Top Assembly

DEIMOS consists of a rotating structure supported by a cradle assembly located on the right Nasmyth platform of the Keck II telescope. DEIMOS can be moved from the science position to a storage position when not in use. The cradle is supported on rollers, driven by a tractor drive during transport and positioned on kinematic mounts for observing. The rotating structure consists of a drive disk supported by a friction drive, a shell supported on the back by a bearing and a front end that protrudes into the elevation bearing of the telescope. There is a cable wrap at the rear of the instrument to provide 730 degrees of rotation of the rotating structure. Top level assembly drawing is D0001. Drawing D0000 is a family tree of the assembly drawings and  numbers. D0002 specifies DEIMOS position angles.

Electronics bays are provided on both the cradle and rotating structure. These bays are actively cooled with glycol heat exchangers. The schematic for the glycol cooling is D0800.

6-Month Preventative Maintenance

Optical Layout

The optical layout consists of the following components:

1)     Front Window

2)     Slit Mask System

3)     Collimator Mirror

4)     Tent Mirror

5)     Grating System

6)     Camera Assembly

7)     Shutter Assembly

8)     Filter Wheel Assembly

9)     Dewar System

Front Window

A BK-7 window and a pneumatic hatch close the front of the instrument. Assembly drawing is D8740.

Slit Mask System

The Slit Mask System consists of a cassette with a capacity of 11 masks, a form to position the mask properly at the focal surface and mechanisms to select and insert masks with control from the DEIMOS GUI. The selection is accomplished by moving the cassette with a scissor actuator. The insertion is accomplished by an air cylinder. The mask is located so that it forms a cylindrical approximation of the spherical focal surface. The mask is located on the cylindrical surface of the form by the insertion “hot dog” pressing against 3 rollers that act as hard points. Other guides and rollers insure that the mask inserts into its proper position. Slit mask assembly drawing is D1100 (1 of 2, 2 of 2).


Collimator Mirror Assembly

The collimator mirror has most of its weight supported at the center by a counterweight cantilevered behind the mirror. The counterweight has rods to support it during assembly/disassembly. The remaining support and positioning is achieved with axial and radial supports at the edge of the mirror. Collimator assembly drawing is D2100.

Collimator Removal for Recoating

Tent Mirror

The tent mirror is supported on two ball supports located on a line through the center of gravity. These supports include flexures that allow for rotation about this axis for flexure control in the dispersion direction. Rotation is controlled by a piezo actuator at the telescope end of the mirror. Tent mirror assembly drawing is D3605.

Tent Mirror Removal for Recoating

Grating System

The grating system consists of three sliders, each containing an optical element. These are translated into the optical path on a Thompson shaft with a ball screw. A moving counterweight is slaved to this translation to maintain neutral rotational balance of DEIMOS during translations. Slider #2 contains an imaging mirror set at fixed tilt. Sliders #3 and #4 contain 6” x 8” gratings mounted in cells that can be removed and replaced. These sliders have tilt mechanisms that can be tilted to any position at any time. When a slider is selected, it is translated to the optical path and clamped against hard pads to position it for use. The clamping system is pneumatic. Grating system top assembly drawing is D4100. The slider assembly drawings are, D4325 Slider #2, D4571 Slider #3 and D4671 slider #4.

Camera Assembly

The camera assembly is an eight element, 15” focal length, f1.29 lens assembly. It is supported in a mount weldment that attaches to the drive disk. The camera mount assembly drawing is D5150. The camera assembly drawing is D100000 in the Schier drawings under D5 directory.

Structure & Rotator

The rotator is a friction drive on the drive disk. There is a 7.9” wheel driving the 8’ diameter drive disk. This is driven by a DC motor through a 260:1 harmonic drive gearbox. The total ratio is 3159.5:1. Rotation encoding is obtained from a Renishaw encoder mounted at the rear of the instrument. The structure assembly drawing is D6390. See D6139 for procedure for replacing the drive motor and harmonic drive.

The rotating structure is balanced with lead weights on bar in front of the drive disk. See Balance Procedure.

Dewar Assembly

The dewar assembly is mounted to the back of the camera assembly. It contains the eight 2k x 4k science CCDs and the two flexure control CCDs. These are on an x translation stage to provide flexure control and a z translation stage to provide focus. There are twoelectronic boxes attached to provide cabling attachments. The cryostate is attached through a flexible cold finger and bellows. It is mounted to the shell assembly. Dewar assembly drawing is D7160 (1 of 5, 2 of 5, 3 of 5, 4 of 5, 5 of 5).

TV Guider System

The TV guider system consists of a PLX camera and a Canon lens viewing a portion of the slit mask and a pickoff mirror in the focal surface. There is a fold mirror between the lens and the focal surface allowing the camera and lens to be placed perpendicular to the instrument axis at the front of the instrument. Assembly drawings are D8712 Front End, D8742 Forward Structure, and D8770 TV Guider Structure.

Shutter Assembly

The shutter assembly is a pneumatic two-blade shutter located between the filter wheel and element 8 in the camera. D5320 is an assembly drawing of the shutter and filterwheel mounted. D5401 is the assembly of the  shutter.

Science Filter Wheel Assembly

The filter wheel is a seven-position wheel driven by a DC motor on a Galil controller. There is a 50:1 Bayside gearbox between the motor and the 5:1 filter wheel drive gears. D5320 is an assembly drawing of the shutter and filter wheel mounted. D5315 (1 of 2, 2 of 2) is the assembly of filter wheel.


The trolley assembly attaches to the back of DEIMOS and moves DEIMOS from the observing position to the storage position with a variable speed DC motor. The assembly drawing is D6968.

Calibration Lamps

Calibration lamps reside in the forward structure and are serviced by removing one of the cylindrical panels from the forward structure. The assembly drawing is D1500.

 Flexure Control System

DEIMOS has a closed loop flexure control system. It is powered by copper argon lamps for use with the gratings and by a white light LED for use with the imaging mirror. These are located in the electronics ring on the rotating structure (D1538) and the light is carried to the slit area by fibers (D1537). The fibers are mounted to the slit mask form with adjustable mounts. The fcs detectors are mounted on the backplane with the science detectors.

Modify Slider #4 to Tilt to 0 Order