Dave, Are you sure it is coma? Many times astigmatism is mistaken for coma. Astigmatism induced by a mirror changes as as you go inside and outside of focus. The stars will be oblong tangentially on one side of focus and oblong radially on the other side of focus. Astigmatism induced by your eye will show oblong stars on both sides of focus, and at focus. The oblong direction and amount will not change with focus, and will be in the same direction across the field. Coma is apparent in the outer portions of the field of view and will be radial on both sides of focus. Mirror induced astigmatism is at its minimum when the image is focused. Coma and eye induced astigmatism are independent of focus position. Brent --- DunnDave@aol.com wrote:
Does anyone have any expirience with Coma Correctors? My newly completed 16" f/5 has coma. I would appreciate any advise anyone might have on how to deal with coma. The Lumicon and TelVue correctors are several hundred dollars so I don't want to just buy one to experiment.
Thanks. Dave
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