A Night at the Telescope

For the double-star observer, there is nothing more inviting than a clear and steady night. After polar aligning, the first order of business is to make sure that your reticle eyepiece's position-angle scale is set up correctly. The inevitable jostling that the telescope is subject to between uses warrants that this be checked at the start of every observing session. Fortunately, the procedure is straightforward. I do this by allowing a star near the celestial equator and on the meridian to trail along one of the two parallel lines of the micrometer's linear scale. This is done with the telescope's drive off and without using a Barlow lens. The micrometer is rotated in the eyepiece holder until the star follows the reticle line very accurately. Once this condition is met, I rotate the position-angle protractor until the 90° mark is indicated by the pointer, taking care not to disturb the eyepiece position. This process is repeated with the Barlow. The pointer is now calibrated with the north point at zero and east at 90°.

With the preliminary adjustments out of the way, you're all set to begin making double-star measurements. The pair being measured should be at least 30° above the horizon to reduce differential refraction and well within the magnitude and resolution limits of your telescope. Begin by locating your first pair and boosting the magnification until the stars span as many reticle divisions as practical. The method I find best for measuring separation with the Microguide reticle micrometer is to carefully rotate the eyepiece until the primary and secondary stars are parallel to the linear scale and the primary is dead center on one of the scale's divisions. Estimate the separation of the pair to 1/10 of a division and convert this into arcseconds using the calibration value for the setup. That's all there is to it — you have just measured the star's separation.

Before you start celebrating, keep in mind that there is a second equally important measurement to make: the position angle of the pair. Begin by rotating the eyepiece until one of the reticle lines of the linear scale bisects both the primary and the secondary at the same time. Now is a good time to note the approximate position-angle reading on the degree scale. This will serve as a reality check to ensure that you haven't made a mistake and that your position angle is near the expected value for the stars being measured.

When you are confident the reticle is correctly positioned, take an accurate reading off the degree circle. Then rotate the reticle 180° and repeat the reading. This procedure is done again for a total of four readings. The results are averaged (remember to subtract 180° from the readings obtained by rotating the reticle). Keep in mind that when the primary star is centered in the field of view, the companion is said to have a position angle of 0° if it is directly to the north of the primary, 90° when directly east, 180° when directly south, and 270° when directly west. Be careful to avoid accidentally offsetting your measurement by 90° or 180°. This is where your reality check comes into play. For greatest accuracy, the measurements of separation and position angle should be repeated on four to six nights and the results averaged.

13 Neglected Double-Star Systems
Name	(2000.0) R.A.         Dec.		Magnitudes		P.A.	Sep.	Year
MLB 639	04h01.1m	+27° 33'	8.7	10.3	267°	7.3"	1930
COU 3244	05h00.0m	+32° 44'	9.8	13.5	87°	2.9"	1973
HDS 817	06h00.1m	+51° 26'	8.7	11.9	251°	17.0"	1991
POU 2367	07h04.2m	+23° 24'	7.6	12.9	166°	27.2"	1907
HJ 453	08h38.6m	+34° 29'	9.0	14.3	102°	19.0"	1903
BAL 2368	09h52.2m	+03° 13'	8.9	9.9	37°	13.2"	1910
HEI 354	10h03.7m	+06° 00'	9.5	13.5	340°	3.6"	1988
LDS 3032	11h16.1m	+54° 14'	9.6	19.8	318°	18"	1960
VYS 5	12h12.3m	+54° 29'	9.8	13.3	0°	15"	1966
STF 1745	13h20.3m	+79° 26'	9.3	11.6	200°	20.3"	1905
ES 1085	14h16.5m	+46° 33'	8.8	11.8	172°	6.1"	1911
KZA 80	15h20.7m	+31° 33'	9.5	10.0	54°	26.8"	1984
AG 348	16h00.2m	+14° 11'	9.5	10.0	26°	16.3"	1893
The double stars listed here have only a single published measure each. That measure and the date of it are provided. What has happened since is anybody’s guess. Some of the observations are quite recent while others are very old. All the doubles are challenging.