Jumping stars in my light frames - Forums

Topic starter

Aug 23, 2025

Hi everyone
i am having some issues with my light fames, my stars look kind weird(dot-trail-dot)
at the beggining of the session the stars look perfectly round, but as the object rises in the sky, the stars suddenly show this weird pattern. Then, when the object is near the meridian, the stars look normal again.
It has happened in different objects in different parts of the sky(lagoon nebula, veil nebula, elephant trunk and rho ophiuci).
My equipment is:
-star adventurer gti
-WO redcat 51 WIFD
-WO uniguide 32mm
-ZWO ASI 120 mm mini
-Canon eos R100
-synscan app, phd2 and nina.
Thanks for your help and clear skies everyone

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bigCatAstro

Frequent contributor

Aug 23, 2025

Have you had a chance to check if any of your imaging train components or the mount clutches are loose?

Anything with your cable management that may be getting caught and pulling as the mount is tracking ascending objects?

What do your PHD2 logs look like? Any spikes in RA?

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Brian Puhl

IOTD Staff

Frequent contributor

Aug 23, 2025

This is definitely a tracking/guiding issue.

There are a multitude of things, but here’s a few that I’d check first:

Make sure guide cam is properly secured.
Ensure dark library is built for your guide camera (so you don’t guide on a hot pixel)

Check for substantial backlash in your RA and Declination axes. You can do this by physically attempting to move the mount back and forth about the axes, it should not feel loose at all.
Check for cable drag. Move the mount about each axis and ensure it’s not stiff.

Like bigCat mentioned, I’d drop either a guide log or some screen shots from your PHD in here. Are you using a USB cable for guiding or ST4?

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AstroÅmazer

Aug 23, 2025

Check for flexture in the imaging train. One culprit could be the Canon to T2 adapter. If it is wiggly, you will see this. Another one is differential flexture between the guide train and the imaging train. Or even the guide train being out of focus. If the guide train is seeing dumbbell shaped stars, it cloud do this, oscillate the imaging train. PHD2 logs will show this with constant corrections on both sides of 0.

Another one is the worm gears in the GTi. How much are they moving under load? There are 2 screws to tighten them. I had to tighten mine till my GTi was stiff then backed off 1/8th to 1/4th turn at a time till it was not wiggly but moving smoothly. This is an unguided experiment with 30s subs with an Askar SQA70 and a full frame Nikon, after cleaning and tightening the worm gears: https://app.astrobin.com/u/Astro%C3%85mazer?i=6vwdwg

Also what length of subs, what percentage of subs show this?

Edit:

My debugging recommendations:

Check without guiding, tracking only. 30s subs. With good polar alignment, it's possible to get round stars with your setup. If this happens without guiding, then it's the imaging train (flexture in adapters, cables dragging etc.) or the mount is loose.

If it does not, it is guiding. Differential flexture or badly focused guide train. Or over aggressive guide settings causing oscillations.

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bigCatAstro

Frequent contributor

Aug 23, 2025

AstroÅmazer · Aug 23, 2025 at 10:52 PM
Check for flexture in the imaging train. One culprit could be the Canon to T2 adapter. If it is wiggly, you will see this. Another one is differential flexture between the guide train and the imaging train. Or even the guide train being out of focus. If the guide train is seeing dumbbell shaped stars, it cloud do this, oscillate the imaging train. PHD2 logs will show this with constant corrections on both sides of 0.
Another one is the worm gears in the GTi. How much are the moving under load? There are 2 screws to tighten them. I had to tighten mine till my GTi was stiff then backed off 1/8th to 1/4th turn at a time till it was not wiggly but moving smoothly. This is an unguided experiment with 30s subs with an Askar SQA70 and a full frame Nikon, after cleaning and tightening the worm gears: https://app.astrobin.com/u/Astro%C3%85mazer?i=6vwdwg
Also what length of subs, what percentage of subs show this?
Edit:
My debugging recommendations:
Check without guiding, tracking only. 30s subs. With good polar alignment, it's possible to get round stars with your setup. If this happens without guiding, then it's the imaging train (flexture in adapters, cables dragging etc.) or the mount is loose.
If it does not, it is guiding. Differential flexture or badly focused guide train. Or over aggressive guide settings causing oscillations.

Good point about taking some unguided test subs. I would only add that if the OP is in a location with high light pollution, they may need to roll back the exposure length a tad.

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The0s

Aug 24, 2025

I’d second checking the worm gears inside the GTi - I have one, had a very similar problem (extremely trailed stars), and almost entirely fixed it by carefully tightening the gears. I now consistently get 180 second subs (and could probably do 300 second subs) with only one or two trailed subs per night.

One word of warning, though: I’ve noticed that if you knock the mount a bit too hard a few too many times, it can loosen up the gears and you’ll have to fix everything all over again.

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Adrimt33

Topic starter

Aug 24, 2025

Hi guys
Thanks for your amazing support.
I will try the various tips/ experiments that you suggested and update with the result.
But for now, i can answer some of your questions.
My T2 ring is the WO M48 T-MOUNT for Canon eos r mirrorless and i always try to make sure is tight.
My guiding i recently tightened it up because my telescope shop suggested this might be the cause of the issue but it still happens.
my guiding cables are the usb that came with the gti and the zwo camera
The gti is moving around 3,5 kg which is less than the 5 kg payload capacity
my exposures are 5 mins from a bortle 7 and light pollution filter antlia quad band.
PHD2_GuideLog_2025-07-22_231942.txt
PHD2_GuideLog_2025-07-15_235542.txt
PHD2_GuideLog_2025-07-16_230010.txt
This are some guidelogs form various session in which this happened, but i don´t know how to analyze them or what to look for.

Again thanks everyone for your support, it has been extremely useful.
Clear skies

Read noise Astrophotography

Frequent contributor

Aug 28, 2025

That dot–trail–dot star shape is almost always a guiding or tracking irregularity, not optics. Since it only shows up at certain sky positions and then disappears, a few likely culprits:

1. Periodic error / gear mesh in RA

The Star Adventurer GTi has noticeable PE. If your guide exposure is too long (e.g. 3–5 s), PHD2 may not catch the rapid gear motion, so stars look like “dot–trail–dot.”
Fix: try shorter guide exposures (1–1.5 s) to catch the gear motion.

2. Declination backlash or stiction

When the mount crosses a certain altitude, the balance on the DEC axis shifts. If there’s backlash, guiding corrections overshoot → small start–stop star trails.
Fix:
- Check DEC balance (slight east or camera heavy bias helps).
- In PHD2, set DEC backlash comp or run DEC in one direction only if possible.

3. Flexure or cable drag

As the mount tilts, cables may tug, especially with a light rig like RedCat + DSLR. That sudden tension = tiny jerks = dot–trail.
Fix: re-route USB/power so nothing pulls when pointing high.

4. Meridian-related guiding geometry

Near horizon: RA motion is slow, DEC guiding more sensitive → guiding errors look worse.
Near meridian: tracking geometry stabilizes, so stars look normal again. This matches what you describe.

What to try next session

Check balance carefully (bias RA slightly east-heavy, DEC slightly camera-heavy).
Shorten PHD2 exposure time to ~1–1.5 s.
Watch PHD2 RA/DEC graphs as the problem starts you’ll probably see sudden oscillations in RA or DEC.
Inspect your cable routing; nothing should hang loose.

Bottom line: The pattern isn’t your optics, it’s the mount. On the Star Adventurer GTi it’s almost always RA periodic error showing up when the geometry makes it harder for guiding to keep up. Faster guide cadence + better balance usually solves it.

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