Charles L. Hull

The Sloan Digital Sky Survey Photometric Camera

We have constructed a large-format mosaic CCD camera for the Sloan Digital Sky Survey. The camera consists of two arrays, a photometric array that uses 30 2048 × 2048 SITe/Tektronix CCDs (24 μm pixels) with an effective imaging area of 720 cm2 and an astrometric array that uses 24 400 × 2048 CCDs with the same pixel size, which will allow us to tie bright astrometric standard stars to the objects imaged in the photometric camera. The instrument will be used to carry out photometry essentially simultaneously in five color bands spanning the range accessible to silicon detectors on the ground in the time-delay–and–integrate (TDI) scanning mode. The photometric detectors are arrayed in the focal plane in six columns of five chips each such that two scans cover a filled stripe 25 wide. This paper presents engineering and technical details of the camera.

The 2.5 m Telescope of the Sloan Digital Sky Survey

We describe the design, construction, and performance of the Sloan Digital Sky Survey telescope located at Apache Point Observatory. The telescope is a modified two-corrector Ritchey-Chretien design with a 2.5 m, f/2.25 primary, a 1.08 m secondary, a Gascoigne astigmatism corrector, and one of a pair of interchangeable highly aspheric correctors near the focal plane, one for imaging and the other for spectroscopy. The final focal ratio is f/5. The telescope is instrumented by a wide-area, multiband CCD camera and a pair of fiber-fed double spectrographs. Novel features of the telescope include the following: (1) A 3° diameter (0.65 m) focal plane that has excellent image quality and small geometric distortions over a wide wavelength range (3000-10,600 A) in the imaging mode, and good image quality combined with very small lateral and longitudinal color errors in the spectroscopic mode. The unusual requirement of very low distortion is set by the demands of time-delay-and-integrate (TDI) imaging. (2) Very high precision motion to support open-loop TDI observations. (3) A unique wind baffle/enclosure construction to maximize image quality and minimize construction costs. The telescope had first light in 1998 May and began regular survey operations in 2000.

The multi-object, fiber-fed spectrographs for the Sloan Digital Sky Survey and the Baryon Oscillation Spectroscopic Survey

We present the design and performance of the multi-object fiber spectrographs for the Sloan Digital Sky Survey (SDSS) and their upgrade for the Baryon Oscillation Spectroscopic Survey (BOSS). Originally commissioned in Fall 1999 on the 2.5 m aperture Sloan Telescope at Apache Point Observatory, the spectrographs produced more than 1.5 million spectra for the SDSS and SDSS-II surveys, enabling a wide variety of Galactic and extra-galactic science including the first observation of baryon acoustic oscillations in 2005. The spectrographs were upgraded in 2009 and are currently in use for BOSS, the flagship survey of the third-generation SDSS-III project. BOSS will measure redshifts of 1.35 million massive galaxies to redshift 0.7 and Lyα absorption of 160,000 high redshift quasars over 10,000 deg2 of sky, making percent level measurements of the absolute cosmic distance scale of the universe and placing tight constraints on the equation of state of dark energy. The twin multi-object fiber spectrographs utilize a simple optical layout with reflective collimators, gratings, all-refractive cameras, and state-of-the-art CCD detectors to produce hundreds of spectra simultaneously in two channels over a bandpass covering the near-ultraviolet to the near-infrared, with a resolving power R = λ/FWHM ~ 2000. Building on proven heritage, the spectrographs were upgraded for BOSS with volume-phase holographic gratings and modern CCD detectors, improving the peak throughput by nearly a factor of two, extending the bandpass to cover 360 nm < λ < 1000 nm, and increasing the number of fibers from 640 to 1000 per exposure. In this paper we describe the original SDSS spectrograph design and the upgrades implemented for BOSS, and document the predicted and measured performances.

Sloan Digital Sky Survey cloud scanner

Astronomers have traditionally monitored cloud cover visually. However, this technique is difficult under dark conditions, insensitive to thin cirrus, and impossible for remote telescope use. A sensitive camera operating in the thermal infrared escapes these problems. Unfortunately, commercially available cameras have smaller than desired fields of view, are not intended for continuous operation, and are expensive. Consequently, a single channel all- sky scanner has been constructed to continuously monitor observatory cloud cover and motions. Operating at a wavelength of 10 microns, it produces a 135 degree(s) X 135 degree(s) image with a resolution of 0.9 degrees. These data are suitable for returning to remote observers and for archiving with astronomical data.

Sloan digital sky survey 2.5-m telescope: light baffles

The Sloan Digital Sky Survey 2.5-m telescope is unique with a 3 degree(s) field of view at f/5. The two-mirror optical design includes two transmitting correcting elements. To avoid excessive central obstruction of the entrance pupil, a conical baffle is necessary in addition to the usual primary and secondary baffles. This conical baffle is suspended approximately midway between the primary and secondary mirrors. In addition, an exterior close-fitting wind and light baffle, not found on most modern telescopes, blocks rays at large angles from the field of view. The baffle design was analyzed using stray light analysis software. Scattered light as a function of source angle was calculated and the dominant scattering surfaces were identified. For sources near the field of view, the uniformity of scattered light over the focal surface was determined.

The Control System for the Apache Point 3.5-Meter Telescope

The control system for the Apache Point 3.5m telescope utilizes small computers (controllers) to control hardware, and a larger computer (the telescope control computer or TCC) to direct the controllers. The controllers provide fast real-time performance and hardware interfacing; the TCC, a DEC µVAX II, provides raw computing power in a standard multi-tasking environment. Major tracking, guiding, and acquisition computations are performed in the TCC, including correction for physical imperfections and computation of slewing paths.

Support and Position Control of Primary and Secondary Mirrors for the Sloan Digital Sky Survey (SDSS) Telescope

The support and position control systems for both the primary and secondary mirror of the SDSS Telescope allow the mirrors up to 12 mm of precisely positioned axial motion, as well as limited tilt and transverse motion. This paper describes the final design and operation of these systems. Some relative strengths and limitations of the components and problems encountered with their implementation are also summarized.

NPT: a large-aperture telescope for high dynamic range astronomy

All existing night-time astronomical telescopes, regardless of aperture, are blind to an important part of the universe - the region around bright objects. Technology now exist to build an unobscured 6.5 m aperture telescope which will attain coronagraphic sensitivity heretofore unachieved. A working group hosted by the University of Hawaii Institute for Astronomy has developed plans for a New Planetary Telescope which will permit astronomical observations which have never before ben possible. In its narrow-field mode the off-axis optical design, combined with adaptive optics, provides superb coronagraphic capabilities, and a very low thermal IR background. These make it ideal for studies of extra-solar planets and circumstellar discs, as well as for general IR astronomy. In its wide-field mode the NPT provides a 2 degree diameter field for surveys of Kuiper Belt Objects and Near-Earth Objects, surveys central to current intellectual interests in solar system astronomy.

Sloan Digital Sky Survey telescope enclosure: design

The telescope enclosure for the Sloan Digital Sky Survey 2.5 meter telescope includes a number of design features that reduce cost and enhance performance. These include: (1) A minimum volume enclosure that rolls away to expose the telescope. The building drive and door systems were chosen from commercially available components for low cost and high reliability. (2) The low profile reduces the visual impact on the environment. (3) The building projects horizontally into the prevailing wind from the top of the mountain. Access to the telescope and instruments is excellent. (4) A closely fitting telescope wind baffle doubles as a component of the telescope light baffling system and has excellent air flushing properties. (5) Attention to temperature control of floor and wind baffle surfaces reduces the production of chilled air near the telescope. (6) Wakes from the telescope and enclosure are small thereby reducing the impact on other telescopes sharing the same site.

Performance of the Apache Point Observatory 3.5-m telescope: II, pointing and tracking

The Apache Point Observatory 3.5-m telescope, loated in the Sacramento Mountains of New Mexico at an elevation of 2.8 km, is awaiting the installation of the primary mirror. The design of this telescope includes a number of innovative features including a lightweight honeycomb borosilicate mirror, friction drives and encoding, and complete computer control to facilitate remote use. A 0.3-m telescope was attached to the telescope mount to monitor tracking and pointing performance. Currently, the telescope tracks open-loop to 0.3 arcsec rms over 10 min and points to 5 arcsec rms. Improvements to the servosystem and encoder mounting should make it possible to meet the goals of tracking to 0.2 arcsec rms and pointing to 1 arcsec rms.