The University of Hawaii Wide-Field Imager (UH-WFI)

Introduction

Wide-field imaging plays a key role in many scientific programs pursued at the Institute for Astronomy (IfA) and is the main complementary function provided by small and midsized telescopes in this era of large telescopes. The University of Hawaii (UH) 2.2-m telescope on Mauna Kea has a Ritchey-Chretien (RC) optical system designed to image large fields with excellent quality. This capability, combined with the flexibility in the scheduling of this telescope, will allow us to conduct search and monitoring programs that require a midsized telescope but that cannot be done at large national facilities due to schedule restrictions. The UH 2.2-m telescope is used for the discovery and monitoring of moving objects such as asteroids, comets, and Kuiper Belt objects (KBOs), and for time-variable objects such as eruptive variables, supernovae, and gamma-ray bursts.

These scientific programs provided the rationale for building the UH 8K mosaic CCD camera, the first large mosaic camera in the world (Metzger, Luppino, and Miyazaki 1995). It provides an 8192 x 8192 field at the f/10 Cassegrain focus of the UH 2.2-m telescope, and uses a field flattener but no other focal reduction.

The use of UH 8K camera at the CFHT prime focus has been replaced at the CFHT by a 8K x 12K system built at UH and purchased by CFHT. This new camera is larger, but is mainly distinguished by having CCDs of better QE and cosmetics. The CFHT camera and a recently commissioned prime focus capability on the Subaru telescope are available to UH astronomers, but are mainly used for targeted observations due to the rather inflexible scheduling of these telescopes. The UH 8K camera on the UH 2.2-m telescope remains the only facility available for major searches and long-term monitoring programs that may require substantial amounts of observing time and innovative scheduling (e.g., 1 hour of observing time each night for several months) because the IfA has unrestricted scheduling flexibility only on that telescope.

The UH 8K camera is currently being upgraded with better CCDs under a grant from NSF (G. Luppino, P.I.). This will give the UH 8K camera significantly better sensitivity per pixel, in particular in the red, and thus will increase efficiency, but will still not allow us to fully exploit the wide-field capabilities of the UH 2.2-m telescope.

To fully exploit the wide-field capability of the telescope optics, we have received an NSF grant (AST-0096833) to build the UH Wide-Field Imager (UH-WFI). This is a large focal reducer lens system combined with a new large filter wheel that will provide a field of view of 31.4' x 31.4'. Since the UH 8K CCD camera already exists, this wide-field capability and the associated gain in observing efficiency can be obtained at a relatively modest cost.

We have explored various ways to increase the area coverage of the camera for search and monitoring programs. Building a prime focus system for the UH 2.2-m telescope is prohibitively expensive and precludes the rapid change of instruments. A simpler and more cost-effective solution is to build a focal reducer/field corrector that allows a much larger field to be observed at a reasonable pixel scale. Such systems have been built for other telescopes, e.g., the ESO/MPIA 2.2-m telescope at La Silla, and are in heavy use.

Optics Design

The design of a wide-field corrector for the UH 2.2-m telescope is constrained by many design features of the telescope and operational considerations. The UH 2.2-m telescope has two bent Cassegrain ports and the standard direct Cassegrain focus position. The direct Cassegrain focus has the largest clear diameter available and is therefore the focus of choice for a wide-field corrector. For UH-WFI, we will remove the existing autoguider system from the telescope, since it would interfere with the optics. For short exposure times, we can rely on the open-loop guiding of the telescope. For very long exposures, we will use one of the CCDs in the camera for guiding.

The optics design for UH-WFI was iterated several times to adjust to the availability of optical glasses. In particular, we were forced to abandon a very good design that used large CaF2 due to the current high cost of the material. Our design now uses a combination of Ohara glasses in two groups. The first group consists of 6 lenses, four of which are Ohara glass S-FPL51Y, the lowest dispersion glass in their product line that could be fabricated in the size required for UH-WFI. Since S-FPL51Y cannot be fabricated in thick plates, the thick lenses of the original design both are split up into two thinner ones within the thickness limit of the raw material. Due to the large number in internal surfaces in the main lens group, we plan on mounting these lenses in an immersion oil filled lens cell.

The second lens group is an individual lens that combines the functions of a field flattener and the dewar window.

Fig. 1: Layout and raytrace of the UH-WFI focal reducer.

The optical design was optimized for the 0.45-1.0 µm wavelength range. Over this range, chromatic aberrations are not significant relative to typical seeing so that very broad filters can be used for search programs. Observations in the 0.35-0.45 µm range are possible but require narrower filters and refocusing between filters. The performance of the optical system remains within specification over the temperature range expected on Mauna Kea. It has been optimized for 0° C and 0.6 atm atmospheric pressure.

Fig. 2: Spot diagram of the polychromatic image quality of the UH-WFI between 0.40 µm and 1.0 µm. The box represents 4 pixels (15 µm each), or approx. 0.90° on the sky. The image quality of the lens system is better than the median seeing of FWHM = 0.70 obtained at the UH 2.2-m telescope f/10 focus. In off-axis field positions, the image quality is actually better than that of the uncorrected but flattened RC focus.

The basic optical data for the f = 13.5 m system described here are a focal plane scale of 65.4 µm/arcsec, resulting in a pixel scale of 0.229 arcsec/pixel for the 15 µm CCD pixels of the UH 8K, and approximately a 31.49' x 31.49' field of view, accounting for some dead space between the CCDs. The very corners of the field are slightly vignetted by the secondary mirror, but the vignetting is well within what is correctable by proper flat-fielding of the data. Grid distortion by the focal reducer is a maximum of 2.8% in the corners of the field and will be corrected in software.

Mechanical Design

The conceptual mechanical design has three components:

Fig. 3: Conceptual mechanical design of the UH Wide-Field Imager, including the lens mount, shutter mechanism, filter wheel mechanism, and the existing upgraded UH 8K dewar.

The filter wheel assembly also serves as the mechanical mounting surface for the lens mount on one side and the dewar on the other. While the position of the filter is not in itself very critical, the filter assembly has to be very stiff to support the lens mount and the dewar within the relatively tight tolerances imposed by the optical power of the field flattener dewar window.