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Confused about how to properly polar allign a CG-5

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#26 orion69

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Posted 06 February 2013 - 04:14 PM



My subs are all 30 min ...


Without guiding?


:D With CGEM?
Guiding of course.

Sorry I deleted my post.

#27 freestar8n

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Posted 06 February 2013 - 04:24 PM

“For best results choose a bright alignment star that is near the Meridian, preferably close to the celestial equator. Try to avoid stars that are close to the west/east horizon or directly overhead because they can be more difficult to center using the mount's altitude and azimuth controls. Also stars too near the celestial pole are less accurate than those further away.”

Is the intersection of the Meridian and the Celestial Equator directly overhead?



Hi-

Now I see the confusion on this issue. The celestial equator only goes overhead at the earth's equator. Anywhere else, and it is tilted above the horizon by 90-latitude. So at 40N it is 50 degrees above the horizon. At 20N it is 70 degrees above the horizon, and you would really lose polar alignment accuracy there, in az. The celestron instructions are fine except for the part about the equator. That will work ok for many people (meridian and equator) but it could cause problems. So I use, and recommend, a star down low and near the meridian.

The fact that the instructions say to avoid a star overhead is a direct indication that they don't really mean to use the equator - since the equator can indeed be way up high for locations where the mount is intended for use - e.g. 20 degrees N.

I am in the middle of a rare, due to weather, imaging session right now - and just did a 2+4/ASPA followed by a 2-star using a star down low and near the meridian.

Frank

#28 cn register 5

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Posted 06 February 2013 - 04:38 PM

It may be better to say where to avoid.
Avoid the Zenith. The azimuth adjustment just rotates the star field and doesn't really move the stars.
Avoid the East and West. The Altitude adjustment just rotates the field and doesn't really move the stars.
Even if you are hight to the east or West the Altitude adjustment just moves the stars horizontally and that's the same direction as the altitude adjustment.
That leaves South or North and not too high. This comes down to what Frank said, as usual he's right.

Chris

#29 Alph

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Posted 06 February 2013 - 05:15 PM

Oh, you forgot to mention to avoid the intersection of the meridian and the celestial equator. When in doubt double check with Patrick Wallace, the author of TPoint, who is unquestionable authority on this subject.

Better yet, check the equations yourself then you will know it from the ultimate authority, spherical geometry. :)

#30 cn register 5

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Posted 06 February 2013 - 05:51 PM

Please stop trying to confuse people.

#31 cn register 5

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Posted 06 February 2013 - 05:52 PM

Please stop trying to confuse people.

#32 Alph

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Posted 06 February 2013 - 06:39 PM

Please stop trying to confuse people.

I hear you.

#33 Alph

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Posted 06 February 2013 - 06:43 PM

Please stop trying to confuse people.

Oh sorry, you really got confused. You double posted.
I recommend that you check first with Patrick Wallace before making insinuations.

#34 cliffy54

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Posted 06 February 2013 - 07:20 PM

You guys sure got me confused, it's like reading one of those Newtonian collimation post. :shrug:

#35 dr.who

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Posted 06 February 2013 - 07:41 PM

The closer you get with that initial rough or what I called "visual" polar alignment (orienting the tripod and mount and sighting up the bore hole) the less movement of the alt and az controls you'll need to do when performing the ASPA. That means less wear and tear on those adjustment bolts and a faster ASPA procedure.


Thank you. That's what I thought.

#36 orion69

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Posted 06 February 2013 - 07:56 PM

You guys sure got me confused, it's like reading one of those Newtonian collimation post. :shrug:


:roflmao:

Just read the manual, you'll be fine.

#37 mpgxsvcd

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Posted 07 February 2013 - 11:33 AM

So I tried this last night. I did my typical polar align with the finder scope. I do this very precisely though. Then I did a 2 star + 4 star align with the 10x crop mode on my camera to make sure that the alignment stars were dead center.

I shot a 4 minute and 16 second sub of the Horse Head. It was blown out even with dark skies and my strongest light pollution filter. However, I thought tracking was acceptable considering I wasn’t guiding and I didn’t do the all-star align or a drift align. This was at an equivalent field of view of a Full Frame camera on a 1600mm scope(2x crop factor + 800mm scope).

I knocked it down to where I wasn’t blowing out the sky at 2 minutes and it still was tracking well. Then I did the all star align with Rigel that was approximately to the south and about 40-50 degrees up.

This time I didn’t have to move the mount much at all. I double checked and it moved Polaris slightly off of the circle in the Polar finder. However, when I tried to image again it actually looked like the Polar alignment was off. Not incredibly so but not as good as it was with just the Polar scope.

I then tried to realign everything again from scratch. I tried to get the Polar alignment as precise as before but it was much harder now that the sun had set and my polar scope is not illuminated.

This time when I tried it my tracking was pretty far off. I was having trouble doing anything more than 1 minute subs.

I guess the moral of the story is if it isn’t broken don’t fix it. If the polar finder gets you the results you need then don’t mess it up trying something else. If you are not getting the results you need then definitely try something else.

Thanks for everyone's help. I learned a lot about what to try when using the finder doesn't work and that happens about half the time.

Single exposure 256 seconds with the full spectrum GF1 - Just Polar Aligned visually with the Polar Finder. No Guiding. It is no replacement for guiding but it works if you can't or won't guide.

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#38 mpgxsvcd

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Posted 07 February 2013 - 11:43 AM

Here is a stack of 8 1 minute subs with no guiding or all star aligning.

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#39 mpgxsvcd

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Posted 11 February 2013 - 01:44 PM

Here is a single 3 minute exposure without any guiding and without using the all star align. This 8" 800mm F4.0 scope was aligned only using the Polar finder visually.

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#40 mpgxsvcd

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Posted 11 February 2013 - 01:49 PM

I did the all-star alignment immediately after I took the first image and then retook the 3 minute image. It has noticeably more trailing after I did the all-star alignment even though I really did not adjust the mount much during the all star align.

Should you wait a certain amount of time before you do the All Star Polar Align? For example does the amount of off-set of the star in the all star aligning process depend on the total amount of time that the mount is tracking?

If your tracking seems good right after you did the visual Polar Align with the Polar Scope should you wait a certain amount of time before doing the All Star align or can it be done accurately right away?

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#41 Raginar

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Posted 11 February 2013 - 01:54 PM

MPG, looks like you've really figured out how to do a polar alignment. I'd recommend not touching your mount ever.

For the rest of us, ASPA with guiding works pretty well.






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