Camera Specifications
CLIC uses the Thermal-Eye 2000B infrared camera to image the night sky. The camera, and a ruggedized enclosure suitable for use outdoors 24/7, is being supplied by Infrared, INC.
A PDF detailing the 2000Bs specifications can be found on Infrared INC's website here. The following lists the specifications most relevant to CLIC.
- 25 mm Lens
- Uncooled Ferroelectric Focal Plane Array (FPA)
- 320x240 pixel array
- 50 μ pitch (pixel width&height)
- Spectral Response Window: 7-14 μ
- NTSC Video Output
- RS232 Controlled gain, level, and polarity
- Protective cylindrical enclosure, 10" in height, 6.5" in diameter.
Reflector Design
The early designs for CLIC's reflectors were based on the Mid-Infrared All Sky Cloud Camera (IRSC) design at the Apache Point Observatory. CLIC's final design, however, is very different.
CLIC uses two reflectors to image as much of the sky as possible onto as much of the CCD as possible. The two reflectors are azimuthally symmetric with hyperbolic cross sections. They are mounted such that they are convex upwards to the sky. The camera is then mounted above the reflectors and points down on them.
The main reflector directs rays from the zenith down to 10° above the horizon to the CCD. Light, however, is obstructed by the camera enclosure, and thus parts of the sky at or near the zenith cannot be imaged with one reflector alone. The secondary reflector is designed to compensate for this effect -- it is offset from the camera and succesfully images the sky near the zenith.
Hyperbolic Design (!)
The surfaces of the reflectors satisfy the equation for a hyperbola offset in the y-direction:
where h is the height of the reflector surface above the ground and r is the radial coordinate measured from the center of the reflector.
It is generally more convenient to solve for h(r):
note that we have taken the convex form by choosing the - sign. The reflector will be bowed outward. The radius of the reflector, R, is constrained such that h(R)=0.
Both the main and secondary mirrors have an extra offset parameter in addition to the parameters that control the shape of the cross sectional hyperbola. This offset parameter (creatively referred to as δ) tracks the position of the center of the reflectors relative to the center of the camera (which is taken to be the origin). δ is a scalar as we only allow for offsets in the x-direction, which is defined by the long end of the CCD.
Constraining the Reflector Parameters
The height of the camera above the ground, H, the focal length of the camera lens f, and the physical size of the CCD are the dominant constraints on the values of the reflector parameters.
this angle constrains the location of the rightmost tip of the main reflector, and the leftmost tip of the secondary reflector. CLIC is designed with the lens placed 40 cm above the ground, so that H=40 cm. The left and right extremes of the reflectors, then, cannot exceed a distance of 23.7 cm from the origin.
CLIC's main reflector is designed to image down to about 10° above the horizon, and to occupy the first 230 pixel columns on the CCD. CLIC's secondary reflector is designed to image a 20° cone centered on the zenith onto the remainding 90 pixel columns on the CCD. These conditions constrain the gradient of the reflector cross section at two points and are sufficient to constrain a, c, δ, and R. b persists as a free parameter whose value does not severly impact the quality of the final CCD image.
The parameters used in the final design are:
| Main Mirror | Secondary Mirror | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| R (cm) | a (cm) | b (cm) | c (cm) | δ (cm) | R (cm) | a (cm) | b (cm) | c (cm) | δ (cm) |
| 12.6496 | 10.8846 | 11.0696 | 16.5282 | 6.5104 | 6.2543 | 1.3219 | 4.6643 | 2.2112 | -12.9257 |
The following images depict the reflector design as visualized in MirRay. The red wireframe mesh depicts the position and size of the camera enclosure
The following plot is a more precise visualization of the reflectors. The scale is in centimeters.
Mount Design
The camera is held up by a set of struts that are mounted onto an aluminum baseplate. The placing of these struts must be done carefully to minimize obscuration of the sky. Of course, the struts must also support the camera and prevent it from buffeting in the wind.
The struts will be designed soon.
Sky Coverage
To understand CLIC's sky coverage, MirRay can perform some basic raytracing to figure out which portion of the sky maps onto each CCD pixel. This allows CLIC to map pixel space onto the sky and can be used to simulate CCD images of the sky.
The following image maps the northern hemisphere onto the CCD, and then maps the CCD back onto the sky. The contribution from each reflector is depicted in the shaded image. Red regions are reflections off the main mirror while blue regions are reflections off the side mirror. Click on the image for the full-size PDF version.
Here's a side-by-side comparison at different altitudes (click for the full-size PDF version).
To ensure that the image of the sky is actually in focus, MirRay can raytrace point sources onto CLIC's CCD. An image can be produced that depicts the degree of defocusing along the CCD. The defocus is not uniform because of the reflector design -- rays reflected off the base are slightly more convergent than rays reflected off the tops of the reflecter dome, thus it is imporssible to have the entire image in focus with a simple converging lens.
The following image is the focus image generated by MirRay for the current CLIC design. Point sources located at different points on the sky are imaged onto the CCD. The point spread function of each point on the CCD reflects the degree of defocusing. Note that certain regions are obscured -- these are the shadows of the struts and the camera enclosure.
Media
MirRay can simulate basic spherical clouds. The following movie is what CLIC would see if a 300°K cloud, 3 meters in diameter, were floating across the sky about 10 meters above CLIC. The cloud first moves across the sky from west to east, then north to south, and then spirals into the center.
Right click on the image below to save the movie to disk -- you'll need quicktime to view it.
CAD Diagrams
These are the CAD designs for CLIC. They are in dxf format and can be opened with any dxf viewer. I recommend downloading QCad -- the CAD program used to create the designs.
A zip file containing all the CAD design is available here.
Alternatively you can download these CAD images in png format. They'll open in your browser -- be warned that they are rather large, and that the lines in the diagrams are rather faint (you'll need to zoom in to 100% to see all the details).
Camera Controller
CLIC will be controlled by a small linux box that we've thrown together. The most important component of the camera controller is the TV card, a Hauppauge WinTV-PVR 250 MCE. The Linux drivers for the PVR 250 are available at ivtv.
The camera controller is running Fedora Core 3, has an Athlon XP 2700+ processor, 512 MB of RAM, a 200GB Hard Disk, and an nVIDIA GeForce FX 5200 video card.
The camera controller will grab sky images from CLIC, analyze them, and then serve the data onto the web.