Welcome to Houston Astronomical Society

Polar Alignment

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:

1. 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.
2. Iterative polar alignment
3. 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:

1. Complete a rough polar alignment of your scope. An “All-Star” polar alignment qualifies for this.
2. Do a one star alignment to establish your pointing model
3. 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)
4. Slew to this star, center it in your camera and synch the mount to the position of this star.
5. Slew the mount to Polaris. Center Polaris in your camera using the Altitude and Azimuth adjustments for the mount. (DO NOT SYNCH)
6. 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.

1. Roughly polar align your mount.
2. Find a fairly bright star on or close to the meridian and as close to the celestial equator as possible.
3. 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.

4. 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.
5. 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.

6. 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

The Garnet Star

by Bill Pellerin, previously published

Object: The Garnet Star (SAO 33693; Mu Cep)
Class: Star
Magnitude: 4.08 (variable 3.4 to 5.1)
R.A.: 21 h, 43 m, 30 s
Dec: 58 degrees, 46 minutes, 48 seconds
Distance: 5,260 LY
Constellation: Cepheus
Optics needed: Naked eye, binoculars or small telescope

Why this object is interesting.
This bright (but variable) orange or reddish star is easy to
spot and is high in the sky at this time of the year. It transits
at 9:13 p.m. on October 20. The star sits on the edge of
IC1396...
The star is very large at 2.4 billion miles across. This is
larger than the orbit of Saturn and 1650 times the diameter
of our Sun. The period of variability is 730 days.
It was Sir William Hershel (1738-1822)
who gave it the name the 'Garnet Star'.
One problem with this description, of
course, is that garnet rock show up
with different colors. Which one did he
mean? There are even reports of the star
appearing to be purple to some observers,
although the star is designated as a 'M'
star. This is how he described the star in
the transactions of the Royal Astronomical
Society:
A very considerable star, not marked by Flamstead, will be found
near the head of Cepheus. It is of a very fine deep garnet colour,
such as the periodical star o ceti was formerly, and a most
beautiful object, especially if we look for some time at a white
star before we turn our telescope to it, such as a cephei, which is
near at hand. (from aavso.org web site)
The Garnet Star's history is typical for a star of this size.
The star burns (fuses) hydrogen into helium early in its life,
releasing energy in the form of light in the process. When
much of the hydrogen has been used up the star expands
to its current red supergiant phase and it is now believed
to be fusing helium into carbon. Fusion continues through
several phases, and the star ends its life as a supernova.
In a supernova, the star's core collapses, then rebounds. A
massive shock wave moves through what's left of the star's
materials and we see a supernova.
The star currently varies in brightness by 1.5 magnitudes
and this variability indicates some instability in the star and
tells us quite a bit about what's going on internally with the
Shallow Sky Object of the Month
The Garnet Star
The location of the Garnet Star
from TheSky v6
star. Stars such as Mu Cep (the Bayer designation) are
called 'Small Amplitude Red Variables'.
There are other red variables that you can enjoy, one
of which has been written about in this series -- Hind's
Crimson Star. This one's in Lepus, just south of Orion,
so look for this one later in the year.
Other examples of SARV's are:
Betelgeuse - Alpha Ori. Most of us don't think of this
star as a variable, but it is.
R Lyr - north of the main part of the constellation
W Cyg

October is here, and that means the weather should start to be a little cooler (maybe), night time comes a little earlier, and more importantly, fall star party season is upon us!  As I write this message, the UBarU star party was held just a few weeks ago, the Okie-Tex Star Party is happening this upcoming week, and in a month, the Eldorado Star Party will be in full swing.  For many of us, it’s a great time to get away under dark skies with many other kindred spirits to observe, take astrophotographs, enjoy great speakers, and otherwise enjoy the time away from the hustle-and-bustle of the big city.

For many people, the lead-up to their first “real” star party can be a bit of a nervous time. “What if my gear malfunctions?”  “What if I forgot a critical piece of equipment and now, I’m hundreds of miles from home?”  Or, perhaps the most intimidating thought of all, “what if I’m the one person who happens to ruin everything by committing the dreaded light violation?”

Star parties can be a bit overwhelming the first time you visit one.  The first time I visited the Texas Star Party, there were hundreds of other astronomers there, all seemingly more knowledgeable about astronomy than I was, and all with much better telescopes than what I lugged out to Fort Davis (this is certainly an exaggeration, but that’s how I felt the first time).  The terrain is dusty, the air is dry, and if the animals around there don’t kill you, the plants certainly seem like they will.

But there’s nothing quite like that first time under Bortle-1 or Bortle-2 skies, where everything is so dark, you can’t even make out the Big Dipper because it’s lost in a sea of stars.  Where the Messier objects you struggle to find with a small telescope or binoculars in Houston seem to “pop” with the naked eye just by looking in that direction.  Or where the fine details of a galaxy arm just look that much more well-defined without the extraneous light pollution that we deal with here every night.  More than this, though, there’s nothing like having that perfectly clear night under the stars that compels you to observe until the sun comes up with friends who are just as excited as you to be observing shortly before the crack of dawn.

For me, it’s that last item that makes these star parties special.  The camaraderie with other fellow astronomers on those dark nights, the sharing of views through an eyepiece, and the knowledge that is gained from others just can’t be beat.  It’s what draws me to these star parties over and over, and what makes our own dark site in Columbus a special place.  For those of us who love to learn by “doing,” these events are our opportunity to become better astronomers.  If you have an opportunity to visit one of these star parties, I highly encourage you to do so.  And if you can’t, you’re getting much of the same experience at our dark site.  Get your dark site certification done and visit at the next opportunity.

To all of you headed to Okie-Tex, the Eldorado Star Party, or to our dark site - happy photon hunting!

This article is distributed by NASA Night Sky Network

The Night Sky Network program supports astronomy clubs across the USA dedicated to astronomy outreach. Visit nightsky.jpl.nasa.gov to find local clubs, events, and more!

Weird Ways to Observe the Moon

David Prosper

International Observe the Moon Night is on October 16 this year– but you can observe the Moon whenever it's up, day or night! While binoculars and telescopes certainly reveal incredible details of our neighbor’s surface, bringing out dark seas, bright craters, and numerous odd fissures and cracks, these tools are not the only way to observe details about our Moon. There are more ways to observe the Moon than you might expect, just using common household materials.

Put on a pair of sunglasses, especially polarized sunglasses! You may think this is a joke, but the point of polarized sunglasses is to dramatically reduce glare, and so they allow your eyes to pick out some lunar details! Surprisingly, wearing sunglasses even helps during daytime observations of the Moon.

One unlikely tool is the humble plastic bottle cap! John Goss from the Roanoke Valley Astronomical Society shared these directions on how to make your own bottle cap lunar viewer, which was also suggested to him by Fred Schaaf many years ago as a way to also view the thin crescent of Venus when close to the Sun:

“The full Moon is very bright, so much that details are overwhelmed by the glare. Here is an easy way to see more! Start by drilling a 1/16-inch (1.5 mm) diameter hole in a plastic soft drink bottle cap. Make sure it is an unobstructed, round hole.  Now look through the hole at the bright Moon. The image brightness will be much dimmer than normal – over 90% dimmer – reducing or eliminating any lunar glare. The image should also be much sharper because the bottle cap blocks light from entering the outer portion of your pupil, where imperfections of the eye’s curving optical path likely lie.” Many report seeing a startling amount of lunar detail!

You can project the Moon! Have you heard of a “Sun Funnel”? It’s a way to safely view the Sun by projecting the image from an eyepiece to fabric stretched across a funnel mounted on top. It’s easy to make at home, too – directions are here: bit.ly/sunfunnel. Depending on your equipment, a Sun Funnel can view the Moon as well as the Sun– a full Moon gives off more than enough light to project from even relatively small telescopes. Large telescopes will project the full Moon and its phases, with varying levels of detail; while not as crisp as direct eyepiece viewing, it’s still an impressive sight! You can also mount your smartphone or tablet to your eyepiece for a similar Moon-viewing experience, but the funnel doesn’t need batteries.

Of course, you can join folks in person or online for a celebration of our Moon on October 16, with International Observe the Moon Night – find details at moon.nasa.gov/observe. NASA has big plans for a return to the Moon with the Artemis program, and you can find the latest news on their upcoming lunar explorations at nasa.gov.

Sun Funnels in action! Starting clockwise from the bottom left, a standalone Sun Funnel; attached to a small refractor to observe the transit of Mercury in 2019; attached to a large telescope in preparation for evening lunar observing; projection of the Moon onto a funnel from a medium-size scope (5 inches).

Safety tip: NEVER use a large telescope with a Sun Funnel to observe the Sun, as they are designed to project the Sun using small telescopes only. Some eager astronomers have melted their Sun Funnels, and parts of their own telescopes, by pointing them at the Sun - large telescopes create far too much heat, sometimes within seconds! However, large instruments are safe and ideal for projecting the much dimmer Moon. Small telescopes can’t gather enough light to decently project the Moon, but larger scopes will work.