Lawrie
Joined: 25 Sep 2008
Posts: 13
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 Posted: Fri Nov 21, 2008 8:23 pm Post subject: Double Stars in Orion |
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Double Stars in Orion
Orion is probably the most recognisable of all constellations. It very prominent now and is observable at any time on any clear night from late autumn throughout the winter. It is also full of interesting objects particularly for visual observers with a modest telescope, the most obvious being the Great Orion nebula (M42) in the "sword handle" of the constellation. However there are other objects well worth finding and studying which are not quite so obvious as M42. Of these double stars are interesting to find and a pleasure to observe. Even if the sky is not crystal clear, they can still be found since they are often quite bright and frequently colourful. Our Victorian forefathers made a great study of them. The first catalogues were compiled in the early and mid 19th century, primarily by Frederick Struve and his son Otto at the Pulkova Observatory in what is now Estonia. Many doubles are referenced to them in atlases and on charts as STFnnn and STTnnn . Other observers followed with their own catalogues. These days the master catalogue is maintained as the Washington Double Star Catalogue which incorporates all the other catalogues. The venerable Norton Star Atlas, particularly in its older editions, reflected the Victorian tradition by devoting only a quarter page each to variable stars and to nebulae, but giving a whole page to a listing of doubles, for each of its charts. In its notes, Norton lists eleven bright doubles in Orion with magnitudes of from 0.3 to 6.2, all well within range of binoculars and a small telescope. Observing these makes for an interesting project for the winter.
To do this systematically a little preparation is necessary using a computer with a planetarium program. On my computer I have two such programs, Sky MapPro, which is excellent, and Cartes du Ciel which is almost as good and is free! I say almost as good because with SkyMap Pro a "target" list of objects can be created which links to the program's search facility. Cartes du Ciel does have an "object List" for each of its displays but the list cannot be created or edited by the user and it has no link to the program's "Find" feature. However I mention the program for another reason.
First I make a list of the doubles as my target list, in my case the eleven doubles in Orion as they appear in my old and treasured copy of Norton's Star Atlas -(1959 edition). Using the target list, I print out a chart for each object from the planetarium program. To do this I first put in a circle representing the Field of View of my telescope at its lowest magnification and "zoom in" on the object until the display shows mostly my FOV and some area around it. Both SkyMap Pro and Cartes du Ciel will print black on white charts but of the two programs I prefer printing the charts from Cartes du Ciel program.
My 150mm Dobsonian reflector has no RA or Dec circles but I have added Altitude and Azimuth scales which makes finding objects vastly easier. To get these coordinates I use my laptop computer at the telescope. Using SkyMap Pro I take each object in turn from my target list and bring it to the centre of the map display. Placing the cursor on the object I get the readout for altitude and azimuth which I set on the telescope. Usually the object is in the FOV but otherwise a small adjustment in Alt or Az and a perusal of the chart soon finds it. It is worthwhile spending a few minutes studying the double, particularly for colours. Then I enter the observation in SkyMap's log facility.
The computer is necessary as the Alt and Az of a star changes with time and even a few minutes between noting the readouts and setting the telescope is quite enough for the coordinates to have changed sufficiently for the object to be out of the FOV. I take care to make sure that the computer's clock is set to UT and the display's time control is set to Real Time or in the case of Cartes du Ciel to System Time.
The printed charts are normally orientated with North at the top whereas the telescope has an inverted view but it is easy to get used to this although it is quite simple to print charts "upside down" in the first place if this is more convenient. Mental orientation i.e. knowing which way is North in the FOV is easily found with a Dobsonian by setting a star in the centre of the FOV and letting it drift which will always be due West. And so when a clear sky occurs all I have to do is check the date and time on the laptop, grab my file of charts and get out there!
There are many more doubles in and around Orion and the ones listed in Norton are the brightest. It is an interesting exercise to find some of your own and to see if they are in the databases of the planetarium programs or in the WDS catalogue. Many are.
Some observers like to make a small sketch of the double star as it appears in the telescope eyepiece and making a display of these sketches for the doubles in a constellation. Others can take things much more serously and make measurements of separations and position angles.
This project approach is of course applicable to other constellations and for that matter to other objects such as variable stars. It is just as you fancy takes you.
Several good books on double stars:
Double and Multiple Stars( and how to observe them). James Mullaney, Springer. A real enthusiasts book!
Observing and Measuring Visual Double Stars. Ed. Robert Argyle. Patrick Moore's Practical Astronomy Series. A thorough amateur and semi-professional guide.
Observing Visual Double Stars. Paul Couteau. MIT Press. A readable professional book by a professional astronomer. Past director of the Nice Observatory.
A Visual Atlas of Double Stars. Mike Ropelewski. The Webb Society. An excellent observers atlas.
Lawrie |
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