Posted by Robin Vann at 00:59 on 2010 Sep 10
Yes indeed. I discovered this yesterday afternoon when I successfully lined up the cross-hairs of my Cheshire with the central marker on the primary by adjusting the secondary, and then aligned the reflection of the Cheshire cross-hairs on the said central marker.What resulted was the secondary mirror appearing slightly off-centre under the focuser tube.Hence I think you are correct David: that’s well diagnosed.I now apparently have the optical axis (which we deem to be slightly diagonal to the focuser tube) centred at the cross-hairs of the Cheshire instead of slightly off-centre. Ideally, I suppose one would centre at the focal plane, to completely compensate for the off-centre focuser tube.Later last night, I performed a star test on Capella at about 50° altitude and succeeded, for the first time, in obtaining a beautiful perfectly symmetrical Airy disc slightly intra-focus and extra-focus, with a little flaring which I put down to seeing. The symmetry was only perfect when the star was centred in a 15′ field of view at 280x, some asymmetry beginning to be noticeable about halfway to the edge of the field.The question remains: should I continue as per the new alignment or return to the original one? I am inclined to continue as present as I am now seeing the Airy disc whereas I wasn’t before, though it worries me slightly that it becomes asymmetrical so close to the optical axis.Thanks again everyone for your help.Robin Vann