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Alignment of the polar scope needed?

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#1 Kieken

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Posted 05 March 2011 - 05:25 AM

Last night I've tried to set up my Vixen Sphinx for some imaging but couldn't get subs longer then 20 seconds at 650mm (usually I stick to 30 seconds but I want to take the next step in imaging and capture more light in one exposure). After polar aligning 3 times and balancing everything a couple of times I realized that the polar scope might be a bit of. When centering Polaris and turning the setting circles I could see the inner cross hair make a circle and Polaris move to the right and back to the center again.

I suppose that this is due to a badly aligned polar scope but I want to be sure before making things worse. Anyone any experience with this? Also, if a want to make any correction, I do this by tightening and loosening the three screws on one of the metal circles on the polar scope?

#2 avarakin

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Posted 05 March 2011 - 09:36 AM

My understanding is that the correct method of checking this alignment is by moving scope around RA axis.
BTW, I don't think word collimation is correct in this context, I would say alignment would be a better word.
As far as aligning the polar scope, some mounts have alignment screws, some not. If they do, most likely it would be 3 screws.
How do you align your mount - just by pointing polar scope directly at Polaris, or you are using more precise method like Kokhab method?

Alex

#3 Kieken

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Posted 05 March 2011 - 10:05 AM

You're indeed correct, alignment would be the better word, not collimation.

I usually align my mount with the polar scope. I was trying to get 1-2 minute subs at 650mm (this should be possible I think). Unfortunately this plan failed when I couldn't get passed 20 seconds of exposure time. I was trying to shoot M65 + neighbours.

I find it a bit strange that 3 weeks ago I didn't notice anything. I shot M81 and some other targets in Ursa Major and did not have any problems to get to 30 seconds of exposure time.

#4 John Carruthers

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Posted 05 March 2011 - 10:27 AM

try ;
http://www.awrtech.c....htm#POLARSCOPE

or
http://galacticfool....align-eq-mount/

#5 avarakin

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Posted 05 March 2011 - 10:36 AM

I find it a bit strange that 3 weeks ago I didn't notice anything. I shot M81 and some other targets in Ursa Major and did not have any problems to get to 30 seconds of exposure time.


This explains it. M81 is very close to pole and M65 is closer to equator. Rotation speed of polar regions is lesser than equator regions, so even though your periodic error was the same, it was impacting images in equator areas more. What you have is very similar to my own experience with a generic CG5 mount - 70% good subs at 30" in UMa and 50% of good subs at 30" in equator areas.

Unfortunately the only way to solve this is to do guiding or have shorter FL.

BTW, I dont think pushing from 30" to 60" will make much of difference, assuming that your histogram peak is at 30% with 30". Check my signature, I have a lot of decent pictures taken with 30" subs. Just need to take many of them...

Alex

#6 Kieken

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Posted 05 March 2011 - 12:12 PM

@ John Carruthers: Thanks for the links, they explain some basic adjustment techniques pretty good.

@ Avarakin: That might have had a big influence too. Still, I'm going to check the alignment of the polar scope the next time I can put the telescope out (hopefully this evening :fingerscrossed:).

#7 solshaker

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Posted 05 March 2011 - 03:36 PM

http://andysshotglas...nmentScope.html

heres another great link for aligning the polar scope.

#8 sctchun

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Posted 06 March 2011 - 01:57 AM

Hi Kieken,

I did something similar with my PAS on my SXW. I centered Polaris on the X in the middle then rotated scope to see the mis-alignment. I then made adjustments to the PAS using the provided allen wrench that you use to lock the PAS into your mount. It takes a while to get it done, but results looks good. I've been able to run 2 to 5 minute unguided shots with my F4 8" Newt (800mm FL).

Steven

#9 Kieken

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Posted 07 March 2011 - 08:52 AM

I've used the mount last night and centered on Polaris before imaging. The polar scope is indeed a bit misaligned but I left it like that. I could take subs of 30s without trailing of the Leo Triplet. Still I'll need to align it someday, probably before the next imaging session.

@sctchun: Nice to here that even 5 minutes is possible with this equipment :).

@solshaker: thanks for the link.

#10 vpcirc

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Posted 07 March 2011 - 09:06 AM

I use the Vixen polar scope and it works so dang well after aligning the first star everything appears in a 12mm ep fov. Are you sure its the polar scope? Maybe leveling? Maybe balance? I regularly image for 15 min without any trailing, for 5-6 hours at a time. I had a very difficult time understanding that polar scope at first, but once I understood how it worked, it's like magic. I've never adjusted or even touched it. No more spending an hour drift aligning. I would also say your setup should be balanced before you ever start messing with alignment. If you're properly balanced (do it with your imaging setup installed) you shouldn't need to touch it again. I have my Sphinx SXD mounted on a pier, and I only have to confirm the polar alignment every few weeks.

#11 Kieken

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Posted 07 March 2011 - 11:30 AM

@ vpcirc: The mount stands on a transportable pier (Skywatcher)and I take great care in getting it as good as my bubble level allows me. Balance shouldn't be a problem either as I balance it with all the cables and accessories attached. then again, how sure can one be if you encounter a problem :question:?

#12 Kieken

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Posted 08 March 2011 - 03:34 PM

I've aligned it last night as good as I could get it that night. For testing I've made an 2 minute exposure of M45 and there's still some trailing visible. So when I get back home I'll probably spend some time in getting it perfect.

Thanks all for the help.






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