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Posted by Robin Vann at 18:38 on 2010 Sep 05
I’m having something happen that I’ve not seen before when I collimate my Newtonian with a Cheshire collimator.I am achieving the position as per the right-hand diagram, but with one difference; the outer cross-hairs of the collimator are slightly offset to the left, only very slightly.Does this indicate that I need to alter something or should I not worry?Thanks in anticipation,Robin Vannp.s. I have now detached the diagrams as mentioned above as the topic is dead.
Posted by David Mottershead at 19:47 on 2010 Sep 06
HelloIt’s a while since I owned a Newtonian, but from when I did, if it really is only very slight, and I mean veryslight, then I would suggest its nothing to worry about. How do the Airy rings appear on a star test? Are targets such as Jupiter clear, sharp and crisp through a range of magnifications? Again, if all looks good, then I wouldn’t worry.
Posted by Robin Vann at 20:48 on 2010 Sep 06
Thank you for your reply, David, and excuse me for being such a rank beginner.I have always had difficulty doing star tests, never really achieving the Airy disc pattern, possibly due to the inadequacies of the rack and pinion focuser on my Skywatcher 150mm. However, Jupiter is sharp as a button, as was a shadow transit of Io last week, Io’s shadow being pin sharp.So I guess I can leave it for now.To be absolutely accurate, I think the problem is that I haven’t perfectly centered the collimator’s cross-hairs over the centre marker on the primary at stage 2 (the centre diagram).Thanks again :)Robin Vann
Posted by Andrea Tasselli at 15:09 on 2010 Sep 07
I shan’t think you should worry. It means, if I understand correctly what you’re saying, that the secondary position isn’t corrected for the required offset. In other words the optical axis is slightly offset from the mechanical center of the focuser. OTOH, if you don’t see the airy disc ever then things are not quite OK. I would check with a star test whether you’re collimated or not. If it is, and the scope has been given proper time to cool off and the seeing is quite ok you should be able to see the airy disc of a bright star.Andrea T.
Posted by Robin Vann at 15:32 on 2010 Sep 07
Thanks Andrea.What you say makes perfect sense. If I move my eye slightly off-centre to the axis of the Cheshire, everything lines up, but I then can’t see the inside surface of the Cheshire on both sides. I have also never seen the Airy disc though I have attempted to many times since starting astronomy in May this year. I have previously put this down to the inadequacies of the coarse rack and pinion focuser but, overall, I now suspect the telescope was delivered with the secondary offset incorrectly in the way you describe.I shall now try to move the secondary so that my Cheshire cross-hairs are correctly centred over the centre marker of the Primary before realigning the primary to centre the reflection of the secondary over said centre marker.Robin Vann
Posted by David Arditti at 01:40 on 2010 Sep 09
I’m not sure about this, but the problem you describe looks to me to be more likely to be due to the focuser not being square to the tube than incorrect offset of the secondary. This is a hard fault to correct, if present, as most focusers have no capacity for adjustment of how they sit on the tube. However, it does not usually lead to serious optical problems.David
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
Posted by Andrea Tasselli at 15:10 on 2010 Sep 11
Actually what matters is that the reflections being centered rather than the actual object, in this case the secondary. If the focuser is tilted with respect to the mechanical axis of the tube, assuming that this can be ascertained with any degree of accuracy, the only way is to use shims (well furnished hardware shops di have them) to de-tilt the focuser. One easy way to assess this is to use a "collimated" laser collimator (some of the cheap ones are not). Once the beam is centered on the primary (I assume the primary is center-marked) moving the focuser up and down the whole length of the drawtube would show shifts in the position of the dot, thus revealing the tilt (and also providing a way to check the correct tilt if shims are used). Obviously this assume the focuser being of a decent sort and thus not bending or shifting while moving.Andrea T.
Posted by Robin Vann at 16:06 on 2010 Sep 11
Thanks again Andrea.Yes, I have thought of shimming it but thought I would use a piece of rubber. Thus one could ‘squeeze’ the rubber as one tightened the focuser nuts back into place and have some variation by way of the degree of tightening, rather than having to calculate the thickness of shim required.And yes, the cross-hairs move relative to the centre marker on the primary as the focuser is moved in and out. However, the focuser is fitted with a locking screw and locking it prior to observing results in the cross-hairs remaining centred whatever the position of the focuser.I think we can close the issue now. Thanks to David and Andrea for your ideas. I feel confident about taking this forward myself now.
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