WWF-Cam (Wyoming Wide-Field Camera)

General Design Considerations

As ever-larger telescopes are constructed, the research role of smaller telescopes (less than 4m) is increasingly directed towards surveys and graduate education.  Similarly, technological developments have expanded the wavelengths accessible to ground-based telescopes.  In particular, the near-infrared region opens up new research opportunities for astronomers.  As a result of these trends a conceptual design for a wide-field, near-infrared camera was developed in 2005/2006.  The Wyoming Wide-Field Camera (WWF-Cam) was driven by the desire to provide Wyoming astronomers with a state-of-the-art, near-infrared imaging capability.  The primary goal of this instrument would be wide-field, near-infrared imaging surveys but a spectroscopic capability was also desired.  Unfortunately, the 2.3-m telescope of the Wyoming Infrared Observatory (WIRO) was originally desigend for mid-infrared wavelengths using single element bolometer detectors.  As a result it has a very high f-ratio at the Cassegrain focus (f/27), making the image scale too high to allow for a wide-field Cassegrain design.  A prime focus design was considered but WIRO's small dome places severe restrictions on the size of a prime focus instrument.  These restrictions proved to be to great and so the final design of WWF-Cam will be of a conventional, Cassegrain design.  This will mean that a new secondary will be needed for WIRO.  In the meantime, WWF-Cam will be built as a traveling instrument so that research with WWF-Cam can begin as soon as possible.

Near-infrared Detector Array

Hawaii-2 Array
Hawaii-2 Near-infrared Array Large fromat (2048 x 2048) near-infrared array built by Rockwell Scientific. The array is sensitive from 0.9 - 2.5 microns and has 18.5 micron pixels.

At the present time the largest single near infrared detector arrays are the Hawaii-2 arrays made by Teledyne (formally Rockwell Scientific).  These detectors have 18.5 um pixels in a 2048x2048 array and are sensitive from a wavelength of 0.9 to 2.4 um.  The high cost of these arrays, approximately $350,000 a piece, severely limits the number that can be purchased by a single university research group.  As a result, WWF-Cam has been designed around a single array.  This means that a large field of view can only be accomplished by reimaging, with a corresponding reduction in the plate scale of the instrument (see below).  The image shows the Rockwell Hawaii-2 array that will be incorporated into WWF-Cam. We will be mounting the array on a fanout board provide by John Geary of Harvard Smithsonian Observatory. This board contains the basic components needed to drive the array.

Optical Design

The design of WWF-Cam is based on a similar instrument known as FLAMINGOS.  FLAMINGOES was designed by Charles Harmer (NOAO) and Richard Elston (Univ. of Florida).  Since the development of this instrument several similar instruments have built for various telescopes around the world.  Charles Harmer was kind enough to rework his original design for FLAMINGOS for WWF-Cam.  The major challenge in the design of WWF-Cam was driven by the desire for the highest practical reduction factor within the insturment such at that WWF-Cam could be placed on a large telescope but still produce the largest possible field of view.  The APO 3.5-m telescope at Apache Point is considered as the likely home of WWF-Cam.  The resulting design is shown in the figures below.

WWF-Cam DesignSpot Diagram

The requirement of a large field-of-view means that the design for WWF-Cam is complex. However, the design should produce excellent images over the full 17-arc min. field of view.  The H-band spot diagram and rms vs. field angle are shown below.
WWF-Cam DesignSpot Diagram

Preliminary Mechanical Design

The mechanical design of WWF-Cam will also be complex.  Since the camera must operate at infrared wavelengths all the components must be cooled to liquid nitrogen temperature (77 K) in order to minimize the thermal background within the camera.  However, since the camera is designed to produce the widest possible field it must be designed to be as small as possible both to reduce weight, and hence flexure, as well as to make it practical to cool to these low temperatures.  The figure below shows the design currently being considered for WWF-Cam.

Optical Bench of WWF-Cam:
Optical Bench of WWF-Cam: A view of the optical bench for WWF-Cam. The baffels, cold shield, and dewar body have been removed for clarity. Light enters from the right and the detector array is on the left, just to the right of the rear bench support bearing. The window-like device on the left side represents a 2X2 array of programable micro-shutters which would allow for multi-object spectroscopy without opening the instrument in order to change out slit masks. This device can be rotated out of the way for direct imaging. To the left of the micro-shutter array are the three elements of the collimator followed by the filter and grism wheels visible just left of the center of the optical bench. Four cryogenic steppr motors will allow the positioning of the two filter wheels, the grism wheel, and a rotating pupil mask. The three camera elements to the left of the filter wheels are followed by the focal plane array and its focusing assembly. The LN2 tank is visible underneath the optical bench. The large mass of the optical bench and the LN2 tank require an additional rear support. This contains a linear bearing to enable the optical bench to contract as it is cooled to 77 K.

Electronics & Software Design

The WWF-Cam electronics can be broadly classified into 3 groups: instrument control, array control, and system monitoring.

1) Instrument control

Control of the WWF-Cam instrument is done through cryogenic stepper motors via a Compumoter 8-axis stepper motor controller, the C6k8, from Parker Automation.  The C6k8 is fully programable and contains a variety of inputs and outputs which are used to sense home/limits and encoder inputs.

2) Electronics & Array Control

The focal plane array is controlled  via a SDSU Gen-II Controller designed and built by Bob Leach of Astronomical Research Cameras.  It also uses a set of pre-amp cards and co-adder boards from IR-Labs.  The combination allows the entire 2048x2048 Hawaii-II array to be read in about 0.1 second.  The Gen-II controller is highly versatile. In particular, custom waveforms for each detector array can be downloaded to the controller using a computer interface. This allows for selectable regions of interest, on-chip binning, etc.  Initial tests and characterization of the controller are now complete and we are currently waiting on delivery of the focal plane array.

3) System Monitoring and Control

(this section under construction)

At the present time we are planning to use LABVIEW for software control of the instrument. LABVIEW has been developed by National Instruments for use as a hardware-control environment. It has been used extensively for astronomical instrumentation.

4) Graphical User Interface (GUI)

(this section under construction)

5) Additional Technical Documents

Here are links to more technical web pages (under development):

Near-Infrared Sky

Cryogenic Test Dewar

Characterization of the SDSU Gen-II Controller

Mechanical Details