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Confused about how to properly polar allign a CG-5

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#1 mpgxsvcd

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Posted 05 February 2013 - 05:00 PM

I am confused about how to properly polar align a CG-5 with the polar finder. I understand that you need to line up the Big Dipper or Cassiopeia in the Northern Hemisphere with where it actually is.

However, I ran into the guide below. I always wondered what the Declination setting circle was for. Is it necessary to go through the procedures below to polar align with the finder scope on a CG-5? Does it matter where those setting circles are for polar alignment? I always thought that if you lined up the big dipper with where it appears to be then you would be fine. From the instructions below it sounds like the Declination setting circle also needs to be set.

It is weird because nothing in the CG-5 manual says anything about adjusting the declination setting circle.


http://www.nightskyi...r_alignment.htm

Also if I do an all star align. In what part of the sky should I choose a star from to get the best results?

#2 rmollise

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Posted 05 February 2013 - 05:19 PM

Why not just use AllStar? Easier, faster.

The declination circle doesn't enter into it.

If you want to use the polar scope, do this:

1. Move the mount in RA until the marker/circle on the polar scope reticle where Polaris goes is on the bottom.

2. Set the RA circle to 0.

3. Look up the current Polaris Hour Angle ("LHA"). Many computer programs will tell you. Cartes du Ciel will when you click on Polaris.

4. Rotate the mount in RA until the RA circle reads this "time." Upper scale for N. Hemisphere. Lock RA.

5. Use altitude and azimuth adjusters to put Polaris is the little circle on the reticle.

You are done. Remember, this is only necessary for imaging. For visual use, centering Polaris in the polar scope, or just the hollow bore without the scope is sufficient. ;)

#3 mclewis1

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Posted 05 February 2013 - 06:20 PM

Also if I do an all star align. In what part of the sky should I choose a star from to get the best results?

Travis,

I use a named star close to the celestial equator and high up in the east or west (just not really close to the meridian ... I obviously don't want to have to flip during the alignment ... and my CGE and CG-5 don't track very far past the meridian).

I get very good polar alignments from a couple of ASPA's and I find that after a little practice they only take a couple of minutes to perform.

I set the mount up and do a "visual" polar alignment/position, then I do a good 2 + 4 initial alignment (usually using the cross hairs in my Mallincam), then an ASPA, and then I might replace/re do one or both of the alignment stars (this seems to depend on how much physical adjustment was required to do the polar alignment).

#4 dr.who

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Posted 05 February 2013 - 08:27 PM

Sorry for the hijack but... Having similar problems with ASPA. Don't I want to rough polar align the mount visually per what Uncle Rod says then do an ASPA?

#5 mclewis1

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Posted 05 February 2013 - 10:36 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.

#6 cn register 5

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Posted 06 February 2013 - 03:57 AM

You will be better with a star that's reasonably close to the meridian because the Alt and Azm adjustments will be at right angles. If you use a star to the East or West you will find that the Azm and Alt adjustments move the star in almost the same direction.

Getting reasonably close to start with seems to me to be less important, I've been so far out that there wasn't enough adjustment from the adjustment so have had to move the tripod to get close and still got a good polar alignment.

I've never redone the ASPA because my understanding is that the ASPA only depend on the current alignment. It's my opinion that all that happens with multiple alignments is that your alignment inaccuracies cause you to make random adjustments but with no overall increase in accuracy.

Chris

#7 mpgxsvcd

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Posted 06 February 2013 - 06:36 AM

Why not just use AllStar? Easier, faster.


So far I haven't been able to get the Allstar align to work. Every time I have done it I have had to make massive adjustments in the Polar alignment to get the star centered even though I had done a very precise polar alignment with the finder scope.

It seemed like I was doing fairly well with the Polar finder alone and it would always be off after I did the all star so I just gave up on it.

I think I was using a star too low in the sky and not close enough to the celestial equator. It just seemed like neither of the polar adjustments would move the star in the direction I wanted it to go.

#8 orion69

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Posted 06 February 2013 - 07:08 AM

Before ASPA, polar align mount best you can. Then do 2 star alignment + 4 calibration stars. After that, polar align mount using star close to the meridian and equator, on the west side of the meridian (that is my procedure, never had any problems).

ASPA is really not than complicated, and it's certainly good enough for 30 min subs @ 900mm...

#9 freestar8n

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Posted 06 February 2013 - 09:11 AM

Hi-

I know that the manual recommends a star near the equator - but I think people will have more accuracy with a star down low and on the meridian - but not too low - maybe 20 degrees above the horizon. The reason is that the polar alignment adjustments are in alt/az directions and have no concept of the equator. This becomes more important the closer you get to the Earth's equator.

Imagine doing ASPA on the Earth's equator - and polar aligning with a star on the celestial equator. It would be at the zenith - and the azimuth adjustments of the eq. mount would not have any effect. It would fail badly.

Although that is an extreme case of a user on the equator - the problem is always there and gets worse as you approach the Earth's equator.

For people in the extreme north or south latitudes the celestial equator should work ok because it is lower in the sky - but there is nothing magical about it. ASPA relies on a star that is sensitive to polar tilt adjustments of the mount - and azimuth adjustments of the mount - and the part of the sky where those adjustments are most precise is down low toward the horizon, for azimuth sensitivity, and near the meridian - for polar tilt sensitivity.

If a user has trouble with ASPA - I recommend trying a star down low, and near the meridian. Make sure the all-sky accuracy is pretty good with a fresh 2+4 alignment - and follow the directions. When making adjustments to center the star by adjusting the mount - don't iterate a bunch of times. Just make a motion in azimuth, then a motion in polar tilt - and lock it down even if it isn't perfect.

Frank

#10 bunyon

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Posted 06 February 2013 - 09:27 AM

Why not just use AllStar? Easier, faster.


So far I haven't been able to get the Allstar align to work. Every time I have done it I have had to make massive adjustments in the Polar alignment to get the star centered even though I had done a very precise polar alignment with the finder scope.

It seemed like I was doing fairly well with the Polar finder alone and it would always be off after I did the all star so I just gave up on it.


This describes my experience almost exactly. Using the polar scope alone, I can do 3-4 minute subs at 1200mm. With ASPA I doubt I could have done 2 minutes at 50mm.

As to where the stars are, if you are on the equator a star on the celestial equator could be at the zenith, or anywhere on the east/west line running through the zenith. The rationale, I assume, behind using a star near the celestial equator is that it is there that the effects of mis-polar alignment are greatest. That is the reason you do drift alignment with stars on the celestial equator, at any rate.

And that may have been my problem with ASPA as well. I can't recall what stars I was choosing but I think I was using stars that showed up in the menu, which only included bright stars, not any on the equator.

While we all have our methods, I'm still not clear how a 2+4 alignment, then an ASPA is faster than sighting Polaris through a polar scope and offsetting for the pole. Takes less than a minute. I suppose my bias here is that I rarely used the GOTO function so am not doing any alignment at all.

#11 freestar8n

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Posted 06 February 2013 - 09:43 AM

The rationale, I assume, behind using a star near the celestial equator is that it is there that the effects of mis-polar alignment are greatest. That is the reason you do drift alignment with stars on the celestial equator, at any rate.



I think that isn't the rationale for using the equator with ASPA, but it is the cause for confusion and why it ends up being recommended - even in the manual.

With ASPA it is all based on goto accuracy and how the alt-az adjustments of an eq mount during polar alignment are projected on the sky, rather than drift rates.

As for the time it takes to do a 2+4 - you would do that anyway if you want GoTo accuracy - and even if you used a polar scope to align. The only real difference in time is the actual time it takes to do the procedure - and that is probably about the same for aspa and a polar scope. And with aspa you don't have to bend down and squint through a little scope.

But - there's no arguing personal preference. ASPA should work, though (as long as your goto accuracy is good after 2+4) - and it should be pretty fast. When you are done, you should be ready to go, but if your goto is a little off you can do another 2-star align with the same stars you used before.

Frank

#12 DaveJ

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Posted 06 February 2013 - 09:45 AM

While we all have our methods, I'm still not clear how a 2+4 alignment, then an ASPA is faster than sighting Polaris through a polar scope and offsetting for the pole. Takes less than a minute. I suppose my bias here is that I rarely used the GOTO function so am not doing any alignment at all.


The situation is that very very few polar alignment scopes are aligned properly, and even if they are, are not oriented to the sky properly. As I responded to another of your posts, ASPA REQUIRES a 2+4 alignment FIRST to build an accurate model of the sky - including corrections for cone error. If that isn't done, the ASPA will not work because it can't hope to function as designed. "Garbage in - garbage out" as we used to say in the software development environment I used to occupy. If you don't want to do the alignments, then, by all means, DON'T USE THE ASPA since it will not work properly. Those of us that do the 2+4 alignments followed by the ASPA are universally thrilled by the process since it results in a much closer-to-perfect polar alignment than the polar scope can.

#13 bunyon

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Posted 06 February 2013 - 09:52 AM

While we all have our methods, I'm still not clear how a 2+4 alignment, then an ASPA is faster than sighting Polaris through a polar scope and offsetting for the pole. Takes less than a minute. I suppose my bias here is that I rarely used the GOTO function so am not doing any alignment at all.


The situation is that very very few polar alignment scopes are aligned properly, and even if they are, are not oriented to the sky properly. As I responded to another of your posts, ASPA REQUIRES a 2+4 alignment FIRST to build an accurate model of the sky - including corrections for cone error. If that isn't done, the ASPA will not work because it can't hope to function as designed. "Garbage in - garbage out" as we used to say in the software development environment I used to occupy. If you don't want to do the alignments, then, by all means, DON'T USE THE ASPA since it will not work properly. Those of us that do the 2+4 alignments followed by the ASPA are universally thrilled by the process since it results in a much closer-to-perfect polar alignment than the polar scope can.



Of course ASPA requires 2 star (or 2+4 - can't find agreement on that) first. When I've tried ASPA, I did the initial alignment. GOTOS were great. ASPA made polar alignment worse in my hands. As I've said, that is probably on me. I don't doubt the hordes of you folks who love it. It clearly works for you and I will endeavor to try it again.

My polar scope is well aligned and there is nothing to "orient". Figure out where Polaris is relative to the pole, center Polaris, adjust alt/az such that Polaris on the little circle where it needs to be. Bang, done.

My argument hasn't been about accuracy but about speed. What I just described is much faster than 2+4, then All-Star. Not that 2+4, All-Star is slow. And, in my hands (and apparently others), it is at least as accurate. One can also drift align. There are many ways to skin the cat and I'm happy if you want to use ASPA.

#14 orion69

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Posted 06 February 2013 - 10:23 AM

Hi-

I know that the manual recommends a star near the equator - but I think people will have more accuracy with a star down low and on the meridian - but not too low - maybe 20 degrees above the horizon. The reason is that the polar alignment adjustments are in alt/az directions and have no concept of the equator. This becomes more important the closer you get to the Earth's equator.

Imagine doing ASPA on the Earth's equator - and polar aligning with a star on the celestial equator. It would be at the zenith - and the azimuth adjustments of the eq. mount would not have any effect. It would fail badly.

Although that is an extreme case of a user on the equator - the problem is always there and gets worse as you approach the Earth's equator.

For people in the extreme north or south latitudes the celestial equator should work ok because it is lower in the sky - but there is nothing magical about it. ASPA relies on a star that is sensitive to polar tilt adjustments of the mount - and azimuth adjustments of the mount - and the part of the sky where those adjustments are most precise is down low toward the horizon, for azimuth sensitivity, and near the meridian - for polar tilt sensitivity.

If a user has trouble with ASPA - I recommend trying a star down low, and near the meridian. Make sure the all-sky accuracy is pretty good with a fresh 2+4 alignment - and follow the directions. When making adjustments to center the star by adjusting the mount - don't iterate a bunch of times. Just make a motion in azimuth, then a motion in polar tilt - and lock it down even if it isn't perfect.

Frank


But CGEM, CGE Pro or other Celestron mounts don't work at Earth's equator... ?

Latitude range: 10° to 70° Northern or Southern Hemispheres

#15 freestar8n

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Posted 06 February 2013 - 10:37 AM

They'll work fine down there if you just lean 'em back a little.

But even at 10 degrees north - using a star on the equator - and on the meridian - would almost be straight up - and you'd be hurting trying to center a star that was off in azimuth. That means the offset of the star would be very insensitive to big errors in azimuth - no different from if you tried to set the polar tilt using a star on the equator - and directly east or west - which is also a desirable thing in drift alignment - but not here.

Anyway - the use of a star high up may be the cause of poor ASPA performance for some people, so I recommend trying one down lower.

Frank

#16 Alph

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Posted 06 February 2013 - 12:44 PM

The rationale, I assume, behind using a star near the celestial equator is that it is there that the effects of mis-polar alignment are greatest.


Ironically the effects of polar misalignment on azimuth error/offset are the smallest at the equator on the meridian. The sweet spot on the meridian is declination = (latitude - 90)/2. It will not hurt if you go lower. A simple rule is to pick a star on the meridian that is as far from the equator toward the horizon as practical/possible.

#17 bunyon

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Posted 06 February 2013 - 12:51 PM

Or use a star near the horizon on the equator.

#18 DaveJ

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Posted 06 February 2013 - 01:27 PM

Or use a star near the horizon on the equator.


It's got to be on the meridian to minimize both azimuth and altitude errors. Near the horizon is the worst since it would force the intersection of the horizon/celestial equator to be near due east or west. The intersection of the meridian and celestial equator is best, by far, to maximize accuracy in both altitude and azimuth.

#19 Alph

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Posted 06 February 2013 - 01:34 PM

The intersection of the meridian and celestial equator is best, by far, to maximize accuracy in both altitude and azimuth.


No. The intersection of the meridian and celestial equator is the worst possible location for adjusting azimuth.

I guess, I should explain why. If you look at the polar misalignment equation in the equatorial coordinates, then you will notice that the contribution of azimuth error to pointing error is 0 (zero) when the declination and the hour angle are both 0.
The sweet spot is at declination = (latitude - 90)/2 and hour angle = 0 because azimuth error = hour angle error at that spot.

#20 freestar8n

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Posted 06 February 2013 - 01:46 PM

The intersection of the meridian and celestial equator is best, by far, to maximize accuracy in both altitude and azimuth.



You could use a land reference to polar align also. Just polar align well, then aim at some fixed object and turn off the mount. When you set it up again, just orient it so the mount is pointing at that object and adjust in alt/az so it is centered.

Since you are moving the mount in alt/az to do this, the best object would be one on the horizon and meridian. Nothing to do with the equator at all.

The same applies to a star. The mount knows where the star should be at all times - and the speed of the star across the sky isn't relevant since its position is always known accurately.

The sensitivity of the az motion just goes as the cosine of the altitude above the horizon. You have no motion at all at the zenith, and maximum motion - and sensitivity - near the horizon.

To avoid refraction and flexure - go up a bit - maybe 20 degrees.

I use ASPA for imaging at around 40N and higher, and I always use a star down low. With cge and cge-pro. Example images are on the MetaGuide site - typically 15m guided with no field rotation.

Again- in many cases the equator/meridian would work ok. But when people say they have poor accuracy with ASPA, I recommend 2+4 and a star lower down - at least to try. No matter what you do want a star near the meridian - for the same reason you want a star near the horizon.

Frank

#21 Cliff Hipsher

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Posted 06 February 2013 - 02:26 PM

Its real easy.

First you have to align the reticule diagram to line up with the counter weight bar. With the mount in the home or starting position (counter weight bar over the NORTH tripod leg) rotate the polar scope so the 'tail' coming off the center cross is aligned with the counter weight bar.

To do a mechanical polar alignment you get the mount level, rotate the mount in DEC to open the view port, and then you rotate the mount in RA so you can place Polaris and Kocab along the upper edge of the counter weight bar.

Look through the polar scope and use the latitude and azimuth adjustments to place Polaris in the circle.

Return the mount to the home or starting position, turn the power on and do a computer aided alignment.

#22 Alph

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Posted 06 February 2013 - 02:53 PM

Or use a star near the horizon on the equator.

That would be really bad for elevation adjustment.

#23 mpgxsvcd

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Posted 06 February 2013 - 03:24 PM

The intersection of the meridian and celestial equator is best, by far, to maximize accuracy in both altitude and azimuth.



You could use a land reference to polar align also. Just polar align well, then aim at some fixed object and turn off the mount. When you set it up again, just orient it so the mount is pointing at that object and adjust in alt/az so it is centered.

Since you are moving the mount in alt/az to do this, the best object would be one on the horizon and meridian. Nothing to do with the equator at all.

The same applies to a star. The mount knows where the star should be at all times - and the speed of the star across the sky isn't relevant since its position is always known accurately.

The sensitivity of the az motion just goes as the cosine of the altitude above the horizon. You have no motion at all at the zenith, and maximum motion - and sensitivity - near the horizon.

To avoid refraction and flexure - go up a bit - maybe 20 degrees.

I use ASPA for imaging at around 40N and higher, and I always use a star down low. With cge and cge-pro. Example images are on the MetaGuide site - typically 15m guided with no field rotation.

Again- in many cases the equator/meridian would work ok. But when people say they have poor accuracy with ASPA, I recommend 2+4 and a star lower down - at least to try. No matter what you do want a star near the meridian - for the same reason you want a star near the horizon.

Frank



I am confused. You say “No matter what you do want a star near the meridian”. Then you say “I always use a star down low”. Doesn’t that imply that you always use a star near the equator and meridian intersection?

It looks like Celstron’s instructions specifically say not to use a star near the Poles. Is that not the case?

“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?

#24 bunyon

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Posted 06 February 2013 - 03:30 PM

<i>Is the intersection of the Meridian and the Celestial Equator directly overhead? </i>

At the equator, yes. At 40 N, it is due south, 50 degrees above the horizon.

#25 mpgxsvcd

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Posted 06 February 2013 - 04:11 PM


My subs are all 30 min ...


Without guiding? At what focal length? 2 minutes would be plenty for me.

I am going to try using the Meridian and Celestial Equator in a few minutes. I have never tried that before. Because of my location at my house I was always forced to use a star near the east horizon or straight up. That was probably my issue.






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