Absolute strip encoder’s type Heidenhain-LIDA are installed in the 3p6 Telescope alpha and delta axis. The strip is mounted in a drum attached to the axis.


40 um marks

4096 interpolation on IK320 board

Alpha resolution: (TB calculated)

Delta resolution: (TB calculated)

Head replacement and adjustments:

To replace the head, protect first the head and the strip by introducing a plastic foil between the head and the strip. Unscrew the bolts holding the head in place and withdraw it.

-         Install the new head in its support, aligning it so its base is parallel to the strip. Adjust the distance using one plastic gauge of 0.7mm.

-         Connect the PWM8 encoder analyser (Heidenhain) to a 12 to 30 volts power (special connector).

-         Connect the head to the INPUT connector of the PWM8 encoder analyser by the special adapter cable that matches the head and the PWM8 box.

-         Switch on the PWM8. With this option you supply +5 volts to the encoder head.

-         Rotate the telescope axis and watch the reading amplitude of both signals (0 and 90). The maximum amplitude is obtained by the optimal alignment of the head grating respect encoder marks (fig. 1). For the adjustment, release a bit the bolts that hold the head, turn t slightly searching the maximal signal amplitudes.

-         Tight the bolts and watch that the signals stays at a maximum. Disconnect the head from the PWM8 and connect it to the IK320 board.

Reference mark adjustment:

Use a memory oscilloscope for display the sinus and cosine signals sum and the zero pulse.

If you do not have a three-channel oscilloscope, you may add the sinus and cosine signals by connecting them together via two 200K ohm resistors to one channel, connect the other scope channel to the zero pulse (reference mark signal). Trigger the oscilloscope with the zero pulse signal in NORMAL mode with 0.6 VDC trigger level.

Move the telescope and until a image is displayed (FIG 1).

The zero pulse amplitude must be between 0.6 and 2,1 V, the position deviation respect the sinus + cosine signals should be less than 20°  (Picture 1). To adjust the phase, rotate the encoder head using the adjusting screw of the head support.

The zero pulse signal amplitude at the zero reference cross point should be 360° +/- 120°, and, as already pointed, it must be centred respect the sinus + cosine signal within +/- 20° .

If we compare it at the Picture 2 (same signal magnified) we have that 360° correspond approx. to five divisions = 72°/division. According to this, our zero pulse has an offset of ~ 14° (1/5 division).

TV1, TV2 and Phase Adjustment:

Connect the head to the PWM8. Adjust potentiometers in the encoder head trying to obtain TV1 and TV2 below 2°.

Confirm that the phase between 0° and 90° signals. The PWM8 box displays the departure from the 90° degrees phase in degrees.

To align the phase, change the head-strip tilt-angle on the head support adjusting screws.

Match the amplitude of both signals by trimming the "Compensate" potentiometer in the head electronics and then adjust the amplitude to 1 Vpp with the "Strom" potentiometer.

As a final check, repeat the whole procedure to verify that all the parameters are OK

 [Observing Facilities and Operations]  [La Silla Home Page]  [ESO]  [Index]  [Search]  [Help]  [News]