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Refractor imaging problem - stars elongated in parts

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Hi all

I have a problem with deformed stars- see attached photo. This is on a newly acquired, second hand Skywatcher evostar 120 ed pro. The pattern is identical with or without a reducer/flattener and in whatever orientation I put the camera and whether the exposure is 15s or 120s. One corner of the image appears almost in focus with round stars, whereas the diagonally opposite corner has elongated stars. The camera and flter wheel plus extension tubes when on my newtonian give pin point stars.  Tracking of the mount is around RMS 0.6. If anyone is familiar with this star pattern and can tell me the solution that would be fantastic!



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Initially I'd say tilt which is easy to fix with a tilt adjustment adaptor. It makes it more difficult to assess you have the correct backspacing if you have this non symmetrical issue. But you've stated it's also like that without flattener.

Does the FOV look the same visually through a wide low power eyepiece?

I hesitate to suggest the possibility of it being astigmatism, having not witnessed it myself I can't be 100pc but it's not something easily fixed if at all. Hopefully your issue is a simple fix like tilt.

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When you say the pattern is identical even after changing camera orientation, I assume you're saying that the right side of the image always shows the same defect, then it's the camera sensor that's tilted with respect to its mounting.

If your camera has a tilt adjuster built in then, adjusting that should correct it. If you use a separate tilt adjuster in the imaging train to correct it then you're introducing tilt of the opposite direction in the scope to correct the camera. This means you can't then rotate the camera for framing as that will cause tilt to reoccur.

It's best to adjust the camera so it has no built in tilt, and if your images still show some tilt, an external tilt adjuster in the imaging chain will correct it and will also allow you to rotate the camera without introducing more tilt.

To adjust the camera tilt, it's simplest to build a test jig of a simple box with a hole in the top and a cheap laser pen as shown later in this thread with various examples. You can adjust it accurately during the day indoors.

Trying to adjust it at night which may mean removing the camera each time, is generally just endless frustration. 😬


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21 hours ago, Elp said:

easy to fix with a tilt adjustment adaptor


8 hours ago, ollypenrice said:

Need a tilt adjuster!


6 hours ago, knobby said:

looks like pretty bad tilt ...

Thanks for all the comments. Tilt is what my initial thought was, but as the fact hat by movng the camera and the very same extension tubes to my Newtonian did not exhibit any form of tilt to me pointed to a problem with the refractor. I rummaged in my box of odds and ends and found a refractor collimating eyepiece and lo and behold the collimation was way out. So its misaligned objective cell and/or focuser. I will report back once all is tweaked.

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  • 4 weeks later...

To close this discussion off the problem has been rectified and all is good once more. The problem was someone had removed the lens cell and slightly cross-threaded it giving it a few millimetres of slant; perhaps Spanish Customs when it came through. Anyway,  I couldn't budge the lens cell so I took it to the official Skywatcher dealer for Spain in Barcelona and they sorted it for 80 euros and collimated it to perfection.

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