Ernesto221 insted of adding rings I need to buy and use a field flattener between the focuser and the camera.
If you are going to only image very small galaxies and crop out the rest of the image, you may not need a flattener. The flattener ensures that the entire field has minimal field curvature, but if you only need a small central area, you don't need it.
In fact, field flatteners will increase the size of the spot diagram for stars that are exactly at the optical axis when compared to not using any flattener. Just look at the published spot diagrams for your telescope.
This for example is the spot diagram of my primary telescope without a flattener:
This is with a 1.01x flattener:
This figure provides the actual scales (i.e., the side of the squares is 100 µm):
On a sensor (or a crop) whose diagonal is less than 10 mm, this telescope (a Petzval) works better with no flattener. Even when it is not a Petzval, the on-axis spot will always be better than using a flattener -- it is a matter of at what distance the spot digram of the no-flattener scope is better than the flattener scope.
Just compute what the size the galaxy would be, compare that to the spot diagrams, and you will find out if you really need a flattener.
Don't buy a single thing in the future until you know what they do. The hobby is full of charlatans and shills who make you go out to buy something, with them profiting from your money.