PDS_VERSION_ID = PDS3 OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = IUE INSTRUMENT_ID = LWR OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "LONG-WAVELENGTH REDUNDANT" INSTRUMENT_TYPE = SPECTROGRAPH INSTRUMENT_DESC = " NOTE: The INSTRUMENT descriptions for the SWP, LWR, and LWP spectrographs are 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 (the LWR (Long-Wavelength Redundant) was used before Oct, 1983). For descriptions see Boggess et al.(1978a,b) and 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 (FULL) High Dispersion (PARTIAL) Low Dispersion LWP 1845-2980 A 2980-3230 A 1910-3300 A LWR 1845-2980 A 2980-3230 A 1910-3300 A SWP 1145-1930 A 1930-2198 A 1150-1975 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, SWP, and LWR Images Dimension LWP SWP LWR Major Axis Trail(arcsec) 21.84+/-0.39 21.48+/-0.39 22.22+/-0.62 Large-Aperture Length(arcsec) 22.51+/-0.40 21.65+/-0.39 23.24+/-0.64 Minor Axis Trail(arcsec) 10.21+/-0.18 9.24+/-0.11 9.88+/-0.42 Large-Aperture Width(arcsec) 9.91+/-0.17 9.07+/-0.11 9.58+/-0.41 Large-Aperture Area(arcsec**2) 203.26+/-9.28 209.74+/-6.23 209.29+/-9.25 Small-Aperture Area(arcsec**2) 6.32+/-0.86 6.58+/-0.86 6.31+/-0.75 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. 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 (Garhart et al. 1997) . 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 LWR -2.3 26.4 26.5 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 (Garhart et al. 1997). 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 LWR 2.66 7.27 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 IUE Camera Users Guide (Coleman et al. 1977), the camera point spread function (PSF) consists of a narrow gaussian-like core having a full width at half maximum (FWHM) of 2 to 5 pixels and a weak long-range tail. The actual resolution in either the spatial or spectral direction can be defined as a function of the FWHM. The Rayleigh criterion of instrumental resolution specifies that two spectra (spatial direction) or two spectral features (spectral direction) can be resolved provided their separation is as follows (Weinstein and Perez 1992): d >- 0.849 x FWHM where d is the distance separating the two features (or spectra). The gaussian fitting routine used in this analysis was GAUSSFITS, taken from the IUE Data Analysis Center software library. This procedure outputs the one-sigma width of the fitted gaussian profile which was then converted to FWHM using the statistical equality (Bevington 1969): FWHM = 2.3548 x sigma --------------------------------------------------------------------------- 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 % or the SWP and 7 % or 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 (Garhart et al. 1997). 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. LWR - Maximum resolution in the large aperture occurs longward of 2300 A, with an average FWHM of 5.3 A, while shortward of this point the FWHM decreases to 7.7 A. Small-aperture resolution is best from 2700-3200 A, with an average FWHM of 5.4 A, and decreases to 7.7 A at 3350 A and 7.5 A shortward of 2400 A. 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 Garhart, 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. LWR - The behavior of the LWR camera as a function of wavelength is similar to the LWP, with the smallest FWHM values for the large aperture of 2.6 pixels (3.9 arcsec) occurring near 3000 A, and increasing to 3.6 and 3.0 pixels at the wavelength extremes. The small aperture, unlike the other two cameras, shows a dramatic decrease in resolution of approximately 10%. 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. ------------------------------------------------------------------------------ * High-Dispersion Mode o Resolution Along the Dispersion o Resolution Perpendicular to the Dispersion ------------------------------------------------------------------------------ Resolution Along the Dispersion A study of the spectral resolution in the high-dispersion mode was performed utilizing several methods. The first measured emission lines from small-aperture wavelength calibration (WAVECAL) images obtained using the on-board hollow cathode platinum-neon (Pt-Ne) calibration lamp. The second measured several features from the emission line sources V1016 Cyg and RR Tel and interstellar absorption line features from the calibration standard BD+75 325. The third method measured absorption features from the calibration standard HD 149757 (Zeta Oph). The WAVECAL images are useful in determining the spectral resolution as they are not affected by the telescope focus nor are they subject to astrophysical broadening. The Zeta Oph spectra are characterized by very narrow interstellar absorption features so they are also useful for measuring spectral resolution. Therefore, the measurements taken from WAVECAL and Zeta Oph images represent the best possible spectral resolution obtainable. LWP - The WAVECAL and large-aperture Zeta Oph resolution data are displayed in Figures 2.21 and 2.24 in the NEWSIPS manual (Nichols-Bohlin et al 1997). The results, along with the associated one-sigma error bars and linear fits (dashed line), are plotted as a function of order number in both wavelength and pixel space. The dotted line in the pixel space plots is the average of the resolution data over all orders. No small- aperture high-dispersion data of Zeta Oph is available. In addition, the standard star, RR Tel, and V1016 Cyg data were too noisy to yield suitable results. The large-aperture Zeta Oph measurements are quite similar to the small-aperture WAVECAL analysis. The spectral resolution in wavelength space is approximately 0.18 A FWHM at order 75 and linearly decreases (roughly) to 0.11 A at order 117. The pixel space data for both WAVECALs and Zeta Oph show the same improvement in resolution between orders 95 and 110. The IUE Systems Design Report (GSFC 1976) lists 15,000 ([lambda/Delta lambda]) as the high-dispersion resolution for the long-wavelength cameras. This yields 0.22 A for order 69, 0.17 A for order 90, and 0.13 A for order 123. These numbers are comparable to the NEWSIPS results of 0.24 A, 0.15 A, and 0.12 A for these same orders. An analysis of IUESIPS spectral resolution was performed by Evans and Imhoff (1985) using FWHM measurements obtained from WAVECAL images. The results are as follows: 0.22 A for order 75, 0.17 A for order 83, 0.13 A for order 96, and 0.13 A for order 116. These figures are very similar to the NEWSIPS results of 0.20 A, 0.14 A, 0.15 A, and 0.13 A. LWR - The WAVECAL spectral resolution measurements are shown in Figure 2.22 in the NEWSIPS manual (Nichols-Bohlin et al. 1997) along with the corresponding linear fit and average. The FWHM trends (wavelength space) below order 80 are quite similar to the LWP figures (i.e., a linear dependence of FWHM on order number). The camera resolution in wavelength space is nearly constant for orders 80 through 115, with a slight degradation in LWR resolution above order 115. This trend is easily visible in the the pixel space resolution plot and is evident from the deviation of the FWHM measurements from the mean (dotted line). Cassatella et al. (1981) and Cassatella and Martin (1982) report a nearly constant FWHM (wavelength space) as a function of order number for WAVECAL images processed through IUESIPS. The average FWHM from their analysis is approximately 0.18 A above order 81; a value which is higher than the corresponding NEWSIPS FWHM of 0.14 A. They report a FWHM of 0.22 A for order 72, which again is much higher than the NEWSIPS results of 0.19 A. Evans and Imhoff (1985) also measured spectral resolution using IUESIPS processed WAVECAL images. They present FWHM values of 0.19 A, 0.17 A, 0.16 A, and 0.15 A for orders 75, 83, 96, and 116, respectively. The corresponding NEWSIPS values for these same orders are: 0.19 A, 0.14 A, 0.15 A, and 0.14 A. Boggess et al. (1978) quote a constant FWHM of 0.19 A for WAVECAL images, regardless of order number. This contradicts all subsequent reports written on this subject as well as the NEWSIPS results shown here. Their analysis was performed early in the life of IUE; perhaps the camera characteristics had not yet stabilized at this period in time. SWP - The WAVECAL, Zeta Oph, and large- and small-aperture stellar source spectral resolution data are displayed in Figures 2.23, 2.25, 2.26, and 2.27 in the NEWSIPS manual (Nichols-Bohlin et al. 1997). As for LWP and LWR, the plots include one-sigma error bars and linear (dashed line) and mean (dotted line) fits to the data. In Figures 2.26 and 2.27, the emission line measurements for orders 111 and above were excluded from the analysis when performing the linear fit to the stellar data because they were highly discrepant. The spectral resolution in wavelength space for the WAVECAL, Zeta Oph, and stellar source images shows no dependence on wavelength within an order and a roughly linear dependence on order number. Unlike the LWP, the SWP resolution from the Zeta Oph analysis (Figure 2.25) is much worse than the corresponding WAVECAL data (Figure 2.23). The stellar source results are somewhat inconclusive for orders 111 and above. The emission line widths are dramatically higher than the corresponding absorption line measurements. This trend was also seen in the analysis by Grady (1985). The IUE Systems Design Report (GSFC 1976) quotes a figure of 10,000 ([lambda/Delta lambda]) for the spectral resolution in high-dispersion mode. This corresponds to a FWHM of approximately 0.2 A for order 66 and 0.1 A for order 125. This same trend is seen in the top plot (Figure 2.23) of the WAVECAL resolution analysis; the spectral resolution is essentially a constant value in pixel space (bottom plot). The stellar source resolution measurements in pixel space (bottom plot of Figures 2.26 and 2.27) show some degradation towards higher order numbers. In addition, the small-aperture data (Figure 2.27) indicates an 8% improvement in resolution over the large-aperture counterpart (Figure 2.26). The general trend of the wavelength-space resolution for the WAVECAL images is approximately the same for every IUESIPS study that has been reviewed (i.e., Boggess et al. 1978, Cassatella et al. 1981, Cassatella and Martin 1982, and Evans and Imhoff 1985). That is, the camera resolution in wavelength space varies roughly linearly with order number and improves towards shorter wavelengths (0.19 A for order 69 and 0.09 for order 106). The results from analysis of WAVECAL images processed through NEWSIPS are almost identical to these figures. Penston (1979) reported SWP large-aperture FWHM values of 0.20 A for absorption lines and 0.24 A for emission lines. These figures are comparable with the average NEWSIPS results of 0.21 A and 0.23 A respectively. However, Penston's (1979) measurements for the small-aperture resolution are no better than the large aperture. This result could be supported by the NEWSIPS analysis as the apparent improvement in small-aperture resolution is less than the one-sigma error of the FWHM average for any given order. Grady (1985) assessed the effects of the two-gyro control mode on high-dispersion data using large-aperture RR Tel spectra. The mean resolution (averaged over all orders) from the Grady analysis (0.22 A) agrees with the average NEWSIPS resolution result. Resolution Perpendicular to the Dispersion The spatial resolution has been determined by analyzing the spectra of high-dispersion standard stars. The FWHM of several pairs of large and small-aperture line-by-line images were measured at five sample positions (viz., 134, 258, 384, 507, and 615). For each sample position, a three pixel wide average cross-cut perpendicular to the dispersion was taken and the widths of the orders measured using the gaussian fitting routine. The results for each image were in good agreement, so we averaged the results to yield a set of spectral widths for each aperture as a function of order number and sample position. The differences in telescope focus between the images were kept small so as to minimize the effect of focus on the resolution measurements (Perez et al. 1990). The database of spectra used for each camera contains a combination of optimally exposed images for the central orders and overexposed (in the central orders only) images for the extreme orders. The spatial resolution data and the one-sigma error bars for each sample position are plotted as a function of order number. The small-aperture data are horizontally offset to the left of the large-aperture data by half an order for clarity. A seventh-order polynomial fit to the data is also provided. LWP - Spatial resolution measurements of the FWHM are plotted in Figures 2.28-2.32 in the NEWSIPS manual (Nichols-Bohlin et al. 1997). The spatial resolution for sample position 384 is approximately 3.5 pixels FWHM at order 69 and decreases to 2.3 pixels at order 80 where it is roughly constant for the remaining orders. The spatial resolution degrades as one moves towards smaller sample positions and improves slightly (as compared with sample position 384) above order 90 for sample position 507. Small- aperture resolution shows an average improvement (over all orders and sample positions) of 4.6% over the large aperture. This difference is most apparent between orders 80 through 100 and at the smaller sample positions where it is as much as 8% for sample position 134. Unfortunately, no LWP high-dispersion spatial resolution studies could be found for IUESIPS data to compare against the NEWSIPS results. LWR - Figures 2.33-2.37 in the NEWSIPS manual (Nichols-Bohlin et al. 1997) show spatial resolution of the FWHM plotted as a function of order number. The resolution trends for sample positions 134 through 384 are quite similar. The FWHM is approximately 3.0 pixels for order 69 and linearly decreases to 2.4 pixels at order 80 where it remains fairly constant for the remaining orders. For sample position 507, the FWHM is around 3.2 pixels for order 69 and linearly decreases to 2.6 pixels at order 80 where it then gradually decreases to 2.3 pixels at order 123. The behavior for sample position 615 demonstrates a rapid decrease in FWHM from 3.8 pixels at order 69 to 2.7 pixels at order 95 where it then gradually decreases to 2.3 pixels at order 120. The small-aperture resolution shows an improvement of approximately 4.7% over the large aperture. The IUESIPS FWHM measurements obtained by Cassatella et al. (1981) using WAVECAL images are somewhat inconclusive. Their data only includes 5 orders (71, 73, 77, 81, and 90) and no mention was made of the sample positions at which these measurements were taken. Their numbers range from 3.5 pixels at order 71 to 2.7 pixels at order 90; values which are around 10% higher than the corresponding NEWSIPS FWHM measurements. The trends seen in the 2-D contour plots made by de Boer et al. (1983) are in good agreement with the NEWSIPS results. They show that for the central sample positions (i.e., 384) the FWHM starts out at 3.1 pixels at low order numbers and decreases to 2.8 pixels towards the center of the camera (e.g., order 90). The slight degradation in resolution seen in the central orders of Figure 2.35 is also apparent in the de Boer plots. SWP - Spatial resolution measurements of the FWHM are plotted in Figures 2.38-2.42 in the NEWSIPS manual (Nichols-Bohlin et al. 1997). The resolution trends, by order number, are, in general, the same for every sample position. The FWHM is around 4 pixels at order 66 (long wavelengths) and decreases to approximately 2 pixels near order 100 (short wavelengths). Unlike the indications from previous IUESIPS studies (e.g., Bianchi (1980), Schiffer (1980), and Cassatella et al. (1981)), this decrease is not linear with order number. A plateau of around 3.0 pixels FWHM occurs between orders 75 and 85. This trend is confirmed by the analysis of de Boer et al. (1983), which displayed the order widths using 2-D contour plots. The FWHM remains fairly constant above order 100 for sample positions 258 and 384. At these sample positions, the higher orders (100 and above) are well away from the edge of the camera. The more extreme sample positions (i.e., 134 and 507) show an edge effect as the resolution dramatically worsens above order 100. The best spatial resolution occurs near sample position 384 and worsens slightly as one moves towards smaller sample positions (i.e. shorter wavelengths within an order). Differences in resolution between the large and small apertures are small. The small aperture shows an average improvement (over all orders) of 2.4% in resolution over the large aperture. As is the case with the low-dispersion resolution studies, the NEWSIPS values show an improvement over IUESIPS measurements. Schiffer (1980) quoted FWHM values of 3.5 pixels for order 75 and 2.4 pixels for order 105. The NEWSIPS results for those orders are 3.3 pixels and 2.1 pixels, respectively. Analysis by de Boer et al. (1983) showed the best resolution of 2.4 pixels FWHM occurring near the center of the camera. The NEWSIPS results indicate a FWHM of 2.0 pixels in this same area (sample position 384). Also, Bianchi (1980) expressed FWHM as a function of order number, regardless of camera, according to the following formula: FWHM = 7.23 - 0.04 X m where m is order number and the FWHM is in pixels. Thus, for order 71, this indicates a FWHM of 4.4 pixels, a figure that is almost 20% higher than the NEWSIPS average measurement for that order. " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "BOGGESSETAL1978B" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "GARHARTETAL1997" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END