The QHY5III462 camera uses the Sixth Generation Sony 2.1 megapixel IMX462 STARVIS CMOS sensor. The pixel size is 2.9u making it the same size and resolution as the sensor used in the QHY5III290 camera that has been so successfully used for planetary imaging by some of the best planetary imagers in the world. Like other cameras in the 5III series, the QHY5III462 is USB 3.0 powered and controlled. No additional power is required.
The IMX462 sensor is back-illuminated and incorporates new technology that gives it some significant advantage over other planetary cameras: First, the IMX462 sensor has sHCG (Super High Conversion Gain) for very low read noise at high gain. This is ideal for stacking hundreds or thousands of short planetary images. Second, it is exceptionally sensitive in the NIR.
In this latest generation of sensors, the photodiode portion of the pixel well is physically deeper than in previous Sony BSI sensors, allowing photons of longer wavelength to penetrate deeper into the substrate. This dramatically increases the sensor’s sensitivity to red and near infrared (NIR) light. The RGB filters over the pixels become transparent at NIR wavelengths, so the sensor displays almost equal peak sensitivity to NIR light as it does to light in the visible spectrum.
One benefit of the back-illuminated CMOS structure is improved full well capacity. In a typical front-illuminated sensor, photons from the target entering the photosensitive layer of the sensor must first pass through the metal wiring that is embedded just above the photosensitive layer. The wiring structure reflects some of the photons and reduces the efficiency of the sensor.
In the back- illuminated sensor the light is allowed to enter the photosensitive surface from the reverse side. In this case the sensor’s embedded wiring structure is below the photosensitive layer. As a result, more incoming photons strike the photosensitive layer and more electrons are generated and captured in the pixel well. This ratio of photon to electron production is called quantum efficiency. The higher the quantum efficiency the more efficient the sensor is at converting photons to electrons and hence the more sensitive the sensor is to capturing an image of something dim.
Extended Near Infrared Sensitivity
Logically, one would think, each generation of Exmor sensor would be built upon and incorporate all of the improvements of the generation immediately preceding. However, this was not the case with the fifth generation Exmor R sensors.
The first back-illuminated sensors used shallower pixel wells (like the third-generation front- illuminated designs) than the physically deeper pixels of the fourth generation. So, while the back- illuminated structure increased the sensitivity in the visible range by 2X, the shallower pixels did not improve the NIR. The answer to this is seen in the latest, sixth generation, Sony Exmor R sensors, like the IMX462. Using physically deeper pixels in conjunction with the back-illuminated structure has dramatically improved the sensor’s sensitivity to both the visible and near infrared wavelengths.
Another advantage of the QHY5III462 is the camera’s “Super High Conversion Gain” capability. By using a lower capacitance, a small amount of charge can be converted to a high voltage resulting in higher sensitivity in low-light conditions. The readout noise of the QHY5III462 in high gain mode is as low as 0.5 electrons!
QHYCCD’s Dr. Qiu compared the signal to noise ratio of the new IMX462 to the highly sensitive IMX385, a fifth generation device. “I know IMX385 is 0.13. And we need to get rid of the image area factor to compare them. The 462 is 2.9um x 2.9um. The 385 is 3.75um x 3.75um The area ratio is 1.67 times. If we convert the 462 to 385 size, it is 0.10. So looks it is better than 0.13. In other words, the 462 has better SNR at the same pixel size.”
The test exposures below demonstrate the low light improvement over the IMX290 sensor. The QHY5III462C image is on the left and the corresponding QHY5III290C image is on the right. The low light conditions and exposures are identical for each top and bottom pair of images and a UV/IR filter was in place for each camera. So this test demonstrates the QHY5III462C’s increase in sensitivity and SNR over the QHY5III290C under the same conditions in the visual light spectrum alone.
Color and Mono Imaging with One Camera
The filter matrix in the IMX462 uses organic dye filters. These filters are very efficient at visible wavelengths but become completely transparent in the NIR. For this reason, good RGB color balance requires an external UV/IR filter that blocks NIR wavelengths.
Many color cameras build this UV/IR filter into the camera or optical window for normal color imaging. However, in order to fully exploit the capabilities of the 462C sensor, in the QHY5III462C camera the optical window is AR coated only with no UV or IR blocking. Instead, the QHY5III462C camera includes two 1.25″ screw-in filters, a UV/IR cut filter to isolate the visible wavelengths for normal RGB imaging and an IR850 filter that will cut the visible wavelengths but pass wavelengths above 850nm.