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

To view this discussion visit https://groups.google.com/d/msgid/open-phd-guiding/d3c87b23-3678-4fff-97dd-07cec96fdfaen%40googlegroups.com.

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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