Digital Astrophotography

DRAFT

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Most astrophotography is now done using digital imaging equipment and techniques. While there are still some people who do astrophotography on film, most astrophotographers prefer the advantages digital imaging offers.

It helps to understand the basics of digital imaging, how cameras work, how color is captured, and the advantages that digital imaging offers over film.

Advantages

One of the key advantages is the fact that digital imaging does not require film, and, therefore, photographers can capture an unlimited number of images and not need to use chemicals to process them. Processing film requires specific equipment, chemicals, and techniques that the average person does not possess. Most people have a computer or access to one, and digital cameras have far surpassed the availability of film cameras.

With film, if you don't have the development equipment and chemicals, you could send the film out for processing, but few processing companies would have the expertise needed to process the images properly to get good prints. Most astrophotographers had to do the work themselves.

With digital imaging, image files are saved as a set of numeric values, and this affords us the opportunity to use mathematics to transform and enhance the data. This above all else changes the game for amateur astrophotography.

Image Sensors

And image sensor in a digital camera is an electronic component that senses light and quantifies it. Fundamentally, an image sensor is a photon counter - counting photons, which are the smallest units of light energy, that reach it.

There are two main types of image sensors: Charge-Coupled Devices, or CCD, and Complementary Metal-Oxide Semiconductors (CMOS). Each has certain advantages and disadvantages, but the way they function is similar. The material of the sensor is a type of semiconductor material. One key property of semiconductors is the ability to act as a switch In the case of image sensors, when a photon of light reaches the surface of the sensor, it causes the charge on that sensor element (i.e. pixel) to increase slightly. Every photon that reaches it will have the same effect. When a picture is taken, the sensor is exposed to the light source, and then, when done, the level of charge on the element is counted and recorded.

An image sensor in a camera is essentially a collection of individual elements like this, known as pixels, each of a given size, arrayed in a grid. When the picture is taken, the processing circuit on the sensor reads each pixel value and records it in a data structure which is then saved to a disk or removable medium as a file.

It's worth noting here that the image sensor does not detect color. No digital camera image sensor that I'm aware of detects color, at least none in regular production cameras. We will circle back to color in a bit. For the moment, what's important is that the sensor simply quantifies the amount of light detected by measuring the charge on the pixel which is increased each time a photon of light interacts with it.

Stepping back a moment, it helps here to understand a few things about light. First, it is well understood that light behaves both as a particle and a wave. There has been a lot of scientific study devoted to this and we won't delve into that. But we will discuss both natures briefly.

As mentioned before, a photon is the smallest unit of light. When a photon interacts with the light-sensitive material of the image sensor, the material acts like a switch to allow the charge to flow. Each photon allows a small amount of energy to flow into the pixel, and at the end of the exposure, the total charge on the pixel is read.

But looking at light as a wave, one of the properties of waves is wavelength: the distance between peaks of a wave. What we call "light" is just a subset of the entire spectrum of electromagnetic radiation. Radio waves are another form. As are x-rays and gamma rays. There are two common ways of measuring electromagnetic radiation: frequency and wavelength. Waves are often diagrammed on a graph.