All images were captured with the same camera, a Digital SLR, with a lens mounted to it. The lens used varied depending on whether I wanted a very wide field-of-view or whether I wanted to zoom in for a bit of a closer view of a specific object. No telescope was used, but for some of the images, I used a ‘star tracker’. I will soon explain what a star tracker is and why it is required.
Some of the objects in these images are so faint that a “normal photograph” – a quick snapshot if you like, would not manage to record them on the camera’s sensor. These faints objects require a long exposure, meaning that the camera shutter is left open for some time, say 30 seconds or longer, to gradually collect light bit by bit as it falls on the camera’s sensor, after having crossed vast distances across space.
Now, over the course of the night (and day), the Earth rotates. As it does so, the stars appear to move in the sky, just as the sun appears to move during the day, rising in the East and setting in the West. A star tracker slowly rotates in the opposite direction to the Earth’s spin, in so doing counteracting the apparent movement of the stars, and thus keeping the camera fixed on them throughout a given exposure. If we left the camera’s shutter open for, say, a whole minute without using a star tracker, the stars would appear as trails of light, not the pin-sharp dots that we want.
Oftentimes, a single 30-second exposure is not sufficient to collect enough light. In such instances, we may either increase the exposure time, i.e. leave the shutter open for, say, a minute or more (depending on how faint the object is), or use another method which I describe next.
Let us say you were trying to take photograph of a very faint object, such that you require a very long exposure of 20 minutes for it to appear bright enough. Rather than taking a single 20-minute image, you can break up the photograph into a series of shorter exposures, e.g. 4 images of 5-minute exposure time each (for a total of 20 minutes). Equivalently, you may choose to take 10 images of 2 minutes each (which still make up a total of 20 minutes), or even 40 images of 30 seconds each. Then, after you have captured your images, you stack them one on top of each other on your computer and take an average. There are different ways to achieve this, but the end result is that you combine the signal recorded in each of your separate frames into a final image that is made of the sum total of your images.
Many of the images in this feature were produced by taking a number of such exposures, then adding them up in this manner until the objects become sufficiently bright. As you may imagine, if you were to carry out the same procedure from a light polluted site, each individual frame would also be recording artificial light (i.e. light pollution), so when you add up all the images, you would not be increasing just the signal from the object, but also that from artificial light. In other words, this method does not help your situation if the sky is light-polluted.