CCD or CMOS
Both chip technologies are based on light-sensitive silicon chips that convert photons into electrons through the photoelectric effect in a wavelength range of 300-1000 nm. The difference between the technologies lies in the way the electron charge is converted into a voltage.
On a CCD sensor, vertical and horizontal charge transport occurs first. The serial charge/voltage conversion of all pixels takes place outside the sensor in the camera electronics. All pixel charges are converted to an analog voltage via an output outside the sensor, amplified and digitized.
With CMOS sensors, on the other hand, the charge/voltage conversion takes place in each pixel of the sensor. According to the activated line, the signal is amplified and digitized via the readout circuit.
Working principles of a CCD and a CMOS sensor (Source: https://www.rauscher.de) (translated)
Both technologies have their advantages and disadvantages, which are listed below.
(Sorces: https://www.stemmer-imaging.com as well as https://www.baader-planetarium.com)
- High homogeneity of the pixels
- Uniform signal with low spatial noise (fixed pattern noise - deviations due to manufacturing tolerances between pixels)
- Low dark current
- Pixels can be very close together → High sensitivity and good signal quality at low light intensities
- Simultaneous exposure of all pixels (global shutter - exposure of all pixels starts and ends at the same time)
- High quantum efficiency (up to 90%)
- Usually higher full well capacity of the pixels compared to those of a CMOS sensor, resulting in a higher dynamic range
- Direct combination of several pixels into one larger pixel (binning)
- 16-bit are "state of the art
- Limited readout speed (disadvantageous for lucky imaging)
- When reaching the full well capacity, electrons get into neighboring pixels and falsify the signal (blooming)
- When reading out the charge carriers, incoming photons can generate additional charge carriers and falsify the signal (smear effect)
- Aging CCD sensors tend to amplifier glow of the outsourced amplifier
- Fast readout speeds possible (advantage if many images per second are acquired, as with lucky imaging)
- Possibility to evaluate only single pixel areas and to treat them separately (ROI - Region of Interest)
- No blooming (skipping of charge carriers when memory is full) or smearing effects (additional electrons generated by photons distort the readout signal), allowing objects with high light intensities to be processed without artifacts
- High contrast capturing with high dynamic range is possible
- Due to the integration of the electronics on the sensor chip, this technology is particularly cost-effective compared to CCD sensors and has a lower power consumption
- Low readout noise values can be achieved at higher ISO/Gain values
- Due to the design, the pixels are not as close together as with a CCD sensor → smaller photosensitive area per pixel
- Each pixel has its own amplifier → higher spatial noise (fixed pattern noise - differences between pixels)
- readout of data via rolling shutter method (line by line readout of object data) → fast objects are displayed smeared, because they move from line to line during readout (for objects in astrophotography this does not matter, but this effect can influence the images due to seeing during lucky imaging) (But: first CMOS sensors with global shutter method are coming on the market)
- The quantum efficiency could be increased to 80% in the meantime by new layer processes, but still does not quite reach that of CCD sensors
- In contrast to the amplifier glow with CCD sensors, there is a sensor glow with CMOS sensors, which is produced by heat radiation of outsourced electronics
- Usually lower full well capacity of the pixels compared to those of a CCD sensor, resulting in a smaller dynamic range
- Only software binning possible, because each pixel can be evaluated only for itself
- 12 to 14-bit are currently common (but: first 16-bit sensors are coming on the market)
Due to the lower acquisition costs, CMOS sensors are playing an increasingly important role in amateur astrophotography. Since the disadvantages of CMOS sensors have been reduced so much in the course of the last development years, they outrun CCD sensors in many areas. Even though CCD sensors still have their justification, they are used less and less in amateur astrophotography.