The ToupTek Astro AE676M monochrome CMOS camera was attached to a Stella Mira 66mm ED APO refractor which was fitted with a field flattener, an Altair V2 2" filter drawer, a camera rotator and a ZWO EAF focuser. A ToupTek GS-100, F=100mm, A=25mm quadruplet PAPO scope was mounted as a guide-scope with an Altair 462M2 monochrome CMOS camera as the guide camera as explained in the previous blog article . Autoguiding was done with PHD2 running on a Fedora mini computer via an INDI to control the Celesdtron AVX mount.
The equipment
The Altair 462M2 camera has an underpowered one stage TEK cooler with a detatchable fan unit that holds in place with strong magnets to enhance the weak cooling. The cooling is powered by the same USB cable that powers and streams data from the camera. Therefore we employed a 4 port SABRENT powered hub between the camera's USB-C to USB 3.0 cable and the USB 3 cable used to carry the signal indoors to the imaging computer. The cooling works but doesn't lower the temperature to those typically achieved by cooled astronomical cameras. Having said that, it did prove to be adequate for the job. It will clearly be better during the cold winter months. For solar imaging, the cooler helps prevent the camera from overheating.
The ToupTek Astro AE676M utilizes the Sony IMX676 sensor, which features a hardware architecture capable of a single-exposure dual-gain readout, often referred to in the industrial/surveillance sector as Clear HDR.
To understand how the HDR mode achieves a 16-bit output by combining HCG and LCG, it helps to break down the mechanics of the sensor's pixel design.
Understanding LCG vs. HCG
In a typical CMOS sensor, every pixel acts as a tiny container storing light energy (photons) converted into electrical charges (electrons). The camera then converts these electrons into a digital number using an Analog-to-Digital Converter (ADC).
LCG (Low Conversion Gain): The sensor sets the pixel container to maximum capacity. This yields a high Full-Well Capacity (around 11,500 electrons for this sensor). It prevents bright targets like the core of a star, the moon, or the sun from instantly clipping into pure white. However, the trade-off is higher readout noise.
HCG (High Conversion Gain): The sensor electronically limits the container size but amplifies the signal intensely right at the pixel. This dramatically drops the Readout Noise (down to about 1.01 electrons), allowing you to see very faint signals buried in the dark background. The trade-off is that the pixel container overflows very quickly on bright details.
The HDR mode is intended for high-contrast targets: This mode should be best for Solar, Lunar, and Planetary imaging, or highly uneven deep-sky objects like the Orion Nebula. In solar imaging, for example, it allows the capture of both the very bright solar surface and faint edge prominences simultaneously.
How HDR Mode Works (Single-Exposure Dual Gain)
The capture software must be set to capture 16 bit data for HDR mode to work properly.
Instead of forcing you to choose between the two, HDR Mode reads each pixel twice simultaneously from a single exposure using two independent readout paths on the sensor:
Simultaneous Capture: When the shutter closes, the camera reads the sensor through the HCG path (optimized for shadows) and the LCG path (optimized for highlights) at the exact same moment. Because this happens in a single exposure, it avoids the "ghosting" or motion blur artefacts caused by taking two separate exposures back-to-back.
On-Board Synthesis: The camera’s internal firmware compares the two readouts pixel by pixel:
• If a pixel is dark or faint, it extracts the data from the clean HCG readout.
• If a pixel is bright and saturating in HCG, it pulls the data from the high-capacity LCG readout.
16-bit Output Mapping: The native hardware ADC of the sensor is 12-bit. However, by stitching the shadow depth of HCG and the highlight headroom of LCG together, the total dynamic range spans past 12 bits. To accommodate this expanded range without truncating the data, the camera mathematically maps the blended information into a 16-bit container (storing values from 0 to 65,535). A number of camera manufacturers have this method available.
In this first part of exploring the Astro AE676M we only used the LCG and HCG modes and will reserve the HDR mode for future testing. The camera was fitted with a Baader UV/IR cut filter otherwise the image would be swamped by IR light to which the camera is very sensitive.
We used AstroDMx Capture to image the Aun in H-alpha light through a Coronado Solarmax II 60, BF 15 H-alpha scope mounted on a Skywatcher Solar Quest solar finding and tracking mount.
A 1000-frame SER file was captured in LCG mode in 8 bits. The best 80% of frames in the SER file were stacked in Autostakkert!4, wavelet processed in waveSharp3 and further processed in PixInsight and GIMP3.
Click on an image to get a closer view.
The Sun in H-alpha light
Even in 8 bits, the prominences could still be seen and captured.
Similarly we used AstroDMx Capture to capture a 1000-frame 8 bit SER file of the Moon through the Stella Mira 66mm ED APO refractor fitted with a Pegasus L-UV/IR cut filter. The best 80% of frames in the SER file were stacked in Autostakkert!4, wavelet processed in waveSharp3 and further processed in GIMP3.
The 93% Moon
Deep Sky imaging in HCG mode
One and a half hours of total exposure were captured by AstroDMx Capture; half an hour's worth of 1 minute exposures through each of H-alpha, O3 and S2 Altair 2" 7nm narrowband filters, plus calibration frames using an Aurora electroluminescent flat panel for the flats.
Electroluminescent flat panel
The camera focused on a bright star using a Bahtinov mask
Screenshot of AstroDMx Capture capturing 60s exposures of M17 in H-alpha
The data were stacked in PixInsight and further processed in PixInsight, GraXpert, SetiAstroSuitePro and GIMP3.
M17 Hubble Palette
Hubble - (H+S)OO L palette blend
HOS Canada, France, Hawaii Telescope palette
HSO palette
OHS palette
OSH palette
SOH palette
Pixelmath images were constructed using Siril
ForaaX palette
Gendler palette
Natural palette
In Part1 we have established that the Touptek Astro AE676M monochrome multi-purpose astronomy camera delivered good results using standard LCG and HCG modes. The results on a quite bright nebula, M17 with 60s exposures in HCG mode was good. Similarly in LCG mode on the Moon and on the Sun in H-alpha were good.
An adverse comment
Whilst the end results on the bright target M17 were quite good, the imaging experience itself was not good and was disappointing. The camera was set to the recommended values for gain and offset yet the preview images were full of huge amounts of fixed noise that didn't change as each new image came in. The noise was so intense that it obscured the fainter details that didn't become evident until after calibration and stacking. We did not use realtime dark-subtraction or live stacking. However, we have never seen this large amount of fixed noise from any camera, cooled or uncooled. This leads to the suspicion that there may be a fault on the camera that we have. We shall test on a fainter object, with longer exposures to determine what happens with respect to this fixed noise. In any event, we intend to contact Touptek to obtain their response. Touptek is arguably one of the very best astronomy camera manufacturers so this type of performance is atypical of their cameras in our experience.
We look forward to Part 2 in which we shall explore fainter nebulae with longer exposures in HGC mode and also HDR 16 bit mode with solar and lunar imaging and also, when it becomes available, the Orion nebula.
