ASPECTOS CLAVES DEL MECI.

The HST is a space telescope mission by NASA launched into low Earth orbit, at a height of _∼550 km, on the 25th _{of April 1990, capable of observing at near-} ultraviolet, visible, and near-infrared wavelengths. The telescope consists of a 2.4 m primary telescope, and at launch there were five scientific instruments; the Wide Field and Planetary Camera (WF/PC), the Faint Object Spectrograph (FOS), the High Speed Photometer (HSP), the Faint Object Camera (FOC) and the Goddard High-Resolution Spectrograph (GHRS). A diagram of the telescope may be seen

Figure 3.3: A schematic diagram of the GHRS.Image Credit: Brandtet al.(1994).

in Fig. 3.2.

The observations in this work make use of the GHRS (Brandt et al., 1994), and as such we will focus our discussion on this instrument. The GHRS was one of the 4 original axial instruments aboard the HST. Owing to an error in the construction of the primary mirror of the HST a spherical aberration was introduced, and in December 1993 HSP was removed in order to fit the Corrective Optics Space Telescope Axial Replacement (COSTAR). With COSTAR in place the spherical aberration which affected the GHRS, the FOC, and the FOS, was corrected allowing these instruments to be used at their highest resolution.

The GHRS1 _{is an ultra-violet spectrometer, making observations from 1150 ˚A–} 3200 ˚A. A schematic diagram of the instrument can be seen in Fig 3.3. Light enters the instrument through one of two apertures, labelled LSA and SSA. The LSA has a 200_{.0 aperture and is used for faint targets. The SSA has a 0}00_{.25 aperture and is used} when the highest spectral resolution is required. Once the light has entered the in-

1_{Much of this discussion is based upon, and much more detail can be found in, the GHRS}

strument a collimator directs it to the rotating carousel which holds the dispersers. By rotating the required element into position spectra can be taken in one of seven modes. Five of these modes are gratings, designated G140L (1100 ˚A–1900 ˚A), G140M (1100 ˚A–1900 ˚A), G160M (1150 ˚A–2300 ˚A), G200M (1600 ˚A–2300 ˚A), and G270M (2000 ˚A–3300 ˚A). In this naming convention “G” indicates a grating, the number indicates the blaze wavelength (in nm), and the “L” or “M” suffix de- notes a “low” or “medium” resolution. The “L” gratings provide a resolution of

R = λ/∆λ = 2,000, and the “M” grating R = 20,000₋35,000. There are two echelle modes also available, Ech-A, and Ech-B, whereby light from the single echelle grating is directed onto one of the two detectors, D1 and D2. These echelle modes provide the highest resolution,R = 90,000₋120,000. The dispersed light then travels to the camera mirror in the case of the gratings, or the cross-dispersers in the case of the echelles (these are required as the orders coming from the echelle overlap unless separated by the cross-dispersers). Finally, the light is sent to the photocathode of one of the two detectors.

The GHRS makes use of two Digicon detectors, differing only in the wave- lengths to which they are sensitive, D1 being sensitive to 1100 ˚A–1700 ˚A (CsI pho- tocathode), and D2 being sensitive to 1700 ˚A–3200 ˚A (CsTe photocathode). The photoelectrons produced by the photocathodes are accelerated to 22 keV and fo- cussed by a 105 G magnetic field onto one of the 500 science diodes. This produces approximately a 5,600 electron pulse per photoelectron. These pulses are read into buffer memory by each diodes dedicated counter. The diodes have slight response irregularities, which are corrected for by deflecting the spectrum across the diodes in the dispersion direction and adding these spectra, hence averaging out the effect of the diode irregularities. In addition GHRS spectra taken with the SSA, as ours are, are intrinsically undersampled. By substepping, or “dithering”, the electron image in the dispersion direction by 1/2 or 1/4 diode critically sampled spectra are obtained (Gilliland et al., 1992).1 _{The background noise in the diodes comes} to_∼0.01 counts/diode/s, with most of this arising from Cerenkov radiation due to cosmic rays.

All of the HST data used in this work was reduced and provided by K. Car- penter (Proposal ID: 6069), however we will provide a brief description of the

standard reduction pipeline which was used to construct from the raw detector counts usable science products. The calibration is carried out using the standard CALHRS pipeline developed GHRS Investigation Definition Team (Soderblom & et al., 1995). This pipeline consists of a number of steps,

• Diode non-linearities and non-uniformities are removed by consulting com- piled tables for the values associated with each of the 500 diodes.

• Photocathode irregularities arise due to the granulation of the photocathode material, and these are accounted for by determining where on the photo- cathode each photoelectron measured at the diode arose, and then applying the tabulated photocathode response coefficient for each point.

• Vignetting (reduction of brightness at the edge compared to the centre) is corrected across the photocathode.

• Absolute wavelengths are determined for each diode by solving the dispersion equation relating photocathode sample position to wavelength (the appro- priate dispersion equation being chosen for the grating in use). A velocity correction is performed to convert the wavelengths into the heliocentric rest frame.1 _{For wavelengths above 2000 ˚A the wavelength is converted to air} wavelengths.

• The background counts are subtracted from the spectrum.

• In the case of the echelles, we must divide the flux value by the normalized echelle efficiency to remove the effect of echelle ripple.

• Finally, the absolute flux is calculated by dividing the flux by the tabulated (pre-calibrated) absolute flux coefficients.

The final, calibrated fluxes are accurate to _±5₋10% (Carpenter et al., 1999). In this work we are concerned with comparing observed and computed spectra, and as such it is necessary to quantify the relationship between the spectrum

1_{Interestingly the Doppler shift caused by the orbital motion of the satellite is compensated}

Table 3.1: Bandpass and wavelength pixel correspondence for each of the optical elements of the GHRS.

Optical Element Range (˚A) ˚A per diode

G140L 1100-1900 0.572-0.573 G140M 1100-1900 0.056-0.052 G160M 1150-2300 0.072-0.066 G200M 1600-2300 0.081-0.075 G270M 2000-3300 0.096-0.087 Echelle A 1100-1700 0.011-0.018 Echelle B 1700-3200 0.017-0.034

emitted by an object, and the broadened spectrum observed by the GHRS. This is known as the Line Spread Function (LSF). The LSF for the GHRS gratings describe the instrumental broadening applied to a delta-function spectral feature by that grating. The SSA, which was used in this work, has a Gaussian LSF with a FWHM of _∼3.7 pixels, and this is independent of grating and wavelength (Gillilandet al., 1992). Table 3.1 contains the wavelength to diode correspondence for each optical element of the GHRS.

In 1997 the GHRS was decommissioned and removed from the HST, replaced by the Space Telescope Imaging Spectrograph.