CES Specifications






Grating turntable resolution

0.1 arcsec.

0.1 arcsec.

Maccon – Inc/Amplif.

Predisperser resolution

0.6 arcsec.

1 arcsec.

Maccon – Inc/Amplif.

Entrance slit resolution



Maccon – step/Driver





Predisperser blue slit



Maccon – step/Driver

Predisperser red slit



Maccon – step/Driver

Calibration lamps



Magnet motor




1. Encoder:


The more suitable encoder for this unit considering price/resolucion vs. requirements,  trying to standarize as far as possible, was the HEIDENHAIN ERO 725 with a resolution of  36,000 pulses /rev.


1.1 Interpolation unit:


The Heidenhain incremental encoder counter unit IK320, provides a 4096 fold interpolation factor, which in combination with the Heidenhain ERO 725 encoder brings an theoretical resolution of ~0.009 arcsec.


2. Positioning accuracy:


The positioning accuracy depends on various parameters and effects which have to be analyzed separately.


-Axial run-out.




For increase the accuracy, and minimize the axial run-out problem considering that the unit rotates only few degrees, it is enough to use two heads diametrically opposed.

Linearity may be compensated by a table, and for reproducibility we have a window of 67 encoder steps for reach the 0.6 arcsec.


3. Block diagram of control system:


3.1 Control description:


For the servo control it will be used the combination that already is in use at the VLT adapter. The MACCON-IK320 set was implemented by B. Gustafsson. The application (sw-wise) is available in the module "adtt".


For the hardware we don't need any special things. The MACCON is configured

to send an interrupt to the CPU when it wants a new position value, and then

the CPU reads the IK320 and feeds the value into MACCON. This is also

supported by the motor library.


Since the unit is not yet available, the parameters for the velocity-servo will have to be tuned according mechanical responses and behavior. For the position-servo loop let’s consider if with ~0.009 arcsec. /step we have to bring the required resolution of 0.6 arcsec. ΰ0.6/0.009=66.6 encoder pulses. We have window of ~67 pulses for the required position.


The reason of the MACCON-IK320 combination is the maximal input pulse frequency (2MHz) of the MACCON controller itself, and the required interpolation factor.

If we would use the MACCON controller, with the 2Mhz maximal input frequency, it would give us the extremely low speed of  0.013rev/sec.

The IK320 encoder counter has a maximal input frequency of 50 Khz. which after internal interpolation permits a maximal rotation of. 1.3rev/sec.


4. Heads positioning geometry:



Since the presdisperser axis may rotate only few degrees (a), two heads mounted according the figure are enough for determine between axis rotation or translation in ‘y’ direction. Comparing the encoder signal phases during rotation, both scan heads will have the same sign, meanwhile if the axis displaces, it will be opposed.


Any eventual displacement in ‘x’ direction does not influence so much the light beam, because it would be quasi parallel to the surface (rotation angle is few degrees).