I have seen very few telescopes considering that I will soon have a PhD in Astronomy, and by soon I mean not soon enough. My work has never required me to actually go to a telescope. Luckily a couple of graduate students who use a local observatory for their work often need an extra body for safety reasons while they observe. I recently joined one of them on a trip to the Large Zenith Telescope (LZT) in Pitt Meadows. I took the opportunity to make some long exposure photos of the telescope and stars. The work they do at the telescope involves shooting a powerful yellow laser straight up, which adds an interesting bit of flair to the shots, and almost makes up for the barn-like shape of the observatory.
The wavelength of the laser light matches that of electrons in the sodium atom, when the light hits the thin layer of sodium in Earth’s atmosphere it excites these electrons, ionizing the sodium. The ionized sodium quickly finds a free electron which emits a photon when it combines with the sodium ion. The telescope collects these photons and because the laser is pulsed the details of the height and density of the sodium layer can be measured. The sodium layer and how it changes with time is very important for a host of scientific fields of research. It has a practical importance in Astronomy since many telescopes depend on the sodium layer for something known as adaptive optics.
I did several exposures ranging from 40 minutes to two hours. I love turning to bulb mode and calculating the correct exposure, it’s good practice, and there’s something very deliberate and satisfying about it. The first step to calculating the correct exposure in this type of situation is to let the camera do it for you, as usual. Set your ISO to something high like 3200, don’t worry about noise, this image is needed only to calculate the correct long exposure. Set your camera to aperture selection, open up the aperture as big as it will go and see what your camera recommends for exposure time. You should be able to find a suitable exposure that’s less than 30 seconds. I find that setting the exposure compensation to +1 or even +2 gives a nice result (I shoot with center-weighted metering).
For example my test shot for the above photo suggested the following settings:
ISO 3200, f/4.0 – 1/3 stop, 13 seconds
I use 1/3 stop increments on my Canon 7D but I don’t like to think about them so I reference them by their relation to the full stop increments. So f/3.5 becomes f/4.0 – 1/3 stop (or f/2.8 + 2/3 stop). Long exposures should always be done at ISO 100 since they are low light shots and are going to have enough trouble with noise. So right away I know that I want to calculate the equivalent exposure for ISO 100.
Stops are factors of 2 for ISO so we have to find out how many consecutive divisions by 2 will get us from 3200 to 100. The sequence is 3200, 1600, 800, 400, 200, 100. So by changing to ISO 100 we’re removing 5 stops of exposure. To get an equivalent exposure we must add this amount to the shutter speed. That is the shutter speed must allow 5 stops more light to enter. Again we can use factors of 2, the sequence this time is 13s, 30s, 1min, 2min, 4min, 8min. I rounded 26 to 30 since it’s only a 1/3 stop shift which won’t make that much of a difference to the shot but makes the math a lot easier. Okay so our equivalent exposure is: ISO 100, f/4.0 – 1/3 stop, 8 min
I wanted a longer exposure so I also changed the aperture to f/11 – 1/3, which is an additional 3 stops. The aperture scale changes by factors of sqrt(2), which is cute but multiplying by 1.414 is not that much fun to do in your head so I suggest just memorizing that scale. The additional 3 stops of exposure time gave me an hour long exposure, perfect for long star trails: ISO 100, f/11 – 1/3 stop, 60 min
You can see the glow from the city at the bottom of this shot. The sign on the door reads “DANGER MERCURY do not enter”. The mercury is liquid but it does evaporate and mercury fumes are lethal. Once the rooftop has been open for a while the concentration of mercury vapor is acceptably low to enter without gas masks. The telescope does not have a conventional glass mirror instead it spins up a small amount of liquid mercury which naturally forms the shape of a parabola. Mercury is highly reflective and makes a very good mirror. This is cheaper then shaping a piece of glass to the type of precision needed for Astronomy. The downside is the telescope can only point straight up, that’s why it’s called a Zenith Telescope.
This last image is a two hour exposure, you can see that the star trails are longer than in the previous shots which were all around an hour long. The shots which include the building were taken on a night with a nearly full moon. The moon is very bright and this allowed me to capture the building as well as the stars, but the moon also makes the night sky bright meaning you see less stars and they don’t seem to shine as brightly. The second shot above was on a different night after the moon had set, the sky is very dark in that shot and many faint stars are visible.
A last bit of advice if you’re doing these types of shots is to turn on long exposure noise reduction. There are many hot pixels that glow bright red, green or blue when the sensor is used continuously. Having the camera measure and remove these will make your life a lot easier. The camera will take a second exposure of the same length and use it to remove these hot pixels from your exposure. This is known as dark frame subtraction, a technique that’s also used in astronomy. Unfortunately this means that your camera is busy taking a dark frame. For example, I started my two hour exposure at 1:30am the exposure was done at 3:12am but I had to wait until 5:00am for the dark frame to be done. So that one shot took four and a half hours!
You can take your own dark frame at another time, but it’s crucial that the ambient temperature be the same. This is also why it’s important not to take the camera inside while it’s calculating the dark frame. With higher temperatures there is more thermal noise and more hot pixels. You could also take one dark frame for the entire night if you’re willing to stick to approximately the same length of exposure. I haven’t tried doing this myself in Photoshop, but I’m guessing that the camera algorithms are doing a better job than I could. However, if you were planning on doing a lot of long exposure photography you would probably want to learn how to do your own dark frame subtraction.