Broadband Detectors for On-Sky Observation with 2D-doped and custom coated n-Channel and p-channel thick CCDs
On-sky observations with broadband delta-doped arrays were planned as part of an effort funded by NASA’s Strategic Astrophysics Technology (SAT). Delta-doped p-channel devices were previously developed in collaboration with Lawrence Berkeley National Laboratory (LBNL). These detectors were processed using MDL’s large-capacity silicon MBE, with a directly applied ALD broadband coating designed to maximize the response over the required range (320–1000 nm). This is the baseline detector for an Explorer-Orion mission concept, and on-sky observation is a necessary step for laboratory devices to showcase their capabilities and demonstrate their viability for future missions. Three prototype LBNL detectors were delivered to Arizona State University for testing at Steward Observatory’s 61-inch Kuiper telescope on Mt. Bigelow.
Palomar: WaSP—Wafer Scale Prime and Zwiki Transient Facility
Earth-based observatories, with their ability to continually improve performance with upgraded imagers, provide new and valuable astronomy. On-sky data on these facilities also serve as performance verification and validation of imaging technology. Custom-packaged 2D-doped charge-coupled devices (CCDs) developed at MDL have now seen first light. These 2kx2k, 15-µm pixel deep-depletion CCD arrays are designed by Semiconductor Technology Associates (STA) and fabricated at Teledyne DALSA. The Wafer-Scale camera for Prime focus (WaSP), a collaboration between Caltech Optical Observatories (COO) and JPL, is the new prime focus imager for the 200-inch telescope, providing multiband color imaging from 320–1000 nm. WaSP was developed to replace the older Large-Format Camera (LFC) with a single high performance focal plane. The 2D-doped, fully depleted high-purity thick silicon devices delivered for WaSP have provided enhanced response across the near-ultraviolet to near-infrared (320–1000 nm). JPL’s delta-doping technology increased the U-band (<400 nm) sensitivity significantly beyond that of commercially available CCDs. Use of high efficiency and high-stability 2D-doped arrays at Palomar-WaSP was partially motivated by the desire to obtain on-sky data as a pathfinder to a space-based UV Transient Astronomy mission concept called ULTRASAT.
In May 2016, the WaSP instrument, with its low-noise 6kx6k science CCD and the JPL-provided guide and focus CCDs, was installed on Palomar’s 200-inch telescope. To achieve the highest image quality, the instrument requires dedicated guide and focus CCDs to provide real-time feedback to the telescope tracking system and focus controls. The guide CCD, in particular, needs to have as high a sensitivity as possible. In response to this need, JPL developed a custom-packaged, custom-coated delta-doped CCD based on an STA 3600 detector where both blue and red ends of the CCD response are significantly enhanced.
Due to the tight space constraints in the WaSP dewar, these custom guide and focus CCDs also needed to be brought as close as possible to the science CCD. This required the development of a custom CCD package to reduce the detector footprint to the physical size of the detector die, with no wire-bonds or detector packaging outside that area. The packaging requirements also called for meeting tight tolerances on the final package thickness, to avoid shimming in the dewar to achieve focus. In the final configuration, all three CCDs (guide, focus, and science) are installed on a flexure mount in the correct focal plane positions within 20 microns across multiple devices, which is impressive.
After the successful delivery of these CCDs, Caltech Optical Observatories funded the production of five additional CCDs for the upcoming Zwicky Transient Facility instrument on the 48-inch Oschin Telescope at Palomar, which will conduct high-cadence sky-surveys for transient astronomical objects.
Palomar’s Zwiki Transient Facility (ZTF; near photograph) and WaSP Wafer Scale Prime camera (not shown) use identical devices provided by MDL for Guide and Focus. Tim Goodsall holding a 2D doped device.
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First light imaging with WaSP broadband optimized 2D-doped STA CCD 3600s. WaSP on-sky image credits to Shrinivas Kulkarni, Director at the time, and Jennifer Milburn, Lead Electronics Engineer of Caltech Optical Observatories.
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Palomar-Cosmic Web Imager
A 2D-doped and AR coated (single layer) 1 k x 2 k EMCCD was processed for on-sky observation at Palomar. Three observation runs at Palomar were completed with this detector behind the Cosmic Web Imager (CWI). CWI is an integral field spectrograph (IFS) designed with 24 slices that covering a 60 by 40 arcseconds area with a resolution R ∼ 5000.
Results from these runs were compared to equivalent observations with the normal CWI detector. We compared both photon counting operation, at a range of gain values, and normal mode operation. The data gathering process has involved many short integrations on sky to track sky lines, and hence flexure during a long exposure; this will allow for higher resolution flexure correction. In addition, these runs were used to verify measured QE results using standard stars, in both normal mode and EM operation. Results from the second run at Palomar behind CWI are shown here.
Image of the NGC 2022 (Planetary Nebula) observed at H-β collected at CWI with 2D-doped. A 2D-doped EMCCD used for on-sky characterization at Palomar Cosmic Web Imager (CWI) spectrograph. Gillian Kyne preparing devices in the lab for CWI tests.
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Steward Observatory: high-purity P-channel CCD
The CHESS flight prototype detector—a delta-doped, p-channel 3508 × 3512, 10.5-μm pixel CCD (SNAP, LBNL)—was incorporated in its dewar at Arizona State University (ASU) and used for on-sky observation and quantitative analysis of its QE and performance. Figure below shows the images of astronomical objects during the expedition.
Detector installed in camera Dewar at Arizona State University and subsequently mounted on the back of the 61-in. Kuiper telescope of the Steward Observatory (shown in center). On-sky images with this CCD.
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Preparation of a 2D-doped and AR-coated 12 Megapixel p channel CCD designed by LBNL and fabricated at Dalsa.
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A delta-doped, p-channel, 8-megapixel (2048×4096) LBNL CCD with high broadband efficiency was used for on-sky tests at Lick Observatory (figure below). While this device sustained some damage during the integration in the dewar at the observatory, it was used to produce on-sky images such as those shown below.
A 2D-doped, 8-megapixel (2048 x 4096), p-channel LBNL CCD. The NGC 4565 “Needle Galaxy”, an image obtained with this CCD at Lick Observatory. Michael Hoenk preparing for a 2D doping run. Galaxy Image Courtesy of Richard J. Stover, UCO/Lick Observatory.
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