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Measuring Double Stars with a Micrometer

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The observation and measurement of visual double stars is an important area of study in astronomy and astrophysics. This type of observing is an area of research well suited for amateur participation. The amateur can still carry out important scientific work, and make a valuable contribution to astronomy. While accurate measurements of visual double stars does not necessarily require the use of an expensive micrometer the process is never the less made easier and highly accurate using such instruments.

To find the apparent separation of a pair of close stars, or double stars as they are called, with a telescope one must use a measuring device such as a micrometer. Micrometers are usually made with adjustable webs, needlepoints, or an eyepiece reticle with graduated lines ruled in the glass. The webs, needlepoints, or reticle lines are positioned at the focal plane, in focus with the image, and magnified. An image can be aligned between the micrometer webs, points, or the ruled lines of the reticle and the separation noted on a dial or scribed on the reticle in either fractions of an inch or millimeters (See Figures 1 through 3).

Figure 1. A Bi-Filar Micrometer produced by Ron Darbinian. A 12mm eyepiece attached to the black box that contains the webs and web holding mechanism. The silver handle to right is the micrometer thimble and spindle to adjust the movable web. A protractor shown is shown behind the black box and is free to rotate 360 degrees to measure the position angle of the double stars.

Figure 2. The Eyepiece Reticle Micrometer. A cross-section view is presented in view (a) and the image of the reticle and Mars is shown in (B).
Figure 3. LEFT: A double star pair as seen and measured in a Bi-Filar Micrometer (webs, ‘T’ for centerline, ‘F’ is the fixed web, and ‘M’ for movable web). RIGHT: A double star pair as seen and measured in an Eyepiece Reticle type micrometer.


First it would be wise to find the orientation of the micrometer in the telescope field relative to the sky. As we know the celestial sky is marked off in the astronomer’s mind and we refer to the direction west and east as Right Ascension (RA). RA is graduated in hours, minutes and seconds, and equated with the longitude of the celestial sky. The north-south direction or Declination (DEC) equates to the latitude of the celestial sky. Usually, double stars are not exactly the same visual magnitude so we consider the primary as the brightest of the two and the companion as the dimmer.

The position angle of a double star pair or polar coordinates are relative to the celestial north, so we must rotate the micrometer so that the two stars drift exactly east to west along the centerline (T) web or etched centerline in the reticle eyepiece field. For this drift check we will use the micrometer with the telescope drive off and select any star at or very near the celestial equator. If the primary, or brightest, star is centered in the field and the dimmer star is on the west side of the primary then the micrometer is correctly oriented to the north (See Figure 4).

Figure 4. Double stars orientated with primary centered in field and companion to right side after north has been established. Position angle of a double star pair indicated by the symbol ‘Theta’.

To translate the separation to some usable angle or linear dimension the image scale of the telescope must be calculated. Image scale is usually expressed in degrees, minutes, or seconds of arc per inch or millimeter. To find the image scale in seconds of arc (arcsec) per millimeter, divide 206,265 (seconds of arc in 360 degrees) by the focal length (F.L.). For example, the image scale for a 16-inch (406.4mm) f/7 aperture telescope with a F.L. of 2844.8 mm is:

image scale = 205,625 / 2844.8 = 72.5 arcsec per mm

Since many of the objects subtend very small angles, usually in the seconds of arc, we must increase the effective focal length (EFL) of our telescope to allow the image to be large enough to be separated by several increments. A large image also results in a higher resolution in the micrometer readings. The Barlow lens is a good way to accomplish this. If a 5x Barlow is used on the above telescope then the EFL will be:

EFL = 2844.8mm x 5 = 14,224mm

With the increase in effective focal length the image scale then becomes:

image scale = 206,265 / 14,224mm = 14.5 arcsec / mm

Let's use a micrometer to measure a double star from The Aitken Double Star catalogue, ADS-111 (RA00:09:21, DEC -27:59.3), 1992 [kappa-1 Scl]. This 6-magnitude pair is separated by 1.4 seconds of arc and position angle 261 degrees. Since this pair is nearly at the same visual magnitude then find the star that appears a little brighter and position it in the center of the field on the centerline (T) web and the fixed (F).

After the image is positioned at the focal plane between the webs or hash markes of the reticle, the telescope drives are adjusted so one star is centered in the movable (M) and centerline (T) webs, and the fixed (F) and centerline (T) webs. The separation is read from the micrometer thimble and spindle, or lines on the reticle and noted (See Figure 5).

Figure 5. Darbinian micrometer thimble and spindle for reading separation in millimeters.

You can find the separation by subtracting the micrometer "zero," that is, the dial reading where the webs or points are centered on each other. To determine the micrometer zero one positions the movable web exactly over the fixed web, then reading the micometer. The Darbainian B-Filar Micometer this author uses has a micrometer zero at 10.6772.  If we measure the separation of the double stars using a web type Bi-filar micrometer with a read of 10.7737 mm -- the separation of the double stars:

separation = 10.7737 - 10.6772 = 0.0965 mm

Given the above image scale of 14.5 arcsec/mm then the size of the image is:

separation = 0.0965 x 14.5 = 1.4 arcsec

Of course, this was just a check to see if this particular double is still situated as catalogued and it appears that it is still separated by 1.4 seconds of arc and at 261 degrees position angle.

In order to join in the fun with other double star observers you may wish to write to Ronald C. Tanguay at 306 Reynolds Drive, Saugus, MA 01906-1533 and ask about the "Double Star Observer" newsletter.


Couteau, Paul, Observing Visual Double Stars, ISBN 0-262-03077-2, The MIT Press, Cambridge, MA.

Webb Society Observers Handbook, Double Stars

Gerald North, Advanced Amateur Astronomy (Edinburgh University Press)

An on-line catalogue of speckle interferometry maintained the Georgia State University is at:

Suppliers of micrometers:

Lyot-Carriichel microrneter (SAF) - write to Edgar Sou1ie’ "Les Dryades," 19 avenue Salengro, 92290 Chatenay-Maiabry, FRANCE

Ron Darbinian, 1681 12th St., Los Osos, CA 93402, U.S.A.

L. D. Reynolds, Unit 4-5, Abingdon Road, Nuffield Ind. Est., Poole,

Dorset BH17 7UG, ENGLAND. (Phone +44 202 C~85&83)

StarGager http://www.grahamoptical.com/filar.htm

EFM-16X Filar Micrometer Eyepiece http://www.adorama.com/PROF1A.html?searchinfo=1A%20Filter&item_no=22


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