Monday, 2 March 2015

Webcasting the Solar Eclipse 20th 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

Northern Lights Photography

Often I am asked how to take photos of the Northern Lights. How does one need to set up a camera to be successful? The answer to this question implies the suitability of a specific camera to the job. There are, of course, other considerations as well, but here I want to concentrate on the three main parameters that determine how sensitive the camera is to light, i.e. ISO sensitivity, exposure time, and aperture.

For nighttime photography of the sky, where focus is at infinity, the aperture should be set to the widest possible value. 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 twice as much light onto the camera sensor as an aperture of f/2.8.

Once the aperture is set wide open, one has to balance between exposure time and ISO value. ISO defines the sensitivity of the sensor, but increasing sensitivity (higher ISO values) implies a grainier or noisier image. Thus one should keep this value low if possible.

But lowering the ISO value implies increasing exposure time to catch the same amount of light. For instance, an exposure time of 4 seconds at ISO 400 is the same as an exposure time of 2 seconds at ISO 800.

However, one cannot, for two reasons, increase exposure time arbitrarily. First, if the Northern Lights are really active and fast moving, one needs to keep the exposure time as low as possible in order to freeze the motion as well as possible. Otherwise the image is just a large blurry blob of green light.

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, you have to multiply the focal length by 1.5 or 1.6 depending on your camera.

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 according to the above rule for a number of popular focal lengths. Check where your lens reaches its limit here at its most zoomed out setting.

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 24mm f/1.4 on a full-frame camera, then you can go to a maximum exposure time of 20 seconds, and an exposure time of 4 seconds at ISO 400 or 2 seconds at ISO 800 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 setup 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.

Good luck!

Photo and graph: Thomas Ulich.