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Perfect polar align and still oblong stars.

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12 replies to this topic

#1 Remaxman

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Posted 12 June 2019 - 10:16 AM

First off I know all about the other options for polar align, not in the budget.

 

Last night I set the Star Adventurer to 12/6 in the reticle. Checked where Polaris is suppose to be and lined it up. Locked everything down. I didnt need to point camera as I already had it set to the area I wanted. Took some test photos and they were off. Nikon d5300 with a 300MM lens 5.6 ap 10/20. And 30 sec. Checked alignment and its perfect. No wind. Very sturdy tripod, one of the best. Took a series of shots at 10 seconds. Still off, even in the centre of the photo. Used Bahintov mask so focus was perfect. 

 

Any ideas?

 

 



#2 Hesiod

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Posted 12 June 2019 - 10:24 AM

Stars shapes are constant between all frames?

If they are, have you checked the collimation of your polarescope?

Last, are sure that the wedge does not move when you lock it, or that does not flex?



#3 GoFish

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Posted 12 June 2019 - 10:37 AM

Like you I would have expected round stars in 10 sec exposures, even though I think 300mm may be a bit long for the SA. The longest lens I’ve used with mine is 185mm and not with 100% success. 

 

Three things to think about:

  • How is your balance? That’s a big camera and lens. 
  • Have you tried a drift align check?  I seem to recall there’s a way to do this with BYN and live view. 
  • Power source?

I’m thinking that a small polar misalignment would not cause oblong stars in 10 seconds. So it makes me suspect RA drive rate error. Whether PE or power issue or unbalanced RA axis or ? I don’t know. 



#4 Traveler

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Posted 12 June 2019 - 10:51 AM

Flexure?



#5 Michael Covington

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Posted 12 June 2019 - 11:02 AM

In what direction are the stars elongated?

I would expect a 300-mm lens with 30-second exposures to show up some periodic error in the Star Adventurer.  At least potentially.  That would be east-west elongation.



#6 Remaxman

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Posted 12 June 2019 - 11:45 AM

Here is a screen grab of the stars. I was shooting straight up. Camera was balanced on the SA. This photo was at 40 seconds iso 400 f5.6 300 mm. I zoomed in for the screen grab. Tripod was level. wedge had slight movement, but after polar align I just snapped the photo via BYN.

 

Thanks guys

 

Screen Shot 2019 06 12 At 12.32.59 PM


#7 Hesiod

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Posted 12 June 2019 - 01:08 PM

Well, a "slight movement" of the wedge could mean that it throws you a couple of degrees off...

Also, before making assumption on the goodness of the polar alignement through a polarscope it should be safer to check if the polarscope is properly collimated and aligned

 

EDIT: by the way, where is the N in your crop? I see also a pattern which could look like atmospheric diffraction: at which altitude was taken the picture?


Edited by Hesiod, 12 June 2019 - 01:10 PM.


#8 Remaxman

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Posted 12 June 2019 - 07:24 PM

Well, a "slight movement" of the wedge could mean that it throws you a couple of degrees off...

Also, before making assumption on the goodness of the polar alignement through a polarscope it should be safer to check if the polarscope is properly collimated and aligned

 

EDIT: by the way, where is the N in your crop? I see also a pattern which could look like atmospheric diffraction: at which altitude was taken the picture?

I checked the collimation and it is spot on. North is at the top of the photo. In Toronto whatever that elevation is.



#9 Remaxman

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Posted 12 June 2019 - 07:27 PM

Well I'm going to say it is the wedge. Both alenkeys were not tight, just one. What ticked me off is that it was the first time out with this tripod. Live and learn tongue2.gif

 

Thanks for all the help!!!!



#10 Alex McConahay

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Posted 13 June 2019 - 05:18 PM

It is virtually impossible to judge "perfect" polar alignment with a polar scope. Even if you think you have it perfect, your eyes, and the precision of the equipment just are not sufficient. 

 

At 300 mm, you are starting to talk about being able to see problems. 

 

Compare your first shot with your last. Do they register perfectly? That is, if you lay one on top of the other, with no adjustment for star positions, are the stars in precisely the same place?

 

If not, you are getting some kind of drift. 

 

It may be field rotation due to mis polar alignment. But it could also be flexure in the equipment, tripod, etc...….And it could be that the tracking motor (speed) is ever so slightly off. And then there is atmospheric refraction. 

 

Alex



#11 Michael Covington

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Posted 13 June 2019 - 05:46 PM

Polar axis misalignment causes drift, usually not in an east-west direction.

 

Severe polar axis misalignment plus corrections causes field rotation, because you can only correct one point in the picture perfectly.  



#12 Jerry Lodriguss

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Posted Yesterday, 08:07 PM

Folks, yall need to get this idea of "perfect" polar alignment out of your heads

There is no such thing because of refraction.

Jerry
  • Michael Covington, tkottary, OldManSky and 1 other like this

#13 t-ara-fan

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Posted Today, 03:02 PM

That is a long lens. Is the camera attached to the tracker, or do you have a lens collar to better balance things? 




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