| Optics: | Refractive, uv-transmitting (10 elements). | |
| Field Lens Aperture: | 208 mm. | |
| Focal Reduction: | Factor of 4, to f/4.34. | |
| Camera Focal Length: | 163 mm. | |
| Detector: | Fairchild 2048 x 2048 Pixel CCD array (15µm per pixel) | |
Imaging Mode: |
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| Field of View | 13.65x13.65 arc-minutes. | |
| Pixel Scale | .39 arc-seconds per pixel. | |
| Wavelength Range | 350 nm to 950 nm | |
| DIQ | 70% encircled energy within 10 µm. | |
| Filters | Up to 15 in three wheels. | |
| Polarimetric Mode: | ||
| Method | Wollaston Prism (wheel 1). | |
| Stokes U, Q: | Rotating half wave plate (wheel 2). | |
| Stokes V: | Fixed quarter wave plate (wheel 3). |
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| Filters: | Up to 5 (wheel 3). | |
| Spectropolarimetry: | Replace filters with grisms. | |
| Spectroscopic Mode: | ||
| Method | Grisms, usually in filter wheel 1. | |
| Resolution | R = 140 to 850. | |
| Spectral coverage | 350 nm to 950 nm in three exposures. | |
| Focal Plane slits | Arbitrary size, number and shape. | |
Above are pictures of M67 before the installation of PRISM (left) and after (right). The red box is the same field as the picture on the left.
The driving requirements in the PRISM design are the wide field of view, the
multi-object slit mask system, and the reconfigurability of the filter wheels
and slit mask system. The optical and mechanical designs can be seen in the
figures. Lowell Observatory possesses a Loral 2048 x 2048 pixel CCD detector
with 15 x 15 micron pixels that will be dedicated to PRISM; this CCD combined
with the optical design will produce a field of view of 13.7' x 13.7' with 0.4"
per pixel.
The optical design can be seen in the next figure. The optics have been chosen
to allow the transmission of light of wavelengths 3200 - 8500 angstroms, which
is the ultraviolet through the red portion of the spectrum. The distortion of
a star's image in different positions in the field of view has been minimized.
The mechanical design can be seen in the figure below in a side view. The two main components of interest are the aperture slider and the filter wheels. The aperture slider is located before the field lens, at the telescope focus. It can hold apertures of different sizes, such as single long slits, multiple short slits or holes, and angled slits, and will be computer controlled. The design of the slider will allow easy exchange of apertures.
The following images are views of the instrument, first looking from the "end" of the telescope where the instrument will be attached, and then from the "bottom" of the instrument, looking up toward the telescope. The first image clearly shows the four aperture sliders, squares that are located at 3, 6, 9, and 12 o'clock. The second image shows the liquid nitrogen dewar on the left side (the peach-colored cylinder) and the last filter wheel (at top center), with six filter positions.
There will be three filter wheels in the PRISM instrument. Some wheels will have filter holders sized for interference and broad-band glass filters. Other wheels will be generally used to hold the polarimetry half-wave and quarter-wave plates and Wollaston prism. These wheels will also be able to hold grisms for slit and multi-object spectroscopy. All of the filter wheels will be easily accessible so that the contents of individual filter wheel positions can be changed at the observer's discretion.
The control software for the CCD electronics and instrument mechanisms is being written by Lowell Observatory.
A graph of the hold time for the dewar can be seen below.
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