Measuring double stars

Let's have a look at three double stars I have observed and recorded below, see how we can do some real science with them. At the top are Mizar and the dimmer Alcor in Ursa Major. Bottom right is Mizar resolved to a higher magnification with a 2X Barlow lens and split to Mizar A and B. And bottom left is Cor Caroli, split to α² Canum Venaticorum, the fainter α¹ Canum Venaticorum.

 

Let's start with Mizar and Alcor. The simplest method to measure binary star characteristics is by using what is called the drift method. This method consists of taking a long exposure image or a video recording of a pair, so that its movement across the skies is recorded. The lines produced by the drifting star images (called the drift lines) define the east–west line of travel of the pair in the image.

Once I captured a video of the binary with my CCD imager, I then extracted the first and last frames of the video and saved them as two separate image files. Merging these two images into one single image produces the image below.

Draw a line joining the primary star from the first frame with the primary in the last frame (and repeat with the secondary star). This produces the drift line which defines the east–west line of travel of the pair (in blue above). Draw another line perpendicular to this (in white here( - this would now point to celestial north. Then draw a line joining the primary and secondary star (in red above). You can now measure the position angle (PA) of the binary directly with a protractor. Knowing the pixel resolution of your optics and using simple O-Level coordinate geometry and trigonometry, you can also now calculate the separation (the distance between the two stars, in arc seconds). Simples!

The visual method described earlier will work well for widely-separated pairs like Mizar and Alcor. However, with closely-spaced pairs and pairs where the stars nearly align with the east–west line, the magnification and scales you will be working with may not give you accurate results. In these cases, you may want to use specialised software that will take a video image of a travelling pair and do all the calculations for you at the pixel level. Let's try this with Mizar A and Mizar B below.

 

One simple (and absolutely free!) software programme you can use is BinStar, by Ed Hitchcock ( see http://www.budgetastronomer.ca/index.php?page=binstar ). Just load your video file into the programme, click on the primary star and then click on the secondary star.

Click on 'GO!' and BinStar will play the video and trace the drift line of the binary pairs as it moves across the sky, while automatically working out the position angle and separation.

An even more sophisticated programme (and free as well) is Reduc by Florent Losse (see http://www.astrosurf.com/hfosaf/Reduc/Tutorial.htm ). Let's now examine Cor Caroli below using Reduc.

You first load in the video file and Reduc will then extract all the frames from the file and save them as individual image files.

Reduc then reduces each frame individually to produce a pretty accurate drift analysis, below.

Position angles and separations are then calculated for each frame - you can save these results and manipulate the data in whatever way you please!