For beginner, we recommend that you set the gain to “unity-gain”. Unit-gain is the gain when system gain is 1 (1e/ADU). This number is shown in the table below, like the unity-gain of QHY168C is 10. In fact, increasing or decreasing a bit doesn’t make a big difference.
You could increase or decrease Gain according to the condition. For example, if your optical system is fast, like F2.2 to F5, or long exposure for more than 5 minutes without narrowband filters, then you can decrease GAIN to achieve a higher dynamic range and make better use of full well capacity. By doing so you can avoid overexposure.
If you use narrowband filter on a slow optical system like F6 to F10, or short exposure time, the amount of photons received will be less. In this case you can increase GAIN to make better use of characteristics of low read-out noise in high GAIN value.
There is no fixed “best value” for OFFSET. To set OFFSET, you should take the bias frame and dark frame at a certain GAIN value, then check the histogram of the frames.
As you can see, the histogram distribution is a peak-like curve. By changing the OFFSET value, this curve will move left or right. We must guarantee the range of the whole curve is greater than 0, and it cannot be chopped off at the end. At the same time, we need to keep a bit of residue on the left side, just over 0 a bit. 100 to couple hundreds ADU are all okay. Don’t be too huge, however.
Pay attention that under different GAIN values, the width of this peak varies. The higher the GAIN is, the wider the distribution will be. So OFFSET value st low GAIN is not suitable for high GAIN, because the curve is very likely to be chopped off.
For those CMOS less than native 16-bits, the AD sampling accuracy doesn’t match perfectly with the full well capacity. At low GAIN level, the system gain will be couple electrons per ADU. The camera loses the ability to distinguish the strength of the signal because of such sampling error.
When GAIN increases, the system gain will decrease. However, increasing GAIN will limit the full charge of the well. If the system gain is 1 for a 12bit CMOS camera, the pixel will be saturated at only 4096 electrons (full well). Some bright stars will be easily saturated. This problem goes worse under fast optical system or long exposure. Over saturated objects cannot be fixed during post processing (unless you shrink stars, like in PixInsight). Also, the color saturation of the star will be affected. As result, the stars will be huge and white washed. We should decrease the gain value in this case, to gain a higher full well capacity.
Under long exposure or using fast optical system, the pixel will receive more photons. The variation of quantized noise from the photon which you can consider as natural dithering of the light intensity, will be greater than the “noise” from the sampling error. Therefore, the effect of the sampling error will diminish. By averaging multiple exposures, this will compensate the lack of depth of the picture because of the sampling error.
If the number of received photons is limited, like using narrowband filters or short exposures, we can increase the GAIN value. It is because the stars will not be easily saturated. At the same time, we limit the noise from the background cosmic radiation. Under this condition, the readout noise and quantized noise are the major factors that affect the ability to distinguish dim light or objects. By increasing the GAIN value in order to decrease the readout noise and quantized noise from sampling error, this would greatly increase the signal to noise ratio.
Model Unit Gain
600M/C 25 (Extended Full Well Mode) *
268M/C 30 (Extended Full Well Mode) *
294Pro 1600 (11MP Mode)
410C 90 (Low gain) 40 (High gain)