OK, the definitive way to determine if you need a different focal reducer (like an Optec) is to measure the image circle on your current FR.
Install the FR and nothing else on the scope. Aim the scope at a bright outdoor scene with the rear of the scope heavily shaded. Easiest way to do this is aim it outdoors through a window at a bright scene. Turn off the room lights and pull the drapes around the scope to keep light from coming in. If that's not possible, you might have to use a blanket over the rear of the scope instead of a darkened room.
Now get a piece of white paper or cardboard and hold it up behind the focal reducer. You should see a circular patch of light. Move it back and forth until the scene is in focus. Put a couple marks on the paper with a pencil on opposite sides of the circle, then measure that to get the diameter.
If it is 28mm or more, then another focal reducer won't help with your stalk positioning.
Dan, I went through this exercise, and while it is hard to be super precise with this, it appears that I'm between a 26-28mm image circle with the current reducer.
The CCDT67 looks like it might present a larger image circle which could be helpful:
- Optimized for 0.67x compression with a fully illuminated circle of 29 mm when the back spacing distance is 85 mm from the rear flange to the camera sensor is used
- Expect a 0.75x compression with a fully illuminated circle of 34 mm when the back spacing distance is 50 mm from the rear flange to the camera sensor
I think my next steps are to do more experimentation at f/10 to see what I can learn. I question how well I'll be able to guide at that focal length, but a real world experiment is in order.
I looked at the flat and light you supplied, and it is certainly not a perfect match.
Something worth considering, is how have you taken your flats? It's possible that your flats are just not optimal. It might be worth taking flats again and/or trying a new method for acquiring to see if you can correct for the shading.
Chris, I'll also do some more experimentation with my flats. The reference image appears to correct almost perfectly. The interesting question to me at the moment is what is different between the various lights that might cause an issue. I actually wonder if there are reflections that are shifting as the night progresses.
Regarding how I take the flats, I use an LCD set to a white image then cover the scope with 2 layers of white t-shirt material. I'll have to do some more experimenting on that front, as what I've succeeded with with the refractors might not scale up successfully here. However, ultimately I think there's some variation in the illumination of the lights that is causing trouble as well.