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With a fast newt I would use a Coma Corrector instead of a Field Flattener
From Cloudy Nights
Coma is an image distortion created by a parabolic mirror. Most telescope mirrors are parabolic and the amount of distortion increases as the focal ratio decreases. The focal ratio is a unitless number derived by dividing the focal length of a mirror or lens by its clear aperture. A coma corrector compensates for this distortion which makes stars appear like a point of light with a fan shaped light cone pointing toward the outside of the field of view. The wider the apparent field of view of an eyepiece the worse the coma will be at the edges of the field of view. Other optical defects may be present but for simplicity I will not mention them now. In theory every possible focal ratio should have its own specific coma corrector lens. In real life that does not happen since they are not a high demand item like contacts or eyeglasses. So for now one coma corrector will have to cover a range of focal ratios.
Mirrors and lenses can have another defect in which the image formed comes to focus not on a flat plane but on a curved surface. One could compensate by adjusting ones focus to get the center of the field of view in sharp focus. Then adjusting a little more to get areas progressively further out to the edge of the field of view in sharp focus while the inner areas become blurry. A field flattener corrects for the field curvature defect or aberration.
in reference to my question of October 8 the field flattner is actually a coma corrector-sorry for the mixup. I am planning to purchase an Astro Tech coma corrector for use on the 8 inch f4. Anybody have one?
I've trying to setup my new Orion 8" newt and Atlas EQ-G and trying to understand the go to hand control. btw I think the whole setup is kind of poorly designed and constructed-Im really not happy with it so far. The dovetail mounting bar is a pain trying to use in the dark-almost dropped the scope trying to get it in the dovetail slot so I put threaded shafts into the bottom of the tube rings and larger holes in the plate and secured the tube with threaded knobs. All this makes the setup much easier in the dark. The finder mount is poorly designed also-I'm making a better one now. I'll post some pictures when I get this stuff done. If anyone has the same scope and mount I would like to hear from them regarding any problems they have had-Thanks!