The Rapid infrared IMAger-Spectrometer (RIMAS) is a near-infrared (NIR) imager and spectrometer that will quickly follow up gamma-ray burst afterglows on the 4.3-meter Discovery Channel Telescope (DCT). RIMAS has two optical arms which allows simultaneous coverage over two bandpasses (YJ and HK) in either imaging or spectroscopy mode. RIMAS utilizes two Teledyne HgCdTe H2RG detectors controlled by Astronomical Research Cameras, Inc. (ARC/Leach) drivers. We report the laboratory characterization of RIMAS's detectors: conversion gain, read noise, linearity, saturation, dynamic range, and dark current. We also present RIMAS's instrument efficiency from atmospheric transmission models and optics data (both telescope and instrument) in all three observing modes.
The Rapid Infrared Imager/Spectrograph (RIMAS) is an instrument designed to observe gamma ray burst afterglows following initial detection by the SWIFT satellite. Operating in the near infrared between 0.9 and 2.4 μm, it has capabilities for both low resolution (R~25) and moderate resolution (R~4000) spectroscopy. Two zinc selenide (ZnSe) grisms provide dispersion in the moderate resolution mode: one covers the Y and J bands and the other covers the H and K. Each has a clear aperture of 44 mm. The YJ grism has a blaze angle of 49.9° with a 40 μm groove spacing. The HK grism is blazed at 43.1° with a 50 μm grooves spacing.
Previous fabrication of ZnSe grisms on the Precision Engineering Research Lathe (PERL II) at LLNL has demonstrated the importance of surface preparation, tool and fixture design, tight thermal control, and backup power sources for the machine. The biggest challenges in machining the RIMAS grisms are the large grooved area, which indicates long machining time, and the relatively steep blaze angle, which means that the grism wavefront error is much more sensitive to lathe metrology errors. Mitigating techniques are described.
The Rapid Infrared Imager/Spectrometer (RIMAS) is designed to perform follow-up observations of transient
astronomical sources at near infrared (NIR) wavelengths (0.9 - 2.4 microns). In particular, RIMAS will be used to
perform photometric and spectroscopic observations of gamma-ray burst (GRB) afterglows to compliment the Swift
satellite’s science goals. Upon completion, RIMAS will be installed on Lowell Observatory’s 4.3 meter Discovery
Channel Telescope (DCT) located in Happy Jack, Arizona. The instrument’s optical design includes a collimator lens
assembly, a dichroic to divide the wavelength coverage into two optical arms (0.9 - 1.4 microns and 1.4 - 2.4 microns
respectively), and a camera lens assembly for each optical arm. Because the wavelength coverage extends out to 2.4
microns, all optical elements are cooled to ~70 K. Filters and transmission gratings are located on wheels prior to each
camera allowing the instrument to be quickly configured for photometry or spectroscopy. An athermal optomechanical
design is being implemented to prevent lenses from loosing their room temperature alignment as the system is cooled.
The thermal expansion of materials used in this design have been measured in the lab. Additionally, RIMAS has a guide
camera consisting of four lenses to aid observers in passing light from target sources through spectroscopic slits. Efforts
to align these optics are ongoing.
The Rapid infrared IMAger-Spectrometer (RIMAS) is a rapid gamma-ray burst afterglow instrument that will provide photometric and spectroscopic coverage of the Y, J, H, and K bands. RIMAS separates light into two optical arms, YJ and HK, which allows for simultaneous coverage in two photometric bands. RIMAS utilizes two 2048 x 2048 pixel Teledyne HgCdTe (HAWAII-2RG) detectors along with a Spitzer Legacy Indium- Antimonide (InSb) guiding detector in spectroscopic mode to position and keep the source on the slit. We describe the software and hardware development for the detector driver and acquisition systems. The HAWAII- 2RG detectors simultaneously acquire images using Astronomical Research Cameras, Inc. driver, timing, and processing boards with two C++ wrappers running assembly code. The InSb detector clocking and acquisition system runs on a National Instruments cRIO-9074 with a Labview user interface and clocks written in an easily alterable ASCII file. We report the read noise, linearity, and dynamic range of our guide detector. Finally, we present RIMAS’s estimated instrument efficiency in photometric imaging mode (for all three detectors) and expected limiting magnitudes. Our efficiency calculations include atmospheric transmission models, filter models, telescope components, and optics components for each optical arm.
The Observational Cosmology Laboratory at NASA’s Goddard Space Flight Center (GSFC), in collaboration with the
University of Maryland, is building the Rapid Infrared Imager/Spectrometer (RIMAS) for the new 4.3 meter Discovery
Channel Telescope (DCT). The instrument is designed to observe gamma-ray burst (GRB) afterglows following their
initial detection by the Swift satellite. RIMAS will operate in the near infrared (0.9 – 2.4 microns) with all of its optics
cooled to ~60 K. The primary optical design includes a collimator lens assembly, a dichroic dividing the wavelength
coverage into the “YJ band” and “HK band” optical arms, and camera lens assemblies for each arm. Additionally, filters
and dispersive elements are attached to wheels positioned prior to each arm’s camera, allowing the instrument to quickly
change from its imaging modes to spectroscopic modes. Optics have also been designed to image the sky surrounding
spectroscopic slits to help observers pass light from target sources through these slits. Because the optical systems are
entirely cryogenic, it was necessary to account for changing refractive indices and model the effects of thermal
contraction. One result of this work is a lens mount design that keeps lenses centered on the optical axis as the system is
cooled. Efforts to design, tolerance and assemble these cryogenic optical systems are presented.