Strange star shape on Zenithstar 73 - Forums

Topic starter

Feb 18, 2022

Since recently, my images taken with the WO Zenithstar 73 have been showing strange star shapes. It looks like the stars have a small "tail" to the left. It is apparent all over the image, not just the center or edges. Previously, the front element was replaced due to pinched optics and the telescope has been performing fine ever since. Nothing changed on the setup in the last couple of months. The only thing I can think of is that outside temperatures have dropped towards a few degrees above 0 Celsius, so I wonder whether the cold might be causing some sort of pinching effect. I verified that the filter is not the problem by taking it out. Also, I made sure there is no flex in the imaging train and back focus is set correctly. Tracking is working nicely with RMS close to 1" and symmetrical in RA and DEC. I am out of ideas as to what might be causing this. Below the full equipment list and a single 120 second image showing the effect (it also appears when using much shorter integration times).

My next step is to test collimation (on request of William Optics). Due to the weather, I will need to use an artificial star but I am waiting for a dry day to do it at the lower temperatures outside. In the meantime, I was wondering whether anyone recognizes this type of deformation?

Equipment:

William Optics Zenithstar 73
William Optics FLAT 73A
William Optics UniGuide 50mm
ZWO ASI 533MC Pro
ZWO ASI 120mm Mini
ZWO EAF
Skywatcher HEQ5 (Rowan belt mod)
ZWO filter drawer
Optolong L-eXtreme filter

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Pedro A. Sampaio

Feb 18, 2022

First question - did this happen to every single sub, or is it something ocasional?

Second - if you rotate your image train, does it follow or maintain the same direction?

Third - you state your RMS is around 1", but how's peak to peak guiding performance?

My first impression is that it seems like you have guiding issues. This looks to me like trailing. Even though your guiding may be around 1", if peak to peak is beyond your resolution in As/Px, you may have subs that look like this.

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Henny Veerman

Topic starter

Feb 19, 2022 · Edited: Feb 19, 2022

It happens on every single sub. I haven't been able to rotate the image train yet but I will test that with the artificial star. Regarding the guiding, below a screenshot from PHD2 Log Viewer. Peak-to-peak is definitely high. But it doesn't look extremely asymmetrical, neither does the scatter plot. I also tested with 10s and 3s exporsures to try to rule out guiding. Both show the same effect but a bit less visible, I guess due to the lower SNR with those short exposure times.

kuechlew

Feb 19, 2022

Pedro A. Sampaio:
First question - did this happen to every single sub, or is it something ocasional?

Second - if you rotate your image train, does it follow or maintain the same direction?

Third - you state your RMS is around 1", but how's peak to peak guiding performance?

My first impression is that it seems like you have guiding issues. This looks to me like trailing. Even though your guiding may be around 1", if peak to peak is beyond your resolution in As/Px, you may have subs that look like this.

Additional question: How does the direction of star deformation relate to the orientation of your scope and your mount? I had a similar effect recently on my mobile setup when the clutch of one of my tripod legs was not fully tightened and the whole setup shifted ever so slightly during exposure ...
You could also try an image with short exposure time. If it's an issue with the optics, it will show with short exposure times too.

With all stars elongating in the same direction and by a similar amount I tend to agree with Pedro that the problem is most likely in the movement of your setup and not in the optics part.

Good luck and clear skies
Wolfgang

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kuechlew

Feb 19, 2022

Didn't see your second posting, sorry. So it seems to be the dynamics and not the optics as assumed earlier …

Pedro A. Sampaio

Feb 19, 2022

Given your guiding stats, I think that would be my first assumption.

In any case, try setting some artificial stars, and see if this artifact persists or not.

andrea tasselli

Frequent contributor

Feb 19, 2022

Doesn't look like mount issue to me, more like issue with the optics. The spike is too spikey, if you see what I mean. To test the hypothesis image a signficantly defocused star inside and outside focus. From that we would know.

Henny Veerman

Topic starter

Feb 20, 2022

Hi everyone,

Thanks a lot for all the feedback, I really appreciate it!

I hadn't really considered the guiding to be the issue because of the scatter plot and lack of any spikes in the frequency plot. I assumed the peak-to-peak variations were due to bad seeing (predictions on Meteoblue were like 2.67"). Still, I looked at the results again to see if guiding could indeed be the cause here. Below a central crop of a 3s exposure I took that same night. While the effect is less pronounced due to the higher noise, I still see the spike/tail on the left of the stars. The second image is a longer exposure (120s) but taken after the Meridian flip. Here the spike points to the right (i.e. same absolute orientation). Interested to hear what you think!

I still need to do the artificial star test, I haven't had the possibility yet unfortunately.

3s exposure (crop):

120s exposure (crop) after meridian flip:

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wittinobi

Feb 20, 2022

looks like this...
https://www.astrobin.com/forum/c/astrophotography/equipment/is-anyone-else-getting-erratic-streaked-stars-from-their-heq5-mount/

Karl Theberge

Feb 20, 2022

Your star look a lot like mine actually.

Seem like the ZS73 get pinched optics from factory....

Look at this video. I would try this first.
https://youtu.be/1gtrVODCepE

After doing this, i got round star using artificial star on a static bench. Off course temperature change and atmosphere would still affect my optics, but now I am sure my scope can produce round star.

Sean van Drogen

Feb 20, 2022

Had this with my ZS73 as well. For me it was a combination of pinched optics and improper backfocus distance setting of the flattener.
The link Karl provided is good to fixed the pinched optics. Used CCDinspector to get the backfocus correct by stacking 5 x 60s exposures in live mode and adjusting the flattener retake 5x 60s to see which way i had to move. Took me about 2 hours to get to proper distance

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Henny Veerman

Topic starter

Feb 25, 2022

looks like this...
https://www.astrobin.com/forum/c/astrophotography/equipment/is-anyone-else-getting-erratic-streaked-stars-from-their-heq5-mount/

Yes , I saw that post. Indeed looks very similar! Only thing is that I also see the effect in very short integrations of 3s. Also, the frequency analysis doesn't show any short period deviations in the guiding...

Henny Veerman

Topic starter

Feb 25, 2022

Karl Theberge:
Your star look a lot like mine actually.

Seem like the ZS73 get pinched optics from factory....

Look at this video. I would try this first.
https://youtu.be/1gtrVODCepE

After doing this, i got round star using artificial star on a static bench. Off course temperature change and atmosphere would still affect my optics, but now I am sure my scope can produce round star.

Yes, very good point. Actually, my Z73 came with pinched optics from the factory as well! Took a while to realize what the problem was. William Optics gave me instructions to do exactly what is shown in the video i.e. losening those screws. However, since this did not resolve the issue they replaced the front lens cell completely. The new one does not show pinching anymore :-)

However, I think that what I am seeing might indeed still be related to some kind of pinching or collimation. Strange thing is that the problem resolved itself over the past days. My current line of thinking is that this might be temperature related since I changed nothing on the setup. The issue appeared when outside temperatures dropped over the last months and now they are rising again the effect is gone. I wonder whether perhaps the lower air temperatures might cause some shrinking of the metal rings in the lens cell which causes pinching or collimation problems. I am going to add a better dew heater to see if this helps when temperatures drop again...

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Henny Veerman

Topic starter

Feb 25, 2022

Sean van Drogen:
Had this with my ZS73 as well. For me it was a combination of pinched optics and improper backfocus distance setting of the flattener.
The link Karl provided is good to fixed the pinched optics. Used CCDinspector to get the backfocus correct by stacking 5 x 60s exposures in live mode and adjusting the flattener retake 5x 60s to see which way i had to move. Took me about 2 hours to get to proper distance

As mentioned just to Karl, I had the same problems with pinching which was resolved by WO. But I think what I am seeing is still some sort of pinching/collimation issue caused by low temperatures... Backfocus should be correct but I will double check with CCDinspector, thanks for the tip!