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2.2 Spectrograph Geometry

Both the long- and short-wavelength IUE spectrographs have two entrance apertures: a small aperture (nominal 3 arcsec diameter circle) and a large aperture (nominal 10 arcsec by 20 arcsec slot). Although the various methods available for determining the fundamental dimensions do not always yield results which agree to within the limits set by the internal consistency of each (see Panek 1982), the IUE Three Agency Committee adopted recommended values for certain dimensions, which are presented in Table 2.1. These values do not reflect the true physical dimensions of the apertures but rather the size as projected on the camera faceplate. As a result, each spectrograph has its own distinct measurement of aperture sizes.

Table 2.1:  Officially Adopted Dimensions for the Apertures in Each Spectrograph, Measured on LWP, LWR, and SWP Images
Dimension LWP SWP LWR
Major Axis Trail Length (arcsec) 21.84$\pm$0.39 21.48$\pm$0.39 22.55$\pm$0.62
Large-Aperture Length (arcsec) 22.51$\pm$0.40 21.65$\pm$0.39 23.24$\pm$0.64
Minor Axis Trail Length (arcsec) 10.21$\pm$0.18 9.24$\pm$0.11 9.88$\pm$0.42
Large-Aperture Width (arcsec) 9.91$\pm$0.17 9.07$\pm$0.11 9.59$\pm$0.41
Large-Aperture Area (arcsec2) 218.17$\pm$10.12 215.33$\pm$6.55 209.29$\pm$9.25
Small-Aperture Area (arcsec2) 6.78$\pm$0.97 6.72$\pm$0.96 6.31$\pm$0.75

Accurate measurements of the trail lengths are necessary, as such information is used to calculate exposure times for trailed images. In addition, knowledge of the effective aperture area is needed to calibrate properly spectra of extended objects.

The camera plate scales have been redetermined (Garhart 1996; LWP 1.5644, LWR 1.5526, and SWP 1.5300 arcseconds per pixel) using the most recent measurements for the small-to-large aperture offsets in pixels (Table 2.2) and FES aperture center locations in arcseconds (Pitts 1988). These latest incarnations replace the oft-quoted plate scale figure of 1.525 arcseconds per pixel (Bohlin et al. 1980), a value that had been used for all three cameras. The aperture separations in the directions along and perpendicular to the dispersion are given in Table 2.2 for low dispersion. The corresponding values for the high-dispersion offsets are obtained by transposing the entries for the low-dispersion offsets along and perpendicular to the dispersion in Table 2.2. Refer to Figures 2.16 through 2.18 to determine the correct sign for the high-dispersion offsets.

Table 2.2:  Standard Offsets from the Small to the Large Spectrograph Aperture as used by low-dispersion NEWSIPS (in pixels)
Camera Along Dispersion $\perp$ to Dispersion Total Offset
LWP -2.3 26.2 26.3
LWR -2.3 26.4 26.5
SWP 0.8 26.1 26.1

These values are defined in a geometrically corrected frame of reference. The total offset is defined as the square root of the sum of the squares of the individual terms. In low dispersion, the offsets along the dispersion have been incorporated into the geometric correction step such that the wavelength scales for the small and large apertures are aligned.

The geometry of the two entrance apertures in relation to the image scan lines and the high and low resolution dispersion directions are shown in Figures 2.16 through 2.18 for the LWP, LWR, and SWP cameras. Note particularly the fact that the displacement between the short-wavelength large aperture (SWLA) and the short-wavelength small aperture (SWSA) is very nearly along the echelle dispersion direction. Therefore, short-wavelength high-dispersion images in which both apertures are exposed will result in nearly complete superposition of the large- and small-aperture spectra (with a wavelength offset). The displacement of the long-wavelength large aperture (LWLA) and the long-wavelength small aperture (LWSA) is less coincident with the echelle dispersion direction in this spectrograph, so that superposition of large- and small-aperture high-dispersion spectra is not as serious in the long-wavelength spectrograph.

Figure 2.16: LWP Geometry

Figure 2.17: LWR Geometry

Figure2.18: SWP Geometry

For the purposes of judging the extent and separation of the apertures in the spectral domain, the scales given in Table 2.3 may be used in conjunction with the quantities in Tables 2.1 and 2.2. Note that in high dispersion a given shift along the dispersion corresponds closely to a constant Doppler velocity shift, whereas in low dispersion a given shift corresponds to a constant wavelength shift.

Table 2.3:  Approximate Spectral Scales in Each Dispersion Mode
Camera Low (Å/px) High (km/s/px)
LWP 2.66 7.21
LWR 2.66 7.27
SWP 1.68 7.72

next up previous contents
Next: 2.3 Instrumental Resolution Up: 2 Description of IUE Previous: 2.1 Raw Image Data
Karen Levay