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 thebest 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.