There are many guidelines and methods for determining whether a telescope of particular aperture can split a particular double star. Obviously very close pairs, and those with large magnitude differences, are more difficult to split than others. However there are many other factors which contribute, most importantly the seeing, magnification and the acuity of the observer. The AAQ has embarked on a programme to observe a wide range of doubles (listed in the monthly newsletter) with many observers and different telescopes, to develop a guide to what a particular ‘scope can do. It’s easy to participate, and your observations can add important information to the study, as well as introducing you to the fun and challenge of observing doubles. Instructions and observing report forms can be downloaded.

Objective of the Survey

The main objective is to have fun and see some beautiful and challenging sights! But in doing so, we can collect data that will allow us to make some statements about how resolution (in simple terms, the ability to see detail) depends on the observer, the telescope and of course the object itself. This is an interesting issue in its own right, but can also help observers decide on what might be possible with their combination of ’scope, observer and conditions. One of the best ways to test resolution is to observe double stars and simply state whether you can see them as separate points of light or not. And that’s what the survey asks you to do.

What To Do

First download, print and complete the Observer's Information Form to create the foundation for your Observations.  Observer Form (73.51 kB)

Second download an introduction and instructions to prepare for observing.  Introduction to the Resolution Survey (28.17 kB)

Then each month, 5 doubles of different characteristics will be identified in Graeme Jenkinson’s observing list in the AAQ newsletter as being the survey pairs for the month. They will also appear below.  You  are asked to observe them and then download, print and complete the Observation Report Form (PDF 434KB).

You will be asked to state whether you have been able to “split” the pair, that is distinguish the two stars (there is more than one interpretation of this – see below). Observe them more than once under different conditions if you like. Every observation is a help to developing the database.

Give the completed forms to Tim Napier-Munn at the next monthly meeting, or Progress reports will be given at meetings and in the newsletter.

Observing Hints

How to interpret the image - The image of a star in a telescope is not a point of light but a light disk surrounded by concentric light and dark rings, called the Airy pattern, due to diffraction. In poor seeing the image is a moving smudge of light. In deciding whether you can separate the two stars, look at the light disk(s) and try and decide whether there is space between them, or if they are touching, or look like a elongated disk, or just look like one disk, something like these:

Separated- clear        Separated - notch effect      Maybe - elongated                Not   

Seeing - “Seeing” is the limitation the atmosphere places on the quality of the image. An easy pair one night may be impossible the next, purely because of worse seeing conditions. In fact observing “standard” difficult doubles is a good way to assess the seeing. But don’t be put off if the seeing is poor. The effect of seeing is one of the variables we want to understand. See the observing form for more details.

Check it – For the more difficult pairs, the best way to confirm whether you have resolved it is to check the position angle (PA) to see if it is about the same as the listed value. The PA is measured from the primary (brightest) star from North in an Easterly direction, so the PA in the diagram is about 135 0. If you are not sure which is North and East, either switch off the drive (the star will drift due West) or nudge the  scope tube in a known direction and see which way the star moves. The orientation will depend on the scope.

Magnification - We are all taught (rightly) that magnification isn’t everything. But it is more important in observing doubles than in most branches of astronomy, because it has to be large enough to allow the eye to resolve the separation, which can be very small (less than 1 arcsecond in extreme cases). The general rule is: for close doubles, build up the magnification from low to high until the separation is apparent (if at all). The maximum magnification that can be sustained will be limited by the seeing. An analysis of the work of a number of double star observers gave this correlation for the most common magnifications they used: 89ÖD, where D is the telescope aperture in cm, eg 347x for a 6”, 449x for a 10”, and 531x for a 14”. And sometimes much more! Picking the best magnification comes quickly with practice. Don’t be put off by published rules of thumb which often advise lower maximum “useful” magnifications. They are usually driven by not wasting the light in the exit pupil, which in our case is not an issue.

Finder Charts for Target Doubles

The following finder charts are available for the test doubles in each of the target constellations:

1.  Finder Charts for Test Doubles in Circinus (88.08 kB) 
2.  Finder Charts for Test Doubles in Eridanus (148.6 kB) 
3.  Finder Charts for Test Doubles in Norma (90.65 kB)
4.  Finder Charts for Test Doubles in Orion (83.69 kB)
5.  Finder Charts for Test Doubles in Scorpius (90.89 kB)
6.  Finder Charts for Test Doubles in Triangulum Australe (201.49 kB)
7.  Finder Charts for Validation Doubles (141.11 kB)