Revised December 10, 1999

M. Radovan

Lick Observatory
 

Triple Wheels:

Each wheel of the triple wheel assembly consists of a metal belt driven wheel with a dual closed loop encoder for position control. The motor and drive system uses a Galil 50-1000 motor to drive a 50 to 1 harmonic drive. Additional drive reduction exists due to the drive wheel to filter wheel diameter ratio. The ratio between the two wheels is 32.9 to 1 (drive to driven). The second loop of the servo system is encoded by a 5 micron Renishaw optical encoder. For the filter wheel dia of 790 mm the Renishaw encoder provides 2.6 arc sec of angular resolution. The primary servo loop which uses the 4000 counts per revolution Galil motor encoder has a resolution of .2 arc sec per motor encoder count.

Collimator Actuators:

The actuators are designed as three independent cartridge assemblies that mount in the lower tubes of the main frame. The line of force of each pair of collimator support struts passes through the node in the main frame when focus is at the nominal setting. When the actuators move from nominal focus the small moments transferred to the main frame have no noticeable effect. Each assembly is mounted to the inside of the tube with six push-pull screws to allow alignment of the actuators to the optical axis of the instrument. A complete cartridge assembly is composed of a THK High Precision ball screw (6-mm lead) slide driven by a Galil servomotor (4000 count/ revolution encoder) through a 100:1 gearbox (figure 8 and 9). Included in the assembly is a 1/10-micron Renishaw linear encoder that forms the second branch of the dual servo loop. The assembly also includes primary as well as secondary limit switches and mechanical hard stops. Due to the large reduction through the gearbox and lead screw, six motor encoder counts equals one Renishaw count. This servo system is capable of repeatable positioning to 0.04 of an arc second of collimator tilt over the entire 50-mm range of focus.

Pre-Dispersion Prism:

The pre-dispersion prism static support uses three tantalum pucks that are bonded to the glass as a connection point for the six struts that support the prism. The support struts use machined cross flexures to insure that the 5 degrees of freedom other that the axial direction of the strut are sufficiently soft to insure that the structure is statically determinate. The cross flexures also provided the necessary range of motion for alignment of the prism. The prism is surrounded by a non-contacting safety cage that catches it if one or all of the bonded tantalum pucks fails

Grating:

The ESI grating is supported in a front defined cell. Adjustable hard stops in the side- wall of the cell provide the defining points for translation of the grating. The grating is held against the defining points (front and side) by spring plungers which act at points on the opposite side of the grating blank. The grating and cell attaches to the Optical Sub Structure with a three point kinematic mount. Each of the kinematics is adjustable to provide tip and tilt alignment of the grating to the spectrograph. Once the kinematic mounts are adjusted for alignment the entire grating assembly can be removed and replaced manually without realigning the grating.

Post dispersion Prism:

The post dispersion prism is support by a determinate space-frame structure that is identical to the support for the pre-dispersion prism. The space-frame structure is mounted to a steel plate that rides on two THK linear slides. The stage drive consists of a lead screw, gearbox (10:1) and Galil motor. The position of the prism is defined by the reference fiducial and the motor encoder. An electro-mechanical brake is used to hold the prism in place. This brake prevents the ball screw and gearbox from back-driving when the slides are in the vertical direction. The brake must be energized to release it. This provides safe operation in case of sudden power failure. Each motor encoder count corresponds to .8 microns of linear motion.
 

Camera Shutter:

The camera shutter uses two blades to insure even exposure times to all pixels on the CCD. Exposure linearity is approx. 1 percent across the CCD. The shutter has two titanium blades that are moved by separate Bimba air cylinders. Shutter logic control is hard wired in the control box to determine which blade to move when a command to change state is given. Typical blade opening or closing duration is approximately 140 milliseconds.

CCD Dewar:

The CCD dewar holds approx. 4 liters of liquid nitrogen to cool the 2k x 4k devices to û120 C. Hold time for the half-full vessel is approx. 20 hours. The CCD is supported in the dewar housing by a hexi-pod structure. This structure provides a very rigid support and thermal stand-off for the CCD. The flexure of the cryogenic container is isolated from the CCD by a flexible cooper braid that connects the CCD to the cold finger. The vacuum housing with cryo container is kinematically mounted to the back flange of the camera. The kinematic mount has 3 adjustable screws to tip, tilt and piston the CCD for alignment to the focal plane. The dewar can be removed from the camera and reattached to the kinematic mount without disturbing the dewar alignment. The alignment of the spectra on the CCD is also adjustable by means of a cam which rotates the dewar about the optical axis. Like the piston and tip/tilt adjustment, this is a one time only adjustment that is preserved if the dewar is removed.

Imaging Mirror:

The imaging mirror is a cell-less optic that is support via 3 kinematic balls that are attached to the mirror by bonded invar pads. The mirror is held loosely is a transport frame by springs which provide the pre-load on the kinematics when the balls engage the v-grooves. The transport frame is attached to a THK linear slide and a cam follower which are propelled by a ballscrew, gearbox, and motor combination. The mirror uses the same drive system and brake as the other two translation assemblies in ESI (Post-dispersion Prism and Low Dispersion prism). The gearbox ratio and screw pitch is the same for all three assemblies. Like the Post-Dispersion Prism the stage encoding is provided via the motor encoder. Safety clips are included to catch the mirror in the event a glue joint should fail.

Low Dispersion Mirror:

The imaging mirror is identical in size and design to the imaging mirror. The drive mechanism and kinematics are identical.

TV Guider:

The guider system for ESI uses a 200 mm Canon lens and a Photometrics PXL camera with a 1kx1k (24 micron pixels) detector to produce a 4x4 arc minute FOV. The guider FOV includes a 1.2X4 arc minute portion of the instrument FOV with the remainder of the guider field being located on a fixed mirror adjacent to the slit masks. Focus of the canon lens is controlled remotely by a motor and timing belt driving the manual focus ring on the camera. The guider system also has an eight-position filter that can accommodate 50 mm square filters. The holders and filter mechanism is identical to the filters systems used in HIRES.

Calibration Source:

The ESI calibration system consists of two separate systems. The first is selectable projector for Neon, Argon, Mercury-Neon and Quartz continuum source. The second is a projection system for the Low Pressure Copper argon Line lamp. Each system consists of a lamp, an optical illumination system feeding a flexible light conduit and a projector system.

Enclosure Hatch:

The ESI instrument hatch slides open and closed on 4 linear bearings that ride on solid hardened shafts. The hatch is moved by a double acting Bimba air cylinder which is controlled by the same skinner 4-way valves used on the camera shutter. The inside of the hatch is coated with a specular paint to reflect the cal lamps into the slit. The hatch is part of the Upper Structure of the spectrograph. This attaches directly to the bearing and does not transfer moments into the science structure of the instrument.
 

Electronics Enclosures:

All electronics for ESI are contained in one of 3 cooled lockers. Each locker contains a radiator equipped with two cooling fans to re-circulate chilled air to cool the electronics. Two boxes contain the Galil servo electronics that control the motion stages. The third electronics locker houses the CCD crate and the electronics for the PXL guide camera.

The boxes are part of the exterior structure of the instrument and form part of the insulated shell of ESI. The mass of the boxes is transferred to the bearing by a structure that is completely independent of the structure that supports the optics.

Cable Wrap System

The cable wrap system presents the electrical, optical and fluid lines to the rotating portion of the instrument by containing them in a single tube that wraps around a track on the instrument. This tube is pulled into a tubular shell by a constant force spring pulling on a wheel. This shell attaches to the cassegrain module and extends out close enough to the telescope mirror cell to clear the telescope structure. A total rotation travel of 540 degrees is obtained by turning ¾ turn either side of the attachment point on the instrument. The total travel is minimized by reducing the path length around the instrument with a triangular shaped spool and by rotating either side of center instead of wrapping 1 » turns in one direction on the spool.

The tube contains 2 power cables, two » in cooling hoses, one 3/8 in air hose and one zipper tubing containing 4 fiber optic and 5 coax cables.