That photo does not represent a good flat.
First, make sure the flats are taken with the OTA at the same focus setting as the focus setting for your stars. This will reflect the actual correction needed.
Second, notice that your average ADU (about the location of the peak of the histogram for a symmetrical curve) is at 3027. That is way too low. Increase your exposure by about a factor of 10 to get the average ADU to be around mid-scale on the histogram, i.e.,push the exposure time until the peak of the histogram is near the center, and the average ADU reads a little above 30000.
At that point, check to make sure there is no odd anomalies in the histogram that causes some peak near to the right side of the histogram. If it histogram fall towards Min ADU to the right side of the histogram, you are good to go. If not, reduce the exposure until there are no large peaks to the right of average ADU.
Check the histogram near to the left side of the curve. If there are anomalies there (instead of a smooth reduction towards Min ADU), increase the exposure.
The Max, Min and Average ADU are displayed in the Preview screen for a reason. (Standard Deviation of the ADU helps with adjusting exposure duration for Light frames.)
Do not use single exposure to judge the final result. Look at the Master flat from the software that you use.
You might also then take some Dark Flats (at same temperature as the temperature that you used to take flats) since the camera that you use is not very stable with temperature (many people avoid that camera because of it). Use the Dark Flat as the dark file for computing flats. It may be the only way to get rid of the amp glow after computing the Master Flat. Many people avoid the ASI294 because the dark current vs temperature curve is not stable.
Check your OTA's specs on what the size of the image circle should be. Remember that if you use a reducer, the image circle will typically become smaller. A mismatched reducer (one that is not designed as a set with the objective) can also give you very small image circles. Try a set of flats without any filter wheel or filter drawer -- if the image circle is larger than the image circle with the filters, you filter is too small and is vignetting the rays.
Chen
