Observations in the near-infrared using large ground-based telescopes are limited by bright atmospheric emission lines, particularly OH lines, which can saturate a spectrograph on the order of minutes. Longer exposures will not contain useful information about the emission lines and also run the risk of detector effects such as bleeding and persistence. By using guide windows on a HAWAII-2RG infrared detector, we demonstrate on-detector suppression of these bright lines in long exposures. This is achieved by periodically resetting detector regions which contain bright emission lines before they have the chance to saturate, while the rest of the detector continues integrating. Used with extended exposure lengths, this could allow for significant reduction of the read noise overhead required for stacking shorter exposures. In addition, through non-destructive reading we are able to monitor the lines which are being reset, allowing us to retain information about the characteristics and variability of these lines. We present the results of a first demonstration of this technique using controlled observations of arc lamps with the 1.2-m McKellar Spectrograph at the Dominion Astrophysical Observatory in Victoria, Canada. We find promising results for the potential future use of this technique.
Persistence effects in HgCdTe infrared detectors cause significant artifacts that can impact the quality of science observations for up to many hours after exposure to bright/saturating sources. This problem will have a substantially greater impact on viable observing modes for infrared cameras on future ELTs due to the leap in sensitivities that is expected. In this paper we present new results from an updated test system that was previously used to prototype “on-detector guide windows” to provide fast T/T feedback to AO systems, interleaved with simultaneous (slow) full-frame readouts for science. We now explore the possibility of continuously resetting these small regions of the detector that are illuminated with a compact source as a strategy for mitigating persistence, using two different detectors. While our results generally show promise for this observing strategy, we found for one of our detectors that the combination of fast localized resets with intense illumination can introduce a potentially problematic persistent change in local reset levels.
NFIRAOS is the first-light adaptive optics system for the Thirty Meter Telescope (TMT). It features a laser guide star wavefront sensor system, which helps to correct atmospheric distortion over a 2 arc-minute field of view. Since the final design review for NFIRAOS in 2018, a trade-study was undertaken to consider a new camera from First Light Imaging called C-Blue One as the LGS wavefront sensor. A redesign of the LGS subsystem of NFIRAOS was undertaken to incorporate this new wavefront sensor into the design with updated AO tolerances, optics, and a new relay system to reimage a high density lenslet-array onto the small sensor.
NFIRAOS (Narrow-Field InfraRed Adaptive Optics System) will be the first-light multi-conjugate adaptive optics system for the Thirty Meter Telescope (TMT). NFIRAOS supports three Near Infrared (NIR) client instruments, and provides exceptional image quality across the 2 arcminute field of view. In 2018, NFIRAOS passed the Final Design Review (FDR), but there have been several substantial changes more recently. The optical enclosure (ENCL) refrigeration design was updated for CO2 refrigerant, and an enclosure wall panel was prototyped. The polar-coordinate CCD for the laser guide star (LGS) wavefront sensor camera was replaced with a commercial C-Blue camera from First Light Imaging. More recently, the NFIRAOS Science Calibration Unit (NSCU), which was previously a separate Canadian contribution, was incorporated into the scope of NFIRAOS, and has progressed to the Conceptual Design level. In addition to these changes, the team has been working to bring the last of the low-risk subsystems to final design level in preparation for the beginning of construction.
As telescopes dramatically increase in aperture, the size and complexity of instrumentation also increases dramatically. Simply scaling up existing spectrograph designs may not be feasible and new approaches must be considered. One alternate approach is multiplexing, where the high cost of large optics is replaced by smaller, lower-cost optics albeit in higher quantities. While the idea of large-scale multiplexed instruments is not new, ELTs push the scaling of instruments into territory where multiplexing may have significant advantages. However, it remains to be seen if optical mounts can be made to meet the ever-tightening optical requirements of modern instruments while retaining the low-cost and ease of use required to make multiplexing a feasible approach.
This paper details the design, prototyping and testing of several types of low-cost semi-kinematic optical mounts at NRCHerzberg from a practical standpoint. Optical mounts tested cover a range of types intended to be easy to manufacture, quick to assemble and suitable for various instrument environments. Assembly repeatability in tip/tilt/focus, assembly time, and wavefront error due to stress birefringence (simulated) are quantified and results presented. Thermal stability, suitability for use in cryogenic environments and ease of manufacture/assembly are also analyzed. Various qualitative metrics are also discussed. Quantitative and qualitative metrics are discussed and presented in a design table to aid in directly comparing lens mounting concepts. Results are also interpreted to determine the direction of future R&D efforts towards creating large-scale, multiplexed instruments.
The Gemini Infrared Multi-Object Spectrograph (GIRMOS) is a powerful next-generation multi-object near-infrared spectrograph incorporating parallel imaging capabilities. In addition to closed-loop Gemini North Adaptive Optics (GNAO) system, each of the four GIRMOS Integral field spectrographs (IFSs) will independently perform additional multi-object AO correction in an open loop. The combined instrument will provide unique scientific capabilities such as simultaneous imaging/spectroscopy modes (for precision spectrophotometry) and interleaved imaging-spectroscopyimaging modes (for characterizing time-variable sources). In this paper, we provide a detailed updates and discussion of the mechanical design and analysis of the imager subsystem of the GIRMOS instrument, which is currently in the Critical Design Review (CDR) stage. We place particular emphasis on the design and testing of two cryogenic mechanisms—the filter wheel and the PRS deployment mechanism—along with the innovative design of cryogenic optical mounts.
The Gemini High-Resolution Optical SpecTrograph (GHOST) instrument is the next generation high resolution spectrograph for the Gemini telescope. The GHOST instrument was developed for the Gemini telescope as a collaboration between Australian Astronomical Optics (AAO) at Macquarie University, the Herzberg Astronomy and Astrophysics (HAA) in Canada and the Australian National University (ANU). The instrument is a fiber fed spectrograph with R<50,000 in two-object mode and R<75,000 in single object mode. The bench spectrograph was integrated at Gemini South from April to June 2022. This paper presents the final integration and alignment of the spectrograph at Gemini South and the measured spectrograph performance at the telescope.
The Gemini High-Resolution Optical SpecTrograph (GHOST) is the next in line instrument being integrated for the Gemini south telescope, in a collaboration between the Australian Astronomical Optics (AAO) at Macquarie University, Herzberg Astronomy and Astrophysics (HAA) at the National Research Council Canada, and the Australian National University (ANU). This paper will discuss shipping considerations and data taken by the NRC-Herzberg and Gemini team to preserve and protect the instrument during a two year hiatus brought on by the COVID-19 pandemic.
The GIRMOS instrument is a multi-object spectrograph with four channels combined with an infrared imager housed within a common cryostat. This instrument will be fed by ground-layer adaptive optics (GLAO) or laser tomography AO (LTAO) corrected light from the Gemini North Adaptive Optics (GNAO) system. The combined instrument will provide unique scientific capabilities such as simultaneous imaging/spectroscopy modes (for precision spectrophotometry) and interleaved imaging-spectroscopy-imaging modes (for characterizing time-variable sources). The National Research Council Canada has recently completed the Preliminary Design of the Imager opto-mechanics. In this paper, we present the driving requirements, as derived from the science cases, and the optical and mechanical designs. The optical design maps a large fraction of the GIRMOS field-of-view onto a single engineering-grade 4Kx4K HAWAII 4RG detector with 21 mas pixels, provided by the Gemini Observatory. The imager produces diffraction-limited image quality across Y, J, H, and Ks-bands across an 85x85” field for an f/32 beam. It includes a location for a full filter complement, an accessible pupil for a cold stop to minimize thermal background, and a pupil imaging mode to align the cold stop to the telescope pupil. The lenses are mounted in cells with rolled flexures or athermalized centering pins and are preloaded to withstand 5g accelerations and provide thermal stability. The filters are housed in a double wheel assembly with cryogenic bearings and roller detents. All of the imager components are connected with a substructure that interfaces with the spectrograph optical bench. This substructure allows for easier testing and integration of the imager, independent from the spectrographs.
The instrument group of the Herzberg Astronomy and Astrophysics has been subcontracted by Australian Astronomical Optics (AAO) at Macquarie University to design and build the bench spectrograph for the Gemini High-Resolution Optical SpecTrograph (GHOST) instrument. The GHOST instrument is being developed for the Gemini telescope and is a collaboration between AAO, the Herzberg Astronomy and Astrophysics (HAA) in Canada and the Australian National University (ANU). The instrument is a fiber fed spectrograph with R<50,000 in two-object mode and R<75,000 in single object mode. This paper presents the i ph and the performance results for the laboratory testing of the spectrograph.
The Gemini High-Resolution Optical SpecTrograph (GHOST) is the newest instrument being integrated for the Gemini telescopes, in a collaboration between the Australian National University (ANU), the NRC-Herzberg in Canada and the Australian Astronomical Observatory (AAO). The GHOST outer enclosure consists of 20 heated thermal panels, forming an encompassing structure with a stationary ‘bridge’ assembly and two removable sections for access. The outer enclosure provides an ultra-stable, dark environment for the bench spectrograph. This paper reviews the outer enclosure construction from a practical standpoint, examining how environmental requirements are met through the thermal panel construction, light seal and dry air system designs. This paper also describes thermal panel production workflow, enclosure assembly methodology, alignment and cable routing challenges. Results of the enclosure's thermal stability verification tests are presented and a list of lessons learned.
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