ChatGPT and oval stars - Forums

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3 days ago

An interesting exchange with ChatGPT regarding a new onset problem with stars across my entire field of view being oval shaped, and all elongated in the same direction. Exchanges are summarized due to the wordy nature of ChatGPT.

What is interesting to me is how much of the solution came not from facts handed down by ChatGPT, but by the back and forth of ChatGPT offering suggestions, and me pushing back on things that I didn’t think made sense. Much like interaction with a person of slightly higher knowledge than myself. I haven’t tested the system out yet, but I think I found a solution. And it was one that I would probably not have gotten around to considering for several days/weeks if it were not thrown in my path.

📷 Screenshot 2026-02-13 145259.jpg

Me: I have a 150 aperture carbon fiber, F/4 Newtonian telescope. When doing astrophotography, the stars across my entire field of view are elongated in most but not all sub-exposures. All of the stars are elongated in the same direction. The extent of elongation is independent of exposure time from 1 to 30 seconds, independent of which camera I use, and independent of what coma corrector I use. Guiding is roughly 0.5 arc seconds. Images with and without elongation often appear within the same acquisition sequence. What are some common causes of astigmatism?

ChatGPT; It’s probably not optical astigmatism if you are sometimes getting subs without elongation. Consider, thermal currents in the tube, primary mirror pinch due to low temperatures, focuser sag, primary mirror flop, and high frequency vibration that is too rapid to be picked up in your guiding graph as possibilities (various tests for these hypotheses added)

Me: Wouldn’t the fact that the elongation is independent of exposure duration tend to rule out mechanical drivers like focuser sag?

ChatGPT: It doesn’t have to be a steady creep. It might be a fluctuation between two states

Me: Wouldn’t one of the two binary states be energetically favored and unlikely to flip during a 10 second sub?

ChatGPT: There are lots of energy sources that could induce a flip. Dithering, wind, autofocus, wind gusts… What happens when you rotate the camera 90 degrees? This will help you localize the problem.

Me: The elongation changes from parallel to the long side of the sensor to parallel to the short side of the sensor

ChatGPT: The elongation must arise in the optical tube or somewhere upstream of the focuser. (Returns to hypotheses of cold-induced mirror pinch and tube currents. Offers various tests to perform)

Me: Here’s another observation prompted by your suggestion of high frequency vibration. I’ve noticed some play in my RA axis, and when I adjust the focuser the image oscillates in a straight line for up to 10 seconds after I take my fingers off the focuser. But I’ve ignored it because the guiding is fine.

ChatGPT: A high frequency vibrational mode due to this play could readily explain your elongated stars. It won’t show up in your guiding if the frequency is high. The fact that the oscillation is in a straight line supports the hypothesis that your elongation is due to slop in the RA axis. (Suggests various discriminating experiments). You need to look at the RA axis bearing load and not at the worm gear assembly. If it were the belt you’d have guiding problems.

Me: I removed the polar alignment scope and its cap from the back of the scope and this is what I see (Photo). What do I adjust?

ChatGPT: (Explains what all the parts are, tells me to loosen the grub screws and turn the bearing retainer bolt in 1/64th of a turn)

Me: I can’t do that. The grub screws are stripped, probably due to a prior operator. But when I adjust these other two screws holding the retaining ring to the outer case, the play disappears. (We argue briefly about the appropriateness of my fix, and ChatGPT eventually concedes).

Tonight we’ll find out who was right.

@Chris White- Overcast Observatory

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Tony Gondola

Frequent contributor

3 days ago

I’m not as “smart” as ChatGPT but I have some simple suggestions. It really looks like a guiding issue to me. RMS doesn’t tell you everything. The first thing I would do is plate solve the image and see if the elongation lines up with RA or DEC. If it lines up with RA, then that’s pretty strong evidence of a guide error. With stars it only takes a momentary shift to get the effect you see.

If it lines up with DEC it could also be guiding. Make sure you have a good PA and that the payload is well balanced.

If it doesn’t line up with RA or DEC then other causes could be, wind, cable snags or pulls, differential flexure to name a few.

I wouldn’t worry about tilt at this point as it would be consistent, unless your imaging train is really loosey-goosey. it doesn’t just come and go.

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John Tucker

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3 days ago

I think it is “guiding” in a sense, just on a time scale that is not picked up in a PhD2 graph or amenable to traditional guiding corrections.

My PhD2 RMS is 0.5 arcsec and my largest transient excursions are about 1.0 arc seconds. At 400mm focal length and with an ASI1600 camera, my pixel scale of 2.0 arcsec per pixel. So I should have round stars. But a typical star is 3 pixels tall and 5 pixels wide. That’s an extra 4 arcsec.

There is movement that is not being captured by PhD comparing the position of a star to its expected location every 2 seconds. At least that’s the best I’ve been able to come up with.

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Tony Gondola

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3 days ago

Have you visually blinked through your subs and watched what the stars do? That would tell you a lot.

Rainer Ehlert

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3 days ago

John Tucker · Feb 13, 2026, 08:29 PM
I think it is “guiding” in a sense, just on a time scale that is not picked up in a PhD2 graph or amenable to traditional guiding corrections.
My PhD2 RMS is 0.5 arcsec and my largest transient excursions are about 1.0 arc seconds. At 400mm focal length and with an ASI1600 camera, my pixel scale of 2.0 arcsec per pixel. So I should have round stars. But a typical star is 3 pixels tall and 5 pixels wide. That’s an extra 4 arcsec.
There is movement that is not being captured by PhD comparing the position of a star to its expected location every 2 seconds. At least that’s the best I’ve been able to come up with.

Have you checked the periodic error of the mount?

John Tucker

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3 days ago

Rainer Ehlert · Feb 13, 2026, 08:43 PM
John Tucker · Feb 13, 2026, 08:29 PM
I think it is “guiding” in a sense, just on a time scale that is not picked up in a PhD2 graph or amenable to traditional guiding corrections.
My PhD2 RMS is 0.5 arcsec and my largest transient excursions are about 1.0 arc seconds. At 400mm focal length and with an ASI1600 camera, my pixel scale of 2.0 arcsec per pixel. So I should have round stars. But a typical star is 3 pixels tall and 5 pixels wide. That’s an extra 4 arcsec.
There is movement that is not being captured by PhD comparing the position of a star to its expected location every 2 seconds. At least that’s the best I’ve been able to come up with.
Have you checked the periodic error of the mount?

I’ve had this mount for 6 years now, so whatever it is, it’s new onset. Have not looked at the unguided periodic error, but PhD isn’t pulsing a lot to keep it on track. I set the PEC a year or so ago but have not looked at it recently.

John Tucker

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3 days ago

Tony Gondola · Feb 13, 2026, 08:40 PM
Have you visually blinked through your subs and watched what the stars do? That would tell you a lot.

Great suggestion!. I don’t see any movement at all except for when it dithers (every 5th sub).

John Tucker

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3 days ago

Tony Gondola · Feb 13, 2026, 08:09 PM
I’m not as “smart” as ChatGPT but I have some simple suggestions. It really looks like a guiding issue to me. RMS doesn’t tell you everything. The first thing I would do is plate solve the image and see if the elongation lines up with RA or DEC. If it lines up with RA, then that’s pretty strong evidence of a guide error. With stars it only takes a momentary shift to get the effect you see.
If it lines up with DEC it could also be guiding. Make sure you have a good PA and that the payload is well balanced.
If it doesn’t line up with RA or DEC then other causes could be, wind, cable snags or pulls, differential flexure to name a few.
I wouldn’t worry about tilt at this point as it would be consistent, unless your imaging train is really loosey-goosey. it doesn’t just come and go.

I should add that when I took the polar alignment scope cover off and removed the polar alignment scope, there was stuff back there that was clearly moving around. I could easily wiggle the large silver disk that the RA axle is attached to by a mm or so.

So I think the bad guiding-like appearance of the stars, the absence of any problems apparent in the PhD2 graph, the loose widget I found at the back of the RA axis, and the long-lived oscillations I was seeing during manual focusing all seem to point toward a high frequency oscillation being the source of the problem. Either way, I’ll know sometime tonight!

The interesting part of this to me was that I think it would have taken me a few more days or weeks to consider the possibility that the elongation was due to a movement that was too high in frequency to be picked up in the guiding graph.

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TiffsAndAstro

3 days ago

I don't see mention of your guiding setup.

Flexure might explain this if say you guide scope is moving and you recently adjusted it.

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Tony Gondola

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3 days ago

John Tucker · Feb 13, 2026, 08:48 PM
Tony Gondola · Feb 13, 2026, 08:40 PM
Have you visually blinked through your subs and watched what the stars do? That would tell you a lot.
Great suggestion!. I don’t see any movement at all except for when it dithers (every 5th sub).

…and the stars are always round?

Tony Gondola

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3 days ago

Prim suspect for a high frequency vibration would be any fans that are running on the rig.

John Tucker

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3 days ago

TiffsAndAstro · Feb 13, 2026, 10:18 PM
I don't see mention of your guiding setup.

Flexure might explain this if say you guide scope is moving and you recently adjusted it.

Sorry, my bad. That should have read “polar scope”. I have edited the comment to correct the error.

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John Tucker

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3 days ago

Tony Gondola · Feb 13, 2026, 11:03 PM
Prim suspect for a high frequency vibration would be any fans that are running on the rig.

There are no fans, which is why I initially rejected the idea of a vibration problem. But what is interesting is this: You know how when you are focusing with a live camera, and the star bounces all over the place for a few seconds after you adjust the focuser? I was seeing that oscillation go on for more than 10 seconds, with the longest lasting component of it being simply bouncing up and down in a straight line. I’ve never seen that before.

And the other thing that put me off was that I have not seen this with my other, much lighter scope, on the same mount. I think the weight of the newt plus the two counterweights really keeps the oscillation going.

Anyway, if the clouds clear tonight as Windy.com predicts, I’ll find out if the adjustment I made to secure the RA axis better fixed it.

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Tony Gondola

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3 days ago

John Tucker · Feb 14, 2026, 12:05 AM
Tony Gondola · Feb 13, 2026, 11:03 PM
Prim suspect for a high frequency vibration would be any fans that are running on the rig.
There are no fans, which is why I initially rejected the idea of a vibration problem. But what is interesting is this: You know how when you are focusing with a live camera, and the star bounces all over the place for a few seconds after you adjust the focuser? I was seeing that oscillation go on for more than 10 seconds, with the longest lasting component of it being simply bouncing up and down in a straight line. I’ve never seen that before.
And the other thing that put me off was that I have not seen this with my other, much lighter scope, on the same mount. I think the weight of the newt plus the two counterweights really keeps the oscillation going.
Anyway, if the clouds clear tonight as Windy.com predicts, I’ll find out if the adjustment I made to secure the RA axis better fixed it.

I believe your camera have a fan.

John Tucker

Topic starter

Frequent contributor

3 days ago

I guess we’ve gotten into a discussion about what caused the problem, but what I really wanted to highlight was the way ChatGPT threw out some ideas I had not considered.

ChatGPT did not SOLVE the problem. It threw out a lot of ideas and aggressively pursued ones that didn’t prove out.

It did draw my attention to a possibility I would likely have not considered for several more days.

Incidentally, after adjusting the loose thingy on the back end of my RA axis, the extended osciillations that were occurring when I adjusted the focus disappeared along with the egg shaped stars. But the situation isn’t fixed. In the aftermath of this the mount goes crazy and does 6 arcsec excursions in RA about every 5 minutes.