Cosmic Origins Spectrograph Research Articles

The cosmic origins spectrograph: capabilities and prelaunch performance

The Cosmic Origins Spectrograph (COS) is an ultraviolet spectrograph scheduled for installation in the Hubble Space Telescope (HST) during HST Servicing Mission 4, currently planned to occur in August 2008. COS was designed to maximize sensitivity to faint point sources, and will provide limiting sensitivities at moderate spectral resolutions (R∼20,000) that are a factor of 2 to >10 times better than those provided by previous ultraviolet spectrographs on HST. Here, we present an overview of the some of the science areas that will be addressed by COS observations and provide a summary of the capabilities of COS and the expected on-orbit performance.

Hubble Due for Fall 2008 Makeover

It's official: In 15 months, NASA will send a space shuttle to service the aging Hubble Space Telescope. Plans to return to Hubble were put on hold 4 years ago after Columbia disintegrated. NASA chief Michael Griffin pledged last year to reverse his predecessor's decision to let Hubble die, but the space agency waited until Atlantis lifted off from its Florida launch pad last week to put the 10 September 2008 servicing flight on the busy shuttle schedule. If the maintenance mission—its fifth—is successful, Hubble will sport two new instruments. The Cosmic Origins Spectrograph and Wide Field Camera 3 will, respectively, measure the structure and composition of matter and examine the universe in multiple wavelengths. By replacing gyroscopes and filling the fuel tank, the mission is also expected to extend the life of the telescope, first launched in 1990, to 2013. That's also when NASA intends to launch its James Webb Space Telescope, Hubble's follow-on.

Ultraviolet Observations of Interstellar Molecules

The benefits of placing ultraviolet (UV) spectrographs in space were first pointed out by Spitzer (1946) and Spitzer & Zabriskie (1959), with much of the emphasis on absorption-line measurements of interstellar atoms and molecules. The first UV observations of the diffuse interstellar medium took place in the early 1970s, using sounding rockets, which were followed by a series of orbital instruments starting in the later 1970s and extending almost to the present time. Many of these observations have provided information on the chemistry of the diffuse ISM, and will be emphasized here. The most abundant molecule in space, H, have been detected through UV absorption-line observations, and many others have been sought unsuccessfully. The Cosmic Origins Spectrograph, expected to be installed aboard the HST in 2007, will have enhanced UV sensitivity and will provide information on molecular abundances in denser clouds than previously studied. One of the high priorities will be the search for electronic transitions of complex organic molecules such as PAHs.

Multilayer resonant subwavelength gratings: effects of waveguide modes and real groove profiles

The boundary integral equation code PCGrate-S(X) is used to analyze diffraction on Hubble Space Telescope Cosmic Origins Spectrograph gratings at different boundary shapes and layer thicknesses. An effect of resonance anomalies excited in nonconformal dielectric layers overcoated on the surface of metallic grating on the efficiency is studied for the first time to our knowledge. Refractive indices (RIs) for bulk MgF2 taken from well-known references are found to be not suitable for thin optical layers at wavelengths between 115 and 170 nm. A method based on scale fitting of calculated and measured grating efficiencies is outlined for derivation of thin-film optical constants at hard to measure wavelengths. The calculated efficiency based on real boundary profiles and derived RIs of the G185M subwavelength grating is shown to fit within 9.6% or better to the measured data.

Algorithms for correcting geometric distortions in delay-line anodes

Time-delay anodes are typically used in conjunction with microchannel plates to create photon counting, two-dimensional imaging detectors. The anode and associated electronics are used to compute the centroid of the charge cloud from the microchannel plate stack. The computation of event position is done in analog circuitry and then digitized to an appropriate number of bits. The analog nature of the time-delay anode makes it susceptible to a wide variety of outside influences resulting in variations in the correlation between physical space and the reported digital value. These variations, both local and global, must be corrected as part of the reduction of scientific data. If left uncorrected in spectral data, for example, these variations result in decreased spectral resolution, inaccurate wavelength identifications, and/or distorted spectral line profiles. This work presents successful algorithms for correcting the dominant distortions present in a time-delay anode. These algorithms were developed as part of the data reduction pipeline for the cosmic origins spectrograph (COS) far ultraviolet channel. COS is a fourth generation instrument for the Hubble Space Telescope.

The FUV detector for the cosmic origins spectrograph on the Hubble Space Telescope

The Cosmic Origins Spectrograph (COS) is a high throughput spectrometer that will be placed on the Hubble Space Telescope (HST) during the last servicing mission in the year 2003. COS will be the most sensitive UV spectrograph ever flown aboard HST and will investigate such fundamental issues as the ionization and baryon content of the intergalactic medium and the origin of large-scale structure of the Universe. The driving design goal for COS is to maximize throughput at a moderate spectral resolution of >20,000 using optics with very few reflections and detectors with high quantum efficiency in two bandpass channels: FUV (1150– 1775 A ̊ ) and NUV (1750– 3200 A ̊ ). The COS FUV detector, a windowless microchannel plate (MCP) detector, consists of two segments each 85 mm×10 mm concatenated end to end with a 9 mm gap between them. The design is based on the Far Ultraviolet Spectroscopic Explorer detectors with identical format and front surface radius of curvature that matches the grating focal plane of the spectrograph. However, enhancements have been made in the design and fabrication of the MCPs, the photocathode, the delay line anode and the readout electronics. We discuss these design enhancements and their significance.

Universal method for holographic grating recording: multimode deformable mirrors generating Clebsch–Zernike polynomials

Recording methods for making aberration-corrected holographic gratings are greatly simplified by use of a plane multimode deformable mirror (MDM) upon one of the two recording beams. It is shown that MDM compensators easily provide the superposition of many interesting active optics modes, which we have named Clebsch-Zernike modes. When we apply only a uniform loading or no loading at all onto the rear side of the MDM clear aperture, the available Clebsch-Zernike modes are made to belong to a subclass of the Zernike modes that includes the three modes of the third-order aberration theory as well as a well-defined part of the Zernike higher-order modes. Such a recording method is considered to be universal, since it does not require the use of a sophisticated optical system such as a compensator. Active optics 12-arm MDM's in the vase form have been designed from the elasticity theory. The design of six-arm MDM's is currently carried out with theoretical results. As an example of the method, the recording of three holographic gratings of the Hubble Space Telescope Cosmic Origins Spectrograph has been investigated. Substantial improvements in image quality have been found by use of a six-arm MDM as recording compensator. The result is that aberrations of much higher order can simultaneously be corrected so that the residual blur images of the spectra occupy areas approximately 10 (direction of dispersion) x 3 (cross dispersion) = 30 times smaller--also in terms of pixel number--than those obtained by our American colleagues. Therefore the active optics recording method appears to provide substantial gains in resolving power and in sensitivity: (i) For all three gratings the spectral resolution would be increased by a factor of 10, and (ii), in addition, for the two higher dispersion gratings, the limiting magnitude on the sky appears to be increased by a magnitude of approximately 1-1.2.

Theory and computation of three Cosmic Origin Spectrograph aspheric gratings recorded with a multimode deformable mirror

The theory of three Cosmic Origin Spectrograph holographic gratings recorded with a deformable plane mirror is presented. Their working conditions are severe, since they have to correct the strong spherical aberration and the field astigmatism of the Hubble Space Telescope. Recorded on aspherized substrates, the gratings produce images that are diffraction limited with regard to spectral resolution.

Illustration of the use of multimode deformable plane mirrors to record high-resolution concave gratings: results for the Cosmic Origins Spectrograph gratings of the Hubble Space Telescope

To illustrate the efficiency of using a deformable plane mirror to record holographic gratings, we have computed the three gratings for the Cosmic Origins Spectrograph. Their working conditions are severe, since they have to correct the residual spherical aberration of the Hubble Space Telescope. Nevertheless, all images obtained are largely diffraction limited with regard to the resolution.

The Observability of Metal Lines Associated with the Lyα Forest

We develop a prescription for characterizing the strengths of metal lines associated with Lyα forest absorbers (LYFAs) of a given neutral hydrogen column density NH I and metallicity [O/H]. This line observability index (LOX) is line specific and translates, for weak lines, into a measure of the equivalent width. It can be evaluated quickly for thousands of transitions within the framework of a given model of the Lyα forest, providing a ranking of the absorption lines in terms of their strengths and enabling model builders to select the lines that deserve more detailed consideration, i.e., those that should be detectable in observed spectra of a given resolution and signal-to-noise ratio. We compute the LOX for a large number of elements and transitions in two cosmological models of the Lyα forest at z ~ 3 derived from hydrodynamic simulations of structure formation. We present results for a cold dark matter universe with a cosmological constant; an Ω = 1 cold dark matter model yields nearly identical results, and we argue more generally that the LOX predictions are insensitive to the specific choice of cosmology. We also discuss how the LOX depends on redshift and on model parameters such as the mean baryonic density and radiation field. We find that the O VI (λλ1032, 1038) doublet is the best probe of the metallicity in low column density LYFAs (NH I ≈ 1014.5 cm-2). Metallicities down to [O/H] ~ -3 yield O VI absorption features that should be detectable in current high-quality spectra, provided that the expected position of the O VI feature is not contaminated by H I absorption. The strongest transitions in lower ionization states of oxygen are O V (λ630), O IV (λ788), and O III (λ833). These absorption lines are all predicted to be stronger than the O VI feature, but even at redshifts 3-4 they will have to be observed in the ultraviolet, and they are extremely difficult to detect with present UV instruments, such as the Space Telescope Imaging Spectrograph (STIS). At lower redshifts, detection of these lines may be possible in STIS spectra of the very brightest QSOs, while one may have to wait for next-generation instruments such as the Cosmic Origins Spectrograph (COS) to detect such lines in a number of high-redshift QSOs. The strongest metal lines with rest frame wavelength larger than 912 A associated with higher column density LYFAs at z ~ 3 are C III (λ977) and Si III (λ1206.5), which peak at NH I ~ 1017 cm-2. Of the lines with rest wavelengths λr > 1216 A, which can potentially be observed redward of the Lyα forest, the C IV (λλ1548, 1551) doublet is expected to dominate in all LYFAs, regardless of the value of NH I. We argue that C IV and C II absorption may peak in different spatial regions, and that comparison of single-phase models of the C IV/C II ionization ratios with observed C IV/C II column density ratios can lead to an overestimate of the ionization parameter in the central parts of the absorbers.