Particularly with deep-sky astrophotography, to be successful you need to have a good grasp on concepts like Polar Alignment and tracking.
When I started with astrophotography, I was at a loss with Polar Alignment: I read articles and asked for info on groups and forums, but ultimately I realized the problem was that too much was taken for granted.
As a result, I was doing things I didn’t understand, and I ended up mixing steps from different methods that were incompatible.
I hoped in vain for something better, to make me understand the task I was supposed to master.
If you feel the same I felt, overwhelmed and unsure about what to do, this comprehensive guide to polar alignment is for you.
In here, you will not find a list of steps to follow blindly, but an explanation of the principles acting behind PA, a guide to master the PA and which accessories will make your life easier.
Polar Aligning And Tracking: Are Those The Same Thing?
No, they are not.
But while Polar Alignment and Tracking are not the same thing, they do work together to produce the best possible images when photographing the night sky.
First, let me remind you that, in the grand scheme of things, everything moves, and nothing stands still.
At this very moment, we on Earth are revolving around the Sun, which is revolving around the galactic center of our Milky Way Galaxy, which is moving with respect to other galaxies, and so on and so forth.
But Earth also rotates on itself once a day. This rotation causes the apparent motion of the celestial bodies (the stars, the planets, the Moon, the Sun, and all the deep sky objects) in the night sky.
Since Earth rotates once every 24 hours, stars look like they move across the sky at a speed of about 15º / hours (i.e., 360º/24hrs).
Tracking is the ability of a mount to move the payload (the telescope and the camera) in sync with the stars, so that your target does not move in the field of view of your instrument.
There are two kinds of tracking mounts: alt/az and equatorial mounts.
Supposing you have it leveled to the ground, Alt/az mounts track the stars with movements in altitude (up and down) and in Azimuth (left and right).
This is good enough tracking for observations and for short exposure astrophotography, like those you can do for the Moon, the Sun, and the Planets. Plus a few other bright objects such as some star clusters.
But deep sky astrophotography aims to photograph much fainter targets, such as nebulae and distant galaxies. In this case, you need long exposures and to photograph the same portion of the sky for hours.
Because of Earth rotation, everything up there revolves in circles around the so-called celestial pole: classic star trails illustrate this fact very nicely.
And because of this, Alt/az mounts can’t be used for deep-sky astrophotography.
The problem is that these mounts will keep the target centered in the frame, but in time, the target rotates and changes its orientation with respect to the frame.
When you hear Alt/az mounts cannot compensate and correct for this field rotation, this is what that means.
Here is where you turn to equatorial mounts. As opposed to the up/down and left/right tracking of the alt/az mounts, the equatorial mount rotates the payload around an axis of rotation.
If the axis of rotation of the mount is aligned with that of the Earth, the mount is able to track the stars across their journey through the night sky while also nulling the field rotation.
High-end equatorial mounts do also track in declination, while star trackers track only in right ascension (more later on the meaning of declination and right ascension).
What Is Polar Alignment?
Polar alignment (PA) is the process of aligning an equatorial mount to the celestial pole, i.e., with the Earth axis of rotation.
This is the most basic and crucial step you must master before moving further into astrophotography.
Why Is Polar Alignment important?
As mentioned before, PA is crucial in deep-sky astrophotography.
A good PA allows you to do the required long exposures without having trailing stars and will enable you to photograph the same region of the sky for hours, without significant drifting (or rotation) of the target.
How Accurate Does Polar Alignment Need To Be?
The rule of thumb is that PA must be as accurate as you can get it. The fact in some cases, you can get away with a less precise PA should not be a reason for you to get sloppy on this.
Having said that, the main factor that determines how accurate the PA must be is the optical resolution of your setup.
This is dependent on both the focal length of the telescope or of the lens you use and the pixel size of your camera sensor.
You can see the optical resolution as the size of the smallest detail your equipment can see:the higher the optical resolution, the smallest the details you can see.
If your optical resolution is sufficiently low, as with a wide-angle lens, the star and its trailing can be recorded by a single pixel, thus short trailing will be “invisible.”
By increasing the focal length, the optical resolution also increases, and now the same trailing can be recorded by two or three pixels, becoming noticeable.
This is at the core of the 500 and NPF empirical rules.
With these rules, you can have a guesstimation for the longest exposure you can make before stars begin trailing noticeably.
As a rule of thumb, the shorter the focal length (and the larger the pixel size), the more tolerant you can be on the quality of the PA.
If you are curious and want to calculate the optical resolution for your setup, this is a nice calculator, providing also some more info.
As a final side note, if you like to print your images, you should know that prints are more forgiving than screens: the resolution is lower, and you usually look at prints from farther away.
I Have A Crop Sensor Camera: Do I Need A More Precise PA?
The Crop Factor affects the field of view, but not the optical resolution. If you were to image with a full-frame camera, providing the optical resolution stays the same, you will not have smaller trails than those you get with your crop sensor camera.
With a full-frame, you have just to zoom in more into the image to see them. With a crop factor camera, trailing stars are just more readily visible as you are “zooming in” into the scene.
For this reason only, you may want to have a better alignment when working with micro four-thirds and APS-C sensor cameras.
If you are interested in knowing how the different sensor types affect your astrophotography, you can read our recent guide on this.
Is Bad Polar Alignment The Only Reason For Having Trailing Stars And Tracking Issues?
While PA plays a significant role in avoiding trailing stars and getting pinpoint sharp images, there are other factors to consider that will degrade your image quality and introduce star trailing.
These are the periodic error of the mount, the stability of the tripod and wedge, and the balancing of the payload on the mount itself.
Here a quick explanation:
- The periodic error of the mount consists of changes in the tracking speed due to small mechanical tolerances in the gears inside the mount. There is not much you can do about it, except correcting obvious plays in the mount.
- A beefy tripod dumps vibrations faster than a weak one, thus it is a crucial piece of equipment for successful astrophotography. Same goes for the equatorial base of the mount (also called wedge): this helps to achieve a precise and reliable PA. Don’t go cheap on them.
- Balancing the weight of your equipment allows for your mount to work in the optimal conditions and to track the sky in a more reliable way. It is often suggested to balance the payload so that it is slightly east-heavy.
Is Polar Alignment The Same For Northern And Southern Hemisphere?
When you do PA, you are pointing your mount at the celestial pole.
If you are in the Northern Hemisphere, you have to align to the Northern Celestial Pole (NCP); if you are in the Southern Hemisphere, you have to align to the Southern Celestial Pole (SCP).
We have always used stars to help us identify the cardinal points (N, E, S, W) and the celestial pole.
In the Northern hemisphere, the NCP is located about a full Moon diameter (or 30 arcmins) from Polaris, the North Star.
Because of the precession of Earth Axis of Rotation, the celestial poles change in time. In the past, about 12000 BC, the North Star was Vega, and it will be the North Star once again in the year 13727.
But even within the human lifespan, Earth’s precession effects in a meaningful way the relative position of Polaris with respect to the NCP, as we will see later on in this article.
NCP And Polaris – The North Star
Polaris, the bright North Star, has been used since ancient times as a navigation beacon, helping sailors and wanderers travel North and orient themselves at night.
And today, it also helps astrophotographers like you and me to align their mount to the NCP.
Polaris is one of the brightest stars in the Northern sky, located in the relatively faint Ursa Minor constellation.
Luckily, you can easily locate it by extending 5 times the distance between Merak and Dubhe in the famous, and readily visible, Ursa Major constellations (also known as the Big Dipper).
SCP And Sigma-Octans
People living in the Southern Hemisphere are not as lucky as those in the Northern Hemisphere, as they do not have a bright star to indicate where the SCP is.
Instead, they have to look for a relatively faint star Sigma-Octans in the faint Octans constellation.
Can You Polar Align Without Polaris (or Sigma-Octans)?
It seems like to PA your mount, you must be able to see either Polaris or Sigma-Octans, depending on the hemisphere you are in.
This would be the ideal situation, but it is relatively easy to PA your equatorial mount even if you cannot see the celestial pole.
The Latitude & Magnetic Declination Method: For Nighttime As Well As Daylight Polar Alignment
This is a coarse PA you can do if you are imaging bright targets with short exposures or if you are doing starry landscapes and star fields with wide-angle lenses.
This method requires you to know the latitude and magnetic declination (the offset between the magnetic North Pole and the celestial pole) for your location.
Let’s do an example. Remember to level your mount to the ground as best as you can and use the wedge to inclinate the mount upwards and to move it left and right.
I live in Brussels, Belgium, and my latitude is 50º49′ N. All I need to do is to inclinate the mount of 50º49′ from the ground.
Next, with an app such as Polar Scope Align Pro (iOS) or Polar Aligner Pro (Android), I can read the magnetic declination for my location. For Brussels, this is 1.6º E.
Next, I use a compass (or my phone) to point the mount in the proper direction: for me 1.6º E from the magnetic North Pole.
The reason why this gives a coarse alignment is the reduced precision of the scale and the fact that the metal of the mount can affect the accuracy of the compass.
Modern phones are often shielded by interferences from metal and electronics, thus being more precise than a classic compass.
Polar Scope Align Pro offers a neat Daylight Polar Alignment tool you can use to align the mount in the day time or at night if you cannot see the celestial pole.
But why would you need to polar align in daylight? One reason would be you want to image the Sun or the early evening Moon with your long focal telescope.
The Drift Method
There are many different kinds of Drift Methods, such as the DARV (Drift Alignment by Robert Vice) and Bigourdan methods.
All drift methods rely on nulling the drifting of stars in the frame observed during long exposures tests shot. This is done by adjusting the mount orientation using the alt/az fine controls of the wedge.
The whole process is iterated multiple times until no drift is observed anymore.
Below is a good video explaining the DARV method more in detail.
As a personal note, I found it difficult to apply this method on a star tracker, but if you have a full-grown computerized equatorial mount, then you should be fine.
While very accurate in theory, in practice, the accuracy of the method can be reduced by large periodic errors of the mount, as you cannot know how much of the drift you see is due to bad PA or to periodic error.
Yet, you can use this method for long exposure deep-sky astrophotography with fairly long telephoto lenses and scopes.
Stars Hopping Polar Alignment
This is a method I came up with, and it is based on the ability to measure differences in declination and right ascension between stars.
Declination (DEC) and Right Ascension (RA) are universal coordinates for stars, referred to as the celestial equator and the celestial pole.
The idea is the following:
- Frame a star with your camera LCD screen. Use the composition guidelines and place the star at the crossing of two lines, to have a precise reference for its position on screen. Note its DEC (DEC1) and RA (RA1) reading them from software such as Stellarium or Sky Guide.
- Pick a second bright star you can see and note down it’s DEC (DEC2) and RA (RA2).
- Now, hop from the first to the second star by rotating your camera in right ascension of an amount equal to RA1 – RA2 and move in declination of an amount equal to DEC1-DEC2.
- If your mount is properly aligned, the second star will now be showing on the LCD exactly where the initial star was at the crossing of the guiding lines. If it is not the case, use the al/az fine-tuning controls on the wedge to manually reorient the mount until the second star is in the right place on the LCD screen.
- Eventually, you can hop back to the first star and check if it is in the same position on the LCD screen as before hopping to the second star.
If you chose stars that are rather close, you can use the motor of the mount to hop, thus reducing the risk of bumping the mount and improving the precision of the method.
The scheme below illustrates the idea.
The accuracy of the method also strongly depends on the accuracy of the DEC and RA scales and on how well you can eyeball the stars positions on the LCD screen.
Polar Alignment With Polaris (Or Sigma-Octans) In Sight
Having the celestial pole in sight allows you to get the best possible polar alignment. Here are some of the most common methods used.
Electronically Assisted Polar Alignment
If you are living in the Southern Hemisphere, this is by far the most simple and precise way to polar align.
And even if you are in the Northern Hemisphere and can use the bright Polaris to PA, this method is the one that will ensure you the best precision.
Confirmed astrophotographers with pro-graded equipment, all do electronically assisted polar alignment, but it requires some extra pieces of equipment and a computer.
In short, a camera such as the Polemaster (or a similar device) or a planetary camera, is mounted on the mount in such a way that the camera is pointing in the same direction of the mount.
The computer software will analyze the images from the camera and guide you to manipulate the mount so as to get it perfectly aligned.
If you don’t want to use a computer in the field, the ZWO ASIAir has a function to help you polar align your mount using your guiding camera.
We have already mentioned some software for electronically assisted astrophotography in this guide, some of which, like PHD2, have some polar alignment functions.
SharpCap is another great software for lunar and planetary imaging. The basic version is free, but the PRO version (10GBP/year) has many more features, one of which is an assisted polar alignment function. Easy and quick.
Note that some of those software does not even require to have the celestial pole visible in the frame.
Some GoTo mounts can also use a star alignment procedure to polar align your mount if you cannot see Polaris or Sigma-Octans.
Manual Polar Alignment
Manually polar align the mount essentially requires to frame the star of reference (Polaris or Sigma-Octans) with a polar scope finder connected to the mount.
And this is the most common type of polar alignment, particularly if you use a star tracker.
From now on, I will refer to aligning to the NCP using Polaris, but the same is true if you live in the Southern Hemisphere. In this case, just use Sigma-Octans instead of Polaris to align to the SCP.
Polar Scope Finders
The polar scope finder can be a simple plastic tube, like that of the Omegon Minitrack LX2, or a more sophisticated optical scope with an alignment reticle inside it.
This can be built-in in the mount, in line with the axis of rotation of the mount, like in the Skywatcher Star Adventurer PRO star tracker or placed aside the mount, like in the Omegon Minitrack LX2/LX3 or the Fornax Lighttrack ii.
For all practical purposes, the location of the polar scope finder does not matter, as long as it is parallel with the axis of rotation of the mount.
With the simple polar finder tube, you look inside the tube and aim to place Polaris at the center of the field of view.
This is rather a coarse polar alignment, only suitable for starry landscapes and star fields with short and medium focal lenses (say < 100mm on full-frame).
For better alignment, you have to turn to optical polar scope finders.
These polar scopes offer you a certain amount of magnification and a built-in alignment reticle to help you place Polaris in a precise position so that the mount is precisely aligned to the true celestial pole.
Either way, you have to manipulate the alt/az controls of the mount’s wedge to bring Polaris in the required position, eyeballing it as best as you can.
Green Lasers
For quick polar alignment, or to help you identify Polaris, some people prefer to use a 303 green laser.
There are adaptors commercially available, allowing you to attach a green laser to the mount.
The precision of the alignment is not better than that you can obtain using a simple polar finder tube, but the process will be much more comfortable.
On the other hand, you better check if the use of such a laser is forbidden by law and, in any case, check for passing aircrafts and never point the laser at an aircraft.
If kids are with you, don’t allow them to use the laser: this is not a toy, and it can cause severe damage to the eyes.
Is Manual Polar Alignment Difficult?
Particularly in the beginning, polar aligning your mount is a daunting and stressful task, as all your results depend on achieving a good PA.
But PA does not need to be difficult if you can understand the concepts behind it.
The best thing you could do is get to choose ONE of the many methods to PA your mount and master it. Also, don’t overthink it.
How Do You Get A Perfect Polar Alignment?
Here is some information to help you get a perfect manual PA with ease.
Understand The Requirements Of The Different PA Methods
If you ask in groups and forums on how to improve your polar alignment, you will get a multitude of answers:
- use the graduated circles on your mount …
- … no use the app …
- … just shine a green laser through your polar scope and you are done …
- … pfft please, buy a polemaster …
- … just do drift alignment …
- … you have to level the mount …
- … no, leveling the mount has no importance for polar alignment …
In principle, all those answers are correct. The difficulty is to know the ones that are correct for the method you intend to use.
Here’s an example: if you use the graduation circles at the back of the Star Adventurer or an app such as Polar Scope Align PRO you need to have the mount leveled to the ground as best as you can.
There is a lot of skepticism about this. In this post, I explain in detail why the Star Adventurer must be leveled.
Other reticles define the position of Polaris with respect to the position of other stars, so one does not need to bother to set the proper date, time and location, nor to level the mount.
Yet, if you rely on other stars to PA, you need to be able to see those stars: they may be occulted by trees or buildings or be washed out in light pollution.
As you see, every method has its pros and cons.
Learn How To Look Into Your Polar Scope
The most challenging task of manual polar alignment is to efficiently look inside the polar finder. Since the mount will point upwards, chances are you will need to kneel before your mount and do a bit of contortionism.
This makes it difficult to look along the axes with the polar scope, a requirement for placing Polaris in the right position in the reticle.
If you are not looking along the axes of the polar scope, you cannot precisely place Polaris on the reticle.
Learn How To Identify Polaris In The Polar Scope
If finding Polaris is easy peasy to the naked eye, at times, I struggle to identify it among the many stars visible in through polar scope, particularly if I am under a dark sky.
If you struggle too, these tips can help you.
First, use the latitude & magnetic declination method or shine a green laser through the polar scope to be sure Polaris is in the field of view of the Polar Scope.
Then look for the brightest star: that should be Polaris. One trick is to increase the illumination of the polar scope illuminator or shine your red flashlight in the polar scope.
This will remove the faintest stars from the view, making it easier to identify Polaris.
And if you are early at your location, you can polar align as soon as Polaris is visible: this way, you will probably see only it in the polar scope.
Remember: tracking is different from PA. Therefore, if you set up your gear when the sky is still bright, don’t worry: you do not need to keep your mount running until dark.
Since Sigma-Octans are fairly faint, these tips will probably not help you if you are imaging in the Southern Hemisphere.
Swaying The Payload
Particularly with lightweight mounts, manipulating and moving the payload to frame your target can make you lose the PA.
It is important you find the way to verify (and eventually re-adjust) the PA once you have framed your target, right before starting your imaging session.
The way you do this depends on the kind of mount and polar scope you have.
An app like Polar Scope Align Pro can be useful.
Equipment That Makes Polar Alignment Easier
Let’s quickly see which kind of accessories can help you to improve your PA.
An Equatorial Wedge (and a sturdy one too)
A Wedge is an equatorial base that, like an alt/az mount, offers up/down and left/right movements. The vertical movement accounts for your latitude, and the left/right movement will help to align the mount to the celestial pole.
Some mounts, such as the Omegon Minitrack LX2/LX3, are sold without a proper wedge. As they are intended for wide-angle astrophotography, you can use a tilt and pan photographic head to align the mount. A proper wedge is available as an accessory.
Star Trackers such as the Star Adventurer Mini and Pro and the iOptron Skyguider Pro do have a reasonably decent wedge. Still, if you need a sturdier and more precise one, you can buy the wedge from William Optics.
Polar Scope Illuminator
Some mounts come with an illuminated polar scope or separated polar scope illuminator.
The Illuminator allows you to better see the reticle, as well as to reduce the number of stars that are visible through the polar scope.
It makes it easier to PA rather than having to shine your red light down the polar scope while juggling to fine-tune Polaris position on the reticle.
Right Angle Viewfinder For Polar Scope
A right-angle eyepiece attaches to the polar scope and makes PA much more comfortable.
Some models even provide a 2x magnification to make it easier to place Polaris on the reticle with higher precision.
The thing to pay attention to make sure the right angle is aligned to the axis of the polar scope, for better precision.
Green Laser
As we saw, a 303 green laser is a great accessory to have with you to close in on Polaris quickly.
You can also use it to create interesting starry landscapes and to indicate objects in the sky to your kids or other people.
Apps For Polar Alignment
Even if you prefer to use different methods to PA your mount, an app like Polar Scope Align PRO is like the swiss army knife for the astrophotographers, as it can help in many tasks.
One great feature of Polar Scope Align Pro is that it can work with different reticles, and for each one of them has a detailed guide explaining how to use them.
How To Polar Align Your Mount: Two Practical Examples
SkyWatcher Star Adventurer Pro
The SkyWatcher Star Adventurer Pro is one of the most popular and capable star trackers on the market.
There are two ways to align the Star Adventurer PRO: by using the graduated scales at the back of the mount or by using an app such as PS Aling Pro.
To understand how to PA a unit, we need to understand how the reticle in the Polar Scope works. The one in the Star Adventurer Pro is shaped like a clock.
Polaris must be placed on this clock, depending on your location, time of the day, and date. The graduated circles at the back of the unit are used for this.
PA using the graduated circles
Level the mount as accurately as you can.
Step 1: Set your offset from the LSTM.
The referent for setting the date and time is your location with respect to the Local Standard Time Meridian, LSTM.
To make the long story short, there is a LSTM every 15º of longitude, and to find the longitude for your LSTM, you have to multiply 15 by your local time zone.
I live in Belgium, and I am in time zone +1, so my LSTM is for longitude 15º E. Since Brussels is at longitude 4º E, I am 11deg west with respect to my LSTM and the Time Zone graduated scale must be set accordingly.
Step 2: Set your local time and date
Next, we align the day of the month to the time of the day.
At the time of writing, for me, it is 6 p.m. on June 4th. Since we have daylight saving time in this period of the year, I would set my Star adventurer to the standard time instead, i.e., 17h.
Step 3: Place Polaris On The Reticle
By setting these gears, we have also rotated the polar scope, so the clock of the reticle is probably not oriented as a normal wall clock. The 6 of the clock, wherever it is, indicates the position of Polaris on the clock.
The little scale with the years tells you on which position Polaris must be placed between the inner and outer circles of the clock.
As you see, I didn’t mention you need to level the mount: this is not a requirement for this method.
PA Using The App
For many, using an app such as Polar Scope Align Pro is the easiest way to PA a mount.
Step 1: Align the reticle with the orientation shown in the app
This is done by setting the star adventurer to midnight October 31st with the meridian indicator set to the 0 of the time zone scale.
Step 2: Level The Mount To The Ground
Carefully level the mount to the ground. Most wedges and tripods have a built-in 3D bubble level you can use to level the mount.
If you fall short in leveling the mount, your PA precision is gone
A leveling platform to put between the tripod and the wedge can make the whole process easier.
Step 3: Place Polaris On The Reticle
Now, and only now, you can place Polaris on the reticle in the position indicated by the app.
The problem is that leveling a tripod is probably the step with the greatest error, making this method suitable for imaging with short focal lenses, but not that great for longer focal lengths.
For more details on using these methods with the Star Adventurer Pro and to be sure the polar scope is properly calibrated, you can read our detailed guide on the use of the Star Adventurer Pro.
The reticle also has marks for PA to the SCP, and this video shows how to PA the mount in the Southern Hemisphere.
PA Using The Omegon EQ-300 Polar Scope
I was using this polar scope with the Minitrack LX2, and it is the type where Polaris is placed on the reticle by looking at the relative position of different stars.
With this type of polar scope you do not have to worry about setting your longitude offset from the LSTM, your date, or your local time. It also does not require having the mount leveled to the ground, although it may help.
The only exception is for the year, which can be used to eyeball the position of Polaris on the reticle if you can’t see δUmi and 51 Cep stars.
Step 1: Orient The Reticle
Look with one eye through the Polar Scope and the sky with the other eye. You should be able to see Ursa Major or Cassiopeia with the eye looking at the sky.
Orient the reticle so that the constellation drawn in the reticle is oriented as the real one in the sky.
Step 2: Place Polaris On The Reticle
Now that the constellations drawn on the reticle are oriented as the real one, you have to refine the alignment by placing Polaris in the gap (indicated as 1 in the image above).
Step 3: Fine-Tune Alignment
If your sky is dark enough, there are two other fainter stars: δUmi in Ursa Minor (gap 2) and 51 Cep in Cephus (gap 3). Try to align those on the same line near the gaps by manipulating the controls of the wedge.
The downside of this reticle is that to achieve a precise PA, you need to be able to see multiple stars that are not as bright as Polaris.
This reticle also has marks for aligning the mount to the SCP: by aligning the marks for the Southern Cross constellation, and α Eridani will put Sigma-octans in the right position
Conclusion
We are at the end of this very long article: thanks for bearing with me and reading until here.
Now you should have a good understanding of the principles and methods used for PA equatorial mounts, as well as learning a few tips about setting up and useful accessories.
With a bit of practice, you will get your PA spot on.