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Basic observing skills.

Identifying the southern and circumpolar constellations

The south celestial pole (SCP) is the point in the sky where the Earth's axis of rotation, extended southward, intersects the celestial sphere. As the Earth turns, therefore, this point remains fixed in the sky; at night the stars appear to rotate around it. The SCP's declination is exactly -90[degrees], and its altitude above the horizon is equal to the observer's geographic latitude.

The chart above shows the stars around the SCP, as seen facing south. To orient the chart correctly, consult the table opposite. First And the most appropriate date in the leftmost column. ln the top row, locate the time of observing. Note the letter found at the intersection. Now turn the star chart so that this letter is at the bottom; the chart will now approximately match the orientation of the stars in the sky.

Finding the south celestial pole

The star chart below, showing stars down to about 7th magnitude, depicts a 30[degrees] wide field centred on the south celestial pole (which is marked with a +). The naked-eye star nearest the pole is 5.5-mag [sigma] Octantis. The trapezium k - [sigma] - n - q is very useful for locating the SCP. Visual magnitudes, calculated from data in the Tycho-2 catalogue, are tabulated below the chart.

Find out more

Become familiar with the southern night sky by using the Southern Star Wheel (a DIY planisphere, with interchangeable star discs, which shows the stars visible at any date and time from southern Africa) in conjunction with the Discover! workbook (a more detailed star atlas covering the sky in 25 charts) to identify the southern constellations. Then get your copy of the ConCards (Constellation Cards), which consist of a complete set of star charts, one per constellation, showing the best deep-sky objects visible with binoculars and small telescopes. These materials are available as free downloads from the ASSA website.

Binoculars

It is often thought that a telescope is essential for observing. Certainly, anything that enhances human vision is extremely useful to the astronomer. But a simple pair of binoculars is an excellent tool for exploring the night sky. Besides being far cheaper than a telescope, they are less cumbersome and are easy to use, especially if sturdily mounted on a tripod.

Choosing a telescope

Choosing the "best" telescope depends on a number of factors, including personal desires, probable viewing locations, storage and transportation and, inevitably, finances. The following 10 tips should help when making your choice.

Be careful what you wish for, you might get it. That 400-mm (16-inch) might be attractive, but unless you have it permanently mounted in an observatory, you will soon lose your observing friends as they tire of helping you to lug it around. And if you cannot handle it by yourself, it will quickly fall into disuse. The best telescope is the one that gets used--and the one that gets used is the one that is convenient to use. Don't be too hasty to spend your own money. Join the activities of your local ASSA Centre or astronomy club, where you will have the opportunity to use a range of equipment, learn observing techniques and discuss actual user experiences of the kind of telescope that you might be interested in. Do Internet research to benefit from others' experiences. Ignore marketing hype. With few exceptions, vendors are unable to provide sensible advice on the products they are pushing. Forget high magnification, it is the least of the issues. The focal ratio of the instrument (written as f/) and the exit pupil (diameter of the beam exiting the eyepiece) are much more important. Less than f/6 mandates the use of more sophisticated (expensive) eyepieces for good image quality. Your first eyepiece should deliver an exit pupil of between 5 mm and 6 mm. Less is more--quality before quantity. A small, good-quality instrument will bring endless satisfaction, whereas a large, poor one will be frustrating--possibly to the extent of turning you off observational astronomy. You will keep a good small scope irrespective of how large your subsequent instruments get. Start simple and work up later as your skills and experience grow. You need to learn your way around the sky. There is no better way than with binoculars, which can be used for many other purposes and are small enough to go with you almost anywhere. So buy binoculars before getting a telescope: their versatility, ease of use and low-power wide-field views are complementary to the telescope's characteristics. Patience is a virtue. There are bargains to be had on the second-hand market. Save some of your initial budget to upgrade accessories later, once you know what works for you. Budget for balance. A good scope on a bad mounting makes for a miserable experience. Rather a few good eyepieces than a whole slew of poor ones. A decent finder scope will enrich your experience more than a high-power eyepiece. A quality focuser is mandatory to get the best out of your optics. Concentrate on these fundamentals rather than unnecessary "features". Buy books, you will learn a lot. A field guide to the sky takes little space and needs no power. lt will tell you what objects are available to look at, as well as where and what they are. While you are at it, get a planisphere. Later, more detailed star charts will be useful. Books on telescopes with a practical approach will assist in your choice of instrumentation, saving you money.

Angular separation

Distances in the night sky are measured in degrees (or finer divisions). From the horizon to the point overhead (zenith) is 90[degrees]. A fist at arm's length covers about 10[degrees] of sky; a fingertip about 1[degrees]. A degree is divided into 60 arcminutes (60'). The Sun and Moon are each or 30', wide. One arcminute is divided into 60 arcseconds (60"), which is about the resolution of the human eye. For example, Jupiter's disc is about 40" across, which is smaller than the human eye can resolve, and it therefore appears like a point of light when viewed without optical aid. In order to see it as a disc, the image needs to be magnified. If viewed with 7-power binoculars (e.g. 7x50), the disc appears to be 7x40" = 280" (or 4.6') wide, rendering it visible.

The aperture of a telescope (the diameter of the lens or mirror) determines the smallest detail that can be seen (if magnified to above the eye's 60" limit). A 60-mm aperture typically has a resolution limit of 2.3", while a 25-cm (10-inch) reaches 0.56".

Dark adaptation and averted vision

One basic technique that observers soon learn is to allow the eyes 20 minutes or more to adjust to darkness in order to become sensitive to faint light (dark adaptation). Bright light spoils this sensitivity. A much-dimmed torch (shielded with red cellophane) will provide sufficient light, once dark-adapted, to read star charts and make notes and sketches. Equally important is the technique of using averted vision. Looking slightly to the side of a faint object, instead of directly at it, gives a much improved view, because the retina is most light-sensitive around the edges. lt is also important to move the eyes around somewhat while observing, because an image kept at the same point on the retina is ignored by the brain after a time.

Limiting magnitude

The brightness of the faintest star that can be seen on a given night, is the limiting magnitude. A number of factors influence this limit, the three most important being aperture, dark skies and experience. The larger the light-collecting area of an optical system is (its aperture), the fainter the stars are that can be seen. The dark-adapted naked eye has an aperture about 7 mm in diameter (the pupil), and from a dark observing site stars down to about magnitude +6.5 can be seen. From within a city, however, the limiting magnitiude may be only +4.5. Under these conditions the night sky appears grey or orangish and the Milky Way cannot be seen.

Binoculars allow more light to be gathered and thus show fainter stars (down to about magnitude +10.5). A telescope allows even fainter stars to be seen: a 60-mm aperture will reach magnitude +11.5 while a 25-cm telescope may reach magnitude +14.5.

The brightness of the sky--a measure of the amount of light pollution--also influences the limiting magnitude. Fainter stars can be seen in the country than in a city (Table 16).

Observing faint objects is also a skill - the eye-brain system benefits from practice and experience. A skilled observer under dark skies can see as much as two magnitudes fainter than a novice under the same observing conditions.

Time

South African Standard Time (as in everyday use and used throughout this Guide) is mean solar time on the 30[degrees] east meridian (which runs east of Johannesburg and just west of Durban) and is exactly two hours ahead of Universal Time (SAST=UT+2). Because the Earth's orbit is elliptical, the time of the Sun's transit above the 30[degrees] meridian varies. Sidereal time, a measure of the rotation of the Earth with respect to the stars rather than the Sun, is given in Table 17 for 30[degrees] longitude at 02:00 SAST. When a telescope equipped with setting circles is aimed on the meridian, its RA circle should read the sidereal time.

The National Metrology Institute of South Africa [http://time.nmisa.org/] currently maintains four cesium atomic clocks, one of which is designated as the master clock for South Africa. Details on how to access a time server will be found at [http://www.time.org.za/].

Astrophotography

Astrophotography is both challenging and rewarding. Difficulties to overcome include faint, low-contrast targets, the turbulent atmosphere, Earth's rotation, and noise in the image. However, modern digital cameras and free specialized software enable even beginners to capture images relatively easily. The key to success is to start simply, with equipment to which you already have access, and then build as your interest and experience grow. There is a natural progression in terms of difficulty and expense:

1. Wide-field scenic images with an astronomical element. To capture star trails, put your camera on a tripod, aim it at a nice piece of sky, preferably with some foreground interest, set it to manual mode and focus on infinity. As a starting point, set the aperture almost at its widest, the exposure to the longest your camera allows, and the lSO to about the middle of the range. Use the self timer to trigger the shutter to avoid camera shake. Trails will be short and curved near the pole, longer and straighter towards the celestial equator. A short focal length gives a wider field and less trailing. Next, with the camera set on Bulb (B) and using a cable release, experiment with longer exposures for longer trails.

2. Moderate-field starscapes without star trails require some form of tracking. You can build a simple barn-door tracker, or attach the camera to a telescope with an equatorial mount.

3. Lunar and planetary work requires a higher magnification. Start with the Moon, as it is very bright and has fabulous detail. Try the afocal method: aim your telescope at the Moon, focusing it by eye, hold your camera (even a cell phone) so it is pointing squarely into the telescope eyepiece, and take the shot. More advanced techniques include using a DSLR at prime focus, and eyepiece projection. Since the atmosphere destroys fine detail, you can use video (even from a webcam) to capture bursts of images over several seconds and use free stacking software to select the best frames and synthesise a single high-resolution image.

4. Deep-space imaging involves long exposures (mandating excellent tracking) as well as many hours of image processing to get satisfactory results. Conventional DSLRs are quite capable of producing very good results but the best results require specialist low-noise high-sensitivity cameras with a cooling system.

Every year brings new developments that make astrophotography easier, but you do need to "pay your dues" in terms of experience to get really good results. And at every level, there is always still more to learn, ensuring a growth path to keep you interested.

Make your own telescope

Consider making your own telescope! Why? Because (1) you can, (2) it is fun, (3) you will learn new skills, (4) it will give you access to the Universe, (5) it may be significantly cheaper than buying one, and (6) you can build something tailored to your specific requirements. The ASSA Johannesburg Centre has conducted a telescope making class continuously since 1993. Find out more: [http://www.scopex.co.za/atm-class.html].

Where to go stargazing

Light pollution is a problem faced by anyone who wants to enjoy a clear view of the night sky. Additional factors such as safety and accessibility further complicate the choice of where to go to look at the stars. Fortunately, several avenues are available for exploration.

* ASSA Centres and other astronomy clubs sometimes have excursions to well-researched darksky locations - not only is there safety in numbers, but also camaraderie. Often, small game parks or dams frequented by fishermen have accommodation or caravan parks, are accessible, not too expensive, and usually not overly illuminated. Joining an outdoors group (e.g. Exploration Society of Southern Africa, Mountain Club of South Africa, Meridian Hiking Club) can put you in contact with people who habitually go to desolate places that are great for astronomy. Generally speaking, the farther you are from a city, the better the sky. Unfortunately, mines and other non-urban industry can illuminate huge areas.

* More important than the sky brightness is ambient lighting in your locale. Glare from dysfunctional fittings like security lights will severely constrain your ability to see anything, because it interferes with your vision. Explain your problem to your neighbours and negotiate a solution (e.g. shielding, realignment of a fitting, turn-off for an agreed time). Finally, the Sun, Moon and planets are excellent targets even from a light-polluted city.

Caption: Figure 23. Common telescope types
Star chart orientation guide

Date     18h   19h   20h   21h   22h   23h   00h   01 h   02h

Jan 01               O     P     Q     R     S     T      U
Feb 01               Q     R     S     T     U     V      W
Mar 01         R     S     T     U     V     W     X      A
Apr 01         T     U     V     W     X     A     B      C
May 01   U     V     W     X     A     B     C     D      E
Jun 01   W     X     A     B     C     D     E     F      G
Jul 01   A     B     C     D     E     F     G     H      I
Aug 01   C     D     E     F     G     H     I     J      K
Sep 01         F     G     H     I     J     K     L      M
Oct 01         H     I     J     K     L     M     N      O
Nov 01               K     L     M     N     O     P      Q
Dec 01               M     N     O     P     Q     R      S

Date     03h   04h   05h   06h

Jan 01   V     W
Feb 01   X     A
Mar 01   B     C     D
Apr 01   D     E     F
May 01   F     G     H     I
Jun 01   H     I     J     K
Jul 01   J     K     L     M
Aug 01   L     M     N     O
Sep 01   N     O     P
Oct 01   P     Q     R
Nov 01   R     S
Dec 01   T     U

Table 15. Popular binocular sizes

Size    Pros & cons

7x35    Easily handheld; excellent wide-field views
        of Milky Way and deep-sky objects. Smaller
        exit pupil restricts dark-sky effectiveness.
7x50    Easily handheld; light-gathering ability
        sufficient for hundreds of objects; best
        choice. Larger aperture may cause sky-glow
        problems in light-polluted urban and
        suburban areas.
10x50   Good choice for urban and suburban users
        who want higher magnification. May require
        a tripod.
10x70   Excellent for clusters, nebulae and
        galaxies. Heavy; tripod usually needed.
11x80   Excellent for faint objects. Heavy; tripod
        needed for extended use.

Source: Phil Harrington (1990) Touring the Universe
through Binoculars. Wiley Science Editions.

Table 16. Sky brightness

Location                                 mag./[arcsec.sup.2]

Sutherland (SAAO observing site)                21.7
Gansvlei Farm, Brandfort (Free State)           21.5
Bonnievale (Western Cape)                       21.5
Britstown (Northern Cape)                       21.2
Paardeberg (Western Cape)                       21.1
Eselfontein Ecocamp (Ceres)                     20.3
Brackenfell, Cape Town (suburbs)                18.7
Military History Museum (Johannesburg)          18.1

Comparison values

Haute-Provence Observatory (SE France)          21.8
Lick Observatory (California, USA)              20.7
Clear sky 30 min after sunset                    15
Clear sky 15 min after sunset                    13
Sunset at horizon                                10
Uranus                                           8.6
Jupiter                                          5.7
Full Moon                                        3.6

Sky brightness, expressed as visual magnitudes per
square arcsecond, as measured at selected locations
in South Africa and elsewhere using a SQM-L meter.

Table 17. Sidereal time

Date     sidereal
2019     time

Jan 01   06h 41m 25.6s
Jan 15   07 36 37.4
Feb 01   08 43 38.9
Feb 15   09 38 50.6
Mar 01   10 34 02.4
Mar 15   11 29 14.1
Apr 01   12 36 15.5
Apr 15   13 31 27.3
May 01   14 34 32.1
May 15   15 29 43.9
Jun 01   16 36 45.3
Jun 15   17 31 57.1
Jul 01   18 35 02.1
Jul 15   19h 30m 13.9s
Aug 01   20 37 15.3
Aug 15   21 32 27.1
Sep 01   22 39 28.5
Sep 15   23 34 40.2
Oct 01   00 37 45.1
Oct 15   01 32 56.8
Nov 01   02 39 58.2
Nov 15   03 35 10.0
Dec 01   04 38 14.9
Dec 15   05 33 26.7
Dec 31   06 36 31.7

The table lists sidereal time at longitude 30[degrees]
for 02:00 SAST. Corrections for other places:
Bloemfontein subtract 15 minutes, Bulawayo--6m,
Cape Town--46m, Durban +4m, East London--8m,
Grahamstown--14m, Johannesburg--8m,
Kimberley- 21m, Port Elizabeth--18m, Pretoria--7m,
Harare +4m and Windhoek--52m.
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Publication:Sky Guide Africa South
Date:Jan 1, 2019
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