A spectrograph spreads out the light gathered by a telescope so that it can be analyzed to determine such properties of celestial objects as chemical composition and abundances, temperature, radial velocity, rotational velocity, and magnetic fields. The Faint Object Spectrograph (FOS) examines fainter objects than the HRS, and can study these objects across a much wider spectral range -- from the UV (1150 Angstroms) through the visible red and the near-IR (8000 Angstroms).

The FOS uses two 512-element Digicon sensors (light intensifiers) to light. The "blue" tube is sensitive from 1150 to 5500 Angstroms (UV to yellow). The "red" tube is sensitive from 1800 to 8000 Angstroms (longer UV through red). Light can enter the FOS through any of 11 different apertures from 0.1 to about 1.0 arc-seconds in diameter. There are also two occulting devices to block out light from the center of an object while allowing the light from just outside the center to pass on through. This could allow analysis of the shells of gas around red giant stars of the faint galaxies around a quasar.

The FOS has two modes of operation PP low resolution and high resolution. At low resolution, it can reach 26th magnitude in one hour with a resolving power of 250. At high resolution, the FOS can reach only 22nd magnitude in an hour (before S/N becomes a problem), but the resolving power is increased to 1300.

Technical Details

The Faint Object Spectrograph has two Digicon detectors with independent optical paths. The Digicons operate by accelerating photoelectrons emitted by the transmissive photocathode onto a linear array of 512 diodes. The individual diodes are 0.31'' wide along the dispersion direction and 1.29'' tall perpendicular to the dispersion direction. The detectors are sensitive over the wavelength range from 1150A to 5400A (FOS/BL) and from 1620A to 8500A (FOS/RD). The quantum efficiencies of the two detectors are shown in Figure 1. The optical diagram for the FOS is given in Figure 2.

Dispersers are available with both high spectral resolution (1 to 6A per diode) and low spectral resolution (6 to 25A per diode). The actual spectral resolution depends on the point spread function of HST, the dispersion of the grating, the aperture used, and whether the target is physically extended. The brightest objects observable with FOS depend strongly upon the type of object and the combination of detector, spectral element, and aperture to be used. Particle-induced FOS detector background is normally the dominant consideration in determining limits on faint sources that can be observed by FOS.

The mapping of the photocathode to the diode array is affected by the changing geomagnetic environment on orbit. An onboard real-time correction (the geomagnetic-image-motion, or GIM, correction) is applied routinely in all data-taking modes except ACQ/PEAK.

The instrument has the ability to take spectra with high time resolution (>= 0.03 seconds, RAPID mode) and the ability to bin spectra in a periodic fashion (PERIOD mode). Although FOS originally had ultraviolet polarimetric capabilities with the FOS/BL G130H grating, the postCOSTAR environment allows polarimetry only for >= 1650A; that is only with gratings G190H, G270H, and G400H and both FOS/BL and FOS/RD.

There is a large aperture for acquiring targets using on-board software (3.7" x 3.7", designation 4.3). Since the diode array extends only 1.3" in the Y-direction, this largest aperture has an effective collecting area of 3.7" x 1.3". Other apertures include several circular apertures with sizes 0.86" (1.0), 0.43" (0.5), and 0.26" (0.3); and paired square apertures with sizes 0.86" (1.0-PAIR), 0.43" (0.5-PAIR), 0.21" (0.25-PAIR), and 0.09" (0.1-PAIR), for isolating spatially resolved features and for measuring sky. In addition, a slit and several barred apertures are available (see Figure 3).

FOS/BL sensitivity decreased by about 10% from launch until 1994.0, but has been stable since that time to the present. A dip in FOS/BL instrumental sensitivity to approximately 50% of pre-COSTAR levels centered at 2000A (which extends with an approximate Gaussian full width from 1600A to 2400A) appeared immediately post-servicing. The FOS/RD sensitivity is now generally stable to within 5%, but was observed to decrease rapidly in Cycles 1 and 2 in a highly wavelength dependent fashion between 1800A and 2100A, affecting gratings G190H, G160L, and to a lesser degree G270H. The flat fields for these 3 gratings changed little between early 1992 and mid1994. Flat fields have been obtained in either the large 3.7" x 1.3" aperture (4.3) or the 0.9" (1.0) aperture for the G190H, G160L, and the G270H gratings approximately quarterly beginning March, 1994 in order to monitor this effect. The sensitivity of both the blue and the red detectors is to be monitored approximately every 2 months in cycle 5. Please refer to Chapter 3 for a more complete characterization of the calibration status of the FOS.

Spectral Resolution The spectral resolution depends on the point spread function of the telescope, the dispersion of the grating, the diode width, the spacecraft jitter, the aperture, and whether the target is extended or is a point source. Figure 3 shows the FOS entrance apertures overlaid upon each other together with the diode array. The positions of the apertures are known accurately and are highly repeatable. The FWHM of FOS line widths does not vary strongly as a function of wavelength.

See the FOS Instrument Handbook

for further more comprehensive documentation.

Summary of Operational History

Instrument Diagrams

• A schematic diagram (28k, gif) of the FOS digicon detectors onboard the Hubble Space Telescope (From the CASS pages).
• An exploded optical diagram (17K, gif) of the FOS (From the FOS Instrument Handbook).
• A schematic diagram (25K, gif) of the FOS appertures projected onto the sky (From the FOS Instrument Handbook).

Digicon Photon Detectors

Pictures

(33k JPEG) The diode array is the heart of the Digicon detector, developed at UCSD. Here, electrons created from the incoming light are converted into an electrical signal which can be counted and ultimately relayed to Earth as science data, generally in the form of a spectrum. There are 512 diodes in the central line seen here. Each is 40 microns wide by 200 microns high. Gold threads connect the diodes to the pattern of leads which feed through the ceramic substrates. The diode array, shown enlarged here, is 1 inch in length.

(19k JPEG) The Digicon tube converts incoming light into useful signals. This side view shows the stacked ceramic ring structure and the accelerator dynode rings. More than 20,000 volts, suitably divided among the dynodes by resistors, accelerates the electrons freed from the photocathode (left) by the photoelectric effect.

(35k JPEG) Here the Digicon has been partially built up with the electron beam steering magnets and has just had pre-amplifier wires welded to the back of the diode array header.

(28k JPEG) A completed Digicon, with HV feed emerging near the rectangular photocathode mask. The knob on the top is a mounting fixture used to hang the detector within the FOS framework. Flat ribbon cables emerging at the rear take 512 channels of spectral information from the pre-amplifiers to the rest of the on-board electronics, including customized FOS computers.

More Digicon Images