Wispi Design Notes
- IMAGING AND COLLIMATING OPTICS
- 400mm f/2.8 Nikor lens: images the sky on the entrance slit.
- 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.
- 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
simplifies the mechanical design.
- 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
- 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 may use sub-pixel corrections to provide feedback to the
tracking drive motors. The video camera provides a live image of the field of view
to a remote operator.
- SPECTROSCOPIC COMPONENTS
- 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.
- 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.
- 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.
- 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
- 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.
- Originally microstepping motors provided slewing and tracking. The current system
is the second generation with DC servo motors controlled by a Galil motion controller.
- Wispi originally had three precision rotating stages. Two stages provided for
offset guiding. A third stage is a grating rotator to permit
precise manual adjustment of the instrument's central wavelength.
In its current version offset guiding is provided by the video camera and those two stages have
- 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
- 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.
- 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 as the
instrumentation control platform. Wispi is now operated using the
XmTel system that was developed for remote astronomy. A driver for Galil
motion controls was added to XmTel for this purpose.
- IMAGE STORAGE AND ANALYSIS
- Image data are stored on a high capacity hard disk in the control
computer, and then transferred to the observatory server at the end of
a run. Spectra and field images are recorded and saved together to provide
a record of the slice of the sky for which each spectrum samples.
- 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
may not be along a row because the image system rotates the spectrum slightly
compared to the the reference horizontal axis of the image. Often simply summing a few
rows of the spectral image will be adequate to allow for this tilt.
Last update: August 16, 2006
kielkopf at louisville dot edu