PDS_VERSION_ID = PDS3 OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = IUE INSTRUMENT_ID = LWP OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "LONG-WAVELENGTH PRIME" INSTRUMENT_TYPE = SPECTROGRAPH INSTRUMENT_DESC = " NOTE: The INSTRUMENT descriptions for the IUE SWP and LWP spectrographs are both included in the following text. The IUE scientific instrument contains two spectrographs which function independently. Each spectrograph has a prime and a redundant camera. The Long-Wavelength Prime (LWP) and Short-Wavelength Prime (SWP) cameras are the standard detectors. More detailed descriptions of the cameras are given in Boggess et al. (1978a,b) and in the Camera User's Guide (Coleman et al. 1977). The Cameras During an exposure the image is integrated in the SEC Vidicon section of the camera. There is no exposure meter so the length of the exposure must be estimated. The duration of the exposure is controlled by the on-board computer (OBC). The exposure length is quantized in units of 0.4096 seconds and can be modified in real-time. At the conclusion of the exposure the camera retains the image until a read is initiated. A read consists of a raster scan of 768 X 768 pixels. The video signal is digitized into one of 256 discrete levels (0 to 255 Data Numbers, or DN) by an eight-bit analog-to-digital converter. Since there is no on-board data recorder, the signal is concurrently transmitted to the ground station in real-time as the read scan is performed. At the highest available telemetry rate, 20 kilobits/sec, the transmission of an entire image and associated engineering data takes 5.24 minutes. The read is destructive, so if something happens to the quality of the received signal or to the ground data-handling system during the read, portions of the image can be permanently lost. After a camera has been read, residual images are erased and a reproducible electronic pedestal of 15 to 40 DN is produced by exposing the camera to a tungsten flood lamp, reading the camera with a defocused beam, and then exposing and reading again. This sequence is called a PREP. Standard and overexposed preps are available. Technical details are given in the IUE Camera User's Guide (Coleman et al. 1977). The Spectrograph With the LWP and SWP cameras, the spectrographs cover the spectral ranges given in the Table below. Gaps in wavelength coverage in high dispersion are caused by truncation of the lower orders by the edge of the camera faceplate. IUE Camera Wavelength Coverage Camera High Dispersion Low Dispersion Full Coverage Partial Coverage SWP 1145-1930 A 1930-2198 A 1150-1975 A LWP 1845-2980 A 2980-3230 A 1910-3300 A Both the long and short wavelength 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 Three Agency Coordination Meeting adopted recommended values for certain dimensions, which are presented in the following Table. These values do not reflect the true physical size of the apertures but rather the size as projected on the camera faceplate. As a result, each spectrograph has its own distinct measurement of the aperture sizes. Officially Adopted Dimensions for the Apertures in Each Spectrograph, Measured on LWP and SWP Images Dimension LWP SWP Major Axis Trail Length (arcsec) 21.84+/- 0.39 21.48+/- 0.39 Large-Aperture Length (arcsec) 22.51+/- 0.40 21.65+/- 0.39 Minor Axis Trail Length (arcsec) 10.21+/- 0.18 9.24+/- 0.11 Large-Aperture Width (arcsec) 9.91+/- 0.17 9.07+/- 0.11 Large-Aperture Area (arcsec**2) 203.26+/- 9.28 209.74+/- 6.23 Small-Aperture Area (arcsec**2) 6.32+/- 0.86 6.58+/- 0.86 An accurate measurement of the trail length is needed, as such information is used to calculate the trailed exposure time. In addition, knowledge of the effective aperture area is needed to calibrate properly spectra of extended objects. For the purposes of image processing, we continue to utilize the previously quoted plate scale of 1.525 +/-0.01 arcsec/pixel (Nichols-Bohlin 1980). With the known separation of the large and small apertures (approximately 40 arcsec in the short wavelength spectrograph and 41 arcsec in the long wavelength spectrograph) and the known geometrical orientation of the apertures (see the later discussion of geometry), the aperture separations in the directions along and perpendicular to the dispersion are given below. Standard Offsets from the Small to the Large Spectrograph Aperture as used by NEWSIPS (in pixels) Camera Along Perpendicular Total Offset Dispersion to Dispersion LWP -2.3 26.2 26.3 SWP 0.8 26.1 26.1 These values are defined in the geometrically corrected frame of reference where the spectrum has been aligned horizontally in the image. The total offset is defined as the square root of the sum of the squares of the individual terms. 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 Fig. 2.16-2.18 in the IUE NEWSIPS Manual (Nichols-Bohlin et al. 1993). The figures are drawn in the geometrically corrected frame of reference with the origin at the upper left. 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 those spectrographs, so that superposition of large- and small-aperture high-dispersion spectra is not as serious in the long wavelength spectrograph. For the purposes of judging the extent and separation of the apertures in the spectral domain, the scales given in the following Table may be used in conjunction with the quantities given in the above tables. 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. Approximate Spectral Scales in Each Dispersion Mode Camera Low Dispersion High Dispersion (A/px) (km/s/px) LWP 2.66 7.21 SWP 1.68 7.72 Instrumental Resolution The instrumental resolution (both spectral and spatial) is determined by the camera resolution, the dispersion mode, the aperture used, the focussing conditions in the telescope, and the pointing stability of the spacecraft. While the dominant effect is the camera resolution, telescope focus and stability of spacecraft pointing also play a major role in defining the resolution. In addition, it is well known that the camera resolution is highly wavelength-dependent. According to the Camera Users Guide (Coleman et al. 1977), the camera point spread function (PSF) consists of a narrow, gaussian-like core with long shallow wings. The actual resolution in either the spatial or spectral direction can be defined as a function of the full width at half maximum (FWHM). Two spectra (spatial direction) or two spectral features (spectral direction) can be resolved. Spatial resolution is specified in pixels, while spectral resolution is denoted in angstroms. --------------------------------------------------------------------------- * Low-Dispersion Mode o Resolution Along the Dispersion o Resolution Perpendicular to the Dispersion --------------------------------------------------------------------------- Resolution Along the Dispersion A study of the NEWSIPS spectral resolution was performed by measuring the FWHM of several features for the emission line sources V1016 Cyg, RR Tel, AG Dra, CI Cyg, and Z And. The analysis indicates a slight improvement in the NEWSIPS resolution (approximately 10 0.000000or the SWP and 7 0.000000or the LWR) over the previous results reported by Cassatella, Barbero, and Benvenuti (1985). Plots of the spectral resolution data are shown in Figure 2.19 of the NEWSIPS Manual (Nichols-Bohlin et al. 1993). The small-aperture data are slightly offset in wavelength from the large-aperture data for clarity. LWP - Large-aperture spectral resolution is best between 2700 and 2900 A with an average FWHM of 5.2 A and decreases to approximately 8.0 A on either side of this range. Small-aperture resolution is optimal between 2400 and 3000 A with an average FWHM of 5.5 A and decreases to 8.1 A at the extreme wavelengths. SWP - The best resolution occurs around 1200 A, with a FWHM of 4.6 A in the large aperture and 3.0 A in the small aperture, and gradually worsens towards longer wavelengths: 6.7 A at 1900 A in the large aperture and 6.3 A in the small. On average, the small-aperture resolution is approximately 10% better than the large-aperture resolution. Resolution Perpendicular to the Dispersion The NEWSIPS spatial resolution has been determined by analyzing the spectra of several low-dispersion standard stars (viz., HD 60753, HD 93521, BD+33 2642, and BD+75 325). The FWHM of large- and small- aperture spectra were measured at several wavelengths and plotted (Figure 2.20 in the NEWSIPS Manual Nichols-Bohlin, et al. 1993). As is the case with the spectral resolution studies, the NEWSIPS values show, in general, an improvement. As is the case with the spectral resolution plots, the small-aperture data are slightly offset from the large-aperture data. LWP - The spatial resolution for the LWP is best near 3000 A where the FWHM for the large aperture is 2.4 pixels (3.6 arcsec), and decreases to values of around 3.0 pixels at the short and long wavelength ends of the spectrum. There is no significant difference between the large- and small-aperture spatial resolutions. SWP - The SWP camera shows the best spatial resolution near 1400 A with mean FWHM values for the large aperture of 2.7 pixels (4.1 arcsec), increasing slightly to 2.8 pixels at 1250 A, and 3.7 pixels at 1950 A. The SWP small-aperture resolution response is approximately the same as the large-aperture resolution. " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "BOGGESSETAL1978B" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "NICHOLSETAL1993" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT OBJECT = REFERENCE REFERENCE_KEY_ID = "BOGGESSETAL1978B" REFERENCE_DESC = "Boggess, A., Carr, F.A., Evans,D.C.,Fischel, D., Freeman, H.R., Fuechsel, C.F., Klinglesmith, D.A., Kruegar, V.L., Longanecker,G.W., Moore, J.V., Pyle, E.J., Rebar,F.,Sizemore, K.O., Sparks,W., Underhill,A.B., Vitagliano,H.D., West,D.K., Macchetto,F., Fitton,B., Barker,P.J., Dunford, E., Gondhaleker, P.M., Hall, J.E., Harrison, V.A.W., Oliver, M.B., Sanford, M.C.W., Vaughan, P.A., Ward, A.K., Anderson, B.E., Boksenberg, A., Coleman, C.I., Snijders, M.A.J., Wilson, R., The IUE Spacecraft and Instrumentation, Nature, 275, 372, 1978B." END_OBJECT = REFERENCE OBJECT = REFERENCE REFERENCE_KEY_ID = "NICHOLSETAL1993" REFERENCE_DESC = "Nichols-Bohlin, J.S., Garhart, M.P., De La Pena, M.D., Levay, K.L., International Ultraviolet Explorer: New Spectral Image Processing System Information Manual: Low-Dispersion Data, 1993." END_OBJECT = REFERENCE END