If you want to inform yourself about Australia, astronomical basics or astrophotography on the internet, you will find an almost unmanageable choice of websites. To make this report stand out from the crowd, I try to combine the three areas Australia, astronomical basics and astrophotography. Much of the following information I acquired before and during my vacation in Australia in 2008. However, this information can be applied not only on the southern hemisphere of the earth. Much can be done at any place on earth. Maybe one or the other astronomically interested globetrotter will be inspired by the explanations and pictures and will take his own astro pictures with his camera on his next vacation?
Orientation at the starry sky on the southern hemisphere
σ Octantis (pronounced Sigma Octantis) has the proper name Polaris Australis and is the southern counterpart of Polaris, being the closest star visible to the naked eye to the southern celestial pole. Around it the stars seem to rotate clockwise. But be careful: Polaris Australies is only conditionally suitable as navigation aid because of its low apparent brightness of 5.47m (observation without disturbing light sources nearby). It is ca. 270 lightyears away from earth and is even shown in the flag of Brazil.
Detail of a star chart with σ Octantis in the constellation Octant
(SMC stands for Small Magellanic Cloud – in dt. Small Magellanic Cloud = neighboring galaxy)
This image is based on the image Octans constellation map.png from the free encyclopedia Wikipedia and is licensed under the GNU Free Documentation License. The author of the image is Torsten Bronger.
Six minutes exposure of the south pole. On the left is the Small Magellanic Cloud, on the top right the Milky Way, on the right in the middle the Coal Sack, just to the right the Southern Cross.
Tip: If you want to photograph as many constellations as possible, you should photograph the objects on the western horizon first, because they are the first to disappear below the horizon. After that you can turn to the remaining objects..
The different time zones
If you are in Australia with a laptop and the program Stellarium, you have to consider the time difference in the program. On the basis of the u.a. Graphics can be used to determine the corresponding time difference.
Australian Standard Time Zones
This image is based on the image Australia-Timezones-Standard.png from the free encyclopedia Wikipedia and is licensed under the GNU Free Documentation License. The author of the image is Chuq.
Australian time zones in southern summer
This image is based on the image Australia-Timezones-Daylight.png from the free encyclopedia Wikipedia and is licensed under the GNU Free Documentation License. The author of the image is Chuq.
Rise and set times
Ayers Rock near Alice Springs in the heart of Australia is one of the most famous and popular tourist destinations. At sunrise and sunset Ayers Rock appears even more reddish. If you want to enjoy this view, you should know when the sun rises or sets. sets. On the website: timeanddate.com, one can look at u.a. inform about it.
Ayers Rock at sunset
Tip: During the southern winter (winter in the southern hemisphere) it gets dark early (ca. 18 o’clock local time) but also bright early (ca. 6 o’clock local time). Good conditions not to take some astronomy pictures too late, because most of the time you have to go on early the next day, because the distances in Australia are huge.
The limiting brightness with and without optical aids
The limiting brightness is the brightness of the stars, which are just still visible. The larger the "opening" is (z.B. the iris in the eye or a lens diameter or. telescope aperture), the fainter stars you can still see. The brightness is given in magnitudes (short mag or a superscript m). The smaller the value is, the brighter the stars are. In a major city, only the stars up to 4 mag can be observed. In the mountains under ideal visibility conditions however up to 6 mag. A difference of one mag corresponds according to Norman Robert Pogson to the factor of . The difference of 6 mag to 4 mag is therefore 2 times 2.512 or 2.512² = 6.31, d.h. in the mountains about 6 times fainter shining stars can be observed!
The limiting magnitude for a telescope can be calculated:
In Australia the finder scope of my telescope was used (SCT LX90). It has 8x magnification and 50mm aperture (8×50). According to the o.g. formula and with a limiting brightness of the eye of 6mm and with an average exit pupil of 6.5mm, I can see stars up to
This is times more light than an observation with the naked eye.
Parallactic travel mount self-made from wood.
In comparison I had binoculars with 5x magnification and 25mm aperture (5×25) with me in Thailand. This corresponds with D=25 (mm)
With the finder scope, I have many times more light available than with binoculars.
Geography meets astronomy
The two turning circles are at approx. 23° northern and southern latitude. They run 2.600 km north resp. south of the equator.
This image is partly based on the image Tropics.png from the free encyclopedia Wikipedia and is licensed under the GNU Free Documentation License . The author of the picture is ErnstA .
Outside the tropics the sun can never be at the zenith. Every solstice (summer solstice in the northern hemisphere 20./21. June resp. Winter solstice in the southern hemisphere 21./22. December) the sun is in the zenith on the tropics. Objects cast only a perpendicular shadow. Within the tropics the sun is at its zenith twice a year (at the equator at the beginning of spring and autumn). But the tropics change their position with the time. At the moment they are moving with 14 meters per year towards the equator. Reason is the nutation, d.h. a periodic fluctuation of the earth axis in the rhythm of 18,6 years.
The northern tropic is also called the Tropic of Cancer. The constellation, which the sun crossed at the solstice, was up to 15 v.Chr. the constellation of Cancer. But due to the precession the summer solstice occurs z. Zt. in the constellation of Taurus. Correspondingly the southern tropic is called "Tropic of Capricorn.
By the way: the Tropic of Cancer limits the range of NATO to the south.
At the end of the vacation (26.09.2008) the sun is perpendicular to the observer at the equator.
The moon in Down Under
Every 18.6 years the Pleiades can be covered by the moon for a longer period of time. The Pleiades are an open star cluster, which can already be seen with the naked eye. Up to seven stars can be seen from the Pleiades, which is why they are also called the Pleiades.
In the night of Saturday, 23. August 2008 on Sunday, the 24. August 2008 seen from Germany the moon covered again the Pleiades. The moon appears shifted in front of the sky background seen from Australia compared to the observation in Germany. This shift was so large that the moon observed from Australia did not cover the Pleiades at all.
Moon near the Pleiades (M45).
In contrast to Germany there is no occultation in Australia.
At the end of 2009 the last occultation took place. After that there will be temporary occultations of the Pleiades by the moon for an observer on earth only from the year 2024 again.
Due to the spherical shape of the earth and the fact that Australia is located on the southern hemisphere of the earth, the moon can be seen as a Venetian gondola.
To the "Venetian gondola moon" Mercury, Venus and Mars are joining them.
The position of the ecliptic and the planets
Most of the travelers have made the experience, that the sun is higher in the sky at noon in the southern vacation destination than in Germany. As the earth moves around the sun in the course of a year, the earth’s orbital plane (ecliptic) also becomes steeper. At the earth’s equator the sun and the ecliptic are perpendicular to the western horizon at sunset (at the beginning of spring and autumn) (analogous to sunrise in the east). Since Australia is not far from the Earth’s equator, in z.B. Alice Springs the sun and the ecliptic almost perpendicular to the horizon. The planets are all always near the ecliptic. Therefore the planets are also perpendicular to the horizon and reach a high altitude above the western horizon at sunset and therefore set later. In comparison the ecliptic in Germany is only in a flat angle to the horizon. The planets do not have a large distance to the western horizon and go u.U. with the sun below, before one could see the planets in the darkness.
An example in Alice Springs, Australia at sunset:
The eastern angular distance of Mercury to the sun rose to proud 25° by the end of August 2008. Nevertheless there was no evening visibility in Germany, because the ecliptic was too flat to the western horizon. Mercury and also Venus set at dusk before it became dark enough to see both planets at twilight.
Simulation of the sunset in Germany on 23. August 2008 with the flat ecliptic
In Alice Springs, however, the ecliptic runs steeply to the western horizon. So there was still an evening visibility of Mercury and Venus in Australia.
Simulation of the sunset in Australia on 23. August 2008 with the steep ecliptic
Sunset in Alice Springs. On the western horizon the planets Mercury, Venus and Mars are visible.
These screenshots were taken from the free planetarium program Stellarium, which is licensed under the GNU General Public License (GPL) for free documentation.
The enlargement
With my finder scope there is only one invariable magnification (8x). Therefore I cannot give any examples to this point. But one or the other star friend may have a travel telescope and can achieve different magnifications: the magnification of a telescope is the ratio of the objective focal length and the focal length of the eyepiece.
Illustration courtesy of Dr. Strickling
The magnification becomes stronger, the longer the objective focal length and the shorter the eyepiece focal length is.
Calculating the size of the field of view
When bright celestial objects are close together, impressive celestial views result. These so called constellations I could observe also in Australia. Once the moon was very close to the Pleiades (in Europe it even covered the Pleiades)!) and another time Venus, Mercury and Mars appeared together as a trio on the celestial stage. If such constellations are found in an astronomical yearbook in advance, the question arises whether the constellation can be seen with the recording device in sufficient magnification (especially with lenses with a fixed focal length). It may also be that the constellation appears much too small in relation to the overall image. This can be checked with the following formula (the optics in front of the camera can be a normal camera lens or a telescope itself):
[For small angles, approximate alpha = b/f ]
If the focal length and the edge length (length and height) of the image chip is known, then the size of the imaged sky area can be calculated. In my case I had as initial sizes a lens with the focal length of 70-300mm and the dimensions of the recording chip of my Canon EOS 350D with length x height: 22.2mm x 14.8mm.
Since I am not using a lens with a fixed focal length, this results in sky areas of different sizes. In the u.a. Table I have listed the smallest and largest focal length. The sizes of the sky areas move in between.
Imaged angle in the sky (in degrees)
Digital SLR Canon EOS 350 D
So if I set my lens to a focal length of 300mm, I can image a sky area of 4.2°x2.8° on my photograph. This is a large enough area of the night sky to photograph lunar-planetary or lunar-star constellations. The full moon, with its 0.5° diameter, would fit into the picture about eight times in a row.
The calculation of the focal length
One of my main goals was that I wanted to capture some constellations almost full-frame. But what focal length did I have to set for it with my lens?
The focal length can be determined as follows:
with the relationshipwe maintain
The scorpion has z.B. 497 square degrees (see Wikipedia). From it the root results in an imaginary square of 22,3° edge length. Let’s put this greatness into the o.g. formula, the result is
With a lens setting of 56mm you can get the constellation Scorpio in full format on the picture.