•What to observe. •Methods used to locate objects. •Using a planisphere. •Angular distance. •Using binoculars. •Using a telescope and a telrad. •Star hopping with a finderscope. •Useful resources – star charts. •So ware that simulates the night-sky. What to Observe The Moon. Diameter about 3474 kms, distance about 384,000 kms.
Comets, asteriods and planets. Example: Jupiter, diameter 140,000 kms, 780 million kms (about 43 light minutes or 0.00008 light years) from the Sun. Jupiter image at right by Chris Turton.
Stars, variable stars and binary stars. Example: the Sun, diameter about 1.4 million kms, distance 150 million kms. You need solar filters or a specialised solar scope to observe the Sun. Example: the binary star alpha Centauri AB, 4.3 light years away.
Open clusters. Example: Jewel Box (NGC 4755), size about 19 light years, distance 6400 light years. Contains around 100 stars. Jewel Box image at right by Rodney Moulder. Globular clusters. Example: M22 (NGC 6656), diameter 97 light years, distance 10,400 light years. Contains approximately 500,000 stars. M22 image at right by Claudio S llo.
Nebulae (clouds of dust and gas). Example: Tarantula Nebula ( NGC 2070) in the LMC, dimensions about 1800 by 1200 light years, distance 160,000 light years. Largest known star-forming region. Tarantula image at le by Rob Keskull.
Galaxies. Example: the barred spiral galaxy M83 (NGC 5236), diameter about 100,000 light years, distance 17 million light years. Contains over 35 billion stars. M83 image at right by Gerry Aarts. Methods Used to Locate Objects The method you use to locate astronomical objects will depend on the type of equipment you are using …
GoTo with motorised mount. Scopes can be set up by doing a two-star alignment. Once aligned, catalogue numbers (e.g. HIP, Messier, NGC) are entered into an a ached hand-held keypad. The RA and Dec is retrieved by the mount’s processor and the mount then moves the scope automa cally to the object. A GoTo motorised mount can also be controlled from an a ached laptop.
Computer-guided Push To. An Argo Navis computer can be used on a Dobsonian. A ached op cal encoders measure the up/down and circular movement of telescope. Once aligned, the computer will use the RA and Dec for a selected object to guide you in pushing the scope to the correct spot (RA 0, Dec 0).
Regardless of the method used, you s ll need to be able to iden fy some major stars to properly set up your GoTo or Argo Navis.
Star hopping (no processor guidance). This is the tradi onal method using maps and star hopping. Most people with Dobsonians do it this way. Using a Planisphere (Star Disk)
A simple “hands on” tool is a planisphere or star disk. It is designed so that it can be used outside at night with a red light torch. Dialing the date and me of night allows you to get a picture of the current night-sky facing north or south. The planisphere (next slide) shows the northern sky for the 1st of Sep at 8pm.
Look for the brightest stars in the sky and iden fy them from the planisphere.
The brightest stars (with the biggest dots) are Spica and Arcturus to the west, Vega and Altair to the North.
From the brightest stars try to locate the stars that form the s ck figures for par cular constella ons such as Lyra, Aquila, Hercules and Cygnus.
If you have a pair of binoculars you could try to find the bright globular cluster M13 in Hercules.
Many of the bright objects on the planisphere can be seen with binoculars.
Angular Distance Astronomers measure apparent distances around the sky are in degrees and frac ons of a degree. 1 cm at a distance of 57 cm marks out about 1 degree.
With your arm held out straight, the distance of your eye to hand is roughly 57cm. The width of your index finger blocks out about 2 degrees of sky. Three fingers block out about 6 degrees.
The long axis of the Southern Cross from Acrux to Gacrux is about 6 degrees. From Altair to Vega is about 34 degrees. You could use hand configura ons of 15+20 degrees to roughly locate Altair from Vega. Using Binoculars
Binoculars are a great way to view the Milky Way and brighter objects in the sky. Using both eyes will give a richer stereoscopic effect. As with telescopes, aperture is important. Larger binoculars will gather more light and let you see fainter objects.
A pair of hand-held porro-prism 7x50mm or 9x63mm binoculars will give a large field of view (about 5 to 6 degrees) and are a good combination of magnification and aperture size.
If you are looking for something in the telescope, binoculars can be used to sweep across the area first and get your bearings.
With binoculars, you should be able to spot at least 30 globular clusters, 120 open clusters and nebulae and 6 galaxies. You may not resolve the stars but you should see some structure. You can also observe the Moon and some mes a nice comet. Using a Telescope and a Telrad
A Telrad is a x1 magnifica on finder used to point telescopes to the right place. It shows the sky the same way as seen in star charts. A Telrad projects three red rings (diameters 0.5°, 2° and 4°) into the air behind the glass window. Before using a Telrad, it should be aligned with your scope using a bright star. Centre the star in your eyepiece then, using the alignment screws at rear, centre the bullseye on the star. Looking at the sky, objects can then be found by shi ing your scope so the bullseye is in the same target area as seen on a map. Whatever is in the bullseye will be in the field of view of your telescope eyepiece. Example: on the map M13 is about 1/3 of the way from eta (η) to zeta (ζ) Herculis. Put the bullseye of the Telrad about 1/3 of the way between the two stars. M13 is about 2.5 degrees from eta (η). The outer ring of a Telrad has a radius of 2 degrees, so eta (η) should be about 0.5 degrees outside this ring. Using a Finderscope Some objects require a more detailed map and the use of a decent finderscope. An 8x50mm finderscope has a field of view around 5.5 to 6 degrees.
A straight-through finderscope will show an inverted image. To centre the crosshairs on an object in the finderscope’s field, you have to move the telescope in the opposite direc on.
Finderscope Cross hairs 6º 3o
6o A right-angle correct image finderscope will show the sky in the same orienta on i.e. the right way up. An object can be centred in the field of view by moving the scope in the same direc on as seen in the finderscope. Ergonomically, right-angle finders tend to be more comfortable to use. Shown is an 8x50 right-angle finderscope on a Skywatcher 100mm refractor like my own. Example: to find the red carbon star V Aquilae with a finderscope. Lambda (λ) and 12 (or i) Aquilae are naked eye and less than 1.5 degrees apart. Move your scope and centre the finder on lambda. V Aquilae can be seen in the finder as a magnitude 7 red star, less than one degree from lambda (λ) or about 1/3 of the way out from the finder’s centre. Star Charts ASTRONOMY 20-- AUSTRALIA A PRACTICAL GUIDE TO THE NIGHT SKY by Glenn Dawes, Peter Northfield and Ken Wallace. An aussie produc on. Atlas of the Southern Night Sky by Steve Massey and Steve Quirk. Another aussie produc on. Has maps of the 81 constella ons seen from southern la tudes and details of interes ng objects for each constella on. Collins Stars and Planets Guide (Fourth Edi on) by Ian Ridpath and Wil Tirion. Uses s ck figures for constella ons. This is an excellent guide for the beginner. It has maps of all 88 constella ons and details of stars and deep-sky objects for each constella on. Sky and Telescope’s POCKET SKY ATLAS by Roger W.Sinno . This is a great resource for amateurs at any level. It has 80 colour maps of the whole sky, 30000 stars to magnitude 7.6 and 1500 deep-sky objects. THE CAMBRIDGE DOUBLE STAR ATLAS James Mullaney and Wil Tirion Not just for the mul ple star enthusiast. It has the same s ck figures for constella ons as the Collins Stars and Planets Guide above. Has 30 colour-coded double-page maps of the whole sky with 25000 stars to magnitude 7.5 and 2400 mul ple stars. In addi on, it has about 900 deep-sky objects. Night-Sky Objects for Southern Observers Rob Horvat Download for free off the web. So ware that Simulates the Night Sky
You can get so ware that acts like a planisphere but does much more. Stellarium is free. There are versions for Mac and Windows. You can use it on a laptop outside but you might need a box around it (open to the front) to prevent damage from dew. A er downloading the program, you must enter your loca on in the Loca on window. The Date/ me window can be used to get a view of the sky for any date and me. The sky view can be dragged up or down, le or right, to centre a constella on. One can also zoom in or out on any object in a region.
You can also purchase other great programs like SkySafari Pro for Macs, iPads, iPhones or Android. Hardware with a built in compass (e.g. iPhone, iPad) will actually show the constella on that the unit is held up to. As with Stellarium, the sky can be dragged or zoomed into. Also, comprehensive informa on is given for any object selected.
The above so ware can also be used to control your GoTo telescope. SkySafari Plus and Pro on an iPad can also control GoTo telescopes remotely using a SkyFi device (a wireless serial and USB adapter) a ached to your scope. Below is a Stellarium screen shot facing north (1st September at 8pm). Below is a SkySafari Pro screen shot facing north (1st September 8pm).