It is barelly visible by the human eye under a dark sky...
The photo is the result of 12x300" exposures with the respective calibration frames.(darks,flats,bias).
There is a slight misalingment of the optics which is visibile in the upper left corner but overall i believe this flattener (TecnoSky 1x) is doing a fine job considering the sensor size.
By clicking on the picture below you may view the 1650x1100 version.