|Field of View:||11° x 8.25°.|
|Exposure:||8 sec x 39 = 5.2 minutes.|
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.
- 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:
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.
I execute this step around sunset so that I am ready for Step 2 when the stars first appear.
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.
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:
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,
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.
|Imaging camera: Polemaster|
|Software: SharpCap, Gimp.|
|Observatory location: 34° South.|
Images © Roger Powell
I’m a founder member of Macarthur Astronomical Society