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Elongated stars at longer exposure times

Astrophotography Equipment Polar Alignment DSO Mount
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#1 balykos

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Posted 06 December 2021 - 11:40 AM

hello,

 

I am testing different settings with my setup (listed below) and I have noticed that the amount of elongation/trailing increased rapidly once I expose for 600". I would expect to see a gradual increase in elongation as I increase exposure times. 

What is it that causes this sudden change in star shape between 5-6 minutes and 10 minutes exposure time? (I'm thinking differences in polar alignment between imaging sessions, or perhaps the periodic error in the mount factors in)

should I limit my exposures to 300" and play it safe?

 

results:

400mm - 240" - pinpoint stars consistently

100mm - 360" - acceptable results stars are round for the most part

100mm - 600" - elongation/trailing seen with all filters, most severe with OIII

 

equipment:

Mount: EQ6-R Pro - PHD2 guiding

Camera: ASI1600MM

Filters: ZWO -  L,Ha,SII,OIII

Lenses:

- Canon 400mm f/5.6L

- Canon 100mm f/2.8L macro



#2 zakry3323

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Posted 06 December 2021 - 11:55 AM

Do you have any photos you could share to show the issue in detail?

 

Without evidence, my first guess would be that your thoughts about periodic error to be correct. A full turn o' the worm usually takes somewhere between 5 and 7 minutes, so that could be it. 



#3 bobzeq25

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Posted 06 December 2021 - 12:01 PM

hello,

 

I am testing different settings with my setup (listed below) and I have noticed that the amount of elongation/trailing increased rapidly once I expose for 600". I would expect to see a gradual increase in elongation as I increase exposure times. 

What is it that causes this sudden change in star shape between 5-6 minutes and 10 minutes exposure time? (I'm thinking differences in polar alignment between imaging sessions, or perhaps the periodic error in the mount factors in)

should I limit my exposures to 300" and play it safe?

 

results:

400mm - 240" - pinpoint stars consistently

100mm - 360" - acceptable results stars are round for the most part

100mm - 600" - elongation/trailing seen with all filters, most severe with OIII

 

equipment:

Mount: EQ6-R Pro - PHD2 guiding

Camera: ASI1600MM

Filters: ZWO -  L,Ha,SII,OIII

Lenses:

- Canon 400mm f/5.6L

- Canon 100mm f/2.8L macro

The mount has a  worm rotation period of about 5 minutes (300").  Once you've reached that, there's no reason why periodic error would make longer exposures worse.

 

Guidescope mounting may be an issue, differential flexure increases with altitude.


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#4 balykos

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Posted 06 December 2021 - 12:29 PM

Thank you both for the quick replies. I am attaching Ha and OIII stretched subs from the same session. camera was pointed at the Heart & Soul nebulas

 

thanks for clarifying the rotation period of the mount/worm gear... so theoretically I would also hit that correction point for some frames even when my subs are 240", but I rarely see problems with those sessions even when using my 400mm lens

Attached Thumbnails

  • oii-600s.jpg
  • ha-600s.jpg


#5 OldManSky

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Posted 06 December 2021 - 12:40 PM

Do you have your PHD2 guide log from the exposures?  That will tell us a LOT more than looking at images -- it will show whether the error was mostly in DEC or RA, and show why the guiding failed to correct it...



#6 scadvice

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Posted 06 December 2021 - 12:44 PM

You may be guiding on a hot pixel. Did you do a dark file for your guide camera in PHD2? 



#7 bobzeq25

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Posted 06 December 2021 - 12:45 PM

Do you have your PHD2 guide log from the exposures?  That will tell us a LOT more than looking at images -- it will show whether the error was mostly in DEC or RA, and show why the guiding failed to correct it...

I'd also like to see a picture of your setup.  The guidescope may be tracking, but the main optics not so much.



#8 balykos

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Posted 06 December 2021 - 01:02 PM

I'm attaching the PHD2 log from that evening. first hour was Ha, second hour was SII, third hour OIII. There were occasional wind gusts but nothing that I haven't seen in other nights with better results.

 

I don't have a picture of my setup and I will admit it's not the most secure given what I've seen from other members on this forum.

I have a vixen dovetail (Dec axis rotated at 90 degrees) on one side I have the rings from the guide scope and on the other side, I have the holding collar for the ASI1600 - the camera supports the filter wheel which in turn holds the Canon lens via the ZWO adapter.

 

when I switch to the 400mm lens... I screw the lens collar onto the vixen dovetail and the lens supports the EFW and camera... same setup for guide scope

Attached Files



#9 OldManSky

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Posted 06 December 2021 - 02:01 PM

From your log...

Your polar alignment isn't great, at 15' off.  Work on improving that.

Your dithers are HUGE -- you can reduce the magnitude (in pixels) of your dithers.

But the real problem...things were going along pretty darn well, just a few errors (that all got corrected) of 5" or so.

Then just after 21:28...your RA goes off the charts with nearly 500" of error.  PHD2 is madly issuing corrections, but nothing happens until nearly 21:30, when it slowly starts to make its way back.  It gets back to somewhat normal around 21:32.  

Do you have a cable snagging, or something sticking?  PHD2 didn't lose the guide star (amazingly!  that's a huge error), and it keeps trying to correct, but nothing happens correction-wise in the graph for a very long time.  That's why your RA error is 37.16" RMS, with a peak of 503.62".  Your DEC is fine.  Something is holding RA from moving.


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#10 balykos

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Posted 06 December 2021 - 04:57 PM

Thanks for the analysis Paul,

 

How were you able to determine from the logs that I'm 15' off in polar alignment? I'm using PHD Log viewer, is it the DEC graph from the Calibration section?

 

I actually have an explanation for what happened at 21:28... the object crossed the Meridian at that time and I had limits enabled in EQMOD... it took me a couple of minutes to realize that and change the setting so I thought I only lost that sub.

 

I reviewed the subs from the entire session and it looks like the stars started off decent (more like eggs) and then progressively got worse as time went by. I'm starting to wonder if bobzeq25 has a point with flexure being an issue, perhaps the lens being supported by the filter wheel is not the best option.


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#11 bobzeq25

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Posted 06 December 2021 - 05:45 PM

 I'm starting to wonder if bobzeq25 has a point

Many people do.  <grin>
 


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#12 Oort Cloud

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Posted 07 December 2021 - 12:13 PM

The mount has a worm rotation period of about 5 minutes (300"). Once you've reached that, there's no reason why periodic error would make longer exposures worse.

Guidescope mounting may be an issue, differential flexure increases with altitude.


I'm pretty sure it's 8m for the EQ6-r, which is moot because it's still between 5m & 10m. Just throwing it out there.

#13 OldManSky

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Posted 07 December 2021 - 12:32 PM

Thanks for the analysis Paul,

 

How were you able to determine from the logs that I'm 15' off in polar alignment? I'm using PHD Log viewer, is it the DEC graph from the Calibration section?

 

I actually have an explanation for what happened at 21:28... the object crossed the Meridian at that time and I had limits enabled in EQMOD... it took me a couple of minutes to realize that and change the setting so I thought I only lost that sub.

 

I reviewed the subs from the entire session and it looks like the stars started off decent (more like eggs) and then progressively got worse as time went by. I'm starting to wonder if bobzeq25 has a point with flexure being an issue, perhaps the lens being supported by the filter wheel is not the best option.

Click on the "Drift" tab next to the "Statistics" near the bottom of the screen.

It will show you your polar alignment error.  It's usually pretty accurate, but in this case it may have been skewed by the mount limits issue.

 

Oh, and about Bob...yeah, he usually does have a point.  And most of the time it's a darn good one smile.gif

Attached Thumbnails

  • phd2_drifttab.jpg

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