Where sCMOS is Wonderful

CCD users have suffered for years with high read noise, snail-like digitization, and download speeds.

It was like that three-hour movie that dragged on. Although CCD detectors benefited from consistent images with a few hot pixels and a column or two, sensitivity across the visible spectrum was only tolerable for mid-priced CCDs. The higher QE meant smaller arrays for a lot more money.

Speed Demon

The newest generation sCMOS APS device used in the SBIG AC4040FSI, AC4040BSI, and AC2020BSI cameras is blazing fast compared to their CCD counterparts, featuring many parallel on-chip amplifiers, multiplexers, and analog-to-digital converters that run at speeds beyond 5x quicker than the CCDs they replaced.

In the sCMOS world, the technology is vastly different – there are 32 parallel analog-to-digital converters multiplexed across the pixel arrays in the AC4040; This brings incredible speed for digitization and download of the sensor data.

Practically speaking, you can focus your images with larger subframes (or whole frames) in real-time and have few gaps between shots, which is essential for transient objects.

Amp Glow vs Logic Glow

At the same time, the trade-offs brought by the radically different sensor design mean that instead of directing a couple of movie stars, you’re managing a cast of thousands.

Unlike the higher operating voltages of CCDs and the characteristic amplifier glow, the lower voltage power supplies of the sCMOS APS eliminate this effect. The negative consequence is that heat will be generated while running two full sets of 16 complete analog / A-D converter chains at the top and bottom of the silicon chip.

Nearly everything shuts down during integration, but significant heat is generated during readout, resulting in a noticeable enhancement of dark current in the affected regions. Unfortunately, the simple “subtract a dark” calibration isn’t sufficient for high-quality images – sCMOS sensor calibration is a bit more work. The good news is that, unlike competing cameras with secret and complex calibration processes, SBIG’s AC4040 provides all the raw data, including over-scan reference pixels.

So, you’re in command of the process, allowing you to take control instead of being stymied by in-camera trickery and opaque algorithms for processing.

Glow is a Limiting Factor

If you are doing 1800-second integrations on a BSI sensor, you will see a glowing gradient in perhaps 300 rows at the top and bottom. Dark subtraction will remove the glow but not the added shot noise; if you are limiting dark current and pushing the signal-to-noise limits to the edge of the envelope, you may want to subframe off a modest number of lines from the top and bottom of the sensor.

In practice, you would only ever encounter this when using a BSI sensor for dark current limited observations. But unfortunately, it is a characteristic of many sCMOS sensors, and the Gpixel shares this.

Double the Data – Dual Gain Channels

This sensor has two parallel tracks, which operate simultaneously during readout. The High Gain (HG) channel is ideal for faint targets, and the Low Gain (LG) channel is excellent for bright objects that would otherwise saturate the detector.

Similar to if you and a friend are watching a movie together, but one of you has sunglasses on. You can control which channels you want to use and simultaneously bring the HG and LG data into the top and bottom of the image data array. Starting with High Gain gets results fastest and is simplest to calibrate.

Don’t Cross the Streams

Unlike the dangerous-to-cross particle streams in a particular 1984 paranormal hijinks movie, there is a “cross”-like feature in the AC4040’s sensor.

It is more subtle in the BSI version and not present in the smaller AC2020BSI. The 4040 sensors have a characteristic quadrant appearance, like how the old four-corner readout CCD sensors looked. The stepper pattern causes this during semiconductor fabrication.

There can be a sub-1-ADU residual pattern after dark subtraction, which looks like a faint cross-like pattern where the four quadrants meet; this isn’t a factor for photometry as it is minimal and gradual; annulus subtraction removes it.

For photographers, you can mitigate this faint residual pattern by using a large dither of 20-30 pixels between exposures, and then use Sigma Clip or SD Mask when stacking.

As a general-purpose astronomy detector, we believe you’ll find the SBIG Aluma AC4040 family of cameras is superior in many ways to its predecessors and competing offerings. As a result, there should be no concerns about managing inherent sensor characteristics and their impact on the output data.

Even the critics are saying good things.

Contact us to be next in line for the best detector under the stars.