Problems Of A Standard ED Refractors

Most of today's refractor be it high end or low ends tends not to give a flat image on the entire field of view unless you incorporate a dedicated field flattener to correct the softness of an image near the edges. Last night, I was able to make a quick and dirty image comparison of the Borg 76ED, an older model 76mm f/6.5 ED refractor with a focal length of around 500mm on the Quarter Moon. I put the Moon at the center of the frame and use a Canon EOS M6 mirrorless camera mounted on the Borg 76ED refractor as well as one image that I intentionally position the Quarter Moon along the edge of the frame to show the image quality difference. The results are below: A.) Moon at center of the frame

B.) Moon at the extreme right edge of the frame.

Putting them side by side for comparison

Its obvious that the edge of the field of view is soft thus a need to install a field flattener to correct this problem. Unfortunately, I don't have a field flattener but I intend to get one in the near future as I mostly use this refractor solar use and the chip sensor of my webcam (ZWO ASI120MM) is small enough that it won't interfere on the edges. But since a colleague of mine has an Orion 0.8x reducer for refractor and loan it to me for testing, I was curious to see if it can help correct a bit the softness of the edges of the field. So I use the 0.8x reducer on the Borg 76ED refractor and the result is below:

The left image was taken at the center of the frame with the Orion 0.8x reducer on the Borg 76ED while the right image below intentionally placed the Moon on the right edge of the frame. The result of course reduced the original 500m f/6.5 system into 400mm f/5.3 system. If compared to the one without the Extender and both on the edge, the Orion 0.8x reducer did correct a bit the sharpness of the edge but not totally though but better than without it. Despite the fact the Orion 0.8x reducer is not a flattener, it still helps a bit to sharpen the edge a bit. I hope to also test the 0.8x reducer on actual stars in the future and hope to be bale to acquire a real field flattener to test.