short stretch of elongated stars

[Dan]

The other evening, I had about a 1 hour stretch of elongated stars in my images. Prior to this time, stars were round and after this stretch stars were round. It also seemed like the elongation grew more severe for about 30 minutes then improved during the remaining 30 minutes until stars were round once again. Could you review my log and see if there is some information as to why this occurred. The time in question is between 1:43 am and 2:54 am 9/14/26.

https://openphdguiding.org/logs/dl/PHD2_logs_zD38.zip

Thank you for your help,

Dan

[Brian Valente]

Dan

I have the same answer I gave you before: I think you have differential flexure. 

If your guiding looks good but your stars are elongated, the issue is differential flexure due to use of a separate guidescope on your sxs rig and use of rings/thumbscrews.  Your logs suggest this is exactly what is going on. You have relatively low total RMS and the RA/Dec RMS are close to equal.

There was a short period of about 8 mins at 21:48 when your total RMS increased to 0.849". RA and Dec were still about equal. Without DF, you would have seen larger stars, but they would still be round. 

The longest run of 50min had a total RMS of 0.77" with RA being 0.561" and dec being 0.527" - so again, relatively close.

What is your image scale on the FSQ? I could give you a better assessment of how your guiding should have impacted your image without DF.

Here's a good article on differential flexure for background:

https://astronomy.mdodd.com/flexure.html 

Brian

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Brian 

Brian Valente

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

[dpdal...@aol.com]

Wow, Brian. I really appreciate you looking at the log.  I thought I would upload it so you could see if there was something I missed. I will work on the differential flexure as we discussed earlier. My image scale is 1.46"/px.

Thank you,

Dan

PS: thanks for the reference article

To view this discussion visit https://groups.google.com/d/msgid/open-phd-guiding/CAJa45i74njXJx5UpZUZp%3DcQDpnQokUHqhBZVwWrnHJf4f8odrA%40mail.gmail.com.

[Brian Valente]

Dan

I just realized i gave you feedback from someone else's logs. I apologize for that. You had about a 6 hour run towards the end?

However, the conclusion is basically the same: your total RMS values are actually a bit lower than what I mentioned before, and your RA and Dec RMS are even closer together.

So... (now correct) guidelogs, still same answer

A couple other small points of feedback:

- your polar alignment is about 4.5 arcmin, you could probably improve that a big

- evaluating your guide RMS impact to your imaging system, you can see the error level of about .7" total RMS and RA/Dec nearly equal has a negligible impact on your imaging system. you are subpixel, at least as far as guiding goes (this would be if you didn't have DF)

To view this discussion visit https://groups.google.com/d/msgid/open-phd-guiding/526600764.674964.1789607264265%40mail.yahoo.com.

[dpdal...@aol.com]

Brian,

Thanks for the added information.

Yes, a 6hr run! I was imaging the Cocoon and B168. From where I am in N Michigan, if I start w Counterweights up, I can get a nice long run on this object. I understand it's not ideal to image so far beyond the meridian but this object is high in the sky for much of this long run and it allows me to sleep until nighttime is over :)

Also, regarding my polar alignment. At the beginning of each imaging session, I run the guiding assistant to determine my MinMo for the evening. If I do an object multiple nights in a row, my polar alignment can vary from night to night. Some nights it will be 4.5 and the next night it might be 0.9 arcmin. My scope is in an observatory, so it is not changing position. I use a QHY polemaster to polar align. I have come to disregard the polar alignment values from PhD since I am spot on with the polemaster and the variability I get from PhD regarding my polar alignment error. Do you have any suggestions or explanation for why the polar alignment error in PhD can be so variable on a fixed telescope?

Regardless, I will run the polemaster again to make sure some squirrel didn't bump my scope.

Any thoughts are appreciated.

Dan  

To view this discussion visit https://groups.google.com/d/msgid/open-phd-guiding/CAJa45i7JDq%2B4FCcAR28OyReQrTyy6_Zw3NaDKheUoevd1buwjA%40mail.gmail.com.

[Brian Valente]

Getting different PA answers is common, and in part because different tools use different approaches.

I tend to rely on actual drift measurements (i.e., PHD results) because it's measuring star movement. Things like Polemaster, Sharpcap, etc. are great, but use platesolving and other proxies to measure PA.

If you have a fixed pier, you might consider doing drift alignment, which is the gold standard for polar alignment. 

To view this discussion visit https://groups.google.com/d/msgid/open-phd-guiding/1381819265.815533.1789658045823%40mail.yahoo.com.

[GAIL DALLOLMO]

Brian, 

Went out to my scope this morning to check things out. Turns out my guide scope was loose (I could actually wiggle it). Holy smokes!!!

Needless to say I tightened everything up. It’s amazing I didn’t have to throw away more images. I’m guessing that when I was tweaking the balance, I minimally moved the guide scope and forgot to clamp it down. Who knows?

Anyway, thanks for your thoughts and help. I will double check PA when conditions allow and let you know if the differential flexure issue is resolved. 

All the best ,

Dan

Sent from my iPhone

On Sep 16, 2026, at 9:32 PM, Brian Valente <bval...@gmail.com> wrote:



Dan

I just realized i gave you feedback from someone else's logs. I apologize for that. You had about a 6 hour run towards the end?

However, the conclusion is basically the same: your total RMS values are actually a bit lower than what I mentioned before, and your RA and Dec RMS are even closer together.

So... (now correct) guidelogs, still same answer

A couple other small points of feedback:

- your polar alignment is about 4.5 arcmin, you could probably improve that a big

- evaluating your guide RMS impact to your imaging system, you can see the error level of about .7" total RMS and RA/Dec nearly equal has a negligible impact on your imaging system. you are subpixel, at least as far as guiding goes (this would be if you didn't have DF)

<image>

To view this discussion visit https://groups.google.com/d/msgid/open-phd-guiding/CAJa45i7JDq%2B4FCcAR28OyReQrTyy6_Zw3NaDKheUoevd1buwjA%40mail.gmail.com.

[Brian Valente]

good catch.

Your guidescope setup is still suspect re: rings and thumbscrews, but tightening up what you already have certainly can improve things.

To view this discussion visit https://groups.google.com/d/msgid/open-phd-guiding/995D4148-60ED-4866-A1E3-A296EFEAC6CE%40aol.com.