Hi Arnold, welcome to the forums and astronomy. There is no need to worry about the coma at the edges of the frame, it can be reduced even eliminated completely by adding a field flattener, coma corrector to the set up. There are several makes readily available via retailers, like this -
https://www.ebay.com.au/itm/204315219061 https://www.bintel.com.au/product/orion-field-flattener.
and may pop up in the classifieds here now and then or just do a search for field flatteners via your browsers. There is plenty of info out there on how to set them up, spacing etc.