What are the High Conversion Gain HCG Multiplier Factors for Altair Cameras?

Some Altair cameras are capable of High Conversion Gain mode. This “HCG” mode can be switched on or off using the ASCOM driver, or your chosen astro imaging software if it supports Altair Cameras.

HCG increases the brightness of an image with minimal impact on read noise, and can therefore be a good setting to use for deepsky imaging. However, because it multiplies the existing gain settings, it should be applied with a low initial gain.

For example let’s assume you set your Hypercam 533C gain at 3.0x brightness (300.0 gain in most apps, or 3.0x in AltairCapture), and then you switch on HCG. The IMX533 sensor has a 3.05x HCG multiplier, which gives you 9.15x gain (915.0 in most apps). That’s a lot of gain, and as you can imagine, sensors can sometimes start to display up strange artefacts or external electrical noise with very high gain. It would also reduce colour saturation, especially around stars resulting in very white stars where the code is saturated. In any imaging situation the best approach is to run your sensor analysis in SharpCap PRO, then run the Smart Histogram feature to determine best settings. Tip: Best done on-target with filter in place to assess the background levels.

Sensor manufacturers do not publish the HCG multiplying factor of some sensors, therefore we have estimated the multiplying factor of some sensors. These are marked with an asterisk in the chart below:

Camera model: Sensor: HCG Multiplier Factor:
Hypercam 61CFX / 61MFX IMX455 2.0x*
Hypercam 24CFX IMX410 4.3x*
Hypercam 26C / 26M IMX571 3.01x
Hypercam 115M IMX492 4.46x
Hypercam 294C IMX294 4.46x
Hypercam 533C IMX533 3.05x