Accuracy of Double Image Micrometer Measurements

In order to give a result for the accuracy that can be reached with the Méca-Précis double-image micrometer (DIM) some of the author's measurements 1998-2001 were compared with speckle measurements4.

The DIM measurements were obtained by using a 32.5-cm f/19 Cassegrain. The limit for clearly resolvable stars is 0'.'40 and the limiting magnitude for the DIM for good accuracy is about magnitude 8.8 for close pairs whilst it is about magnitude 10 for the secondaries for distances more than 1'.'2. The limiting magnitudes given above refer to the Hipparcos brightnesses given in the Fourth Catalog of Interferometric Measurements of Visual Binary Stars.5 However, the limiting magnitudes are only reached when seeing conditions are good (about 25% of all nights) and the zenith distance does not exceed 40°.

Since orbit grading is a difficult matter and the measurements have been done for many orbits with grade 3, 4 or 5, a subset of the DIM measurements was compared with speckle measurements given in the Fourth Catalog of Interferometric Measurements. Single measurements on pairs which could not be separated and measurements which are four years or more off were not considered. Corrections for orbital motion were applied to perform the comparison for the same epoch of observation. Two comparisons are given:

1. Alzner (DIM measurements) minus speckle measurements on telescopes smaller than 1 m, almost all having been measured with the 66-cm refractor at US Naval Observatory in Washington, DC. The results of this comparison are given in Table 13.1. The distribution of the 119 means as a function of separation used for this comparison is presented in Figure 13.5.

2. Alzner (DIM measurements) minus speckle measurements on telescopes larger than 1 m (most measurements by Aristide, Docobo, Hartkopf, Horch, Mason and Scardia4). The results of this comparison are given in Table 13.2. The distribution of the 63

Table 13.1. Alzner (DIM) - Speckle <1 m comparison, 105 binaries

Parameter

Number or Result

Means DIM

119

Means speckle

119

Observations DIM

290

Observations speckle

307

Observations/mean DIM

2.4

Observations/mean speckle

2.6

Mean of AO (degrees)

+0.02

a of AO

±1.07

Mean of Ap (arcseconds)

+0.018

a of Ap

±0.038

Figure 13.5. The distribution of 119 means used for comparison with speckle < 1 m as a function of separation.

means as a function of separation used for this comparison is presented in Figure 13.6.

Table 13.1 indicates the DIM separations as being about 0'.'02 larger than speckle <1 m separations. One

Table 13.2. Alzner (DIM) - Speckle >1 m comparison, 59 binaries

Parameter

Number or Result

Means DIM

63

Means speckle

63

Observations DIM

151

Observations speckle

73

Observations/mean DIM

2.4

Observations/mean speckle

1.2

Mean of AO (degrees)

-0.34

a of AO

±1.32

Mean of Ap (arcseconds)

-0.002

a of AO

±0.030

Figure 13.5. The distribution of 119 means used for comparison with speckle < 1 m as a function of separation.

Figure 13.6. The distribution of 63 means used for comparison with speckle > 1 m as a function of separation.

possible reason could be the fact that most DIM measurements were done with magnifications of 620 and 496. These are comparatively low magnifications for separations below l'.'O. There may be a tendency to set the two images of the pair a little further apart in order to see the images better. For pairs larger than l'.'O (62 means, comparison with speckle <1 m) the offset mean Ap (arcseconds) was evaluated as being +0'.'005.

Table 13.2 indicates the DIM angles being rotated by -0.34° versus the speckle >1 m angles. However, if the angle for the difficult pair STF 248 is not taken into account, the offset reduces to -0.2°. The comparison of pairs with separations between 0'.'40 and l'.'OO (distances with DIM always measured, not estimated!) with speckle observations >1 m (34 means) gives a mean Ap (arcseconds) of +0'.'011.

Prospective users of this micrometer are recommended to read the paper by Christopher Lord6 which thoroughly discusses the properties and use of the micrometer.

Figure 13.6. The distribution of 63 means used for comparison with speckle > 1 m as a function of separation.

References

3 Agati, J.-L. and Huret, R.-G., 1988, L'Astronomie, 482-9

4 Alzner, A., Micrometer measurements from 1998.02 to 2002.92, 2003, Webb Society Double Star Section Circular, no 11.

5 Hartkopf, W.I., Mason, B.D., Wycoff, G.L. and McAlister, H.A., 2002, Fourth Catalog of Interferometric Measurements of Binary Stars (see http://ad.usno.navy.mil/wds/ int4.html)

6 Lord, C.J.R., Measuring Close Double Stars with a Lyot Micrometer, (http://www.brayebrookobservatory.org) The Méca-Précis Lyot micrometer is available from: Méca-Précis, Zone Industrielle des Sables de Beauregard, 36700, Chatillon-sur-Indre, France. Tel: +33-2-54-02-3636. Fax: +33-2-54-38-94-54 (e-mail: mecaprecis36 @wanadoo.fr).

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