Introduction to Observing Double Stars.

I few years ago, I looked through a lovely long refractor . Up until then I’d used Newtonian’s , our school proudly had the only 6″ Newt in the county ! I was completely blown away by the view of the “double double” in Lyra. Not only were the stars pin sharp marbles, but the gaps between them were dark and well defined. Something I had never seen. That hooked me into longish achromatic (f8-f11) refractors and observing double stars. I hope this brief introduction may kick off a few adventures in an often overlooked area.

About half the stars that we see are doubles. The majority were born as triples, doubles and less than a third were single stars. The simplest of equipment will show double stars , their colours and the background field view . Here is the orange and blue of Albireo,


There are optically aligned doubles and binaries which orbit around a common centre of gravity. In addition there are spectroscopic binaries which may , in eclipsing give variable stars. Some of these are so close they react and pull material across the void.
Every constellation , however poor in deep sky objects has an attractive array of easily observable stars. These are ideal for light polluted skies and looking away from the Moon. There is a great deal of history here,reflected in the Herschel , Struve and Burnham catalogues. You can tread in their footsteps and discover very rewarding targets.
Diffraction discs.
Your scope produces Airey rings or diffraction discs of stars at focus. It’s not actually the surface of the star, but the best disc that your scope can produce.
Separation.
Is measured in arc seconds (“). This is an angular measurement equivalent to 1/3600 of a degree.Some quoted separations are widening and some narrowing. The WDSC ( Washington double star catalogue) is continually being updating by amateur observers measuring separations. The close limit is down to the optics, the observer and the seeing (stability). We got down to 1.1″ on 36 Andromedae in both a 5″ and 4” refractor. This is a good minimum for our skies.
Colour.
This is very subjective, stars change colour with age, going through to red carbon stars. However it’s possible to observe colours from lilac to green ! Larger apertures will bleach out colour. 4″ of refractor aperture is the most efficient optical system producing maximum colour. Short refractors will add their own colour fringing due to chromatic aberration (CA), this can be avoided by using longer refractors. A semi apo filter can be added to eliminate ça, there is little advantage in using an apochromatic Triplet for observing. They are harder to make,heavier and more expensive.

The enjoyment of binary stars is the challenge of finding and splitting them and their lovely colours, often there can be great contrast in brightness. Some are multiple groups, such as Meissa.
PA.
Often quoted is the position angle between the star and it’s companion. It’s a useful indication of where to look .
By eye.
There are a few that can be spotted, the optical Alcor Mizar and the pair in the Hyades.
Sources.
Star charts will denote doubles by means of a line through the star. Often they are named as a Struve (Σ) , Otto Struve (ΟΣ) etc. Many named stars are binaries. The following sites are useful,
200 most beautiful stars   http://users.compaqnet.be/doublestars/#a08268
Star splitters.   https://bestdoubles.wordpress.com/
Sky and Telescope,” 2100 double stars for small telescope” by Sissy Haas.
Cambridge “Double Star Atlas”.

Some current doubles.
Gemini.   Separation.  Mag.1. Mag2.   Observations.
Wasat      5.8″.       +3.6.   +8.2    Yellow and a delicate speck of a companion.
Castor.     4.2″.       +1.9.   +3.0.    Further companion at 71″.
Mebsuta.   110.6″.     +3.1.   +9.6.    White and blue.
Kappa Gem. 7.2″.       +3.7.   +8.3    Orange and pale blue. (SAO 79653)

Leo.
Regulus.   176″.       +1.4.   +8.2.    White and delicate pale purple.
Algieba.   4.6″.        +2.4.   +3.6.    Pair grapefruit orange.
90        3.4″-67″.     +6.3-+9.8.       Lovely triple. (SAO 99673)

Ursa Major.
65.      3.9″-63.2″.     +6.2-+8.3.       Triple. (SAO 43945)
57.      5.5″.          +5.4.   +10.7.   White and violet.
53.      1.7″.          +4.3.   +4.8.    “Alula australis ” showcase.

Cancer.
Iota Cancri.   30.7″.       +4.1.  +6.0    The winter Albireo.
Tegmine.   1.1″-5.9″.      +5.1-+6.3      Bright challenging triple. (SAO 97645)

Canes Venatici.
Cor Caroli.    19.3″.      +2.9.  +5.5.    White and blue green.(SAO 63257)

There are plenty to observe and various challenges and projects, such as the “Double Star challenge”, which I made a start on observing and recording from smaller apertures. http://www.billboublitz.com/Haas_Project/Database.html
I hope this has started a few journeys into double stars, under
Clear skies !
Nick.