A Question of Accuracy

The question of what is an acceptable level of accuracy for a double-star observer's measurements is one without an easy answer. From the information given to me over the years by professional observers, I have formulated a rough guide that helps judge the accuracy of double-star measurements. Take a few nights to measure a variety of relatively fixed pairs and compare your results with those given in the Double Stars for Calibration table on page 3. For pairs in the 1.0-arcsecond separation range, measurements of separation should not differ more than about ±10 percent and position angle not more than about ±5.0° from the published values. For wider pairs that span around 100 arcseconds, your separation measurements should not vary more than about ±1 percent and position-angle measurements not more than about ±0.5° from the WDS values.

This is only a rough guide, since two highly skilled observers can obtain measures that differ more than the limits set above. If your measurements typically fall within these limits, your results will be of scientific value. If, however, you consistently obtain measures that are far removed from the WDS data, then you should check such things as the optical quality and alignment of your telescope, quality of the micrometer, prevailing seeing conditions, and your measurement technique. The reticle micrometer, just as with any type of visual micrometer, requires a certain degree of proficiency to be used correctly. You should not expect accurate results at first. Practice, in conjunction with meticulous methods, will eventually yield quality results.

Science in the Numbers

It is a common belief among some observers that the CCD camera has rendered visual micrometry obsolete. Nothing could be further from the truth. One of the properties lost with the use of CCDs for double-star measurement, whether by speckle interferometry or integrated imaging, is dynamic range. It is very difficult for CCDs to measure doubles with components of substantially different brightnesses or faint pairs that are very close. For these challenging pairs, visual micrometry still remains the best measurement technique.

Because so few professionals remain in this field, qualified amateurs are badly needed. Double-star micrometry has always had a significant percentage of amateur observers in its ranks. For example, Paul Baize, perhaps the greatest double-star observer who's ever lived, made 25,000 measurements and calculated 200 orbits! With several thousand visual double stars within range of 3- to 8-inch telescopes, today's amateurs are in a unique position to make contributions to double-star astronomy that they previously may not have thought possible.

10 Interesting Doubles
Name	(2000.0) R.A.        Dec.		Magnitudes		(1999.0) P.A.	Sep.	Notes
b Mon AB	06h29.8m	–07°02'	4.7	5.2	132°	7.2"
b Mon AC	06h29.8m	–07°02'	4.7	6.1	125°	9.9"
a CMa	06h45.1m	–16°43'	-1.5	8.5	162°	4.3"	1
a Gem	07h34.6m	+31°53'	2.0	2.6	66°	3.8"	2
z Cnc AC	08h12.2m	+17°39'	5.1	6.2	72°	6.0"	3
i Cnc	08h46.7m	+28°46'	4.0	6.6	307°	30.4"
s2 UMa	09h10.4m	+67°08'	4.9	7.9	354°	3.8"	4
g Leo	10h20.0m	+19°50'	2.6	3.8	125°	4.4"	5
a CVn	12h56.0m	+38°19'	2.9	5.6	229°	19.4"
a Sco	16h29.4m	–26°26'	1.0	5.4	273°	2.9"	6
Notes: 1 Due to the large magnitude difference, the white-dwarf companion, Sirius B, is an elusive and challenging target. 2 There are actually six known components to Castor! The faint C & D components are 72.5" and 204.4" away, respectively, at position angles 164° and 222°. The A and B components are both also spectroscopic binaries. 3 For a challenge look for z Cnc AB, a 0.8" 60-year binary at about 90°. 4 Observers have claimed a marked color difference and a secondary that changes in brightness. Both claims are uncertain. 5 This system has been increasing in separation and slowing down since its discovery by William Herschel in 1782. 6 Antares is a double system consisting of a cool giant and a hot dwarf star. You should see a significant color difference if you can overcome the large magnitude difference.