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Peter, it's a little counter-intuitive, yes. You are correct that I set the guide pulse frequency by changing the "Camera Exposure Duration" pulldown. However, I then set the properties of the ASI120MM to a different exposure, and so achieve the desired true exposure and frame stacking.
So the 'exposure' for PHD2 is also the actual frequency of guide pulses, whereas the 'exposure' for the ASI120MM is the WDM exposure setting of the camera. So if I set the camera exposure to 500ms, and the PHD2 exposure to 5 sec, then when guiding, PHD2 will report a stacking of 10 frames, as expected.
This 10-frame stack is useful, as I noted, for getting a better 'averaged' estimate of the centroid, compared to a single long WDM exposure, and also avoids guidestar bloat etc. Once can control the FWHM of the guidestar nicely by playing with the WDM exposure time (and gain, if necessary), without mucking about with the actual guide pulse.
I think this only works for video stream type guiders like the ASI120MM, the QHY-5LII and the new Orion Starshoot AG. I don't think it works for CCD-based guide cams, like Lodestar & ST-i.
There is another way to control the guide pulse frequency, and that is to set the Time Lapse (ms) in the Advanced settings.
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