The CHIPS Instrument

Contents:
1. Scientific Requirements
2.1. Overview of the CHIPS Instrument
2.2. Entrance Slits, Diffraction Gratings, and Optical Bench
2.3. EUV Filters
3. Sky Coverage, Spectral Resolution, and Sensitivity
4. Data Analysis, Data Products, and Archiving

1. Scientific Requirements

In order to study the key spectral features of the local hot interstellar plasma, we require a nebular spectrograph with a peak resolution / of ~100 or higher and a sensitivity better than 20 LU (photons cm^-2 s^-1 sr^-1). If the plasma temperature is near 10⁶ K, the various cooling models can be distinguished and the plasma luminosity well characterized by an instrument with a bandpass from about 160 to 260 Å. At slightly higher temperatures, emission lines at shorter wavelengths become important. An extension to shorter wavelengths also provides overlap with the Beryllium X-ray band, which extends from about 115 to 185 Å. CHIPS spans the 90 - 260 Å range. Exceeding these limits is impractical, because shorter wavelengths require shallower graze angles, while longer wavelengths are highly absorbed by the local neutral interstellar medium and will be overwhelmed by the bright plasmaspheric emission at 304 Å.

Figure 1: A 3-D layout of the CHIPS spectrograph. The wire frame exterior represents the volume available within CHIPSat.

2.1. Overview of the CHIPS Instrument

The overall CHIPS layout is shown above. Light enters the spectrograph through the array of nine entrance slits shown at lower right. Inside six of the slits, small flat pickoff mirrors steer the beam and coalign the fields of view (in one dimension) with the three central channels. Each slit illuminates a single diffraction grating. Each grating is cylindrical, so the light is focused only in the plane of dispersion. All nine gratings disperse their spectra onto a common, flat detector plane. Thin-film filters near the front of the detector attenuate out-of-band stray light. Zero-order light does not strike the detector and can be baffled separately. Unlike a classical Rowland spectrograph, which offers poor performance and requires a highly inclined detector in grazing-incidence geometries, we vary the spacing of the grooves to provide aberration control and to flatten the focal surface (Harada-91). The EUVE and ORFEUS instruments used such variable line-spaced gratings with great success.

Each of the nine slit/grating channels has a speed of about f/10. The gratings are aligned in one dimension on the sky, leading to a total field of view of about 5° × 26.7°. The rotated orientation of the off-axis channels introduces a slight mismatch between the ideal focal surface and the common detector plane at the ends of the bandpass. The resolution curve even for the central channel is fairly narrowly peaked, however, so the marginal loss caused by channel multiplicity is small. CHIPS provides no significant angular resolution within its field of view. Point-source imaging is not necessary at CHIPS wavelengths, because the integrated flux from stellar point sources is far below the expected diffuse flux level. The continuum from HZ 43, the brightest white dwarf in the sky, is less than the detector background. The well-known flare star AU Mic in outburst (Katsova-97) produces lines corresponding to ‹ 0.5 LU in diffuse flux when observed with CHIPS. We summarize the top-level CHIPS instrument parameters in Table 1.

2.2. Entrance Slits, Diffraction Gratings, and Optical Bench

Near the center of the band, the finite spectral resolution is dominated by the projected slit width. This gives us great flexibility in the resolution / sensitivity tradeoff, which can easily be fine tuned if new observations or theoretical predictions arise during the instrument-design stage. The presence of true entrance slits (rather than wire grid collimators) is a tremendous benefit. Contamination, stray light, and safety concerns are greatly minimized when the instrument has a well-defined interior and exterior.

Design parameters of the diffraction gratings are summarized in Table 1. Our baseline plan is to utilize nine identical replicas of a common mechanically-ruled master grating. The manufacture of these gratings is less challenging in virtually every respect than those previously fabricated by Hitachi Ltd., an experienced vendor, for the ORFEUS project. The groove density is lower by a factor of 3, the radius of curvature is larger by a factor of 1.5, the active area is smaller by a factor of 8, and the surface figure requirements (~5 ) are looser. We baseline zerodur replicas, again following the ORFEUS design. Each grating is about the size of a credit card. In the mass budget and optical layout we have provided for a 4 mm thickness, relying on a conservative estimate from Hitachi Ltd. The small size and mass of the gratings will facilitate their mounting. We will investigate bonding as well as more conventional clamping techniques.

The baseline metering structure (optical bench) consists of a 3-D space frame constructed of graphite cyanide ester composite tube sections, panels and end fittings. This extremely lightweight system provides great structural rigidity and excellent dimensional stability in fabrication and over wide variations in operational temperature. This type of custom bench can be fabricated by many aerospace vendors and has been implemented in different variations on the ASTRO-SPAS, FUSE and HST instruments. The optical instrument will be encased in a sealed, flexible contamination and stray-light barrier. This barrier will prevent contamination in-flow from other hardware on the satellite as well as providing a cavity/enclosure for pure purge gas retention at an over-pressure of a few psi. The whole enclosure has a remove-before-flight hermetic cover with an associated breather valve located on the top of the optical bench to provide a contamination barrier system that will allow CHIPS optics to remain clean throughout storage, transportation and integration activities.

2.3. EUV Filters

Without filters, scattered light from bright emission lines of geocoronal and interplanetary origin would exceed the detector background count rate. Hydrogen Lyman (1216 Å), with a typical flux of 3500 Rayleighs (Chakrabarti-84), is of particular concern. For wavelengths between 210 and 260 Å, aluminum is the best filter material. The in-band transmission of a 1000 Å Al / 300 Å C filter (much thinner films are not structurally practical) is ~45%, and Lyman is strongly rejected. For wavelengths between 90 and about 150 Å the natural choice is 720 Å thick Polyimide, with a 500 Å Boron layer to help attenuate Lyman . Both filters provide sufficient attenuation of other bright out-of-band features such as plasmaspheric He II at 304 Å.

At intermediate wavelengths, each material offers advantages. The transmission of aluminum falls sharply below 170 Å, providing a powerful tool to distinguish instrumental background from astrophysical continuum, and aluminum is opaque to second-order (soft X-ray) light. The CIE plasma models indicate that second-order light should be faint compared to the first-order spectrum, but in this virtually unsurveyed band it is wise to take precautions. The critical Fe line complex is spanned by both filter materials. Although the transmission of Polyimide is lower, it has no sharp edges in this region, and a well-understood instrumental response will be important in interpreting the various line ratios. The aluminum panel will be the more sensitive at wavelengths above 170 Å. There is sufficient coverage below 170 Å to set a good baseline for continuum limits. Wavelength intervals blocked by filter bars in one panel are spanned continuously in the other. Differences in the spectra from the two panels will serendipitously constrain the second-order diffuse flux from about 83 to 102 Å at high resolution.

An exciting new filter material is Zirconium, which offers substantially higher throughput than Polyimide and even aluminum across much of the band, and better out-of-band rejection. If our on-going analysis confirms its flightworthiness, we will substitute Zirconium for the Polyimide panels. If this confirmation occurs sufficiently early in the schedule, we will reoptimize the overall filter design to take greater advantage of Zirconium's transmission properties.

These filters will be held within a thin metallic rectangular holder attached to the front face of the CHIPS detector in a manner similar to that used successfully with all 7 detectors on the EUVE mission. The CHIPS EUV filters will be fabricated by LUXEL Corp. to our specifications, as derived from throughput models developed for the EUVE filter program (Vedder-93). The filters will undergo a series of vibration tests to demonstrate their mechanical integrity. Laboratory lifetime tests and on-orbit data from the EUVE satellite indicate that filter degradation due to aging over the 12-month CHIPS mission will produce at worst only localized count-rate enhancements, easily recognized and removed in data processing.

3. Sky Coverage, Spectral Resolution, and Sensitivity

Based on our experience with the EUVE spectrographs, CHIPS should be able to observe during both orbital day and night. The first six months of normal operations will yield a sky map containing approximately 316 resolution elements (resels) observed for about 40,000 seconds each (see Figure 2). After completing the preliminary map, investigators may select areas of the celestial sphere which warrent further investigation or continue sky mapping to increase the integration times. During nominal operations, the instrument will remain active with a nearly 100% duty factor. The spacecraft will reorient the instrument twice per orbit, with each slew requiring roughly one minute.

Figure 2: The CHIPS field of view can tile the sky in approximately 316 pointings.

To characterize the theoretical resultant spectral resolution (/) we adopt the spectral width within which 76% of the energy is enclosed. This measure of the performance is equivalent to the FWHM when the distribution is Gaussian, but the FWHM can be unrealistically optimistic when the distribution contains a narrow central peak. We show the system resolution, based on a full 9-channel system raytrace and detector resolution effects, in Figure 3 (top panel). The peak resolution comfortably exceeds the minimum desired performance, providing significant margin for unexpected problems in the component or alignment error budget. In Figure 3 (bottom panel), we show the 3 sensitivity curve for a CHIPS integration time of 100,000 seconds. This assumes the efficiencies and scattering parameters obtained with the EUVE gratings, together with other relevant instrument/detector parameters listed in Table 1 or discussed above.

The CHIPS sensitivity to diffuse emission can be illustrated by comparing its predicted performance with that of EUVE. With EUVE, a diffuse emission feature with 120 LU flux was only 1% as bright as the general instrumental background. With CHIPS, a 120 LU feature is twice as bright as the background. Even faint features in the 3-14 LU range create a modulation of at least 3% of the background signal.

4. Data Analysis, Data Products, and Archiving

The unprocessed (raw) science data from CHIPS will consist of time-tagged photon lists (X, Y, and pulse height) generated by the on-board photon-counting MCP detector. In addition, we will record the attitude of the satellite such that the instrument boresight and roll orientation can be calculated to enable investigators to determine the field of view for each CHIPS observation. The basic steps in extracting calibrated spectra from these raw detector images are

• screen the data to eliminate any high-background portions and poor-quality packets;
• correct event coordinates for thermal drift in the detector electronics if necessary;
• estimate and subtract the detector background (derived from count rates lying beneath the filter bars on the detector);
• correct for detector flat-field effects;
• correct the 2-D spectra for geometric distortions;
• optimally extract 1-D spectra from the 2-D images;
• apply wavelength and flux calibrations to the extracted spectra.

• Sky maps with a spatial resolution of 5° × 26.7° degrees in each of four to six principal emission lines. The exact wavelengths will be determined after a preliminary analysis of the data set.
• Higher S/N sky maps in broad spectral bands. The use of broader spectral bands increases S/N at the expense of wavelength resolution.
• Individual spectra of all sky resels (approximately 316), each spanning 90 to 260 Å at a peak resolution R = 150 and a S/N between 20 and 45.

The following raw data products will also be provided to the archive:

• Time-tagged photon data (X, Y, pulse height) from the entire mission;
• A record of the instrument boresight and roll orientation from the entire mission;
• Complete calibration files and software tools to enable users to manipulate the photon data and to extract spectra subject to user-defined constraints.