How to Find the South Celestial Pole

Field of View:11° x 8.25°.
Exposure:8 sec x 39 = 5.2 minutes.
Image date:2021-01-09.

The above image, taken with an 11° wide field astro-camera, shows the star field in the South Celestial Pole (SCP) region. For practical purposes, this region is visually devoid of stars. The brightest stars are a very dim 5th magnitude – only about 10% of the brightness of The Northern Pole Star.

What is a Celestial Pole?

The North and South Celestial Poles are imaginary points in the sky about which the stars appear to rotate. They are extensions of the rotation axis of the Earth.

To take long astro-images, astronomers need to counter this rotation and this requires us to align the axis of the telescope’s rotation with the axis of the Earth’s rotation. This means aligning with one of the Celestial Poles.

Late last year I posted the following star trail image which illustrated how far the stars rotate over a 25 minute period. To the visual observer the rotation is not detectable.

Southern Star Trails
7th December 2020
©Roger Powell

The Problem

  • in the Southern Hemisphere, we don’t have a bright pole star;
  • the surrounding stars are so dim that you can barely see them; and
  • the stars rotate about the SCP but this is visually undetectable.

So how do amateur astronomers south of the Equator locate the SCP?

Some very approximate methods are available here but are of no use for polar aligning a telescope.

I’ve experimented with other methods of polar alignment, achieving varying degrees of success – but now I’ve developed my own sequence which gets an accuracy of less than ten arc-seconds. This is how I do it, in three steps:

Step 1

Rough Align

For any location in the Southern Hemisphere, the orientation of the SCP is True South (not Magnetic South) and above the horizon by the same angle as the geo-latitude.

I’m at latitude 34° South, so when I try to aim my mount at 34° above the horizon and towards True South I might get an alignment accuracy of 1° – 3°, which is not good enough for astro-photography – but it’s only the first step.

A magnetic compass is used to find magnetic South, remembering that True South is offset by the known magnetic deviation at the observing location. At my location (and at this point in time), True South is offset by about 12.5° towards the East of Magnetic South. This places the tripod legs facing roughly South.

How to find True South (TS).
Magnetic South (MS) is offset b
y an angle specific to your individual location.
Drg: R.P.

I execute this step around sunset so that I am ready for Step 2 when the stars first appear.

Step 2

Getting Closer

I then connect to a wide-angle polar camera, knowing that Step 1 will always bring the SCP somewhere within its 11° x 8.25° field of view.

The polar camera is not mounted on the movable telescope, it is fixed to the axis of the equatorial mount below it – and always points in the direction of the mount’s axis of rotation. The aim is to align the mount’s axis with the Earth’s axis.

PoleMaster camera by QHYCCD fitted to the polar axis of my Skywatcher EQ6-Pro equatorial mount.
No matter how much the telescope is rotated, the camera always points in the direction of the telescope’s rotation axis.
True polar alignment is achieved when this axis is adjusted to align with the rotation axis of the Earth.
(Image RP 2018-09-29
)

As darkness descends, the distinctive pattern of the four brightest (fifth magnitude!) stars, shaped like a trapezium (and joined with yellow lines below) will always be the first to pop out on my laptop monitor:

Finding the South Celestial Pole
Image taken 2021-01-09 75 minutes after sunset.
Orientation varies with time and season.
Image and gif © Roger Powell

As the sky gets darker, familiar star patterns appear and I can quickly adjust the mount so that the vicinity of the SCP appears close to the centre of the pole camera’s field of view.

This step only takes a minute, because once it becomes dark enough to identify star patterns and roughly tune the polar alignment to an accuracy of less than about 20 arc-minutes, the pole-cam has served its purpose. I don’t waste any more time with it,

Step 3

Accurate alignment

At this point, more stars are becoming visible in the fading twilight and my astronomical skills are replaced by….. magic!

Now that I know the polar alignment is close (but not accurate), I revert to the 1.5° x 1.0° astronomical camera on the telescope itself and use the amazing polar alignment feature of my imaging software – SharpCap.

I wrote near the top of this article that the rotation of the stars is visually undetectable – but rotating an almost aligned telescope by 90° is enough for the software to determine the axis of the mount and compare it with the axis of the Earth. It does this by plate-solving and comparing two images. It then prompts me how much to adjust the Alt-Az bolts on the mount to align the Telescope’s axis precisely with that of the Earth.

It guides me to a polar alignment accuracy of between 0 and about 10 arc-seconds of the SCP, which is pretty awesome when you consider there are 3,600 arc-seconds in one degree!

As my proficiency at polar alignment improved, the problems with using a guide scope evaporated.

Obtaining a good polar alignment will facilitate smooth guiding of the telescope during the exposure, with no star trailing, even in exposures lasting several minutes.


Camera Details

Imaging camera: Polemaster
Software: SharpCap, Gimp.
Observatory location: 34° South.

Images © Roger Powell

I’m a founder member of Macarthur Astronomical Society


21 Comments

  1. Hurray for Polaris!
    Thank you for the star trails photo. Because of the brighter stars at the edge and fainter stars nearer the pole, I can tell my brain that I’m looking down a tunnel into the distance. Like a vortex. – bob k

    Liked by 1 person

    1. It is a vortex – of blackness!

      You Northerners are lucky to have Polaris. I understand that many still use a polarscope to find it and polar align. I tried that method here – on my knees, leaning my neck backwards, looking for mag 5 stars that are mirror reversed….

      I soon gave up on that!

      Liked by 2 people

      1. I’ve never had the ‘vortex’ feeling looking at northern sky trails photos. I think not having a bright star like Polaris in the middle helps me get that sense of a ‘tunnel’ view extending off into the distance. Yes, we are lucky for Polaris. I think so much astronomy here is European-centric, we don’t get the challenges and thrills of the Southern Hemisphere skies. Seeing the center of the Galaxy overhead is quite a sight (so I’ve heard).

        Liked by 1 person

        1. It is strange how the SCP region is so sparse. I don’t think any of my images fully indicate that, although you immediately picked up on it. It does look a bit like a vortex.

          The difference in the north is there is a light at the end of the tunnel.

          Liked by 1 person

  2. I’m impressed with that accuracy. Doing those high quality images you share involves a commitment of time and procedures. The payoff is obvious. I’m glad we have Polaris.

    Liked by 2 people

    1. One of the problems with astro-imaging is the amount of calibration time needed each night after the first stars appear. Getting each procedure ticked off quickly is not as important as achieving a good degree of accuracy but every minute saved is a minute more imaging time before packing up time. Something I need to work on.

      Liked by 1 person

  3. . . . along with poisonous everything, another reason for not moving down under . . . not that I make use of Polaris but, if I ever wanted to, it’s there.

    Nice explanation and methodology.

    Like

    1. Yes, funnel web spiders and sharks etc. I get your meaning but I think we humans have done our share of that. At least I no longer need a pole star, even though lately I wouldn’t see it even if there was one.

      Liked by 1 person

    1. I understand your frustration about the clouds, we have clouds in the Southern Hemisphere too.
      In fact right now I’m inclined to the considered opinion we currently have most of them, … 😨

      Liked by 1 person

  4. Nice write up Roger, I have seen you in action setting up you mount first (plumb bob and level) and then your scope. I have to say the systematic approach you have is a very good one and as said before it shows up in your pics. I do think a Pole Master is a good investment, do you think that for most up to step two is enough?

    Liked by 1 person

    1. Richard, the Polemaster has been a pretty useful tool for me. Initially I used the Polemaster camera – with it’s own software routine – as the principal method of polar alignment. It was touted as getting 30 arc-second accuracy. I never achieved that but the arc-minute accuracy it provided enabled my guiding system to operate with less problems than before and did enable longer exposure astro-imaging.

      The main reason I stopped using the Polemaster software was because it would not function on the new laptop I bought specifically for astronomy, so I tried the SharpCap routine and discovered it guarantees arc-second accuracy. However I soon realised that the Polemaster camera would still be very useful to retain in my toolbox.

      Polemaster and Sharpcap use the same principle of aligning the rotation axis of the mount with the rotation axis of the Earth. SharpCap does it with the main imaging camera through the telescope, so the higher resolution and narrower field brings improved accuracy.

      To answer your specific question, if you are using a guide-scope, then getting a high degree of polar alignment accuracy – whilst satisfying – is not necessarily the prime objective. What you really want to achieve is smooth guiding so you can take pictures with round stars. Polemaster should help achieve that most of the time.

      Like

  5. Roger: I’m both mesmerized and awe-struck with your simply-beautiful (yes, enticingly complicated (to me!)) solution to what is a most difficult, and necessary aspect of accurate guiding, especially in the Southern Hemisphere. It DOES underscore (no pun intended) the convenience of having a roughly +2 mag. star a half a degree or so from the Northern Celestial Pole. I recall, back in the olden days, quickly swinging over to Polaris, and then a little to the side, and that would do it. BUT we were just doing that so the visual observing would be better. For me, time lapse photos were (especially) out of the question for deep sky objects. Thanks for the fascinating details. M 🙂

    Liked by 1 person

    1. As you say, with visual observing one can make do with an approximate polar alignment. With imaging it is very important to get greater accuracy – and achieving the skill to do that took me some time. The important thing is not to waste too much time trying to get a perfect 00° 00′ 00″ because that is not the objective. As you would know, getting smooth performance from the guiding system is the only thing that matters.

      Liked by 1 person

  6. Wow, thanks for the explanation, Roger. It’s interesting how much of a, maybe, underappreciated luxury we have here in the north. I hope someday I’ll have to hunt around and use your instructions.

    Like

    1. Polaris is undoubtedly a luxury and an aid to approximate location of the NCP, especially for visual observers, as Marty mentioned above. However, if I moved back to the Northern Hemisphere I would be utilising Polaris only in my Step 2. These days software alignment routines are what provide the precise accuracy for smooth guiding.

      Liked by 1 person

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