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Hello guys. I just acquired a Sky-Watcher 150/750 PDS Newtonian and I am new to the whole Newtonian thing, I watched videos and read articles about how to collimate and I follow through all the techniques (Cheshire eyepiece, collimation cap, laser collimator), and then when I go out imaging I even do a star test and it looks perfect. However, as soon as I take an image in focus immediately in the corners only on one side I can see distorted stars. I use a DSLR Canon 250D modified with a MPCC iii Baader coma corrector. Obviously I am doing something wrong. Any Idea what is going on here? Can anyone suggest a good book for more advanced collimation that could possibly help? Thank you guys. Here is how it looks like.  |
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| Sounds like tilt and/or focuser sag. |
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| You haven't go a problem with collimation but with tilt and the MPCIII isn't especially tollerant of tilt in my experience. This said, that CC isn't the best around anyhow. The source of the tilt can be anywhere within the image train; focuser sag, tilting within the compression ring with the CC, the CC to camera intrface and so on. Given that is pretty much uni-directional shimming might do the trick but isn't going to be an easy job. |
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andrea tasselli: shouldn't tilt be eliminated when collimating on a star? I mean isn't this the point? |
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| Looks like tilt of some sort. Try rotating the camera and take several exposures to see how it changes. I have the same issue and also use Newtonian along with MPCC. Like others mentioned, the tilt can come from anywhere so it takes a lot of experimenting... |
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andrea tasselli: They would need to know how to collimate in focus with all the imaging train in place, which isn't easy for anyone and requires dedicated software and hardware to do it right. And is no gurantee that once you put on your DSLR things won't change. Not for the faint of hearts! Never mind that if the focuser is sagging then the is no redress possible unless this is corrected first. But let's assume that the scope is collimated (or collimated enough for imaging purposes) then you would see soemwhere within the image the locus of zero (or near zero) coma, the optical axis. If there is tilt then the tilt components add to the deteroration of the image two-fold: one by adding the tilt's own aberration and secondly by shifting the optical axis away, in this case outside the sensor boundaries. |
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With the two screws that the camera is clamped to the focuser/2" adapter, I always experienced mild to strong tilt (APS-C DSLR or 533MC), and as the MPCC is quite short, ASTAP generally complained. In the end, I have switched to a Baader Clicklock clamp and am very satisfied. According to ASTAP only 1-3% tilt which is fine for me. The only disadvantage with the 150PDS and default focuser - I can no longer get into focus with the guide camera when using the ZWO OAG. That's why I'm guiding with the separate guidescope again. But with the 750mm focal length, no big deal i guess. |
