P & C,
Note also that when using a reducer on a Newt, coma will also become more apparent around the edges, and there will be other aberrations visible too which may include field curvature and spherical aberration (due to the reducer, not the scope if it's using a parabolic mirror).
If this set up is for astrophotography, this is not the right tool for the job, be it a Newt or any other scope. In a Newt you will need a coma corrector. In a refractor a reducer/field flattner for the specific apochromatic scope you are using (there are not scope specific reducers/flattners for achromatic refractors). SCT's, those specifically made for the optical design used by your SCT, standard or Edge or ACF - all are different. There may be a reducer for Maks - I seem to recall something along those lines, but as Maks are typically so long in focal ratio, it is somewhat futile to use a reducer with them.
If it is for video astronomy for an on-screen fix, then you may be happy to tolerate the coma with a Newt, or a little bit of field curvature in a refractor when using one of these inexpensive reducers. Or you may still prefer to look into using the appropriate corrector unit.
Note also that there is a significant difference between "reducer" such as this inexpensive 0.5X item, and a "reducer/flattner/corrector". An inexpensive reducer is just a simple lens arrangement. This is why these are so cheap. A reducer/flattner/corrector is more complex as it also produces a flat focal plane that is necessary when projecting the scope's image onto the flat chip our cameras have. This is even more important as chip size gets larger and larger. As a result these items are much more expensive and very scope specific as these need to be designed for the very specific curved focal plane each scope produces, which includes scope design, focal length & focal ratio, all of which mean very exacting design requirements. A coma corrector does the same field flattening job but for Newtonians.
And here we all are when we start out in Astro that this is all easy!
It can actually very quite technical and complex... if you want it to be.
Thanks for noting your experience with the modification you did to your reducer and when you used it in your Newt. I will do this mod too and see what results I get with a Newt, a fast achromatic refractor and an ED80. I only indulge in video astronomy by way of imaging, but if I can find ways to better control aberrations, then I'm all for it
Alex.