By Don Selle
After a brief hiatus (life seems to intrude on my astronomy) this installment of Astrophotography Corner concerns one of the two important mechanical requirements for getting good image data, polar alignment. Next to autoguiding (which we cover in the next installment), getting your mount well polar aligned is essential.
So what exactly is polar alignment? Simply put, it is the process whereby one of the two axes of your mount is aligned as closely as possible to be parallel with the Earth’s axis of rotation. The term polar alignment comes from the fact that this axis, by definition, runs through the Earth’s north and south poles. The axis of rotation also points at the locations in the sky that (also by definition) are known as the North and South Celestial poles (NCP & SCP).
For polar aligned observers in the Northern Hemisphere, this means the line which runs through the center of rotation of your mount axis, and which is perpendicular to the plane of its rotation, will point at the NCP. For a German Equatorial Mount (GEM) this is typically known as the RA axis. For an alt-az mounted scope such as an SCT mounted on an equatorial wedge, it is the azimuth axis which is aligned with the NCP. While the concept is pretty simple, the why’s and how’s take a little more explaining.
We polar align so that only one axis of our mount needs to track at “sidereal rate*” to keep our object of interest in the same spot on our imaging chip. Ok, but why all the fuss? An alt-az mount that is well leveled can do this too. It does so by making continuous adjustments in both the altitude and azimuth axes. Tracking and keeping an object centered is only half the problem though. Field rotation is the other half.
The image in the scope using an alt-az mount, while staying centered, will rotate over time** (see figure below) due to the geometry of the mount and the movement of the night sky. Field rotation is not acceptable for a camera taking long exposures, as the rotation of the object will lead to smearing of the image.
So how closely polar aligned does your mount need to be? If you are doing visual observing, a rough polar alignment is just fine. The built-in “All-Star” polar alignment routines on many GoTo mounts will get you close enough to keep the object in your eyepiece, but it is unlikely to be accurate enough for imaging. Don’t fall in the trap of using this polar alignment routine and wondering why your stars aren’t pinpoint! Autoguiding is also not a reliable substitute for an accurate polar alignment. If you are autoguiding, you can be well out from polar aligned and still keep a star on the imaging chip for exposures of 5mins or longer. You can still, however, get oval stars and blurred images due to field rotation.
While it is possible to calculate*** how accurately you need to be polar aligned to keep field rotation from rotating 1 pixel width or less, keep in mind that blurring due to seeing conditions will also mask the effects of field rotation, so the results of the calculation will be conservative.
In general, polar alignment of less than 5 arcminutes error will be adequate for most imaging. Longer exposure times, higher focal lengths, and to a lesser extent, smaller camera pixels will require more accurate polar alignment.
So what is the best way to polar align? There are several general methods all of which require that your mount, OTA and camera have a clear line of sight to Polaris. These are:
- Use an accurate polar alignment scope that is installed in the center of the RA axis or with the polar scope mounted on a separate bracket and aligned parallel with the mount’s polar axis.
- Iterative polar alignment
- Polar alignment camera and software combination
Only the drift alignment method can be done without Polaris being visible.
Using a polar alignment scope. A polar alignment scope has a special reticle which is designed so when the reticle is rotated to the correct angle to match the date and time, placing Polaris in the designated position completes your polar alignment. In order to be more accurate, the scope must be collimated with the RA axis.
Additional accuracy can be gained if you make an adjustment from Standard Time (ST) on your watch to the Local Apparent (LAT) time of you location and use the LAT time to match the date and time of the polar alignment scope. Since 15 degrees of Earth’s rotation equates to 1 hour of ST, each degree of longitude difference is 4 minutes difference in LAT. If you are east of a ST meridian, correction is added to ST, if you are West, it is subtracted. For Houston, the difference between is about -20 minutes from ST as 90 deg W is the time zone meridian for Houston while about 95 deg W is the actual longitude.
The polar scope on the Takahashi mount I have been using for 15 years has an adjustment built into it that accounts for this adjustment to Standard Time. With this polar scope I routinely am able to align the mount better than 5 arcminutes from the NCP in about 5 minutes.
If your polar scope does not have this function built in, do not despair. There’s an app (actually several) for that! I typically use Polar Finder on my phone in order to polar align my camera tracker. It finds my position from the phone and then recreates the reticle in my finder scope and simulates where Polaris needs to be place. Works well every time.
Iterative Polar Alignment. Back when I was first starting in astrophotography, I was using a fork mounted SCT on an equatorial wedge. The only two ways I could get polar aligned was to use this method or the drift method. Iterative polar alignment took me less time to complete so I got pretty good at it.
You can use this method with today’s GoTo GEMs. The steps are as follows:
- Complete a rough polar alignment of your scope. An “All-Star” polar alignment qualifies for this.
- Do a one star alignment to establish your pointing model
- Find a reasonably bright star (mag 3 or brighter) rising in the east that is about 40 degrees above the horizon and which is north of but nearby the Celestial Equator (ie Dec is + single digits up to may +20 degrees)
- Slew to this star, center it in your camera and synch the mount to the position of this star.
- Slew the mount to Polaris. Center Polaris in your camera using the Altitude and Azimuth adjustments for the mount. (DO NOT SYNCH)
- Repeat steps 3, 4, and 5 for 3 or 4 iterations. You will see the amount of distance you need to move the mount on both stars decrease significantly. When the distance is small you are polar aligned.
Polar alignment camera and software combination. These days high sensitivity cameras are relatively inexpensive, and imaging software has evolved so much that plate solving (ie analyzing an Astro-image and comparing the stars in it to match them with the known position of stars in a database) to find where the camera is pointing have become commonplace. These technologies can be combined into a slick way to dial in your mounts polar alignment.
Accessories like the QHY PoleMaster or the Ioptron iPolar can be mounted on the polar axis of your mount and act like electronic polar alignment scopes. They take and plate solve images and annotate them to show you where to place particular stars in the field by moving the mounts altitude and azimuth adjustments, in order to have the camera pointing at the Celestial Pole. These work great in the northern hemisphere but are ideally suite for the southern hemisphere where there are few bright stars near the SCP.
There are also imaging oriented programs like SharpCap and PHD2 which have built in functions to assist you in polar aligning your mount by using your guide camera instead of a dedicated “electronic” polar scope type camera. Once you get the hang of how these devices and software work, polar alignment can be done quickly and with relatively good accuracy.
Drift Alignment.**** All of the above require that you are able to see Polaris with your polar scope, or your OTA and camera. What can you do to polar align if your northern horizon is blocked? Do a drift alignment.
Drift alignment is an age-old approach to getting your mount polar aligned. It is based on the fact that if your scope is out of polar alignment, stars tend to drift in Declination – either north or or south depending on where they are in the sky. Adjustments to the mount’s azimuth and altitude until this drift is minimized or practically eliminated.
- Roughly polar align your mount.
- Find a fairly bright star on or close to the meridian and as close to the celestial equator as possible.
- Using a reticle eyepiece or your camera watch the star drift for at least 5 minutes or more. If the star drifts South the telescope’s polar axis is pointing too far East. If the star drifts North, the telescope’s polar axis is pointing too far West. Adjust the mounts azimuth accordingly.
Repeat this process for the same duration and make adjustments until very little or no drift is seen.
- Now switch to a star rising in the east which is at least 20 degrees above the horizon and on or near the celestial equator.
- Using a reticle eyepiece or your camera watch the star drift for at least 5 minutes or more. If the star drifts South the telescope’s polar axis is pointing too low. If the star drifts North, the telescope’s polar axis is pointing too high. Adjust the mounts azimuth accordingly.
Repeat this process for the same duration and make adjustments until very little or no drift is seen.
- Now go back and repeat steps 2 through 5 to ensure that any azimuth adjustment has not changed the altitude and vice versa.
I would suggest that you learn one or more of the polar alignment techniques and practice them during evenings when sky conditions do not favor good images. With a little work and practice, you will soon be able to get your mount well polar aligned quickly and efficiently without wasting too much of your precious dark time!
Want to learn more about Astro-imaging or have a specific question? Contact me at: [email protected]. If you would like to share your experience with others by submitting an article for the AP Corner by all means let me know!
* You would think that sidereal tracking rate would equal 1 revolution per 24 hour day, since our clock time is based on one full rotation of the Earth per 24 hour day. In fact, the sidereal rate is slightly faster than this. Since the Earth moves slightly in its orbit each day it, the stars return to their same positions after 23 hours, 56 mins and 4 secs or about 3 mins and 54 seconds earlier each day. That’s why the sky changes from month to month and season to season.
** It is possible to add a “field rotator” to an alt-az mounted OTA to counter this effect, however this adds more complexity (3 axes to coordinate precisely etc.). This is done on some professional telescopes. It was also tried on amateur scopes over a decade ago, but did not catch on.
*** Here is a calculator you can uses http://celestialwonders.com/tools/rotationMaxErrorCalc.html
**** You can find a more detailed procedure here: https://explorescientificusa.com/pages/polar-alignment-using-the-drift-method