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Perfect polar align and still oblong stars.

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#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

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#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 14 June 2019 - 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
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#13 t-ara-fan

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Posted 15 June 2019 - 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? 



#14 Remaxman

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Posted 15 June 2019 - 04:23 PM

It was balanced with the counter weight. 



#15 17.5Dob

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Posted 15 June 2019 - 09:59 PM

It's a Star Adventurer...on a photo tripod...with the camera on a ballhead.....and  a 300mm lens.

There are 20 points of failure ++++++

Most prominent and likely is that you didn't sight on Polaris....

 



#16 17.5Dob

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Posted 15 June 2019 - 10:07 PM

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.

sml_gallery_276970_10724_302730.png

 

Thanks guys

 

That's dang good tracking for 40" at 300mm !!!!

Your lens is absolutely junk though....and I thought my Nikkor 180 f2.8 had bad CA wide open


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#17 Cajundaddy

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Posted 15 June 2019 - 10:21 PM

Try again with a good 100mm prime.  That 300 zoom is riddled with CA (note the blue flare on the left and red flare on the right of each star) and probably to much FL for the star adventurer.


Edited by Cajundaddy, 15 June 2019 - 10:23 PM.


#18 Remaxman

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Posted 15 June 2019 - 10:26 PM

It's a Star Adventurer...on a photo tripod...with the camera on a ballhead.....and  a 300mm lens.

There are 20 points of failure ++++++

Most prominent and likely is that you didn't sight on Polaris....

 

No its on a very heavy duty tripod. No Ball head. And I was dead on Polaris. 



#19 MikeECha

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Posted 16 June 2019 - 03:49 PM

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

 

Check if there is any play/flexure between the camera mount and the lens. I get star shapes like that when I add a 2x TC to my lens. In my case, the weight of the camera plus the 2x TC causes the old Adaptall bayonet to flex about .5mm. The problem goes away when I use my smaller 1.4x TC.



#20 premk19

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Posted 16 June 2019 - 04:18 PM

No its on a very heavy duty tripod. No Ball head. And I was dead on Polaris. 

Dead on Polaris is not good polar alignment.


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#21 moxican

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Posted 17 June 2019 - 10:43 AM

I'd like to highlight that it is really easy to bump that mount out of alignment. For example, you polar align, then release the clutch and put your equipment on the mount, is ample enough to loose polar alignment. 

Not to mention that a lens at that focal length is a bit too much for hat mount. 



#22 Remaxman

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Posted 17 June 2019 - 10:50 AM

Dead on Polaris is not good polar alignment.

 

I meant the offset position of course :)


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#23 Remaxman

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Posted 17 June 2019 - 10:52 AM

I'd like to highlight that it is really easy to bump that mount out of alignment. For example, you polar align, then release the clutch and put your equipment on the mount, is ample enough to loose polar alignment. 

Not to mention that a lens at that focal length is a bit too much for hat mount. 

A tuck could hit this mount and it wouldn’t move. I took some photos last night with the wedge locked down and they were pretty good. 



#24 vidrazor

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Posted 17 June 2019 - 12:15 PM

Can you align to Polaris with your setup fully in place with the Skywatcher? I was under the impression you can't, which is why I got a Skyguider. Aligning to Polaris and then mounting the gear can throw you off.

Make sure the polar scope is properly aligned. The scopes on these lower end models are known to come misaligned from the factory. I lucked out, mine looks like it's near perfect.

If the scope is good, keep practicing alignment. I've been practicing alignment for the past 2 weeks using a 400mm Sigma APO macro on a Olympus E-M10 Mk II micro four thirds camera (800mm equivalent), and so far I've managed 20 and 30 second exposures, which I imagine is as good as I'm ever going to get unguided. All my shorter lenses should be cake in comparison. :)

I'm gonna spend another week practicing setup with this arrangement before finally going out to better skies to actually shoot subs with the Sigma and shorter lenses.

#25 jeff.bottman

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Posted 17 June 2019 - 02:13 PM

Your stars look not that bad to me.  Question is; how does the composition look at full monitor screen size?  If stars look OK, who cares if pixel peeping show slight imperfections?  Or if you are going to make a print, look at the screen at print scale and see how it looks.  I get the sense that many astro-pic fanatics are quite anal (in the Freudian sense) as far as technical perfection.  That ain't the only thing that makes a great image, although blatantly out of focus is unacceptable.




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