C8 corrector rotated - help me fnd the right posit
Posted 01 December 2005 - 11:50 AM
I believe that the corrector plate on my C8 was rotated out of position by the previous owner. Is anyone familiar with the procedure to determine the correct rotational position of the corrector? Is there an "easy" way, such as positioning it so that the Celestron name on the secondary holder is horizontal? Thanks for any help - GD
Posted 01 December 2005 - 12:37 PM
Posted 01 December 2005 - 04:39 PM
Posted 01 December 2005 - 05:09 PM
You would have to remove it to see what is going on. That is a bit of an odd issue for sure.. Coma... Meaning field curvature correct?
I'm not good enough with SCT's to advise you further in all honesty. I'll bet some of the real SCT guru's will chime in here given some time.
Posted 01 December 2005 - 08:09 PM
there seems to be a bit of "coma" effect that I can't quite get dialed out
If it's off-axis, that's normal. The secondary is spherical or near-spherical, so those SCTs are not corrected for coma.Roughly, for the linear field, it should be similar to the coma level in f/6 Newtonian. If it is in the center and can't be removed by collimation, then the corrector is either decentered or tilted. Orientation doesn't matter.
Posted 02 December 2005 - 01:19 AM
Posted 02 December 2005 - 07:49 AM
Posted 02 December 2005 - 10:06 AM
Posted 02 December 2005 - 02:08 PM
is the easy part. If that doesn't do, I'd check out how much of a play there is in positioning of the corrector in the opening and the secondary on the corrector. It shouldn't be much, especially the latter. If the secondary is in its proper place at the corrector, it still shouldn't be too hard. In that case center coma is likely caused by decentered corrector/secondary, and they are decentered in the direction of the comatic extention. They should be moved slightly in opposite direction, by manipulating edge shims.
Some units have corrector/secondary purposely decentered in the tube to compensate for not quite centered primary. Maybe the pervious owner had this and tried to have it "fixed". Whichever position results in no center field coma should be good. Hopefully, everything is OK mechanically.