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Teardrop star issues


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Equipment: EQ6R-Pro; Explore Scientific 80ED Triplet; ZWO ASI120MC guide camera; Canon 250D imaging camera; OVL field flattener. Software: NINA; PHD2 (multistar).

I cannot solve this issue with the stars in my images. It started some weeks ago, and basically I am not able to image at all. Image 1 shows a 10sec exposure on the star Sirius, with the teardrop shaped stars around. Image 2 shows 120sec exposure on NGC7522, same night, same setup, again teardrop stars. On both images you can see the tracking is lower than 1, and on a windy night. Both images here show the exact same type of distortion to the stars. I did try a 240sec exposure and that was the same. Image 3 shows PHD2 on Sirius, changing any of the settings doesn't work. This was after a new calibration was carried out, which was very good (see image 4).

Any ideas please?


Image 1.jpg

Image 2.jpg

Image 3.jpg

Image 4.jpg

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Where is the guider mounted?

If its on the OTA then I don't think its a mount issue, your guiding results would be bad.

If the guider is mounted separate from the OTA then differential flexure might be an issue - something loose on the 80ED side.


If you rotate the OTA+camera 90 degrees does the shape rotate too - if so its an optical element. I would try imaging without the flattener in place and see if the problem goes away/changes.


I see that the RA rms is more than twice the DEC rms - that will lead to oval stars at longer focal lengths but at your focal length I would not expect that to be visible since your scope wont resolve it.

Edited by paul
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Thanks Mechanoid, but even a 10sec exposure with Rigel looks the same. Tested again tonight an it's still the same. Tracking works fine with average of 0.75. Tried two different field flatteners and both show the same result, even no flattener and using a 1.25 connection still shows teardrop stars. 😁

Edited by Tony
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Did the tear drop rotate with the OTA?

If it's not your flattener then

1. Has the Camera M42/48 adapter slipped off square (sometimes they have an inner screw thread adapter held by grub- screws) 

2. Do you have LP filter that is not lying flat?

3. A cross threaded joint.

4. Basically, some part that is misaligned.

Looking at your Sirius image the tear drop is not aligned to RA or DEC so I'm pretty certain it's in the optics.

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Thank you Paul

I tried two different field flatteners and nothing changed, I also changed the 2in to a 1.25in connection and that was the same. I varied the back focus and again nothing changed. I going to strip the rig down and re-build it, making sure all the connections are tight and properly positioned. Hope this does the job, but no clear skies tonight🥵

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I had an issue with elongated stars that was caused by the mirror in my Newtonian being slightly pinched in the cell. I know you're using a refactor, but could something have changed inside your optics. It doesn't take much. Maybe even a differential thermal expansion effect ?  Perhaps one of the triplet lenses has moved off axis somehow. You could try shaking it to hear if something rattles !  One thing is eliminated - it definitely isn't your tracking, the like of which I only get on really good days.

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I see from pictures on the web that this scope, (which I guess from your equipment list is an Explore Scientific 80ED), has a clamping ring that allows rotation.  I assume that all the triplet elements are closely grouped at the front, but might there be some further field element close to this ring that gets distorted when the ring is clamped up with the screw ?

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