The peak values are in units of ADU (i.e., the digital numbers that come out of the A/D converter).
What you see is a red curve superiposed on a small sub-image from the guide frame. The red curve is supposed to be a 2D slice of a 3D point spread function of the guide star that is shown in as the background image in that mini-image.
The point spread function of the star with an OTA that has circular aperture is the Airy Disk. See first figure here:
https://www.telescope-optics.net/diffraction_image.htm
If you are familiar with the sinc (sin(x)/x) function in time-domain filters, the Airy Disk is the 3D equivalent of it. The 2D slice of the Airy Disk actually does look like the sinc function, but is not (it is made up of Bessel functions).
Another difference is that that the point spread function (Airy Disk) does not go below zero, unlike the sinc function -- and that is because the camera sensor measures the intensity, and thus there is no such thing as a negative ADU (i.e., no such thing as a negative photon; well there is, but you can't see it on our simple sensors :-).
So, you take what looks like the sinc function, find the central peak, and the measured ADU units is what is shown as the "peak" value. The ASIAIR guide interface uses 8 bits -- so the ADU values shown will be in between 1 and 255). For best multi-star guiding, try to adust the gain of the guide camera to get lots of stars whose ADU is greater than 100 but less than 255.
With very steady skies, that ASIAIR mini-image should show something like what you see in the first few images here (Airy Disks):
https://en.wikipedia.org/wiki/Airy_disk
Chen
