these numbers by 1.5 or 2, respectively, and the resulting number should stay below 50 mm.
For nighttime photography of the sky, the camera needs to let in as much light as possible. Therefore start by setting the aperture as widely open as possible. Apertures are given as ratios like f/2.8. The aperture is largest, when the so-called f-number is smallest. For instance, an aperture of f/2.0 will let more light onto the camera sensor than an aperture of f/2.8. In other words, start by choosing the smallest f-number.
Exposure Time and ISO
Next, you have to balance between exposure time and ISO value, which are directly coupled together for a fixed aperture. For the same scene, doubling ISO requires halving exposure time, or halving ISO requires doubling exposure time. For example, an exposure time of 4 seconds at ISO 400 is the same as an exposure time of 2 seconds at ISO 800.
In other words, ISO determines how light-sensitive your camera is. The problem, however, is that the higher you set the ISO sensitivity, the noisier and grainier the image will become. It depends on your camera model – and your taste/judgement – how much image noise you want to accept. Take your camera out at night and try it with different settings for a few scenes to find out what the highest acceptable ISO value is for your camera.
Also for exposure time there are upper limits. First, if the Northern Lights are really active and fast moving, one needs to keep the exposure time as low as possible, otherwise all the motion will make the northern lights in the photo look very blurry without any structure.
Secondly, at some point the rotation of the Earth will cause the stars in the image to appear as lines rather than dots. A commonly used rule states that one should keep the exposure time below a value of 500 divided by focal length (in seconds). For example a lens with a focal length of 50 mm will allow for a maximum exposure of 500/50 = 10 seconds. Note that, if you use a crop sensor (APS-C), you have to multiply the focal length by 1.5 or 1.6 depending on your camera.
Balancing Exposure, ISO and Aperture
The diagram above (click it for a larger version) will help with this balancing act. The dashed horizontal lines at the top give the maximum exposure time that will keep the stars as dots for a number of popular focal lengths. Check where your lens reaches its limit here at its widest field of view (shortest focal length).
Next, choose the diagonal line with the f-number of your widest possible aperture. Then you can move along this line to see which combination of ISO values and exposure times to start with when photographing the Northern Lights.
For example, if your lens is a 28mm f/2.8 on a full-frame camera, then you can go to a maximum exposure time of 15 seconds, and an exposure time of 4 seconds at ISO 1600 or 2 seconds at ISO 3200 would work fine.
The photo above was taken at only 0.5 seconds exposure time at ISO 800 using a 24mm f/1.4 lens. This shows that the graph is not a definite rule. The graph provides a starting point, meaning if you set your camera using these values, you will succeed taking photos of the Northern Lights. But if the lights are bright, you might get away with shorter exposure times or lower ISO values, thus you can freeze faster motion or get less sensor noise.
Finally, this chart will tell you also if your equipment is enough for the job. Just check where your camera specifications would be on this chart. Furthermore, modern camera sensors get more sensitive all the time, and some cameras nowadays get decent results with ever higher ISO values. Thus I encourage you to give it a try and see how far you can push your camera.
Photo and graph: Thomas Ulich.