The Need-To-Know Specs:
Sensor
The make and model of the actual sensor inside the camera.
Resolution
Image resolution is size of the resulting images produced from the camera, usually measured in megapixels (millions of pixels), e.g. 16 megapixels. It is also sometimes measured in width x height of the total pixels, e.g. 4944 x 3284.
Sensor Size
The sensor size is the physical dimensions (in millimeters) of the sensor’s effective image area. The larger a sensor is, the wider the field of view it has, and vice versa — a smaller sensor will have a narrower field of view. This figure is sometimes expressed as full frame (approximately 36x24mm), APS-C/crop sensor (approximately 24x16mm), Micro 4/3 (approximately 18x12mm), and other common consumer camera sensor sizes.
Sensor Diagonal
The sensor diagonal is the physical measurement of how many millimeters are between the opposite corners of a sensor. When choosing a camera for deep sky astrophotography, it is important to know what the image circle of your telescope or additional optics like a reducer/flattener. Make sure the sensor diagonal is smaller than your image circle. If you don’t, it will likely result in elongated stars towards the corners of the image and possibly vignetting.
Pixel Size
Pixel size is the physical size of the pixels, measured in microns (µ). For deep sky astrophotography, larger pixels (like 5µ or higher) are usually better as they gather more light per pixel, but this comes at the tradeoff of lower resolution. For planetary imaging, a smaller pixel size is usually better, but it can depend on the telescope used.
Back Focus Distance
The back focus distance specification on a camera is the distance (in millimeters) from the sensor to the threads where the camera attaches to the imaging train. When using corrective optics such as a reducer, field flattener, reducer/flattener, or coma corrector, back focus spacing is essential to keeping the focal plane flat and ensure round stars throughout the image. First, find out the distance of back focus that your corrective optics require (e.g. 55mm), and then subtract the camera’s back focus distance (e.g. 17.5mm) from that to figure out how much spacing you need (e.g. 37.5mm).
The Nerdy Specs
Quantum Efficiency (QE)
Quantum efficiency is how overall efficient a sensor is at converting the incoming light into a signal/image that you can see. The higher the QE percentage, the better it is for low light and deep sky astrophotography. This number does not matter quite as much for planetary imaging.
Full Well Capacity
Full well capacity is how much light/charge each pixel can absorb (measured in electrons) before becoming purely white and unable to record more detail. The higher the full well capacity, the better. A higher full well capacity means that you can expose for longer before losing detail, and as a result, higher full well capacity cameras will have better dynamic range.
Read Noise
Read noise is a common type of noise, measured in electrons per pixel, that is generated during the process of converting the signal from analog to digital in the camera’s electronics. The lower the read noise, the better. Read noise occurs independently of the incoming signal, and therefore can occur in images taken even with the dust cover on.
Capture Speed
An important specification for planetary imaging, capture speed is how many frames per second a camera can capture. For deep sky astrophotography, this specification is not very important as exposures are usually many seconds or minutes long. For short exposure planetary imaging, the higher the number, the better.
Sensor Illumination
Sensor illumination differentiates whether a sensor is front-side or back-side illuminated. Generally speaking, back-side illuminated (BSI) sensors are better as they have a higher quantum efficiency.
Bit Depth
Bit Depth is the range of luminance values that each pixel can record. A camera with a higher bit depth per pixel, like 14-bit, will be able to produce smoother gradations between areas of varying brightness in an image. A lower bit depth camera, like 10-bit, may suffer from banding, or noticeable lines on parts of the image with gradients. The higher the bit depth, the better.
Cooling Temperature
Cooling temperature is how much cooler (measured in Cº) the camera can get than ambient air temperature running the cooling fan to keep the sensor cool. The lower the temperature below ambient, the better.
Color Filter Pattern
In color sensor cameras, the color filter pattern is the order in which red, green, and blue pixels repeat after one another to produce a color image. Nearly all consumer and astronomy cameras use the Bayer Filter, which repeats in a Red, Green, Green, Blue (RGGB) pixel pattern.
Shutter Type
Shutter type has two definitions. 1. In astronomy, particularly in CCD cameras, it can differentiate electronic shutters from mechanical shutters. For almost all imaging purposes except some CCD imaging, an electronic shutter is preferred. 2. It can also differentiate whether a camera has a rolling shutter, where the image is read out one line of pixels at a time, or a global shutter, where the image is read out all at once. For all deep sky imaging purposes, a global shutter is not needed.
