Instrument Description

The Four Star infrared camera is a 1-2.5 micron near infrared camera for the Magellan Baade Telescope. The instrument utilizes Four Rockwell HAWAII-2RG imaging arrays in a close-packed arrangement to achieve a 4096 X 4096 equivalent pixel imaging area. The projected field size on the sky is 10.9' X 10.9'. Four Star is a facility instrument for the Magellan Consortium and is being built by Carnegie Observatories in Pasadena. Funding for FourStar has been provided by the National Science Foundation and the Carnegie Observatories.

Optical Design

Four Star will reside on the Magellan Nasmyth platform and utilize the F11 telescope secondary. A refractive optical system was chosen with all elements except the front window/lens being cooled to cryogenic temperatures and in a vacuum.

The Four Star optics were designed by Carnegie Staff member Stephen Schectman who also provided a similar refractive design for the PANIC IR camera. This optical design may be classified as a seven element design with three groups. The first group is a pair of field lenses which serve as a type of collimator to reconverge the beam. This group consists of two 16" diameter fused silica field lenses (L1, L2) which form an exit pupil just in front of element L3, reducing the beam waist and providing a useful location for a cold pupil stop. Element L1 also serves as a vacuum window. A cold and continuously adjustable cold stop is placed at the exit pupil. This cold stop blocks most stray thermal radiation from striking the detector. Elements L3, L4, L5, and L6 form the second group which we refer to as the Camera Module. The Camera Module essentially reconverges the telescope beam to F3.6 at the focal plane. Just before the focal plane the third group, consisting of a field flattener and filter performs additional aberration correction, field flattening, and filtering. There are three field flattener lenses, one for each IR filter, and the filter thicknesses are adjusted to match a given field flattener. This allows optimal aberration correction and optimal coating effectiveness in a part of the optical train where this is critical (near the focal plane). The disadvantage of this approach is that additional expense and care are required to carefully match filter and flattener characteristics. The final plate scale delivered by the F3.6 beam is 0.16"/pixel accommodating a total field of 10.9' X 10.9'.

The Four Star lenses are all spherical except for L3 which is a CaF2 element with an aspherical surface front surface. The instrument will initially be deployed with three field flattener lenses covering the J,H, and Ks bands. The optomechanical design can accommodate four field flatteners if desired. There are two filter wheels, accommodating ten filters total. As initially deployed FourStar will have broadband J,H, Ks filters and four filters that subdivide the H band (HA-HD) for Methane and Water band for studies of Brown Dwarf and cool star atmospheres.

Mechanical Design

The Four Star Dewar is designed to provide a bi-modal cryogenic temperature environment whereby the bulk of the optics are cooled to about 200 K-deg whereas the imaging arrays and its environs are cooled to liquid nitrogen temperature (77 K-deg). We have utilized this dual temperature approach before on the PANIC IR camera and the WIRC IR camera at Baade and the duPont 100-inch, respectively, with good success. The dual temperature regime is advantageous since problems associated with thermal stress, thermally induced changes in optical properties, and thermally induced changes in glass optical properties are reduced to more manageable levels.

The Four Dewar Sections

The Four Star Dewar in Cross Section

The Four Star Dewar consists of three main sections: I, II, and III. As seen in the solid model above (described from left to right) a conical section adapts Four Star to the Magellan Nasmyth rotating ring/guider assembly. Light expanding from the telescope focal plane enters the field lens pair (lens Group 1) and travels down a radiation shield/light baffle to the Camera Module (lens Group 2). The Camera Module lenses are supported in an Invar cell held by three flextures radiating outwards to an external ring, penetrating the radiation shield, and attaching to the vacuum vessel. The outer surface of the ring serves as a rolling surface for astatic support. The Camera Module lenses are assembled and aligned as a unit. Just to the right of the Camera module the radiation shield transitions to a mating shield in section II which is at liquid nitrogen temperature (LN2). The joining of these two shields is by adjustable thermal shunts tuned to balance the heat load on the section I shield and maintain it close to 200 K-deg.

              Filter and Flattener Wheels                                                                Filter/Flattener Wheel Detail

Section II contains the detectors, field flattener wheel, filter wheel, and internal ASIC cold PCB cards, all enclosed in a LN2 temperature shield. These structures mount to a cold plate that is the front surface of an internal LN2 tank designed to cool the array and these associated structures. Behind this tank is a larger tank in Section III that holds the LN2 reservoir for the radiation shields.

The Dewar and its associated support components are designed for ease of operation to help reduce burden of routine support and operations. The larger LN2 tank which is under the largest heat load and will boil off its LN2 first before the small can. It is therefore imperative that this tank be keep well-filled at all times. To facilitate this, a fill tube from this tank is co-linear with the rotational axis of the instrument so that a fill "stinger" from an automated fill system can remain permanently in place. Also, to minimize handling overhead, all electronic instruments supporting the dewar will be attached to, and travel with, the instrument. There is a control computer that moves with Four Star (on its handling cart) and camera is both fully usable and self contained, on or off the telescope. The Four Star handling cart incorporates the astatic support roller mechanism to unload weight from the Nasmyth rotator bearing. Once assembled in Chile, the cart will always remains with the dewar.

Four Star Electronics and Data System

In stark contrast to the proceeding generation of infrared cameras, the required external electronic support for Four Star is minimal. Rockwell has supplied application specific integrated circuits (ASIC's) which are deployed at liquid nitrogen temperature inside the cryogenic dewar. All array biasing, clocking, and analog-to-digital conversion are provided by the ASIC's. The only additional electronics to facilitate readout are small external boards fastened in blister boxes just outside section II of the dewar which hold field programmable gate arrays (FPGA's) that "glue" the ASIC logic interface to USB 2.0. Thus each array is a USB device, very analogous to a consumer digital camera. Remarkably even array power is derived through USB. The end user must supply only a computer with USB ports, Windows XP, and appropriate drivers and software.

Our data system is entirely IP based: every device that touches hardware has an IP address. All communications links are using either 10, 100, or 1000 BaseT or BaseFX (fiber optics) depending upon the required bandwidth and electrical isolation desired. Thus temperature monitors, temperature controllers, motor controllers, the telescope interface, workstations, and the array readout computers are all IP devices. Among the many advantages

of this approach are that highly standard software protocols, hardware devices, and cabling can be used to facilitate communications. This scheme lends itself naturally to a distributed data system which can evolve as needed.

The camera electronics are mostly confined to two 3U high 19-inch racks symmetrically located on the diameter of section I of the dewar. By keeping the electronics with the rotating dewar we avoid the problem of implementing a cable wrap. The only connections from the dewar to the inertial reference frame are one fiber optic cable, one thin hose for water-glycol cooling, and AC power. These cables will all be routed through an existing cable wrap used by the telescope guider. The electronic units in their racks generate 482 Watts of power most of which is removed by fans internally recirculating air past glycol heat exchangers.

The computer system for Four Star consists of seven computers. There are four Array Servers on the dewar, one dedicated to each HAWAII-2RG array. These are essentially autonomous controllers that accept requests from clients to read the arrays, format the image data into sensible data frames with header information, and transfer the image frames to end user work stations. All observer interaction will occur using Mac OS-X based computers. The observer will nominally operate the camera with the Control Room Mac and reduce

data with the Data Server Mac both located in the control room. One additional Mac, the Nasmyth Mac, resides with the camera at the Nasmyth platform and is used when working with the dewar during setup or maintenance operations. This computer stays with the dewar and is always available.

Each computer is generously equipped with disk space, memory, and screen real estate, but the Data Server Mac more so than the other two Mac's. This computer will be available for such computationally intensive tasks as running real time quick look reduction packages as well as concurrent pipeline operations. The Data Server Mac will be especially well-provided with disk space and mass storage capability. The data system software will be implemented with a Mac OS GUI (i.e. not X11). It is designed to be run from any Mac on the network Four Star network segment, and possibly as well, by Macs located outside the Four Star segment by implementing a Virtual Private Network.