Astrophotography of the Solar System: the lucky imaging method
We generally associate astrophotography or night photography with long or very long exposures. However, there are certain occasions, usually because the object we want to photograph is very bright, where this is not entirely true and we have to use faster shutter speeds. For example, if we want to observe the Moon, our shutter speed will be between approximately 1/200 and 1/500 seconds, as Mario Rubio already commented in his article on the technique to photograph the Moon in issue 3 of the magazine.
However, in addition to how bright the object is, there is another factor that is even more important and that will condition us to choose very short exposures. This factor is the atmosphere. The atmosphere is fantastic, however we would not be here talking about astrophotography if it were not for it, it provides us with wonderful photos of sunrises and sunsets, but at the same time, unfortunately, it makes astrophotography from Earth a little more difficult. In order to counteract the negative effects on our images, astronomers invented a technique called lucky imaging (or lucky image in its literal translation into Spanish). As we are going to see now, the idea of lucky imaging is to take a lot of very short duration photographs.
Before we dive into the necessary equipment and technique, let’s talk a little about its history and why we need it.
Lucky imaging: what it is and why
The lucky imaging method was born between the 1950s and 1960s using cinematographic cameras for the observation of planets. Due to the level of technology of this type at that time, the results were very limited and difficult to achieve. So fast forward 30 years, to the 1990s and the advent of the Internet and web cameras. Before the proliferation of CMOS sensors, those that we all have in the cameras of our current mobiles, the first webcams came with CCD sensors, much more sensitive in low light conditions. It was then that the amateur astronomer community began experimenting with astrophotographic web cameras, connecting them to telescopes and obtaining surprising results. Suddenly, a normal person in the backyard of his house could obtain images of planets as sharp as a professional astronomer in an observatory.
The method itself tries to take hundreds or thousands of exposures in a row of no more than 0.1 seconds in length. The idea is to quickly take many images of the same object to counteract the fluctuations that the atmosphere suffers and that causes us to lose sharpness when observing the sky. The quality of the hundreds / thousands of images generated will always be slightly different and therefore we can choose the best among them and discard the worst, by using special software.
Typically, around 50-60% or even a higher percentage of the images are usually discarded. This will give us the ability to create a much sharper image.
Now let’s talk a little about the atmosphere and its fluctuations to better understand the need for the lucky imaging technique. Even on the calmest days and with the most constant weather the atmosphere may not be the best for astrophotography. For example, if it is very hot, the middle / lower layers of the atmosphere will be very turbulent, which would greatly impoverish the clarity of our astronomical images. Go outside tonight and look at the stars. The constant scintillation they show is a direct cause of the turbulence in our atmosphere. Be careful, we will always see twinkling in the stars, even on the best of days, but on those nights when the atmosphere is particularly bad we can see that the twinkling is much more pronounced.
To avoid the turbulence of the atmosphere, the main observatories of the world are located at high altitude. For example, the observatories of El Teide in Tenerife and that of El Roque de los Muchachos in La Palma, are located about 2400 meters above sea level. This height not only allows us to be above the clouds almost every day of the year, but it is also high enough for the atmosphere to be as stable as possible. Depending on the time of year, in the Canary Islands, the atmosphere begins to stabilize above 600-1000 meters and therefore it will show much less fluctuations than if we observed from sea level. Although it seems little, that difference of 400 meters is very significant when observing, obtaining great differences in the final results.
When a beam of light from a celestial object enters the atmosphere, it begins to interact with it. If the atmosphere is stable, the light beam will pass without being seriously affected and what we will see in our camera will be seen with some clarity. Instead of a point, we will see something a little thicker giving us the feeling that it is blurry, but we will not have lost much information. If, on the other hand, the atmosphere is turbulent, the light beam will begin to interact with each of the layers of the atmosphere, dividing as it passes layers. The end result is that in our camera we will have lost almost all spatial resolution (amount of detail) and the object will appear weaker in our image. For example, a bad atmosphere can cause two nearby stars to appear as one in our image.
Another thing to keep in mind is the height at which the celestial object is. The closer to the horizon, the more atmosphere your light beam will have to pass through, therefore the more it will be affected by it. The ideal is to observe objects that are closest to the zenith (the highest point in the sky located just above our heads) where the atmosphere will be more stable and narrow.
When to use this technique
Now that we know a little more about how the atmosphere affects our observations, let’s talk about what celestial objects we can use lucky imaging on. As we have said, we are going to need lucky imaging when we require a high control in the quality and quantity of detail of our image. A clear example of this is when we go to photograph the surface of the Moon (see figure 2). In general, considering that exposures are going to be at most 0.1 seconds (100 milliseconds), celestial objects have to be bright enough for us to detect them. Thus, in addition to the aforementioned Moon, we can (and must) use lucky imaging to photograph the Sun, Venus, Mars, Jupiter and Saturn.
We already know what we are going to observe with the lucky imaging technique. Let’s see now what is the equipment necessary to make the observations. If our first objective is to photograph the Moon, then we can start with the most basic equipment of all, a reflex camera, a 200mm lens or more, and a tripod. So basic. If the SLR camera is a full format (or full frame in English) camera such as the Canon 5D or 6D, for example, then I would recommend starting with a somewhat longer lens, such as 300mm or 400mm. With my Canon EOS 500D that has an APS-C sensor (cropped format) and my lens at 300mm I get good images, yes, at the cost of limiting the resolution of my final image to 1000 × 1000 pixels, so those images are They are somewhat small if you wanted to print and hang them at home.
To this basic equipment I would also add a remote shutter release, to avoid vibrations in the camera and / or a cable to connect the camera to the computer. As the images that we are going to take are very fast, it does not matter if the Moon moves between image and image as long as it is well framed in its entirety in the sensor. We will already worry about aligning the images in the processing.
In the event that the Sun has sunspots on its surface and we know that these are very large (on the order of 5 times the size of Earth or larger), then we could also use our basic equipment to point at the Sun and see some structure. on its surface (see figure 3).
It is very important to note that in the case of wanting to observe the Sun, you must always use a solar filter in the mouth of the lens. To find out if the Sun has sunspots on its surface and their size, I recommend visiting the page spaceweather.com. Unfortunately, the Sun is entering its calmest part of its activity cycle, so the number of sunspots will continue to decrease for the next two to three years. The next peak of activity will not be until about 2025. However, the Sun can always sporadically surprise us with a large sunspot like the one shown in figure 3.
In issue 2 of the magazine, Raúl López Ramírez recommended as a telescope for astrophotography of galaxies and nebulae, a refractor of about 80-120mm in diameter. If we have a similar telescope, in these cases, we will also have a tripod with tracking capacity. Such a telescope and a reflex camera attached to the telescope offer us great equipment for photographing the Moon and the Sun since they enter our sensor completely and we will not have to make mosaics. Again remember that to observe the Sun you always have to use a solar filter in the mouth of the telescope. Additionally, if we are very interested in observing the Sun, we can also buy a specific telescope for solar observation. I currently have a Lunt LS60THa which is a 60mm diameter refractor with a special lens with an alpha hydrogen filter (see figure 4). This filter blocks almost all sunlight and only lets through a very small part of the light. By observing the Sun with a hydrogen filter, not only will we be able to see sunspots, but we will also be able to differentiate other structures on the solar surface such as filaments. In addition, if the Sun is showing prominences (see figure 5) on its edge, we can also see them clearly. It is important to note that we should never wait to see prominences when using a normal sunscreen. These can only be seen with an alpha hydrogen filter. Also, many of the prominences have lifetimes of only hours, so don’t be discouraged if it’s gone by the time you’ve managed to aim the telescope.