Observing double stars is a very attractive and interesting occupation. There is an enormous amount of double (and multiple) stars. Some of them are so close, that their binary signature can only be discovered by means of spectrography. Binary stars that are sufficiently apart so that they can be separated visually, are called 'visual double stars'. These stars are particularly interesting for amateurs. More, this is an area where amateurs can contribute scientifically with relative cheap and simple methods.

To observe double stars, you don't need a splendid and very dark sky. Light pollution is less disturbing and the transparancy of the sky is less important too. There is no problem observing double stars during full moon.

The most important properties of a double star are :

  1. Separation : the (apparent) separation between both components ( in arc seconds)

  2. Position Angle : the angle of the vector from the main star to the companion ( degrees ). This angle is measured from N (0 degrees), through E (90 degrees), S (180 degrees) and W (270 degrees).

  3. The color of both stars.

There are different ways to observe a double star.

  1. You can simply observe it visually and enjoy the sight. Many doubles have a nice color contrast because the nature (spectrum) of both stars is quite different. At the same time, you can try to estimate the position angle. Estimating the separation is not always easy, but the position angle can ( after some experience ) be estimated with an accuracy of 5 degrees ( or even better ). Also, try to have an idea of the color of both stars. Don't be surprised if you see the color differently than others do. This is a normal phenomena, since seeing colors in stars ( apart from a number of very obvious cases like Albireo ) is not that simple and is quite subjective. It also depends on the eyes (and brain) of the observer and the optics used. It is great fun however to exchange your results with others.

  2. It is also possible to measure the separation and the position angle. There are different ways to do this:

    1. The 'microguide eye piece' : this is an eye piece with a short focal length and a special illuminated reticle. This reticle contains a linear scale to measure the separation and a round scale with marks to measure the position angle. With such an attribute, it is possible to make measurements that are quite accurate.

    2. Measuring separation and position angle is also possible with a CCD camera ( or another digital camera with the possibility to make exposures up to about 30 seconds ). The principle is simple : put a double star in your field of view. Best is to position it at the East side of your image. Start an exposure of about 30 seconds. After 3 or 4 seconds, switch OFF the right ascension motor of your mount. The double star will now start to drift to the West. The aquired image will finally look like 2 small balls with 2 long star trails. Export the image as a jpg file. Finally, separation and position angle can easily be measured using a simple program called "starposit". For Starposit : see here.

Double stars

Observing double stars


©1993-2013 Andromeda - Vereniging voor sterrenkunde van de Dendervallei

The most frequently used catalog of double stars is the WDS ( Washington Double Star Catalog ). This catalog contains about 100000 visual double stars! To make a selection from this enormous amount of binaries is not always that easy. This is the main reason why we developed BINSTARS

The package contains the complete WDS catalog along with the reference and 'notes' files. With Binstars you can do the following:

  1. Make a selection of double stars from the entire WDS catalog. You can select on:

    1. Constellation : The WDS contains a lot of data like the coordinates of a binary, but there is nothing about the constellation where it can be found. (because this is in fact not scientifically relevant). That is why we made a routine that finds the constellation by using the coordinates and the borders of the constellations.

    2. Minimum separation of the components ( in arc seconds )

    3. Maximum magnitude of the main star.

    4. Maximum magnitude of the companion star.

    5. Maximum magnitude difference between both components.

    6. Catalog: In the WDS there is also a reference to a subcatalog ( mostly the discoverer ). The designation of a double starts with an abbreviation that refers to this discoverer. Stars beginning with 'STF' for instance are doubles from the catalog of Struve.

  2. Once you made a selection, it can be saved and called back later. This way, there is no need to search the entire WDS again.

  3. The selection can also be exported as a text file. This file can then be opened in another program or printed.

  4. For each double star, Binstars will show you all relevant data, such as : the separation and the position angle of the first observation ( at time of discovery) and the latest observation, the number of observations within this period, the magnitude of both stars, the name of the discoverer (from the reference file) and (eventually) some notes that can be found in the WDS 'notes'.

  5. For each star, Binstars can print a finder chart for you completely automatically. This finder chart can be printed in direct mode or mirrored ( in case you have a diagonal prism on your finder scope ). The finder chart is printed with just one button! The chart will always consist of 2 parts on 1 page : The upper part has a field of about 45 degrees and is meant for the naked eye or a 'telrad' pointer. This part is never mirrored. The bottom part is a detailed map with stars up to magnitude 10. This one can be used for your finder scope and can be mirrored or not. At the top of the page, you will find the data you have chosen in the 'options' part.

  6. It is also possible to print a finder chart for any object without selecting a double star. Just give the coordinates of the center and give a title for your map.

  7. For each star, you can display a map on the screen to show the position of the star.

  8. All your observations can be logged in Binstars.

  9. In case you logged an observation with a measurement of the separation and the position angle, you can compare your result graphically with the results of the WDS.

  10. By means of the 'html' button, binstars will automatically make html pages of all observations. These pages will be ready to publish on the web. You can have your choice : observations of all observers, or, only observations of a particular observer ( yourself for instance).

  11. In order to exchange observations between different observers, it is possible to export observations from a particular observer (or all) and over a specific period. The result will be a text file which can then directly be imported by someone else.

  12. etc ...


This is an example how Binstars generates the html files. Click here

Remark : The text of the observations here are in Dutch language. If your Binstars is switched to 'English', it will generate English html files. The text of the observations themselves however remain in the original language of the observer.

Observations from Binstars

The complete Binstars package can be downloaded FREE OF CHARGE

Proceed as follows:

  1. Download the zip file.

  2. Make a directory on your computer and unpack the zip file there.

  3. That's all folks! The program itself is called 'binstars.exe'. There is a complete English manual in the file binstars_e.doc ( a Microsoft Word document ).

The default language of this program is Dutch. However, simply click the flag in the upper right corner to switch over to English!

The most recent version of Binstars is 4.15

Click here to download the latest version! Enjoy!

Binstars was written by Patrick Mergan.

Downloading Binstars

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