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Have you heard about Diffraction’s StackPro™ in-camera stacking feature?

Before we get to describing all the fantastic things it does, let’s first talk about some of the challenges that necessitate it.

Modern CMOS active pixel sensors have all the analog processing built into the sensor. Certainly, this is an advantage compared to the older, more mature CCD technology—especially in terms of simplifying the camera’s analog circuitry and digitization speed.

Unfortunately, CMOS APS devices also have a few disadvantages.

The downside of CMOS APS sensors

In a CCD sensor, the pixels are clocked out one-by-one into a very high-quality analog-to-digital converter (A/D Converter, also known as an ADC) system.

This has the advantage that every pixel is treated identically, but it is relatively slow and requires complex external analog electronics. Some CCD sensors can clock out pixels in all four corners—which provides a significant speed boost—but this requires even more external electronics.

A CMOS APS sensor, in contrast, has all the analog electronics built into the sensor. To achieve the exceptionally fast readout speed, each column in the sensor has an associated A/D converter. That means there are thousands of complex A/D converters built right into the sensor! You can see where the speed advantage comes; however, there are also compromises.

On the other hand, typical CCD cameras use a 16-bit converter. This is a mainstay of camera design using a separate component, especially when there is only one of them. But unfortunately, converters get much more complicated and difficult to design as the bit depth increases. A 12-bit converter is relatively simple; but a 16-bit converter requires a lot more transistor hardware and finesse to make it work correctly.

As a result, most CMOS sensors are generally restricted to 12-bit output, which can only produce numbers from 0 to 4095, compared to 0 to 65535 for 16-bit converters. This limits the dynamic range of the sensor.

Some sensors have two 12-bit converters per column and operate in a so-called High Dynamic Range (HDR) mode. One converter is run at a lower gain, which allows the sensor to effectively record 16 bits of dynamic range.

Unfortunately, this also results in much more complexity. If the combination of the two outputs is done externally, the calibration process involved is exceedingly complex, and there is a substantial risk of nonlinearity if this is done incorrectly.

Some sensors do the combination internally to provide direct 16-bit output, but there are still potential impacts on linearity that need to be examined carefully for science applications. Any calibration problems in the combination process cannot be corrected after the fact.

Is there an alternative to this?

In one word, yes.

A benefit of modern CMOS active pixel sensors is extremely low read noise. This suggests an alternative solution: you can stack 16 individual 12-bit images to produce a single 16-bit image.

This increases the dynamic range 16X, and also increases the net read noise 4X. Since the read noise is often only 1 to 2 electrons, even with stacking 16X this is still on par with typical CCD sensors.

The upshot for the observer is that they need to take 16X as many exposures, with 1/16th the exposure time.

Sounds great, doesn’t it? Regrettably, with modern high megapixel sensor arrays, this starts to become a serious data management problem! Transferring and processing thousands of huge images is a real chore.

The solution

To solve this problem, Diffraction Limited developed our StackPro™ in-camera stacking feature.

Simply dial in your desired total exposure time—and the camera automatically divides it into sub-exposures, captures the data, stacks it in its internal memory store, and then downloads a single stacked frame to your computer!

You have all the advantages of high dynamic range without the complexities of HDR or the massive data management problem that comes with using conventional cameras. To be clear, an image created with StackPro uses a mere 6.2% of the storage that a conventional CMOS camera generates to provide the same exposure length. This isn’t image compression; this is storage efficiency. Over the course of a six-hour observing session shooting 16-minute exposures, a 16-megapixel sensor would generate perhaps 786 megabytes of data in high gain mode. Competing cameras would generate between 9 to 12.5 gigabytes of data. Advantage: SBIG StackPro.

Typical tasks like stacking multiple images take up tons of valuable personal and computer processing time. Conversely, using StackPro, you can enjoy a potential time savings of 93%! Meanwhile, StackPro goes one step further, helping you automatically determine the sub-exposure duration. All you need to do is tell StackPro the shortest sub-exposure duration you want, and StackPro automatically figures out the rest.

The result? StackPro makes taking CMOS images a breeze and provides the best linearity and dynamic range—with the least amount of hassle. And remember—StackPro is only available through Diffraction’s SBIG family of CMOS detectors.