The Micrometer

I use a RETEL micrometer to make the measures (Figures 21.2 and 21.3). There are three wires in the field of view of the eyepiece. Two are fixed and perpendicular to each other; the third moves in two directions and, used in conjunction with the fixed wire parallel to it, measures the separations whilst the other wire is used for position angles. The movable wire is controlled by an engineering micrometer screw which has a range of about 11.5 mm and which can be read to 1 micron using the fitted vernier. The wires have a diameter of 12 microns, which translates to 0'.'85 in the focal plane. As the telescope will resolve pairs about 0'.'55 apart this is plainly unsatisfactory. This can be easily overcome by means of a Barlow lens. In this case I employ a x3 Barlow which triples the effective focal length and reduces the apparent size of the wires in the eyepiece to about 0.3''. In conjunction with the 18 mm Kellner eyepiece supplied with the micrometer this gives a magnification of about x450 and this is used for all measures.

Figure 21.1. The

8-inch Thorrowgood dome. (D.W.Evans)

Figure 21.2. The

RETEL micrometer and its illumination power supply. (R. Sword (IOA)).

Figure 21.2. The

RETEL micrometer and its illumination power supply. (R. Sword (IOA)).

The field is illuminated by a single red LED which can lead to parallax problems if the illumination is set too high. A way out of this is to locate an LED in the telescope dewcap thus illuminating the field more evenly. On bright stars it is best to turn the illumination down or even off to set the wires since they can be seen in shadow against the star disks. Using the manufacturer's illumination I can measure wider pairs down to about V=10 and for faint, close pairs then STF 1280 (magnitudes 8.9 and 9.1 at 1.2'') represents the limit for the 8-inch refractor.

Although it is clearly better to have a micrometer residing permanently on the telescope, in my own case this is not possible since the telescope is often used for other observations including solar projection. Hence it must be fitted and removed for each observing session. I therefore have to check the instrumental position

Figure 21.3. The

RETEL micrometer and Barlow lens mounted on the 8-inch Thorrowgood refractor.

Figure 21.3. The

RETEL micrometer and Barlow lens mounted on the 8-inch Thorrowgood refractor.

angle of standard pairs at the beginning and end of the night. I also measure the separations of the same pairs to give a determination of the scale of the micrometer by taking a mean of the two determinations which usually agree to within 1%.

Whilst the micrometer is being fitted to the telescope, the dome is opened to allow the inside air to come to the same temperature as the air outside. As the dome is fairly small this does not take very long. A note is made of the dome temperature at the beginning and end in case refraction corrections need to be made and to check whether any scale variation in the micrometer with temperature is discernable. In practice I don't do this. For pairs < 30'' in separation the correction is very small.

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