Sensor analysis comparison

[giraud]

Hello Robin,

I have juste performed the sensor analysis my new 26C camera from Altair Astro and the results are quite different at low gain wrt read noise, when comparing the test results and the Altair Astro charts (https://www.altairastro.com/altair-hype ... 6451-p.asp).

Those charts show a low read noise (lower than 1e-) in HCG mode, while the test I have been running show values above 3e- from gain of 100 to 300, and then the read noise goes down to about 1.5e-. I was planning to use the very low gains to take benefit of a high full well but that doesn't seem to work in real life if I want to keep low read noise.

I am wondering if this difference could come from the management within Sharpcap of the various gain modes the camera offers?

Another question somehow related: does the smart histogram also considers as input the full weel caracteristics of the camera to propose the optimum exposure and gain?

Many thanks.

Cheers.
Eric.

[admin]

Hi Eric,


SharpCap auto switches into HCG for Altair cameras (and ToupTek cameras and related brands), meaning that the sensor data chart will look different to one created for an application that lets the LCG/HCG switch happen manually. You will find the chart more similar to one for a ZWO camera which also auto-switches into HCG (but measures gain differently).

For the 26C in SharpCap, from gain 100 to 299 the camera is in LCG mode with the normal camera gain set to the value shown in SharpCap. When you set gain 300 or above, SharpCap switches the camera to HCG mode (which provides a gain boost of 3x) and sets the normal camera gain back to 100. This gets you an image 3x brighter than minimum gain with significantly lower read noise (but roughly the same FWD) as if you stayed in LCG mode with 300 gain set on the camera gain.

Essentially this approach is designed to put the camera into the best operating mode for the total gain selected without the user having to worry about understanding LCG and HCG. This is particularly important for some models (for instance those based on the IMX294 sensor) where setting HCG with minimum gain will give issues with the sensor never saturating the output values no matter how overexposed it is. SharpCap will automatically keep Altair camera users clear of problem settings in this case.

cheers,

Robin

[giraud]

Thanks Robin, very clear, as always.

I am still wondering if the FWD is considered by the smart histogram when it proposes an optimal exposure and gain solution. Or if this is a criteria that is left for the user to decide whether to follow the proposition or choose another one considering FWD among other things?

Cheers,
Eric.

[admin]

Hi Eric,

the FWD is considered (or rather, the ratio between the FWD and the read noise) when you select the option to aim for 'maximum dynamic range'. The FWD divided by the read noise is considered to be the dynamic range of a single image, that value is then multiplied by the square root of the number of images you could take in an hour to estimate the dynamic range of the final stacked image.

cheers,

Robin

[giraud]

Thanks RObin.

Cheers.
Eric.