ZWO ASI CAMERAS: WHICH ONE TO CHOOSE
Posted on February 04 2020
Monochrome, RGB, DSO camera, planetary camera, cooled, uncooled; these are only some of the different features of the ZWO cameras. To choose the right camera can be complicated so the two tabs below will assist you to get an easy insight of all the ZWO camera features at first glance.
|ZWO Specification sheet in PDF format|
As you can see, there are two "families" of sensors: cooled cameras for deep Sky Objects (DSO) and uncooled for planetary imaging. For bright objects like planets, the moon and the sun, a small uncooled sensor is all you need to capture a short video of the object and then process the frames with the technique of the lucky imaging. For certain planets with a fast rotation such as Jupiter and Saturn, the video captured should not be longer than 30-60 seconds. So to get a sufficient number of frames, you will need a fast computer equipped with USB3.0 port and SSD drivers.
RGB or Mono?
How to choose between a camera with a colour or monochrome sensor? For planetary cameras this decision is determined by the pixel scale and the elevation of the object above the horizon. In fact, for small pixel/arc second ratios in the order of 0.25"-0.1"/pixel, the atmospheric dispersion could be relevant with objects below 65° of elevation. Colour cameras will show a red and blue rim on the opposite side of the objects and it is almost impossible to correct it during post processing. The atmospheric dispersion corrector however generates a chromatic dispersion equal and opposite to the atmospheric dispersion, nulling the effect. Therefore this accessory could be quite complicated to use, especially in Newton telescopes.
Monochromatic cameras need coloured filters and a filter wheel but here the atmospheric dispersion can be corrected in post processing by aligning the colour channels. Also, monochromatic sensors have the advantage to be highly sensitive in the infrared spectrum and many planets reveal the majority of their details in this spectrum interval. In same cases such as during the Mars opposition in 2018 where all planetary details were obfuscated by a dust storm, imaging in the IR channel was the only way to reveal some details.
Overall colour sensors are better for small telescopes and do not require a long post-processing as there is only one channel. Monochrome sensors would produce better results but they need a filter wheel and a longer post-processing procedure.
Regarding deep sky objects cameras, RGB sensors are more versatile and they also require less post-processing. Monochrome sensors though allow to use narrow band filters with the great advantage to increase the details in emission nebulae. Also, narrowband filters drastically reject the majority of the light off the band and that allows to capture deep sky objects even in bright urban skies.
Sensor and pixel size
As a rule of thumb, the combination of telescope and sensor should result in a sampling between 1"-2"/pixel. To calculate this value, use the following formula:
Without using this formula for all the pixel sizes and focal length combinations, we can say that sensors with small pixels (2.4µm for example) are suitable for refractors that generally have a short focal length. Larger pixels are ideal for Schmidt Cassegrains that have a long focal length (2000mm - 4000 mm).
Clearly, the larger the sensor, the wider is the framing area in the sky so generally bigger sensors are better, and you can always crop the image at the end of the processing. Not all the telescopes however have a field that is fully corrected for the full size of the sensor so you may consider a coma corrector or a field flattener to obtain pinpoint stars over the full area of the sensor.
The Analog to Digital Converter measures the ability of the sensor to convert the photons in voltage. A higher ADC means higher dynamic range which measures the number of colours or shade of greys in an image. A higher dynamic range means more natural looking images and the post-processing can be "pushed more" without producing unnatural results.
The read noise should not be confused with the thermal noise that is caused by electrons, generated spontaneously within the silicon chip. The read noise is generated by the sensor and its electronic components and it cannot be reduced.
Full well capacity
It measures the maximum charge that a pixel can hold. It is important in images where bright stars are close to faint nebulae: when a pixel collects the light of a bright star, it gets saturated generating a higher voltage and the surrounding pixels will be affected causing the star to look larger than it should.
Although it is not important for DSO cameras, it is critical for planetary cameras. Fast Frame Per Seconds is the key success to collect as much frame as possible during planetary imaging. The FTP can be increased when a smaller ROI (Region Of Interest) is selected, for example around the small disk of a planet. Note that the exposure may slow down the FPS as well as the writing speed of your hard disk.