for quick testing of Electronic Autocollimators For Ident.-No.: 223 244 Version date 05.05.2004
1. Basics The is designed for quick testing of accuracy of electronic autocollimators. If a light beam passes an optical wedge, the beam will be deflected by an angle, defined by the mechanical wedge angle α of the glass plate and the refractive index of the glass material. If a mirror is placed behind the wedge the light beam will be deflected twice. The deflection angle in double pass is calculated as follows: δ = 2( n 1) α (1) and can be measured with an autocollimator. As the mechanical wedge as well as the refractive index of the glass are constant, the absolute deflection angle is constant, too. By rotating the wedge the value of the deflection angle does not change, only the direction of the deflection is changing. This property is used for the. 2
The consists of an optical glass wedge with a mechanical wedge angle α of approx. 60 arcseconds. Behind the glass wedge a mirror is positioned that reflects the light beam. If using an autocollimator, the emitted light bundle is deflected by the wedge, reflected by the mirror and is deflected again by the wedge (double pass) and projected into the image plane of the autocollimator. Angle calibrated autocollimators like the ELCOMAT don t show the deflection angle but the tilting angle of a mirror that would produce the deflection angle. According to the law of reflection the tilting angle of the mirror is by a factor of 2 smaller than the deflection angle. Therefore the angular value displayed by the ELCOMAT is also smaller by a factor of 2 than the deflection angle of the prism in double pass. When the glass wedge is rotated by 180 the position of the autocollimation image changes by δ (and not by 2δ). Therefore, in the ideal case the angle change should be exactly δ. 3
2. Testing of electronic autocollimators Index mark - Fix the autocollimator test wedge on a breadboard with 4 screws M6. - Fix the autocollimator in the adjustable holder or fixture D65 ( Height of optical axis must be 100 mm). If the height of the optical axis of the autocollimator is greater than 100 mm, then a spacer block must be used under the Autocollimator Test Wedge. - Adjust Autocollimator with Laser attachment (219 757) such that the reflected laser spot coincides with the emitted laser spot. After that, remove the laser attachment. - Adjust the Autocollimator with adjustment screws of the adjustable holder such that the reflected crosshair is in the middle of the testing area (for example at 30 arcsec if testing the range between 0 and 60 arcsec) - By rotating the knob of the 12 times the autocollimation image describes a circle. - For testing of accuracy of the autocollimator in X-direction the Autocollimator Test Wedge is to be rotated until the X-value is approximately at its minimum. Now, turn the knob such that the index mark on it coincide with the one on the housing. (the y- value should be in the range of some arcsec). The actual displayed value are to be set to 0 (relative mode). After that the wedge must be rotated 180 (6 rotations of the knob). Note both, the x- and the y-value (the y- value should be in the range of some arcsec, again). Use a calculator to calculate the value of the 2 2 expression: x + y.this value should be equal to the certified angle δ within the given uncertainty. - For testing of accuracy of the autocollimator in Y-direction the Autocollimator Test Wedge is to be rotated until the Y-value is approximately at its minimum. Now, turn the knob such that the index mark on it coincide with the one on the housing. (the x- value should be in the range of some arcsec). The actual displayed value are to be set to 0 (relative mode). After that the wedge must be rotated 180 (6 rotations of the knob). Note both, the x- and the y-value (The x-value should be in the range of some arcsec, again). Use a calculator to calculate the value of the 2 2 expression: x + y.this value should be equal to the certified angle δ within the given uncertainty. IMPORTANT: For exact measurements it is recommended to come to the Index mark all the time from the same rotation direction to eliminate backlash. 4
3. Error analysis The is designed for quick and simple testing of accuracy of autocollimators. It is not an instrument for calibration as the intended use of this instrument is to test if after transportation, long storage time or operation the performance of the autocollimator has changed. The accuracy of the measurement with depends on: - Accuracy of autocollimator itself - Environment conditions (vibration, temperature, inhomogeneities in the air) - Setting accuracy of Minimum/Maximum of X- or Y-angle If using the for testing of electronic autocollimator ELCOMAT 3000, the measurement uncertainty will be 0.5 arcsec. 4. Dimensions 5