KEYWORDS: Mirrors, Optical alignment, Imaging systems, Point spread functions, Vacuum ultraviolet, Calibration, Tunable filters, Sensors, Signal to noise ratio
The Carruthers Geocorona Observatory is a NASA Heliophysics mission designed to study the variability of Earth’s hydrogen exosphere. Launching in 2025, the Carruthers GeoCoronal Imager (GCI) will observe the exosphere at Far Ultraviolet wavelengths from an Earth-Sun L1 vantage point. The GCI consists of two coaligned imagers that simultaneously provide both wide field observations of the entire exosphere, in addition to high spatial resolution observations near the Earth’s limb. The optical prescription for both the narrow field imager (NFI) and wide field imager (WFI) is discussed, including critical analyses that were performed during the design phase of the project. A deterministic alignment approach was adopted to verify performances of the imagers at visible wavelengths prior to verification in the vacuum ultraviolet. The details of this alignment plan, along with opto-mechanical considerations and requirements are discussed in detail. Finally, we discuss the imaging performance of the system in the ultraviolet utilizing a ground calibration facility previously developed for another NASA spaceflight mission.
The Keck Planet Finder (KPF) is a fiber-fed, high-resolution, echelle spectrometer that specializes in the discovery and characterization of exoplanets using Doppler spectroscopy. In designing KPF, the guiding principles were high throughput to promote survey speed and access to faint targets, and high stability to keep uncalibrated systematic Doppler measurement errors below 30 cm s−1. KPF achieves optical illumination stability with a tip-tilt injection system, octagonal cross-section optical fibers, a double scrambler, and active fiber agitation. The optical bench and optics with integral mounts are made of Zerodur to provide thermo-mechanical stability. The spectrometer includes a slicer to reformat the optical input, green and red channels (445–600 nm and 600–870 nm), and achieves a resolving power of ∼97,000. Additional subsystems include a separate, medium-resolution UV spectrometer (383–402 nm) to record the Ca II H & K lines, an exposure meter for real-time flux monitoring, a solar feed for sunlight injection, and a calibration system with a laser frequency comb and etalon for wavelength calibration. KPF was installed and commissioned at the W. M. Keck Observatory in late 2022 and early 2023 and is now in regular use for scientific observations. This paper presents an overview of the as-built KPF instrument and its subsystems, design considerations, and initial on-sky performance.
We present a compact, double-pass cross-dispersed echelle spectrograph that is tailored specifically to cover the 383 nm to 403 nm spectral range and record R∼16,000 spectra of the stellar chromospheric Ca II H and K lines. This ‘H and K’ spectrometer was developed as a subsystem of the Keck Planet Finder (KPF), which is an extremely precise optical (440 - 870 nm) radial velocity spectrograph for Keck I, scheduled for commissioning Fall 2022, with the science objective of measuring precise masses of exoplanets. The H and K spectrometer will observe simultaneously with KPF to independently track the chromospheric activity of the host stars that KPF observes, which is expected to dominate the KPF measurement floor over long timescales. The H and K Spectrometer is fiber fed from the KPF fiber injection unit with total throughput of 4-7% (top of telescope to CCD) over its operating spectral range. Here we detail the optical design trade offs, mechanical design, and first results from alignment and integration testing.
State of the art astrophysics demands extremely stable wavelength measurements, e.g. few cm/s scale Doppler radial velocimetry for Earth-like planet detection or multi-year cosmic redshift drift measurements. We present new techniques for 500 − 1000× improvement in stability using an interferometer in series with a spectrograph to form an Externally Dispersed Interferometer (EDI). When the received spectrum suffers a wavelength jitter, the phase of the moir´e pattern from the interferometer delay shifts in opposite directions for two signal paths, nonfringing and fringing; with appropriate weightings (“crossfading”) the net phase reaction cancels, stabilizing the spectrum. We present an improvement to our previous technique of multiple delays, using a single delay to crossfade, and demonstrate stabilization of ≳ 500× on existing Hale Telescope data. Single-delay EDIs are easier to construct and operate than those with multiple delays, and the EDI ensures that exactly the same pixels are used for the science and calibration signals, and in constant proportion under intensity fluctuations, greatly easing positional requirements.
We built an externally dispersed interferometer (EDI) testbed for exploring methods of improving high resolution spectrograph performance. We tested the EDI on the Keck Planet Finder (KPF) spectrograph May 11, 2022 measuring a Fabry-Perot (FP) etalon back lit by white light. This is also the first time an EDI has been used to measure a periodic source. The data shows that the EDI is useful for diagnosing the point spread function (PSF) width and shape, in particular the asymmetry of the PSF. This EDI ability comes because EDI can simultaneously measure both the conventional nonfringing spectrum, and the fringing derived spectrum. A conventional spectrograph resolution is limited by slit blur– the EDI resolution is not. A heterodyning effect shifts the fringing sensitivity peak to arbitrarily higher frequency, set by the interferometer delay value, and thus its resolution can exceed the spectrograph used alone. By comparing the Fourier transforms of the two measured signals, we can compare the phase shift changes, which gives information about the small asymmetry of the spectrograph blur, independent of the much larger asymmetry of the FP source spectrum. We show that comparing the phases of the nonfringing and fringing components versus harmonic number in the Fourier Transform is a useful method for measuring the asymmetry in the PSF. We report the first measurements of an EDI measuring a periodic source, which is a Fabry-Perot (FP) interferometer back illuminated by white light. The periodic character of the FP source created spikes in the Fourier transform, which was very convenient for analysis.
The Keck Planet Finder (KPF) is a fiber-fed, high-resolution, high-stability spectrometer in development at the UC Berkeley Space Sciences Laboratory for the W.M. Keck Observatory. KPF is designed to characterize exoplanets via Doppler spectroscopy with a goal of a single measurement precision of 0.3 m s-1 or better, however its resolution and stability will enable a wide variety of astrophysical pursuits. Here we provide post-preliminary design review design updates for several subsystems, including: the main spectrometer, the fabrication of the Zerodur optical bench; the data reduction pipeline; fiber agitator; fiber cable design; fiber scrambler; VPH testing results and the exposure meter.
The Keck Planet Finder (KPF) is a fiber-fed, high-resolution, high-stability spectrometer in development for the W.M. Keck Observatory. The instrument recently passed its preliminary design review and is currently in the detailed design phase. KPF is designed to characterize exoplanets using Doppler spectroscopy with a single measurement precision of 0.5 m s−1 or better; however, its resolution and stability will enable a wide variety of other astrophysical pursuits. KPF will have a 200 mm collimated beam diameter and a resolving power greater than 80,000. The design includes a green channel (445 nm to 600 nm) and red channel (600 nm to 870 nm). A novel design aspect of KPF is the use of a Zerodur optical bench, and Zerodur optics with integral mounts, to provide stability against thermal expansion and contraction effects.
We present the design and test results of a double-scrambler and fiber agitator system for the Keck Planet Finder (KPF) spectrograph. The mechanical agitator for modal noise suppression is constructed from two linear stages with the fibers mounted in a “W” curve. When driven back-and-forth at different rates, the stages change the position of the fiber curves, and hence vary the modes propagating through the fiber. Near-field temporal centroid shifts caused by modal-noise are reduced by a factor of 100 by the agitator, while mid-range spatial frequencies have reduced power by a factor of ∼300 in the near-field, and ∼1000 in the far-field. The scrambling system incorporates two octagonal fibers, and a scrambler consisting of two identical cemented lenses ∼20 cm apart, which exchanges the optical near- and far-fields of the fibers. The scrambler shows scrambling gains >16,000 in the near-field, and >40,000 in the far-field. The measured throughput efficiency of 99.3% of the expected maximum demonstrates that scrambler-induced focal ratio degradation (FRD) is minimal. The scrambler also serves as the feed-through into the vacuum chamber where the spectrograph is housed, thereby removing concerns about stressing the fibers, and introducing FRD, at this interface. Our illumination stabilization system, consisting of two octagonal fibers, a two lens scrambler, and a mechanical agitator, produces very homogeneous fiber output in both the near and far-fields. When coupled to the Keck Planet Finder spectrograph, this system provides illumination stability corresponding to a velocity of 0.30 m s−1 .
High-resolution broadband spectroscopy at near-infrared (NIR) wavelengths (950 to 2450 nm) has been performed using externally dispersed interferometry (EDI) at the Hale telescope at Mt. Palomar, with the TEDI interferometer mounted within the central hole of the 200-in. primary mirror in series with the comounted TripleSpec NIR echelle spectrograph. These are the first multidelay EDI demonstrations on starlight. We demonstrated very high (10×) resolution boost and dramatic (20× or more) robustness to point spread function wavelength drifts in the native spectrograph. Data analysis, results, and instrument noise are described in a companion paper (part 1). This part 2 describes theoretical photon limited and readout noise limited behaviors, using simulated spectra and instrument model with noise added at the detector. We show that a single interferometer delay can be used to reduce the high frequency noise at the original resolution (1× boost case), and that except for delays much smaller than the native response peak half width, the fringing and nonfringing noises act uncorrelated and add in quadrature. This is due to the frequency shifting of the noise due to the heterodyning effect. We find a sum rule for the noise variance for multiple delays. The multiple delay EDI using a Gaussian distribution of exposure times has noise-to-signal ratio for photon-limited noise similar to a classical spectrograph with reduced slitwidth and reduced flux, proportional to the square root of resolution boost achieved, but without the focal spot limitation and pixel spacing Nyquist limitations. At low boost (∼1×) EDI has ∼1.4× smaller noise than conventional, and at >10× boost, EDI has ∼1.4× larger noise than conventional. Readout noise is minimized by the use of three or four steps instead of 10 of TEDI. Net noise grows as step phases change from symmetrical arrangement with wavenumber across the band. For three (or four) steps, we calculate a multiplicative bandwidth of 1.8:1 (2.3:1), sufficient to handle the visible band (400 to 700 nm, 1.8:1) and most of TripleSpec (2.6:1).
The Ionospheric Connection Explorer (ICON) is a NASA Heliophysics Explorer Mission designed to study the ionosphere. ICON will examine the Earth's upper atmosphere to better understand the relationship between Earth weather and space-weather drivers. ICON will accomplish its science objectives using a suite of 4 instruments, one of which is the Extreme Ultraviolet Spectrograph (EUV). EUV will measure daytime altitude intensity profile and spatial distribution of ionized oxygen emissions (O+ at 83.4 nm and 61.7 nm) on the limb in the thermosphere (100 to 500 km tangent altitude). EUV is a single-optic imaging spectrometer that observes in the extreme ultraviolet region of the spectrum. In this paper, we describe instrumental performance calibration measurement techniques and data analysis for EUV. Various measurements including Lyman-α scattering, instrumental and component efficiency, and field-of-view alignment verification were done in custom high-vacuum ultraviolet calibration facilities. Results from the measurements and analysis will be used to understand the instrument performance during the in-flight calibration and observations after launch.
We describe demonstrations of remarkable robustness to instrumental noises by using a multiple delay externally dispersed interferometer (EDI) on stellar observations at the Hale telescope. Previous observatory EDI demonstrations used a single delay. The EDI (also called “TEDI”) boosted the 2,700 resolution of the native TripleSpec NIR spectrograph (950-2450 nm) by as much as 10x to 27,000, using 7 overlapping delays up to 3 cm. We observed superb rejection of fixed pattern noises due to bad pixels, since the fringing signal responds only to changes in multiple exposures synchronous to the applied delay dithering. Remarkably, we observed a ~20x reduction of reaction in the output spectrum to PSF shifts of the native spectrograph along the dispersion direction, using our standard processing. This allowed high resolution observations under conditions of severe and irregular PSF drift otherwise not possible without the interferometer. Furthermore, we recently discovered an improved method of weighting and mixing data between pairs of delays that can theoretically further reduce the net reaction to PSF drift to zero. We demonstrate a 350x reduction in reaction to a native PSF shift using a simple simulation. This technique could similarly reduce radial velocity noise for future EDI’s that use two delays overlapped in delay space (or a single delay overlapping the native peak). Finally, we show an extremely high dynamic range EDI measurement of our ThAr lamp compared to a literature ThAr spectrum, observing weak features (~0.001x height of nearest strong line) that occur between the major lines. Because of individuality of each reference lamp, accurate knowledge of its spectrum between the (unfortunately) sparse major lines is important for precision radial velocimetry.
High-resolution broadband spectroscopy at near-infrared wavelengths (950 to 2450 nm) has been performed using externally dispersed interferometry (EDI) at the Hale telescope at Mt. Palomar. Observations of stars were performed with the “TEDI” interferometer mounted within the central hole of the 200-in. primary mirror in series with the comounted TripleSpec near-infrared echelle spectrograph. These are the first multidelay EDI demonstrations on starlight, as earlier measurements used a single delay or laboratory sources. We demonstrate very high (10×) resolution boost, from original 2700 to 27,000 with current set of delays (up to 3 cm), well beyond the classical limits enforced by the slit width and detector pixel Nyquist limit. Significantly, the EDI used with multiple delays rather than a single delay as used previously yields an order of magnitude or more improvement in the stability against native spectrograph point spread function (PSF) drifts along the dispersion direction. We observe a dramatic (20×) reduction in sensitivity to PSF shift using our standard processing. A recently realized method of further reducing the PSF shift sensitivity to zero is described theoretically and demonstrated in a simple simulation which produces a 350× times reduction. We demonstrate superb rejection of fixed pattern noise due to bad detector pixels—EDI only responds to changes in pixel intensity synchronous to applied dithering. This part 1 describes data analysis, results, and instrument noise. A section on theoretical photon limited sensitivity is in a companion paper, part 2.
High resolution broad-band spectroscopy at near-infrared wavelengths has been performed using externally dis- persed interferometry (EDI) at the Hale telescope at Mt. Palomar. The EDI technique uses a field-widened Michelson interferometer in series with a dispersive spectrograph, and is able to recover a spectrum with a resolution 4 to 10 times higher than the existing grating spectrograph. This method increases the resolution well beyond the classical limits enforced by the slit width and the detector pixel Nyquist limit and, in principle, decreases the effect of pupil variation on the instrument line-shape function. The EDI technique permits arbi- trarily higher resolution measurements using the higher throughput, lower weight, size, and expense of a lower resolution spectrograph. Observations of many stars were performed with the TEDI interferometer mounted within the central hole of the 200 inch primary mirror. Light from the interferometer was then dispersed by the
TripleSpec near-infrared echelle spectrograph. Continuous spectra between 950 and 2450 nm with a resolution
as high as ~27,000 were recovered from data taken with TripleSpec at a native resolution of ∼2,700. Aspects
of data analysis for interferometric spectral reconstruction are described. This technique has applications in im- proving measurements of high-resolution stellar template spectra, critical for precision Doppler velocimetry using conventional spectroscopic methods. A new interferometer to be applied for this purpose at visible wavelengths is under construction.
Externally dispersed interferometry (EDI) uses a hybrid spectrometer that combines a Michelson interferometer in series
with a grating spectrometer. EDI provides a means of deriving spectral information at a resolution substantially higher
than that provided by the grating spectrograph alone. Near IR observations have been conducted using the Triplespec
spectrometer mounted on the 5m Hale telescope. Spectra have been reconstructed at a resolution of ~27000 where the
resolution of Triplespec is ~2700. Progress in the development of the EDI technique is reported herein emphasizing
studies related to the accuracy of the reconstructed spectra.
We describe the fiber optics systems for use in BigBOSS, a proposed massively parallel multi-object spectrograph
for the Kitt Peak Mayall 4-m telescope that will measure baryon acoustic oscillations to explore dark energy.
BigBOSS will include 5,000 optical fibers each precisely actuator-positioned to collect an astronomical target’s flux
at the telescope prime-focus. The fibers are to be routed 40m through the telescope facility to feed ten visible-band
imaging spectrographs. We report on our fiber component development and performance measurement program.
Results include the numerical modeling of focal ratio degradation (FRD), observations of actual fibers’ collimated
and converging beam FRD, and observations of FRD from different types of fiber terminations, mechanical
connectors, and fusion-splice connections.
We describe a non-contact optical measurement method used to determine the surface flatness of a cryogenic sensor
array developed for the JDEM mission. Large focal planes envisioned for future visible to near infra-red astronomical
large area point-source surveys such as JDEM, WFIRST, or EUCLID must operate at cryogenic temperatures while
maintaining focal plane flatness within a few 10's of μm over half-meter scales. These constraints are imposed by
sensitivity conditions that demand low noise observations from the sensors and the large-field, fast optical telescopes
necessary to obtain the science yield. Verifying cryogenic focal plane flatness is challenging because μm level
excursions need to be measured within and across many multi-cm sized sensors using no physical contact and while
situated within a high-vacuum chamber. We have used an optical metrology Shack-Hartmann scheme to measure the
36x18 cm focal plane developed for the JDEM mission at the Lawrence Berkeley National Laboratory. The focal plane
holds a 4x8 array of CCDs and HgCdTe detectors. The flatness measurement scheme uses a telescope-fed micro-lens
array that samples the focal plane to determine slope changes of individual sensor zones.
An optical technique called "interferometric spectral reconstruction" (ISR) is capable of increasing a spectrograph's
resolution and stability by large factors, well beyond its classical limits. We have demonstrated a 6-
to 11-fold increase in the Triplespec effective spectral resolution (R=2,700) to achieve R=16,000 at 4100 cm-1to 30,000 at 9600 cm-1 by applying special Fourier processing to a series of exposures with different delays
(optical path differences) taken with the TEDI interferometer and the near-infrared Triplespec spectrograph at
the Mt. Palomar Observatory 200 inch telescope. The TEDI is an externally dispersed interferometer (EDI) used
for Doppler radial velocity measurements on M-stars, and now also used for ISR. The resolution improvement
is observed in both stellar and telluric features simultaneously over the entire spectrograph bandwidth (0.9-2.45
μm). By expanding the delay series, we anticipate achieving resolutions of R=45,000 or more. Since the delay is
not continuously scanned, the technique is advantageous for measuring time-variable phenomena or in varying
conditions (e.g. planetary fly-bys). The photon limited signal to noise ratio can be 100 times better than a
classic Fourier Transform Spectrometer (FTS) due to the benefit of dispersion.
The Cosmic Hot Interstellar Plasma Spectrometer (CHIPS) observatory launched on 12 January 2003, and
was the first and only successful GSFC UNEX (NASA Goddard Spaceflight Center University Explorer
class) mission. The UNEX program was conceived by the National Aeronautics and Space Administration
(NASA) as a new class of Explorer mission charged with demonstrating that significant science and/or
technology experiments can be performed by small satellites with constrained budgets and a limited schedule.
The purpose of the observatory was to examine details of the local bubble thermal pressure, spatial
distribution and ionization history. The observatory was also used to observe solar spectra, both scattered
from the Lunar surface and via a fortuitous 2nd order scattering path. CHIPS confirmed that spectral features
within the 90-260Å band were much dimmer than was predicted by contemporary theories, and operated four
years beyond its design lifetime. The observatory was placed in an extended safe-hold mode in April of 2008
for budgetary purposes. The spectrometer consisted of six spectrograph channels which delivered >λ/100
resolution spectra to a single detector. Cost constraints of UNEX led to a design based on a traditional
aluminum structure, and an instrument with a large field of view (5° x 26°). All optical and optomechanical
systems on the spectrometer performed flawlessly on orbit. We discuss the challenges, difficulties and
lessons learned during the design, fabrication and execution stages of the mission.
The Cosmic Hot interstellar Plasma Spectrometer (CHIPS), successfully launched on 2003 January 12, provides astronomers with an observatory dedicated to observation of the hot interstellar medium in the extreme ultraviolet. We describe here the otpical and photometric performance of the spectrograph based on calibrations of the individual components, end-to-end vacuum tests, and in-orbit observations of the Moon.
The CHIPS observatory was launched on 12 January 2003, and is the first UNEX (NASA Goddard Spaceflight Center University Explorer class) mission. It is currently on-orbit and performing diffuse spectroscopy in the 90-260Å wavelength band. The instrument is integrated with a custom 3-axis stabilized mini-satellite, designed for roughly one year of operation. The purpose of the observatory is examination of details of the local bubble thermal pressure, spatial distribution and ionization history. The spectrometer consists of six spectrograph channels which deliver >lambda/100 resolution spectra to a single detector. Cost constraints of UNEX led to a design based on a traditional aluminum structure, and an instrument with a large field of view (5° x 26°) for the dual purpose of increasing sensitivity in the photon-starved 90-260Å band, and to reduce requirements on spacecraft pointing. All optomechanical systems on the spectrometer, including coalignment, thermal, front cover and vacuum door release are performing well on orbit. We discuss design, test and operational performance of these systems, as well as launch loads and thermal system considerations.
We describe the design and development of the CHIPS microchannel plate detector. The Cosmic Hot Interstellar Plasma Spectrometer will study the diffuse radiation of the interstellar medium in the extreme ultraviolet band pass of 90Å to 260Å. Astronomical fluxes are expected to be low, so high efficiency in the band pass, good out-of-band rejection, low intrinsic background, and minimal image non-linearities are crucial detector properties. The detector utilizes three 75mm diameter microchannel plates (MCPs) in an abutted Z stack configuration. A NaBr photocathode material deposited on the MCP top surface enhances the quantum detection efficiency. The charge pulses from the MCPs are centroided in two dimensions by a crossed-delayline (XDL) anode. A four panel thin-film filter array is affixed above the MCPs to reduce sensitivity to airglow and scattered radiation, composed of aluminum, polyimide/boron, and zirconium filter panes. The detector is housed in a flight vacuum chamber to preserve the hygroscopic photocathode, the pressure sensitive thin-film filters, and to permit application of high voltage during ground test.
CHIPS is a NASA UNEX mission designed for diffuse background spectroscopy in the EUV bandpass from 90-260Å. The spectrometer is optimized for peak resolution near 170 Å, in order to study diffuse emissions from cooling million degree plasma. Details of local bubble thermal pressure, spatial distribution, and ionization history are the goals of CHIPS observations. We discuss the opto-mechanical design adopted to meet the throughput, signal to noise, and spectral resolution requirements within the mass, volume, and budgetary constraints of a UNEX Delta-II secondary payload. Mechanical tolerance requirements for the six spectrometer channels are discussed, along with details of the lightweight mounting scheme for CHIPS diffraction gratings, front cover slit mechanisms and thermal design. Finally, visible light and vacuum alignment techniques are discussed, as well as with methods employed to minimize stray light.
We present a status report on CHIPS, the Cosmic Hot Interstellar Plasma Spectrometer. CHIPS is the first NASA University-Class Explorer (UNEX) project. CHIPS was selected in 1998 and is now scheduled for launch in December of 2002. The grazing incidence CHIPS spectrograph will survey the sky and record spectra of diffuse emission in the comparatively unexplored wavelength band between 90 and 260 Å. These data will provide important new constraints on the temperature, ionization state, and emission measure of hot plasma in the "local bubble" of the interstellar medium.
The Extreme Ultraviolet Explorer (EUVE), launched June 7, 1992, conducted an all-sky survey in the extreme ultraviolet wavelengths (70-760 angstrom) for 6 months and is now performing spectroscopic pointings for Guest Observers. The seven microchannel plate (MCP) detectors used on the instrument (four for imaging in photometric wavebands and three for the spectrometers) have operated successfully throughout the mission. The long-term (780 days) performance characteristics such as quantum efficiency (QE), gain, and background count rate, will be reviewed along with other interesting unexpected effects noted during the mission. Because the QE has remained constant, the background has been reduced, and other effects have been minimized, the EUVE instruments currently operate better than after launch and will probably continue to do so.
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