Star Streak Troubleshooting

[jdh_astro]

Hello! I'm troubleshooting some problematic star shapes in my images and wanted to see if my guiding performance could identify the problem. My guiding RMS is consistently below my imaging resolution of 1.74"/px (mostly less than half my resolution), but my stars still show the same "egg" shape across the whole frame, regardless of guiding performance (see attached screenshot, full image is too large to attach). The egg shape is present in most if not all the frames no matter the location in the sky I'm imaging. It's not an optical defect since short exposures show perfect stars across the whole frame. 

In my guide logs below, you'll notice a very large spike in the frequency analysis around 116s (1/3 period of the CEM70 worm gear), could this be the primary cause of the issue? Or is it more likely to be differential flexure? I would like to rule out every other possibility before blaming it on differential flexure, as that will be challenging to address since my setup is remote. 

Also, can the 116s periodic error be addressed via changing guide settings? I've tried manually specifying the PPEC period as 116s, but then the 348s periodic error spike increases. 

Setup:

CEM70 (2021 model)

William Optics Cat 91 (448mm focal length)

ZWO ASI 2600mm Main Camera

SVBONY106 60mm guidescope

ZWO ASI 120mm mini guidecam

Guiding from 9/12 where images show streaked stars:

https://openphdguiding.org/logs/dl/PHD2_logs_QH2o.zip

Baseline Guide Results from 9/15:

https://openphdguiding.org/logs/dl/PHD2_logs_xkgh.zip

Thanks in advance for any help that can be provided!

Jonathan Hill

@jdh_astro

[Bruce Waddington]

I don't see anything in the guide logs that would easily explain the amount of star elongation in your screenshot.  In the interest of efficiency, I think you should turn the problem around and quickly test for differential flexure because I think that's what is is.  This can be done easily using the procedure described in the appendix of this document (page 13):

https://openphdguiding.org/tutorial-analyzing-phd2-guiding-results/

This doesn't have to be a disaster even in a remote location, it can easily be caused by something that's gotten loose in the guiding assembly.  Once that's resolved, you can come back to the questions about periodic error correction.  In general, short-period errors cause more problems than long-period errors assuming the amplitudes are similar. That's because the "ascent" and "descent" sections of the periodic error curve are steeper and require more aggressive guiding.  At some point, they become too steep to be handled by guiding. If your mount supports permanent periodic error correction, the best approach is to program a PE correction curve using an app like PemPro.  If you can't do that and want to try letting PHD2 PPEC attack the problem, set the period length to 116 seconds and disable the check-box for 'auto-adjust'.  But again, I think this is all secondary to your problem with elongated stars.

Good luck,

Bruce

[Jim Bachesta]

For what it's worth, I had an issue where I had star trails that occurred on short and long exposures. The trails were consistent in length. I identified the problem to be caused by fan vibration on my ZWO2600mm Pro. I put shock mounts on the fan and replaced the fan with a low noise fan. 

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[Bruce Waddington]

Great comment, Jim, thanks for posting it.  Fortunately, I think the diagnostic procedure I mentioned should be able to distinguish this from differential flexure, so it will be interesting to see what develops.

Bruce

[jdh_astro]

Hi everyone,

Thank you for the replies and the great feedback. I stacked my blue filter images from last night with no registration or pixel rejection and got the linked image. I also included a single subframe for reference. 

There is clearly a good amount of drift across the ~25x 300s frames (dithered every 5 frames). Does this pass or fail the differential flexure test?

https://drive.google.com/drive/folders/1aucN-eHRZMKoymBJxY04i2i4uKkN792E?usp=sharing

Thanks for your help!

Jonathan

[Brian Valente]

Jonathan

I hope you evaluated the results and made your own assessment? 

DIff Flexure is not a pass/fail assessment. It's a 'how much is there and is it acceptable' 

Dithering causes movement of the image in arbitrary directions.I realize the instructions said take pictures as you normally would, but it complicates the picture to assess DF. It also looks like there was some jump in the middle, and caused the stars to run on two separate tracks?

How much elongation is there in each frame? if your image scale is roughly 1.8"/pix (guesstimating here) and over three hours you drifted approximately 28 pixels (total wag here) , I would expect to see about 1 pixel elongation per 5 minute exposure. You can assess all the individual frames and measure the eccentricity. The single image you showed has eccentricity of 0.58, so that seems to match up. 

Let me know if all that makes sense, i'm not sure how versed you are in this

A picture of your setup showing the guiding configuration would help as well

As a final thought, regarding the initial "....and is it acceptable', the elongation is really only visible to me at 100% or closer. Something like BlurXTerminator would easily handle this. You can decide how much time and effort you want to expend on chasing this. 

Brian

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--

Brian 

Brian Valente

astronomy portfolio https://www.astrobin.com/users/bvalente/ 

[Bruce Waddington]

Jonathan - I'm afraid you missed the point of the exercise.  If the intent is to measure unwanted star movement from one frame to the next, dithering is going to ruin the test.  You can't do this as part of an imaging session that is going to be dithering, you do it like this:

1.  Figure out the maximum exposure time you can use that doesn't show star elongation or other types of trailing.  Pick an exposure time that is approaching the upper limit, don't just pick 1-sec or something.  Decide for yourself how much is too much, but be sure you don't get unacceptable elongation with your selected exposure time.

2.  Without any kind of dithering or re-centering operations, take a series of exposures using the exposure time you determined from step 1.  Don't change filters, just use the luminance filter.  Point the scope somewhere close to Dec=0, but it doesn't matter exactly what you're pointing at - just a field of stars is fine.  You want to take a minimum of about 6 exposures or  15 minutes of elapsed time.

3.  When you're done, blink through the images or stack them without first aligning them.  When blinking them, differential flexure will cause each individual star to move from one frame to the next, always in the same direction.  If you stack without aligning, you will see substantially elongated stars.

Bruce

[jdh_astro]

I've evaluated my results and have no other ideas for cause of problem other than differential flexure. This seems to be the fallback explanation for most people in similar scenarios, so I guess I'll assume the same. 

The consistent amount of trailing I get in my images does not meet my imaging quality standards, so if the issue is differential flexure, should I switch to my uniguide 32 guidescope since it has a much more rigid construction? This will put my guidescope resolution at 6.45"/px with my main rig at 1.73"/px, making my guide ratio 1:3.72. I've read a range of 1:3 - 1:5 is acceptable for guide resolution ratios, what do y'all think of this? I've had good and relatively consistent results guiding at 1:5.16 ratio with my 800mm newtonian, Orion 50mm guidescope, and same mount, so could I expect something similar here since I am now using a 448mm refractor?

Otherwise, if I want to keep my 60mm guidescope, it will take a lot of trial and error to isolate and remove the flexure since I don't know if its coming from the tube rings (I haven't found any good tube ring alternatives online) or the helical focuser (which I can't do anything about). 

Jonathan

[Brian Valente]

>>>I've evaluated my results and have no other ideas for cause of problem other than differential flexure. This seems to be the fallback explanation for most people in similar scenarios, so I guess I'll assume the same. 

you don't seem to be very convinced by this conclusion? 

Given your comment re: "does not meet my imaging quality standards" it sounds like you are a stickler for eccentricity, in which case i'd suggest looking at an OAG. A  more rigid guidescope can certainly help, and it be worth a try. who knows it may offer enough improvement to satisfy your needs, but it's a pretty coarse image scale and it's still a separate guidescope. So again you are looking at not eliminating diff flexure, but reducing it. 

Brian

To view this discussion visit https://groups.google.com/d/msgid/open-phd-guiding/124261eb-2432-44b7-87d3-e9fae9d67eden%40googlegroups.com.

[Bruce Waddington]

Yes, I don't understand this comment either.  Did you actually do the test I described?  And when you blinked the images, did you see the stars in the field moving consistently from one frame to the next in the same direction?  If so, this isn't just a guess or a "fallback explanation", it's a firm diagnosis.  On the other hand, if you didn't run the tests or can't interpret the results, that's another matter.

Bruce

[Jonathan Hill]

Hi guys, 

I did end up doing the test and it did show the constant field movement. Could an alternative explanation be just poor tracking or drift from my mount? Just trying to rule out everything else. 

I would love to switch to an OAG, but unfortunately I'd have to remove the rotator from my imaging rig, which, in my opinion, is essential for my framing preferences. 

BlurX can definitely fix my stars in post (it has done a great job so far), but I'm a firm believer in curing the disease if possible instead of treating the symptoms. Plus, when galaxy season rolls back around, this magnitude of trailing will blur fine galaxy structure in my images. 

I've put in a maintenance ticket with the observatory to switch to the other guidescope, so we'll see if that improves the performance. I will add that when I used this scope previously, it interfaced with the main scope via a 3D printed adapter (which I've since learned is a no-no). I've since replaced this with a metal dovetail bar, so maybe that will fix the issues I was seeing before switching to the 60mm guidescope.  

Thanks again for your patience and feedback. 

Jonathan

On Sep 21, 2025, at 00:24, Bruce Waddington <bw_m...@earthlink.net> wrote:



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[Brian Valente]

>>>  Could an alternative explanation be just poor tracking or drift from my mount? Just trying to rule out everything else. 

it can be multiple things, so it's still a good idea to address DF

Looking at your original guidelogs, there is a high periodic error at 115 seconds that is causing some issue for you. in your PPEC settings, i see you have the period set to 347 but the auto-adjust is on. Not sure what you are chasing there, but based on the below, i would set your PPEC period to 115 seconds, auto adjust off and increase your prediction gain to about 80

Also FYI I see you at times fiddling with PPEC aggressiveness: that kind of change and small increments doesn't do anything and will just waste your time. 

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[Bruce Waddington]

It's highly likely that the test results are showing differential flexure, this is a pretty reliable conclusion.  That doesn't say there aren't other things you can do to improve, it just says you probably have a baseline problem of DF to start with.  I don't really know your entire setup in terms of guide scope + imaging scope but it sounds like the imaging setup has a coarse image scale.  In that case, it should be possible to use a separate guide scope if you pay close attention to the rigidity of its mounting details.  Things like stalk mounts and thumb-screw fasteners can all contribute to differential flexure.

Bruce

[Jonathan Hill]

Bruce & Brian, 

Thanks for your time and advice in helping me diagnose and fix my flexure problem! Swapping to the more rigid, wider field guidescope has reduced the flexure greatly. My guide numbers are very comparable to before with the longer length scope, but my stars are almost perfectly round. I also fixed my PPEC period length to 116s as that's consistently been the largest peak in the guide log frequency analysis. The drift test only shows a VERY small amount of movement between exposures, which is also barely noticeable when blinking through dithered and undithered frames. 

These results have been very consistent over the last 4-5 nights over several regions of sky, so I'm happy with the fix. Thanks again for your time and patience in assisting me! 

Jonathan

On Sep 22, 2025, at 16:12, Bruce Waddington <bw_m...@earthlink.net> wrote:



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[Brian Valente]

Thanks for the update, good to hear you stuck with it

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[Bruce Waddington]

Thanks for letting us know, Jonathan.  You've also demonstrated for yourself how differential flexure behaves.  With a separate guide scope, there will always be some amount of DF, but the question is how-much-is-too-much for your imaging requirements.  So, for example, if you suddenly switched to using 20-minute exposure times, you'd probably decide the DF was, again, too large.

Cheers,

Bruce