AAQ Special Interest Sections

Double Stars

Welcome to the Double Star Section of the AAQ, dedicated to the enjoyment of observing double and multiple stars.

You can take in an introduction to Double Stars, and then jump to further information on Double Stars for a list of books, journals and web references.

When you are ready to practice what you've read we'd invite you to take part in the AAQ Resolution Survey described below. By simply using a telescope, your eyes and some easy to learn techniques you can discover more about your equipment and the sky, and contribute to some useful astronomical research.

AAQ RESOLUTION SURVEY Observing Double Stars

Latest Targets -

• A special challenge - difficult pairs to generate independent data (PDF 142KB) to test the accuracy of the new AAQ model for predicting double star resolution.

• February - Test Doubles in Vela

• March - Test Doubles in Leo

Submit your Resolution Survey observations -

• download a Observation Report Form (PDF 434KB) and the instructions (PDF 29KB)

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.

First download, print and complete the Observer's Information Form (PDF 75KB) to create the foundation for your Observations.

Second download an introduction and instructions (PDF 29KB) to prepare for observing.

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.

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.

RESOLUTION SURVEY TEST DOUBLES

STT 517 (RA. 05 13.5 Dec. +01 58) mags. 6.8 & 7.0, sep. 0.7”, pa. 241˚ (2004). Easier than it looks so persevere, particularly if you have an 8” scope or larger. High magnifications required (>300x).

KNT 3 (RA. 05 26.8 Dec. +03 06) mags. 4.6 & 8.6, sep. 2.9”, pa. 327˚ (1991). There is a strong magnitude contrast but it should be accessible to most scopes. Averted vision can help find the secondary.

STF 728 (RA. 05 30.8 Dec. +05 57) mags. 4.4 & 5.8, sep. 1.2”, pa. 46˚ (2004). A combination of some magnitude difference and close separation makes this pair a bit tricky. Use high magnification, and seeing will really count.

STF 774 (Zet Ori, Alnitak) (RA. 05 40.8 Dec. -01 57) mags. 1.9 & 3.7, sep. 2.6”, pa. 166˚ (2002). Should be easy but the primary is very bright so there is some glare to contend with. Don’t be confused by the dim star much further away (about 1’).

STF 795 (RA. 05 48.0 Dec. +06 27) mags. 6.0 & 6.0, sep. 1.1”, pa. 216˚ (2003). This is the classic Dawes test, as both components are magnitude 6 which Dawes defined as his test condition. His formula suggests that a 106mm scope should just split this pair.

Download a PDF print version of the eyepice finder charts (91KB).

HJ 3527 (RA. 02 43.3 Dec. -40 32) mags. 7.0 & 7.2, sep. 2.1”, pa. 43˚ (1993). Should be possible in most telescopes. Good test as the stars are of similar magnitude.

HJ 3527 (RA. 02 43.3 Dec. -40 32) mags. 7.0 & 7.2, sep. 2.1”, pa. 43˚ (1993). Should be possible in most telescopes. Good test as the stars are of similar magnitude.

The next two pairs represent a triple star. Try and record both pairs.

 HJ 3556 (RA. 03 12.4 Dec. -44 25) mags. 6.4 & 8.8, sep. 3.7”, pa. 190˚ (2002). Easy.

JC 8 (RA. 03 12.4 Dec. -44 25) mags. 6.4 & 7.4, sep. 0.7”, pa. 170˚ (2002). This is easier than it looks so persevere but use high magnifications – preferably over 500x.

JC 1 (RA. 03 19.5 Dec. -21 45) mags. 4.0 & 9.5, sep. 5.9”, pa. 289˚ (1987). Moderately difficult, due to the magnitude difference, but most scopes should get it. Don’t be confused by another star 180˚ to the companion and about 6 times further away. Use fairly high magnifications to get separate the secondary from the primary.

BU 12 (RA. 03 24.4 Dec. -14 00) mags. 7.0 & 9.1, sep. 2.4”, pa. 281˚ (2001). Most scopes should get this, but a little tricky with the mag difference and separation.

Download a PDF print version of the eyepice finder charts (91KB).

HLD 124 (RA. 15 45.0 Dec. -50 47) mags. 6.6 & 8.4, sep. 2.2”, pa. 194˚ (1991). Should be possible in most telescopes in good seeing.

HJ 4813 (RA. 15 55.5 Dec. -60 11) mags. 5.9 & 8.4, sep. 4.4”, pa. 100˚ (2000). Easy.

Iot Nor, HJ 4825 (RA. 16 03.5 Dec. -57 47) mags. 5.3 & 5.5, sep. 0.5”, pa. 295˚ (2000). For larger ’scopes. There is an 11th mag. companion 8” away at pa 243˚, but it is the bright one we want to split. I have done it in the 14” SCT at magnification 861x.

I 558 (RA. 16 15.6 Dec. -56 41) mags. 8.2 & 9.7, sep. 1.7”, pa. 50˚ (1991). Moderately difficult, due to a combination of the primary magnitude, the magnitude difference, and the separation. Seeing will be important here.

COO 197 (RA. 16 25.3 Dec. -49 09) mags. 8.1 & 8.2, sep. 2.1”, pa. 109˚ (1991). A good test for smaller ’scopes because the stars are of similar magnitude. Dawes says a 2” ’scope can split this, but that is based on stars around 6th magnitude. The current AAQ model predicts a 50% probability of splitting with a 4” ’scope at average seeing (3).

Download a PDF print version of the eyepice finder charts (91KB).

BU 36 / 2 Scorpii (RA. 15 54.0 Dec. -25 21) mags. 4.7 & 7.0, sep. 2.1, pa. 269˚ (1991). Moderately difficult but many scopes should get it.

Xi Scorpii, STF 1998 (RA. 16 04.4 Dec. -11 22) mags 4.9 & 5.2, sep. 0.9, pa. 352˚ (2007) A good test for sub-arcsecond resolution. It’s easier than it seems, but needs high magnification. There are two pairs in the same wide angle field (a lovely view) but it’s the brightest star you want; it has a dimmer star an easy 7.6” away.

Nu Scorpii (RA. 16 12.0 Dec. -19 28) mags. 4.3 & 5.3, sep. 1.3” pa. 2˚ (2003) The famous double double. The dimmer of the two pairs should be fairly easy at 2.4” separation. The one we want is the brighter and closer pair to the south.

Alpha Scorpii, Antares (RA. 16 29.5 Dec. -26 25) mags 1.0(v) & 5.4, sep. 2.5”, pa. 274˚ (1997) One of the most famous of all. The problem is that the secondary tends to get drowned out by the great red supergiant. Persevere with this one. Seeing is important.

I 99 (RA. 16 49.5 Dec. -43 57) mags. 8.0 & 8.7, sep. 1.0”, pa. 65˚ (1991) A tough one.

HDO 260 (RA. 16 47.5 Dec. -45 28) mags. 7.7 & 8.8, sep. 0.5”, pa 356˚ (1991) There are two similar magnitude stars in the same high magnification field; the target is the one NE. This one is only for those of a strong disposition! (aka big scopes). Dawes’ Limit says that a 9” should just resolve it (not allowing for the magnitude difference). I have resolved it in my 14” SCT in very good seeing at 861x. As usual, “No Split” reports are important.

Download a PDF print version of the eyepice finder charts (93KB).

HJ 4632 (RA. 13 59.0 Dec. -65 50) mags. 6.4 & 9.5, sep. 6.5”, pa. 14˚ (1991) The orange primary has a wide but faint companion. Should be OK for most scopes.

DON 680 (RA. 14 50.1 Dec. -67 06) mags. 7.5 & 9.8, sep. 1.5”, pa. 248˚ (1991) With a magnitude difference more than 2, and a close pair, this will be a challenge for smaller scopes.

WDS I 329 (RA. 15 14.0 Dec. -61 21) mags. 6.7 & 7.7, sep. 0.9”, pa. 339˚ (1991) The magnitudes are similar so this is a good test for the sub-arcsecond resolution capability of medium size scopes; persevere with this one. This double is located in a straight line with HDO 246 and WDS I 428 (see above) and they are all located just inside the same wide angle field of view.

B 1777 (RA. 15 16.6 Dec -60 54) mags 5.8 & 8.6, sep. 1.3”, pa 164˚ (1991). This is a tough one, and will probably need good seeing and at least a moderate size scope. Note that there are two similar magnitude stars in the same field. The one you want is the westerly of the two. It has a faint (mag 11.6) companion at 11” and 315˚ pa, but this is not the test ! It’s a much closer companion that we’re looking for.

CPO 16 (RA. 15 30.4 Dec. -58 22) mags. 7.0 & 8.0, sep. 2.4”, pa. 32˚ (1991) This should be accessible to most scopes in reasonable seeing.

Go to finder charts for the Circinus stars or download a PDF print version (90KB).

I 240 (RA 15 36.2 Dec. -65 07) mags. 7.2 & 10.8, sep. 2.5”, pa 187˚ (1991). Quite hard with large magnitude difference and close.

RMK 20 (Hartung 708) (RA. 15 47.9 Dec.-65 27) mags. 6.2 & 6.4, sep. 1.8”, pa. 147˚ (1991). Similar magnitudes; good direct test of resolution.

HJ 4813 (RA. 15 55.5 Dec. -60 11) mags. 5.9 & 8.4, sep. 4.4”, pa.100˚ (2000). Magnitude difference of 2.5 but sufficient separation to be resolvable in most scopes.

HDS 2352 (RA 16 37.8 Dec -64 15) mags. 7.7 & 10.3, sep. 10.9”, pa 148˚ (2000). Should be easy despite the faint secondary.

HDO 257 (RA 16 46.7 Dec -67 07) mags. 5.1 & 11.5, sep. 25.0”, pa 123˚ (1900). This is testing the limiting magnitude of your scope and conditions. And its an interesting one. There are two secondaries of similar magnitudes close to each other; the one we want is probably the closer of the two and I think the brighter, so if you see either then count it as a hit. The two measures in the records are 30” at pa 180˚ (1891) and 25” at pa 123˚ (1900). My measure of the closer one is 28.5” at pa 194˚, so there may be some confusion of identity in the historical measures; it is unlikely to have moved 57˚ in 9 years so I think at least the 123˚ is wrong.

Go to finder charts for the Triangulum Australe stars or download a PDF print version (202KB).

Epsilon ( e ) Hydrae (RA. 08 46.8 Dec. 06 25) mag. 3.5 & 6.7, sep 3.0”, pa. 299 (2003). Test of large contrast in magnitudes (more than 3) at a fairly close separation.

BU 590 / 29 Hydrae (RA. 09 27.3 Dec. -09 15) mag. 7.8 & 11.3, sep. 10.7”, pa. 171 (1999). This tests whether you can see the dim secondary. Use averted vision if necessary.

BU 411 (RA. 10 36.2 Dec. -26 43) mag. 6.7 & 7.8, sep. 1.3”, pa. 310 (1998). A tough one ! Use high powers – at least 300x, and more if you can (500x plus). Don’t forget that “Can’t see” reports are also very useful.

HJ 4455 (RA. 11 36.6 Dec. -33 36) mag. 6.0 & 7.8, sep. 3.4”, pa. 241 (1991). Should be splittable in most scopes with sufficient power.

54 Hydrae/H 3 97 (RA. 14 46.0 Dec. -25 27) mags. 5.1 & 7.3, sep. 8.1”, pa 124 (1998). Should be straightforward.

Omega / Struve 1356 RA. 9 28 52, Dec. 9 01 28. mags. 5.7 & 7.3, pa. 88 0, sep. 0.6” (2002). A difficult test for larger ‘scopes.

 49 / Struve 1450 RA. 10 35 26, Dec. 8 36 42. mags. 5.8 & 7.9, pa. 157 0, sep. 2.2” (2003). Two magnitudes difference and fairly close – a good test !

55 / BU 1076 RA.10 56 06, Dec. +00 41 49. mags. 6.0 & 9.0, pa. 59 0, sep. 1.0” (1999). Tougher – more magnitude difference and a close double.

KUI 56 RA. 11 14 26, Dec. 8 01 10. mags. 5.8 & 11.8, pa. 252 0, sep. 22.9” (2000). This tests the limiting magnitude of the ‘scope and conditions.

 i (Iota ) RA.11 24 18 , Dec.10 29 30. mags. 4.1 & 6.7, pa.125 0, sep.1.5” (2003). Quite hard but should be possible for some ‘scopes in good seeing.

I67 Vel (Hartung 337) Mags 5.2 6.2 Sepn. 0.7” PA 137 0 (last measured 1998).

RA.08 22.5 Dec. -48 29. A test for larger apertures.

h 4104 Vel (Hartung 340) 5.5 7.3 3.0” 250 0 (1999)

RA 08 29.1 Dec. -47 56 This is a pretty triplet but concentrate on the close double. Should be visible in most ‘scopes.

R87 Vel (Hartung 368) 4.9 7.7 2.6” 335 0 (1998)

RA 08 56.3 Dec. -52 43 Nearly two magnitudes difference.

h 4188 (Hartung 381) 6.0 6.8 2.8” 281 0 (1992)

RA 09 12.5 Dec. -43 37 A direct test of resolution with two very similar magnitude stars.

See 118 Vel (Hartung 410) 6.7 12.0 15.2” 142 0 (1934)

RA 10 15.1 Dec. -55 33 This tests the limiting magnitude of the ‘scope and conditions.

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