Nice looking image. I'm having a go at this one myself right now.
You can work this out with a plate solve if you like. The image you posted is 45.66 x 30.29 arcminutes.
Using FOV (degrees) = 57.3 * Sensor Size (mm) / focal length (mm)
45.66/60 = 57.3 * (7.8/1000)*3110/fl
so fl= 1826mm (I think
)
If the aperture is 200mm then it seems to be an f/9
Of course this would be more accurate on a single uncropped sub.
Strange how QHY and Theo don't publish the sensor x/y size - just the diagonal size.
James