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All Star Polar Alignment

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#1 Subidai

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Posted 19 January 2021 - 12:13 AM

Hi, When I perform an ASPA on my celestron CGX Mount I find that I can get the star aligned in the centre of the eyepiece but when I slew to an object such as Neptune or even Jupiter / Saturn the item is not in the centre of the eyepiece but off near the edge. Does this have to do with the level of the mount not being level enough? Any other ideas?



#2 rkinnett

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Posted 19 January 2021 - 12:46 AM

Welcome to CN!

 

The workflow with ASPA is:

1) rough align using PA scope

2) turn on mount, do 2 or 3 star alignment

3) run ASPA

4) repeat steps 2 and 3 until no further corrections are needed.

5) finish with 2 or 3 star alignment

 

Leveling doesn't matter in the slightest, at least not to final pointing and tracking performance after you finish the alignment routine because the alignment routine mitigates tilt.  I bought a CGEM secondhand with a broken bubble level and wrote to Celestron who essentially responded along the lines of "who cares, you don't need it".  Leveling your mount does affect the first round of your alignment routine, however;  the further off your mount is from level, the further you'll have to bump over to each alignment star.  Roughly level your mount but don't spend more than a minute or so on it and don't worry about getting the bubble dead center.

 

Even with perfect alignment, you're not guaranteed that go-to will center a target in your eyepiece or camera view.  Go-to precision is affected by many things.. your choice in alignment stars, whether you're working on the same side of the mount as your alignment stars, stiffness of your image train, etc.  If your focal length is very long, e.g. a Barlowed SCT for imaging Neptune or Jupiter or Saturn, you will usually have to hunt for your target.



#3 Subidai

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Posted 19 January 2021 - 12:51 AM

Great, thanks for that advice. I will try it. Is there any way to slightly adjust the mount after your go to has arrived at the object. That is, if the object is off centre then why doesn't the mount allow for slight corrections to centre an object?



#4 Noah4x4

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Posted 19 January 2021 - 03:07 AM

Admittedly, I was using an Alt -Az with wedge, but I found two of the stated steps were highly material to this question of subsequent GoTo accuracy.....

 

1.  The "Rough" Align.

 

The closer you (initially) got to being accurate the better as inevitably the lesser the initial accuracy, the greater the polar alignment error (PAE) to be eliminated during the ASPA step. . "Rough" is a misnomer. I could not see Polaris, so I used a compass to derive that initial N-S accuracy (n.b. don't forget Magnetic North differs from North). 

 

3 Run ASPA (but you must do an EQNorth/two star align before and after).

 

This tip follows on from (1) in many respects. The bigger the PAE left by step 1, the greater the adjustment necessary. The greater the adjustment, the increased likelihood of another error creeping in. This was highly material with wedge as the greater the number of wedge knob turns required the more likely another mechanical error crept in (like tripod legs sinking in wet grass). With a GEM the process is much easier, but the same physical principles apply. If your initial "Rough" Align is better, then it should be all round better. 

 

In summary, a polar scope must be a great help. But if you can't see Polaris due to obstructions, then use a compass. Lastly, if your GoTos remain out, no harm in repeating the process. 



#5 Subidai

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Posted 19 January 2021 - 03:24 AM

Great, thanks again. I am in the southern hemisphere. So no Polaris.



#6 mclewis1

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Posted 19 January 2021 - 09:49 AM

It sounds like we're confusing the initial alignment (solar system, one star, two star, SkyAlign, etc.) with the physical polar alignment utility (referred to as the All star polar alignment or ASPA).

 

Initial alignment determines the pointing (goto) accuracy.

ASPA determines the tracking accuracy of an object pointed to.

 

ASPA requires that a good accurate initial alignment has already been done. ASPA guides you to moving the mount to improve the physical polar alignment. Many folks will re do the initial alignment after performing the ASPA, particularly if the Alt or Az adjustments had to be moved quite a bit.



#7 robbieg147

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Posted 19 January 2021 - 04:16 PM

For the sake of a few minutes I would always level the mount myself, I know it does not matter in the end but it will make things a lot easier when you adjust the PA using the bolts if the mount is on a true plane. 


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#8 skaiser

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Posted 20 January 2021 - 11:23 AM

When doing alignment, you can start off using a low power eyepiece to make it easier.

But for final pass at alignment, you should use a high power eyepiece so you are getting the stars finely centered.

If you don't have a high power eyepiece, defocus the stars so you have a large donut  and center this in the eyepiece. Makes it easier.

Take care


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