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Must star alignment procedures always start with polar alignment?

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#1 mikedent

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Posted 27 February 2021 - 09:24 PM

Hello- I'm sure this is a beginner question that should have an obvious answer, but instruction books are not always clear. With Ioptron cem25p and other such goto mounts, the hand controller offers various star alignment options like one, two, three, etc star alignment menu options, in addition to polar alignment. Is it always necessary to do a polar alignment first before these extra steps are done? Or can those extra options act as a substitute for a polar alignment? ( I really don't see Polaris very well in my location, very dim and it does not really show up in the built-in polarscope.) My manual does not make that clear. I do mostly visual observation so I can slew and align to a visual planet or star, but that does not seem to work well for astrophotos that I have tried. Even with using a Sidereal tracking setting ie for stars, I will get star movement/trails on the photo. Also noticed that with visual observing, slew and align, sidereal tracking, that the star will move away from the center of the eyepiece after a while. So to get best results, is polar alignment required to be done first? Or what am I missing? I do not use a computer for this, just the mount's hand controller and its goto functions. Thanks for advice!



#2 John Tucker

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Posted 27 February 2021 - 09:31 PM

Well, generally yes.  Especially with an equatorial mount, the whole system is set up for one axis of the scope to be aligned with the rotational axis of the earth.  Also, some GoTo programs assume that the mount is properly polar aligned, and won't give you good pointing if that is not the case.


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#3 barbarosa

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Posted 27 February 2021 - 09:36 PM

The previous post nailed it. The CEM (nice mount) is an equatorial mount. An EQ mount works by having one axis, the right ascension (RA) axis pointed toward the celestial pole. Once the mount is polar aligned you do a star alignment to adjust the star map (the position data) in the hand controller, to match your time date and location. Once that is done the mount will track a star without any image rotation (we hope).

 

Some EQ mounts, I do not know if this includes iOptron, can be aligned in alt az mode, though I cannot see any reason to do so.

 

A view toward Polaris makes alignment to the NCP easier but there are other methods such as drift alignment.



#4 ravenhawk82

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Posted 27 February 2021 - 10:29 PM

Yes, the 1/2/3 star alignment should always be done after polar alignment when using an EQ mount. How precise though depends on what you're doing. For visual work, I can usually get away with pointing my mount generally in the direction of polaris and then doing a 3-star alignment. That'll put all my targets somewhere in the eyepiece, and since I scan around a bit as I look I'm not too worried about how dead on it is. Field rotation is a non issue for visual observation.

For a photographic setup though, it needs to be as precise as possible prior to doing your star alignments. Partly because it's harder to find a target in a sensor than in a wide field eyepiece most of the time, but mainly because you'll end up with progressively worse field rotation the farther off you are. The best autoguider setup in the world won't keep your stars looking tack sharp in a long exposure if your polar alignment is no good to begin with. You do your polar alignment before your star alignment because if your mount doesn't have a decent frame of reference it'll have no idea where to point when you feed it a target to slew to.



#5 sg6

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Posted 28 February 2021 - 03:41 AM

Yes, The goto alignment is different to the polar alignment.

If you do the goto alignment I would expect it to fail as too many errors and when you do the polar alignment then that would invalidate the goto alignment. Even if it was successful.

 

If your start position was say 5 degrees out and the goto managed to compensate then it has a compensation determined for a 5 degree error. You then polar align and the error is now say 0.2 degrees but the goto is compensating for initial 5 degrees because that is what it measured and determined.

It is sort of obvious.



#6 Phil Sherman

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Posted 28 February 2021 - 10:35 AM

As stated in previous responses, polar alignment must be done before sky alignment. If you have a short focal length (ie <600mm) scope and limit your image exposure times to less than 10 seconds, then you probably won't see star trails. A visual polar alignment is rarely good enough to support photos with a longer focal length or longer exposure time.

 

Polar misalignment causes star trailing in images. Longer exposures also will show star trails caused by periodic error, a characteristic of any mount that's driven by gears. you can purchase/lease hardware/software tools to improve polar alignment or you can use your camera to perform a photographic drift alignment. You can do a visual drift alignment but that usually requires an illuminated reticle eyepiece.

 

Periodic error is usually handled by using guiding. At a minimum, this requires a separate guide camera and either a guide scope or an off axis guide attachment. If you're guiding and imaging, using a computer to run everything, including the mount generally makes things easier because all of your controls are consolidated to a single device.



#7 kathyastro

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Posted 28 February 2021 - 10:58 AM

Star alignments (1-star, 2-star or 3-star) are for the goto system only.  They do not allow the mount to track the moving sky accurately.  They are only for finding objects.

 

Most goto systems have enough margin of error to accommodate a poor polar aligment.  So the goto alignment will appear to be successful and may initially give good results in finding targets.  But the poor polar alignment remains, and the accuracy of the gotos will decrease during the session as the mount continues to track in a direction the electronics were not expecting.

 

The polar alignment is a mechanical adjustment, and it must be accurate before you can align the goto system.



#8 base16

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Posted 28 February 2021 - 11:53 AM

For visual even if you can’t see Polaris just roughly point towards Polaris is sufficient, it doesn’t need to be spot on. Then use the star alignment to improve pointing (GoTo) accuracy.

If you’re doing imaging, previous comments are correct in that you would do PA to improve GoTo and Tracking accuracy, and follow up with Star Alignment to refine the pointing accuracy.

Edited by base16, 28 February 2021 - 11:54 AM.


#9 kel123

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Posted 28 February 2021 - 01:06 PM

First of all, I think this post should be moved to the beginner astrophotography forum.

I also think that some previous posters don't possibly understand your peculiar situation of not having access to polaris and have not also considered the equipment you are using, hence can get you further confused. I can understand because we are in the same boat of not having access to polaris. And sometimes, I also use mount software assisted polar alignment method like Celestron's All Star Polar Alignment.

However, like they rightly pointed out, you need fairly good polar alignment to get any fairly good astrophotography image. And technically, polar alignment is done first before star alignment.

But there are other factors at play here. It is easier to do a polar alignment first when you can sight polaris but when you cannot, other procedures are necessary.

The reason your mount has 1,2,3 star alignment before polar alignment is because it recognizes that you might not have access to polaris and hence offers to assist in polar alignment after star alignments.

How it goes is that you do a 3-star alignment for better accuracy, then you use your hand control assisted polar alignment procedure to align to the pole with a star different from polaris.

After you have successfully aligned to the pole and your handset confirms success,you then repeat the 3-alignment because your adjustments of the altitude and azimuth knobs have knocked of the previous 3-star alignment.

Most mounts which have polar alignment procedure without polaris , including Celestron's ASPA requires you to do the star alignment before it can assist you in the polar alignment, but you will need to re-do the star alignment after the polar alignment. In fact, ASPA does not offer you the option of polar alignment until you have completed a star alignment.

Hence, since you cannot see polaris, use a compass to point the polar axis of your mount roughly to the north like you already know how to do, do a 3-star alignment, then polar alignment, then re-do the 3-star alignment.

As far as you get your mount's polar alignment procedure correctly, you are good to go. I hope this helps.


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