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.