A tower and access platform were fabricated and erected for the Hobby-Eberly Telescope (HET) Differential Image Motion Monitor (DIMM). The 5 meter high tower exhibits a fundamental frequency of 43 Hz. The cost of the tower system, including access platform and enclosure was $12K US. The enclosure includes a unique application of an off-the-shelf truck accessory that serves as a shutter. The enclosure is ventilated, and incorporates a windshield to reduce vibrations that have proven problematic in the Meade Telescopes used for the DIMM system.
The Hobby-Eberly Telescope (HET) is a fixed-elevation, 9.2-m telescope with a spherical primary mirror and a tracker at prime focus to follow astronomical objects. The telescope was constructed for $13.9M over the period 1994-1997. A series of extensive engineering upgrades and corrective actions have been completed recently, resulting in significantly improved delivered image quality and increased operational efficiency. The telescope's Spherical Aberration Corrector (SAC) optics were recoated with a highly reflective and durable broadband coating at Lawrence Livermore National Laboratory. The software mount model that maintains optical alignment of the SAC with the 11-m primary mirror array was recalibrated and improved. The acquisition and guiding optics for both the High Resolution Spectrograph (HRS) and the Low Resolution Spectrograph (LRS) were reworked and improved, allowing for better focus and SAC alignment monitoring and control. Recoating of the primary mirror segment array was begun. Telescope images of 0.82 arcseconds have been recorded for sustained periods in preliminary testing following the engineering upgrade, an improvement of 50% over previous best performance. Additional engineering upgrades are scheduled to consolidate these performance gains and to continue improving delivered image quality, throughput, and telescope operational efficiency. The HET is now capable of the science performance for which it was designed.
KEYWORDS: Telescopes, Mirrors, Image segmentation, Domes, Mars, Image quality, Simulation of CCA and DLA aggregates, Sensors, Electronics, Optical instrument design
The Hobby-Eberly Telescope (HET) is a fixed-elevation, 9.2-m telescope with a spherical primary mirror and a tracker at prime focus to follow astronomical objects. The telescope was constructed for $13.9M over the period 1994-1997. A number of telescope performance deficiencies were identified and corrected following construction. Remaining problems included: 1) Dome seeing, 2) inadequate initial mirror segment alignment accuracy, and 3) mirror segment misalignment with time. The HET Completion Project was created in May 2001 to attack these problems and to identify and solve the next tier of problems. To address dome seeing, large louvers were installed and in operation by May 2002. Efforts are also underway to eliminate or suppress heat sources within the dome environment. To address segment alignment accuracy, a prototype Shack-Hartmann device, the Mirror Alignment Recovery System (MARS), was built and is in routine use at HET. The Segment Alignment Maintenance System (SAMS) is in early operation and has markedly improved telescope performance. Two Differential Image Motion Monitor (DIMM) telescopes were brought into regular operation in July 2001 to quantify atmospheric seeing at HET. As these improvements have been implemented, telescope image quality has improved significantly. Plans are in place to address additional performance issues.
The Hobby-Eberly Telescope (HET) enclosure is receiving a series of modifications to improve dome seeing, including removal of residual heat loads from the optical path, increased insulation of the enclosure, and ventilation of the enclosure ring wall and dome. Analysis indicates that the contribution of dome seeing with the new system will be less than 0.05 arc seconds. The design of the HET enclosure lends itself to very large ventilation openings in the stationary portion of the enclosure also referred to as the "ring wall", with supplementary openings in the rotating dome. The ventilation design implemented has opened approximately 58% of the ring wall, and will open 8% of the dome, in the coming year, in order to achieve natural ventilation of 22 air changes per hour at the minimum design wind velocity of 3.5 mph. A system of ventilation louvers similar in design to the Kitt Peak Observatories 4-meter telescope was determined to be the most practical and cost effective design for use in the ring wall. Conventional off-the-shelf louvers are proposed for the dome, due to constraints in its design. Special considerations for retrofitting an operating facility included a custom hoist mounted on the dome for installation of the louvers (5000lb./ea.), and an inflatable curtain to protect the telescope during foul weather. The ring wall ventilation system has been in full operation since early April 2002 and is part of a program in progress to substantially improve the HET dome seeing.
A long term program to quantify the intrinsic site seeing at McDonald Observatory, using two differential image motion monitors (DIMMs) has been initiated on Mt. Fowlkes where the Hobby-Eberly Telescope (HET) is located. Raw DIMM data are corrected to the zenith and to a uniform 10msec integration time. Nightly median seeing measurements (FWHM) along with the max/min range are presented for 186 nights over the 13 month period between July 2001 and July 2002. A definite seasonal effect is present in the dataset with the median seeing in the spring-summer-fall months (0.93±0.18 arcsec) being significantly better than the winter months (1.24±0.33 arcsec). The measured seeing was better than 0.70 arcsec about 9% of the time. Since DIMM units were operated at ground level these data are not quite lower limits to the site seeing performance. Even so, the seeing of this West Texas continental site at 6,650ft (2,027m) elevation in the Davis Mountains is superior to what has been assumed in the past, based on less direct seeing measurements.
Future plans are described for moving a DIMM telescope to a tower mounted, semi-automated observatory to sample the site seeing at an elevation above the ground similar to the HET mirror.
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