WISPI Design Notes

• IMAGING AND COLLIMATING OPTICS
  1. 400mm f/2.8 Nikor lens: images the sky on the entrance slit.
  2. Precision bilaterally adjustable slit. The slit defines the spatial region, and also provides the limiting stop for the spectrograph. The optical system maintains image quality along the slit length so that each row of the image is a spectrum of that spatially selected region of the sky.
  3. 180mm f/2.8 Nikor lens: collimates the light from the slit. We decided to use commercial lenses for the widest field unaberrated field. The trade-off is transmission because of the multiple surfaces, and possibly poor focus in the extreme limits of the CCD response. The use of commercial lenses with internal focus mechanisms also greatly simplifies the mechanical design.
  4. A 200mm f/2.0 Nikor lens: forms an image of the spectrum and the sky on the CCD detector. The fast lens provides a large front aperture that eliminates vignetting at the limits of the spectrum.
  5. Auxiliary imager: Originally a 3.5-inch Questar catadioptric telescope was used solely for tracking. The auxiliary optical platform on WISPI now accomodates a short focal lens and a guiding/imaging video system that uses sub-pixel corrections to provide feedback to the tracking drive motors.
  
  
  
• SPECTROSCOPIC COMPONENTS
  1. The Milton Roy diffraction grating is blazed for the near infrared. It may be used in the second order for shorter wavelengths. The groove spacing was chosen to minimize the the need for order-sorting filters. Typically it is used in combination with inexpensive gelatin photographic filters inserted in the filter carrier of the 400 mm lens. It may be used without any filters, but there will be some spectral contamination.
  2. A Spex precision bilateral slit is the field stop for the wide field image, and the entrance slit for the spectrograph. The slit is mechanically coupled to the imaging lens, and to the collimator, by Nikor lens mounts. In this way the slit anchors the mechanical assembly and is in rigid alignment with the optical axis. Additional support is provided for the front elements of the lenses.
  
  
  
• DETECTORS
  1. The original Tektronix 1024×1024 liquid nitrogen cooled CCD from Princeton Instruments was selected to provide exceptionally low noise as a state-of-the-art CCD for its time (1995). The 25-micron square pixel of this detector matches the resolution of the lenses. It was replaced in 2002 by an Apogee SITE CCD camera in order to reduce the complexity of the CCD controller, particularly in view of the obsolescence of its Windows operating sytem. The Apogee camera can run under Linux, which allows us to utilize pointing and camera software we have developed for other applications requiring remote operation. Since the camera platform is modular, any large-area CCD with a Nikon lens mount can be accomodated. For most applications, spectral quality is limited by photon statistics rather than dark noise, so the improved performance of liquid nitrogen cooling is not actually utilized.
  2. For tracking, WISPI originally used an 11-micron square pixel CCD in the SBIG ST5 camera. The ST5 was serially interfaced, slow, and very prone to frosting. It was replaced first by an ST8, and more recently by a high sensitivity real-time video camera. The auxiliary optics platform can accept most CCD cameras coupled to standard camera lenses, but the use of a video camera simplfies operations. Techniques of implementing control system feedback from the video camera are being developed. Alternatively, an SBIG STV can be installed, but this is only a short-term solution because the STV is no longer manufactured.
  
  
  
• MOUNTING AND DRIVE COMPONENTS
  1. The mounting was designed by us for this instrument. It was machined in the Physics shop by our instrument maker Keith Gowen. The components are primarily aluminum, brass, and stainless steel. The counterweights are steel.
  2. Microstepping motors provide slewing and tracking. The current system is the second generation, replacing an industrial controller with motors that have built-in drivers. Stepping motors with coarse (arc second scale) steps are adequate for the intrument because the slit width projected onto the sky is several seconds of arc. An additional improvement would be to incorporate encoders and possibly servo motors, and this design improvement is under study.
  3. WISPI has three precision rotating stages. Two stages provide for offset guiding, a feature we added when the decision was made to improve tracking performance. A third stage is used as a grating rotator to permit precise manual adjustment of the instrument's central wavelength. The mounting bayonets for the Nikor lenses are also included in this line.
  4. Two 10-inch worm gears, one for each axis, were obtained as a package from Edwin R. Byers Engineering, a company with long experience in manufacturing telescope tracking gear systems. The gears are "solar-rate" gears, intended for tracking the Sun using synchronous motor drives. With 360 teeth, these gears adapt very readily to computer control.
  
  
  
• INSTRUMENT CONTROL COMPUTER
  1. Originally the system ran on a PC under Windows, using Forth for the telescope control and the proprietary camera software for image acquisition. This system was used to acquire data on Hale-Bopp and Hyakutake, but it was unstable because of resource limitations and the interaction of the camera software with the operating system.
  2. When it was clear that the camera controller and software would no longer be supported by the manufacturer, and with the high cost and inconvenience of liquid nitrogen cooling, the detector system was replaced. This change permitted us to move to Linux on a PC as the instrumentation control platform. We are seeking design modifications that will permit running WISPI from a single laptop.
  
  
  
• IMAGE STORAGE AND ANALYSIS
  1. Image data are stored on high capacity hard disks in the control computer. Archives are made to DVD disks as needed. Spectra and field images are recorded and saved together to provide a record of the slice of the sky for which each spectrum samples.
  2. Image analysis of the spectra are very straightforward given the low distortion of the optical system. In some cases it is adequate to use linear spectral fitting, and spectra can be extracted from individual rows of the CCD image following a rotational transformation to align the spectrum with the virtual image row. In the raw data, the spectrum is not along a row because the image system rotates the spectrum slightly compared to the the reference horizontal axis of the image.

Last update: August 16, 2006
kielkopf@louisville.edu