Mdopp

On 18/11/2020 at 23:47, gorann said:

No reducer can work for a full frame chip (=43 mm diagonal) with the Esprits since they are already full frame (40 - 44 mm imaging circle with Esprit 100 - 150) so a reducer would only cause a dark ring along the edges with a full frame camera and loss of resolution, so utterly useless. For Esprit 150, the image circle with the new reducer would be 44mm x 0.77 = 34 mm, so it would just about fill the frame of an APS-C.

I came across this rather old thread when I was looking for more information on this reducer. Maybe other people find it useful so I would like to share my experience with the 0,77 reducer on an 150ED and a full frame camera (ZWO ASI 6200MM).

Of course, there is noticeable vignetting. But this can be corrected nicely by applying flats. The stars in the outmost corners are somewhat elongated but not too much. There are no dark areas in any of the corners. All in all, I think the reducer works quite OK with a full frame camera.
This may be because the first lens of the reducer is actually placed inside the telescope‘s tube much further to the front than the camera sensor or the normal 1x field flattener. I assume that at this position the field is actually larger and so the reducer can indeed provide a full frame image.

I am attaching an image that was shot with this setup. I only cropped some 10 pixels around the edges to remove the effects of dithering. Otherwise, this is the entire full frame.