TerryFogg It occurred to me that with a read noise of only 1.0e-03
The 0.001 number which you quoted is actually the dark current noise (current density in electrons per pixel per second), and not the read noise; which is a much larger value of 1.5e- RMS (independent of exposure time).
A low dark current basically allows you to expose for a long time before the noise reaches the read noise. Short exposures don't really take good advantage of the low dark current because you still need to read the data off the chip, and at that point, run into the read noise.
Assuming that you guide with a 1 second sampling period, and a subframe is 256 seconds long, consisting of a simple accumulation of these 1 second frames, the contribution of extra read noise would amount to 24 dB. I.e., I don't think it is going to work for the DSO case.
There are specialized sensors that have non-destructive reads and have been designed for exactly what you are describing ... read at >1 fps for guiding, and then finally after N seconds, a single read for the actual DSO frame, so that the DSO frame does not suffer from the accumulated read noise from >N frames. For example:
https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10709/107090V/A-CMOS-visible-image-sensor-with-non-destructive-readout-capability/10.1117/12.2314144.short?SSO=1
The current commercially available CMOS sensors that ZWO use all have destructive reads, unfortunately. So, two sensors.
Chen
