Can someone explain the two gain settings for an AC4040 camera? There is a high gain and a low gain setting. What exactly does each of these do? Do they raise or decrease dynamic range? And are these settings dependent on readout modes? Lastly, anyone have any suggestions for photometry settings or a place to start? Thanks, Mike
You can select between the High Gain setting and the Low Gain setting using the Readout Mode selector. There is also an HDR mode, which simultaneously reads out the chip in high gain and low gain. The usual readout mode that most people use is High Gain Stack Pro. For astronomy, I wouldn't use Low Gain unless perhaps you were shooting the moon. The initial settings on the camera are those recommended by the sensor manufacturer for HDR mode. It may be advantageous to use the High Gain mode with a slightly lower gain setting, to give you more dynamic range. If you reduce it too much you'll start increasing the effective noise floor (due to sampling noise), but reducing it somewhat will give you more dynamic range. I heartily recommend reviewing the User Manual to see how all this stuff works.
Thanks Doug. I have been through the manual and information relating the gain settings is sparse. There is a high gain and a low gain setting, am not real clear as to what each of these does. I assume reduce the gain (sensitivity) for both ends, i.e., dim and bright, when they are lowered. Lowering the high gain will increase the dynamic range if I understand. However lowering it a lot will increase the noise. The issue is am doing photometry and am saturating at a 30" exposure (17" telescope/AC4040/very dark sky/V-filter). We thought that lowering the gain might help with this but the information or settings for this are sparse. What about high gain stackpro, with perhaps 2" sub exposures? So for a 30" exposure, it will take 15 two second exposures and combine? Also lowering the gain a bit. Do I have this right? Anything else you might offer?
There are two readouts, which always operate simultaneously in the sensor. This isn't like a CCD - you can read out each pixel more than once. In this case, the sensor reads out every pixel twice, at two different gain settings. The Readout Mode selector chooses where you get one (High Gain), the other (Low Gain), or both (HDR). Stack Pro uses in-camera stacking, which is much easier to work with than HDR. I recommend you read the user manual on this. I could explain it here but I'd literally just copy/paste the user manual. Most people use High Gain StackPro mode. The two readouts can be adjusted separately, in the Camera Settings. The default settings are: High Gain 1650 = 16.5X = 0.819 e-/ADU, linear full well capacity 3,100 e-. Adjustable from 100 to 1650. Low Gain 280 = 2.8X = 18.69 e-/ADU, linear full well capacity 72,600 e-. Adjustable from 40 to 660. These numbers were chosen to optimize HDR mode. They may not be the best choice for High Gain mode. The sensor is rated at 3.7 e- dark noise, corresponding to 4.5 ADU with the default High Gain setting. If you reduced the gain to 800 aka 8.0X, that would be 1.7 e-/ADU and full well of 6,430. You would get a 10% increase of noise, i.e. 4.1 e-, which is negligible. Well worth doing. As for StackPro, bear in mind that stacking increases the noise by SQRT(N). So stacking 2 subexposures increases your noise by 41%. Stacking 4x doubles your read noise. Stacking 16X quadruples your noise floor. That might sound bad, but even quadruple is similar to the performance of the old CCD sensors. An STX-16803 typically had read noise around 11 e-. If you go with gain 800 and 10X StackPro, then your read noise will be 12.9 e- and full well will be 64,300 e-. It's up to you to select the parameters that gives you the best compromise between noise and dynamic range.
Given your feedback, I've just added some information to the User Manual. Page 17 - https://diffractionlimited.com/downloads/SBIG_Aluma_AC_Series_Users_Manual.pdf
