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High and low dispersion small-aperture spectra of the on-board hollow cathode platinum-neon calibration lamp are used to determine wavelength as a function of position in the image. These wavelength calibration (WAVECAL) exposures are obtained approximately every two weeks and are usually a combination of the calibration spectrum and a tungsten flood-lamp (TFLOOD) exposure. The TFLOOD exposure was originally added to raise the DN level of the fainter emission lines but is also currently used to allow reseau marks to be located on the low dispersion WAVECAL images. These "found reseau" positions are used to perform the geometric correction as described below.

It is important to point out that the geometric distortion characteristic of raw IUE images (Section 4) requires that the fitting of analytic dispersion relations (Section 6.2) be performed in geometrically corrected image space. Consequently, although spectral data images are no longer explicitly geometrically corrected in standard IUESIPS processing, WAVECAL images are geometrically corrected and all derived dispersion relations refer to geometrically correct space. The G-1 geometric mapping function Section ( 4.4.1) is then used throughout the software system to transfer the dispersion relations to raw-image space.



Each set of calibration images is processed to provide analytic relations between wavelength and the line and sample position of a pixel in geometrically correct space. Prior to 6 January 1984 the geometric correction for WAVECAL images was applied using the reseau positions measured on a separate TFLOOD image which was acquired close in time to the low and high dispersion WAVECAL images. As mentioned above, the geometric correction is now applied using reseau positions measured on the low dispersion WAVECAL images. The applications program RSFIX is used to "fill in" reseau positions (using bilinear interpolation of neighboring reseaux) which cannot be accurately determined because of superimposed emission lines. Although this procedure could in principle be also used with the high dispersion WAVECAL reseaux, it was felt that the larger expected number of bad reseau positions would limit any improvement in the geometric correction. (See, however, Section 6.6 on special wavelength calibration procedures).

The pixel locations of the platinum-neon emission lines are measured on the geometrically corrected WAVECAL images by a cross-correlation search algorithm (see Turnrose and Perry, 1977) like that used to find reseau positions. The starting search positions are determined from the current set of mean disper sion constants corrected for the temperature and, in the case of LWR and SWP, the time of acquisition of the particular WAVECAL image (see Section 6.3.1). The measured platinum-neon line positions are then combined with laboratory values for the wavelength and order number of each emission line and used in a regression analysis to determine a set of dispersion constants (A and B) relating wavelength and order number to pixel location according to the following relations:


sample number = A1Z1 + A2Z2 + A3Z3 + ............ + A7Z7

line number = B1Z1 + B2Z2 + B3Z3 + ............ + B7Z7

Z1 = 1
Z2 = m lambda
Z3 = (m lambda)
Z4 = m
Z5 = lambda
Z6 = m2 lambda
Z7 = m lambda2


m = order number
lambda2 = wavelength (Å).

For low dispersion the spectral format is represented by a linear relation (only terms 1 and 2 are used, where m = 1). For both dispersions the analytical dispersion relations are accurate representations of the emission line positions located using the cross-correlation search algorithm in geometrically correct space to within one pixel (formal standard deviations of the regression analysis are typically between 0.25 and 0.45 pixels in both the line and the sample directions). In high dispersion, approximately 135 platinum-neon lines are typically used in the final regression; in low dispersion between 12 and 18 lines are typically used in the final regression. The lines available to the regression analysis form a set referred to as a line library and are selected on the basis of the tested ability of the automatic cross-correlation routine to find them unambiguously. A detailed description of the high dispersion line libraries can be found in Turnrose and Bohlin (1981). The low dispersion platinum-neon line libraries are listed in Harvel, Turnrose, and Bohlin (1979).


The dispersion relations calculated from individual calibration images are no longer implemented per se in production processing. Starting on 18 July 1980 mean dispersion constants were implemented in which approximately 25 individual dispersion relations (calculated from standard biweekly tungsten flood-lamp and platinum-neon images obtained between 1 June 1979 and 1 June 1980) were averaged together term by term (Thompson et al., 1980). As more WAVECAL images became available for analysis, updated sets of mean dispersion constants were implemented in production processing as shown in Table 6.l. The implementation of mean calibration data eliminated the biweekly discontinuities in the way the IUE data were formerly reduced and also made possible the further improvements described in Section 6.3.1 of correcting the dispersion constants for temperature variations and secular effects. Continual monitoring of the biweekly calibration images is conducted to determine whether the implemented mean dispersion relations should be modified.

Table 6-1:   History of IUESIPS Mean Dispersion Constants
ImplementationNo. of ImagesEnd DatesCorrections 
Low Dispersion
 3-03-81413-31-791-01-81THDA & Time2
 9-21-82461-01-808-10-82THDA & Time3
 6-20-841057-15-783-07-84THDA & 2nd Order Time5
 3-03-81403-31-791-01-81THDA & Time2
 9-21-82441-01-808-10-82THDA & Time3
 6-20-841079-30-783-11-84THDA & 2nd Order Time5
High Dispersion
 5-19-81413-31-791-01-81THDA & Time2
 9-21-82471-01-808-10-82THDA & Time3
 6-20-841039-30-783-07-84THDA & 2nd Order Time5
 5-19-81413-31-791-01-81THDA & Time2
 9-21-82451-01-808-10-82THDA & Time3
 6-20-841099-11-783-11-84THDA & 2nd Order Time5
1Thompson, et al. (1980)
2Thompson, Turnrose and Bohlin (1982a)
3Thompson and Turnrose (1983)
4Thompson (1983b)
5Gass and Thompson (1984)


The wavelength calibration exposures are made using the small aperture since it provides the sharpest possible images of the calibration lamp emission lines. The dispersion constants derived from these images define a dispersion relation for the small aperture. When the large aperture is used to take data, it is necessary to modify the A1 and B1 terms of the dispersion relation to translate the dispersion line by the separation of the two apertures. The offsets in line and sample between these apertures have been measured to an accuracy of several tenths of a pixel unit (Turnrose et al., 1979; Turnrose, 1983). Note that as of 1 August 1979 telescope operations and image processing have both used the "physical center" offset values referred to in the references above. A complete discussion of these offsets and the offsets used earlier can be found in the listed references. The current offset values have been given in Table 2-3 in Section 2.2.


In this section are discussed the corrections applied to the mean dispersion relations before spectral fluxes are extracted (Section 7). These corrections are based on the characteristics of each individual image.


Variations in spacecraft temperatures cause shifts in the location of the spectral format with respect to the reseau grid. Additionally, it has been found that the location of the spectral format shifts with time, apparently independent of the temperature variations. Correlations of the systematic behavior of these motions as a function of time and camera head amplifier temperature (THDA) were first found for the LWR and SWP camera as are described in Thompson, Turnrose, and Bohlin (1982a). For the LWP camera, correlations with THDA have been determined, while correlations with time are still being evaluated (see Thompson, 1983b).

Corrections for thermal and temporal shifts based on the derived correlations were first implemented as part of the standard LWR and SWP data processing at GSFC on 3 March 1981 in low dispersion and on 19 May 1981 in high dispersion by a new IUESIPS applications program, TCCAL. Corrections for thermal (but not temporal) shifts for LWP images were implemented on 12 April 1983.

The corrections are based on the THDA temperature at the end of the exposure (denoted by T in the equation below, and expressed in centigrade degrees), and the time (t) expressed as the total number of elapsed days since 1 January 1978. Both values are generally obtained from data extracted from the image header label. The THDA at the end of exposure is normally extracted from the camera snapshot section of the image label, but if that THDA value is not available, then the THDA at the time of image read is extracted. If neither THDA is available, then the processing defaults to the mean dispersion constants unless either (a) a specific THDA value is manually specified by the image processing specialist when the image is processed, or, (b) for LWR and SWP images processed after 20 June 1984, a reasonable date of observation can be extracted from the image label in which case a correction for time only is applied. The last option above was implemented to improve the calibration of images which do not contain valid temperature data (e.g., history playback images and images obtained prior to about March 1979). See Section 9.3 for information about how these data are documented in the image processing history label.

The correction terms Ws and Wl representing a uniform shift applied to the mean dispersion constants are defined by the general expression


Ws = W1s + W2sT + W3st + W4st2
Wl = W1l + W2lT + W3lt + W4lt2
where Ws and Wl are the corrections to be added to the A1 and B1 terms of the dispersion relation respectively. The current correlation coefficients are shown in Table 6-2. As was shown in Table 6-1, the corrections that have been applied to the mean dispersion constants have changed along with the updated mean dispersion constants. The previously implemented correlation coeffi cients can be found in the references cited; the overall corrections actually applied in production have been documented in the image label as described in Section 9.3.

Table 6-2: Coefficients Defining the Dispersion Relations For the Small Aperture  
A15.873462158066862E 03-4.568022566378104E 035.240320204548078E 02
A41.595428893061642E 013.70636790765387E-022.400037009830254E-01
B11.722851374444825E 031.567990956548678E 04-7.171777625701399E 03
W1(S)-7.430500388145447E-015.459306716918945E 00-2.977794647216797E 00
W3(S) -1.768400659784675E-032.857662504538894E-03
W4(S) 3.070972525165416E-07-5.223851076152641E-07
W1(L)-4.000792503356934E 00-8.628579139709473E 00-2.841607093811035E 00
W3(L) 1.599742565304041E-037.730186916887760E-04
W4(L) -3.199881462023768E-07-6.993195711402223E-08
A11.046282942865237E 03-2.992355784397701E 029.833223402481985E 02
B1-2.722748512318324E 02-2.647551045134080E 02-2.633234804632572E 02
W1(S)-7.578814029693604E-015.142534255981445E 00-3.452352523803711E 00
W3(S) -1.864231890067458E-033.721332177519798E-03
W4(S) 1.824748778744834E-07-6.585678420378827E-07
W1(L)-2.995339393615723E 00-8.595767974853516E 00-1.659444808959961E 00
W3(L) 2.750693820416927E-032.752062573563308E-05
W4(L) -5.675888132827822E-078.504440529577550E-08

The major effect of the temperature and time corrections is to shift the location of the spectral format in the direction approximately perpendicular to the dispersion direction in low dispersion and approximately along the dispersion direction in high dispersion (Thompson, Turnrose, and Bohlin, 1982a). Since only the zero-point or offset terms of the dispersion relations are corrected, there is no change to the scale of the dispersion relations. Accordingly, the component of the correction along the dispersion direction corresponds very nearly to a constant velocity shift in high dispersion and a constant wavelength shift in low dispersion.

Based on the statistical analysis of the WAVECAL images used to establish the time and temperature dependencies, the corrections described above reduced the average relative (i.e., intrinsic) error in high dispersion wavelength assignments to a velocity equivalent of less than 3 km s-1. Larger relative errors may be expected for specific wavelengths near the tube peripheries or affected by residual uncorrected geometric distortion (see Section 6.5) and for images exposed during times when large variations in spacecraft temperature exist. There is limited recent evidence that overall differential (i.e., non-uniform) motion of the spectral format may occur, which may also increase the expected errors. This as yet ill-understood motion has been observed to introduce non-uniformities of up to approximately 1 pixel. In addition, extrinsic errors caused, for example, by spacecraft pointing limitations (target decentering due to small errors in the initial acquisition, spacecraft roll drift during long exposures, etc.) and the uncertainty in centroiding spectral features in extracted spectra may exist which contribute to the absolute error in IUE wavelengths. A more detailed discussion of overall errors is presented in Section 6.5.


The time-and/or-temperature-corrected dispersion relations, as mapped into raw image space by the G-1 function discussed in Section 4, are used by the spectral extraction routines described in Section 7 not only to assign wavelengths but also to determine the position of the extraction slit as it is passed numerically by the computer along each spectral order of the image. Accurate gross and background flux levels therefore require proper registration of the dispersion relation with the actual spectral orders, particularly in the region of the closely-spaced orders in high dispersion. Since the corrections described in Section 6.3.1 for determining the location of the spectral format are statistical in nature, automatic and manual registration methods are used to modify further the dispersion constants on an image-by-image basis to remove residual registration errors in the direction perpendicular to the dispersion. See Section 9.3 for information regarding documentation of registration shifts in the image label. Automatic Registration

Several modifications have been made to the automatic registration routine DCSHIFT, as described chronologically in Turnrose and Harvel (1982) and Turnrose, Thompson and Gass (1984). The current version which was implemented at GSFC on 24 November 1981 is described in detail in Thompson and Bohlin (1982). The basic technique involves using a cross-correlation spectral- order-finding algorithm to sample the raw image at 12 discrete locations and to determine the line and sample offsets of the spectral format relative to the corrected dispersion relations, perpendicular to the orders themselves. The 12 discrete search areas are now all chosen in the same spectral order, as shown in Table 6-3. In high dispersion the registration shifts are first calculated in the region of the closely spaced orders (order 108) where precise registration is most critical. If these shifts do not pass certain built-in program constraints (described in Thompson and Bohlin, 1982), up to three progressively lower orders are searched in the same way. With the average of the 12 offset values, IUESIPS adjusts the A1 and B1 terms in the dispersion relations, in effect tailoring the dispersion relations for the particular target image in question. Algorithms currently exist for the automatic registration of both high and low dispersion point-source spectra and of low dispersion trailed spectra.
Table 6-3:   Search Area Wavelengths
Low dispersion wavelengths (Å)
ong wavelengthShort wavelength
High dispersion wavelengths (Å)
Long wavelengthShort wavelength
m= 1081008677m= 1081008277
2139.52310.5 268730051275137716791789.5
2148.52319.5269930171281138316881798.5 Manual Registration

In the event that DCSHIFT fails to determine an adequate shift or if requested by the Guest Observer for a weak spectrum, a manual registration shift will be calculated. In this mode, the raw image is displayed on the Experiment Display System (EDS) screen with a wavelength overlay generated using the (time-and/or-temperature-corrected) dispersion constants. If the dispersion lines of the overlay are observed to fall to one side of the spectrum, the image processing specialist will manually enter a shift in the sample direction which will cause the overlay to be superimposed on the data. IUESIPS then converts the entered sample direction shift into line and sample offsets which correspond to an equivalent shift perpendicular to the dispersion. These offsets are then added to the A1 and B1 dispersion constant terms as described above.

Note that the GO can greatly facilitate the processing operation by specifying on the Observatory Record Sheet (see Section 1.4) that the manual shift routine should be used for those images which would not be suitable for the automatic algorithm such as weak or emission-line spectra. Registration Accuracy

The errors expected with the automatic registration of point-source spectra are generally less than 0.1 - 0.3 pixel and are slightly larger for trailed spectra. Because of the possibility of differential motion of the spectral format referred to in Section 6.3.1, it is possible that even perfect registration in order 108 could leave registration errors as large as a pixel in the lower orders (since the differential motion error generally changes linearly across the image). Since the lower orders in high dispersion are farther apart and since the extraction slit is intentionally made slightly longer than the width of the spectral orders, these registration errors should not seriously affect the extracted spectral data. Although the errors involved in the manual registration routine will depend upon the accuracy of the image processing specialist and the quality of the raw spectral image, they are probably typically less than 0.5 pixel.

It may be noted that although the dispersion relation describes the spectral format location in geometrically correct space, the correction applied refers to a shift measured in raw image space. Theoretically the raw image space registration shift should be converted back to an equivalent geometrically correct shift to compensate for any expansion, contraction, or rotation of the image that would occur in the geometric correction process. Test results indicate, however, that this error is in fact less than a few percent of the calculated shift and is therefore insignificant.


Following the extraction of spectral fluxes (Section 7), two additional corrections to the assigned wavelengths are routinely made. These corrections are described in the following subsections in order to consolidate the discussion of wavelength topics within Section 6.


As of 10 November 1981 at GSFC, wavelengths are routinely reduced to a heliocentric frame of reference for all high dispersion spectra except on- board calibration lamp exposures and images for which the target coordinates or the time of observation are not available. An observer can determine if the correction has been performed by examination of record 0 of the MEHI file (see Section or the image header label as described in Section 9.3.

Using the time of the midpoint of observation, the velocity components of the earth and IUE in a righthanded rectangular equatorial coordinate system (+x is toward the vernal equinox, +z is toward the north celestial pole) are computed using the routines described in Harvel (1980). Harvel has shown that by using the fixed orbital elements listed in that reference, the spacecraft velocity calculation was accurate to 0.25 km s-1 over the first 3 years of IUE operation. (Schiffer (1982) has indicated that an observed evolution of the IUE orbital elements may increase the error in the spacecraft velocity calculation by about 1 km s-1. Most of the change is in the Z component. The operations software now stores current IUE orbital elements in the science header; see Figure 9-1b.) Additional spacecraft velocity errors can result from errors in calculating the time of the midpoint of the observation. Currently the midpoint time is set equal to the end-of-exposure time minus half of the total exposure time, as read from the raw-image label. Depending on the camera procedures used, this method could cause an error in the spacecraft velocity calculation. However, since the magnitude of the spacecraft velocity is small compared to that of the earth's motion (4 km s-1 versus ~ 30 km s-1) the error will in general have a small effect on the overall wavelength correction.

The computed net radial velocity of the IUE spacecraft toward the object is then:

V = Vx cos( d ) cos( a ) + Vy sin( a ) cos( d ) + Vz sin( d )
Vx = Vx (earth) + Vx (IUE)
Vy = Vy (earth) + Vy (IUE)
Vz = Vz (earth) + Vz (IUE)
a = right ascension of the object
d = declination of the object.

The extracted wavelengths are then corrected by:
lambda corrected = (1 + V/c) lambda uncorrected
where lambda corrected is in the heliocentric reference system and c is the speed of light.

Note that the calculation is such that a positive net radial velocity correction indicates a net approach of the IUE spacecraft toward the target, following the standard convention. The individual IUE and earth velocity components and the net radial velocity correction used are documented in the image processing history label as discussed in Section 9.3.2.


The wavelengths obtained using the platinum-neon calibration lamp are vacuum wavelengths. Since it is customary in the ultraviolet literature to list air wavelengths for lines longward of 2000 Å, a vacuum-to-air wavelength correction is applied in the processing of all images for lambda >= 2000 Å. In the case of high dispersion spectra, this correction is made after the heliocentric correction described in Section 6.4.1. The formula used for wavelengths lambdavac equal to or greater than 2000 Å is:

lambda air = lambda vac/f( lambda vac)


f( lambda ) = 1.0 + 2.735182 × 10-4 + 131.4182/ lambda 2 + 2.76249 × 108/ lambda 4

At 2000 Å, the magnitude of the correction is ~ 0.65 Å, and it increases with wavelength to ~ 0.90 Å at 3100 Å.

Note that under the old reduction software (i.e., prior to 3 November 1980 for low dispersion and 10 November 1981 for high dispersion) the vacuum-to-air correction was only applied to data from the long wavelength spectrograph (cameras LWR and LWP). It is now applied for all cameras at wavelengths greater than or equal to 2000 Å.


The final uncertainty in the wavelength measurement of a spectral feature in an IUE spectrum is a combination of various intrinsic and extrinsic errors. The analysis of WAVECAL images used to establish the time and temperature dependencies also establishes the average intrinsic error in the wavelength calibration process. Table 6-4 lists these average intrinsic errors for the various time/temperature compensation methods. (Note that for SWP and LWR, the column relevant to current production techniques is that headed "THDA & 2nd Order Time", whereas for LWP, which currently has no time dependence applied, the relevant column is that headed "THDA Only." These errors pertain to the limitations of modeling the behavior of dispersion relations fitted to the rather homogeneous set of WAVECAL images. The absolute accuracy of applying this internal wavelength scale to actual spectral images depends on a number of other, largely extrinsic, factors. Such factors include the extent to which the temperature and time pertinent to a given image fall within the correlation range defined by the WAVECAL data, the extent to which spectral- image-specific reseau motion and geometric distortion can be compensated, the extent to which the limited-term polynomial defining the dispersion relations can compensate for small-scale deviations from a smooth wavelength scale, the extent to which the laboratory wavelengths assigned to the WAVECAL spectra are correct, the extent to which the target object is centered in the aperture, and the extent to which a feature in an extracted spectrum can be accurately centroided. These various factors are addressed below.

Table 6-4: Average Relative (Intrinsic) Error (1 sigma in pixels) for Various Corrections to the Mean Dispersion Constants  
Dispersion DirectionNo CorrectionTHDA OnlyTime Only THDA & TimeTHDA & 2nd Order Time
SWP High     
SWP Low     
LWR High     
LWR Low     
LWP High     
LWP Low     

Prior to the implementation of temperature and time corrections (reference Table 6-1), systematic errors were introduced for images exposed during extreme spacecraft temperatures or at times much different than the average time for the mean dispersion relations. Further, now that the overall time dependence has been observed to be nonlinear (i.e., a second-order time dependence is used), it is apparent that systematic errors were also introduced by extrapolating the linear time correction beyond the end dates of the input calibration data (see Table 6-1) such that the greater the extrapolation in time, the larger the systematic error.

Because the dispersion relations, which are determined in geometrically corrected space, are now applied to spectral images in raw image space, errors in the mapping function G-1 represent an additional error in the assigned wavelengths. Although studies have shown that reseau positions vary with both temperature and image intensity (see Section 4.2, Oliver (1979), and Thompson 1983a), IUESIPS currently uses only mean reseau positions for LWP and LWR images and only temperature-corrected mean reseau positions for SWP images (see Section 4.3). The lack of any correction for image intensity can cause errors of 1 pixel or more in the assigned reseau positions (Thompson 1984c). Since the error occurs primarily in the line direction, it may result in a position-dependent wavelength error corresponding to up to f 0.7 pixels. The mean reseau positions currently used in IUESIPS are all based on an analysis of 60-percent UVFLOOD images. Although the data set included images taken over a fairly wide range of temperatures, they were all at approximately the same DN level. Since the 60-percent UVFLOODs have a central mean DN of approximately 120, the largest errors may occur for images with higher or lower exposure levels (and at extreme spacecraft temperatures for the uncorrected LWR and LWP cameras).

A possible source of absolute wavelength error is the inability of a limited-term polynomial to compensate for small-scale deviations from a smooth wavelength scale. Small-scale image distortions, when occurring in the direction perpendicular to the dispersion, have been observed and described as residual curvature; such distortion must certainly also occur along the dispersion and would result in localized small-scale (sub-pixel) wavelength errors.

Another possible explanation for errors in the assigned wavelengths, suggested by Ayres (1984), is that errors may exist in the library of laboratory wave lengths. In addition, Ayres suggested that the non-uniform distribution of emission lines used in generating the dispersion relation may be such that the fit favors the regions which contain the most emission lines. Recent work by de La Pena (1984) shows that the SWP high dispersion wavelength assignments show a wavelength-dependent variation of ~ 2 km s-1 which could be explained by Ayres' suggestion. It should be noted that all the above results were based on studies of WAVECAL images and therefore would not be subject to reseau-position errors due to the beam-pulling effects mentioned above.

Target de-centering within the aperture can lead to appreciable absolute wavelength errors, particularly in the large aperture, where the centering error can typically be about an arcsecond, even larger in the case of blind-offset acquisition. A pointing error of 1 arcsecond along the dispersion direction is 0.66 pixel, or about 5 km s-1 in high dispersion. As mentioned in Section 6.3.1, spacecraft drift during long exposures can also contribute to de-centering error. The small size (about 3 arcseconds) of the small aperture should result in better absolute wavelengths due to reduced target de-centering effects.

The ability to centroid a spectral feature in an extracted spectrum is a final limitation to the accuracy of assigned wavelengths. As is pointed out in Thompson, Turnrose, and Bohlin (1982a), for an isolated narrow feature with the instrumental resolution of 2.5 pixels in the cleanest IUE data, the best possible estimate of the measurement accuracy would be a 0.25 pixel, which in high dispersion corresponds to a 2 km s-1 velocity uncertainty. In many cases, the spectral data will not be of sufficient quality to approach a 0.25 pixel measurement error.

Finally, it should be remembered that the errors referred to in most of the above discussions are AVERAGE errors, particularly those in Table 6-4. Recent experimental results have, in general, confirmed these average errors although it was found that the quoted averages can be misleading if used to describe the accuracy of measuring a single spectral feature. One analysis that was performed involved extracting several LWR and SWP high dispersion WAVECAL spectra using standard production processing techniques and measuring the wavelength assignment accuracy of the Pt-Ne emission lines in several orders (Heckathorn 1984 and Thompson 1984a). It was concluded that although the average errors found were small (i.e., the overall mean wavelength error for each of six images was always less than 3 km s-1, larger errors were found in measuring the wavelengths of individual lines (± 6 km s-1).


The study of reseau motion and spectral format shifts has shown that the most accurate wavelength assignments (particularly in high dispersion) are obtained when (a) the calibration images are taken close in time to the spectral image to be extracted, (b) the spacecraft temperatures remain stable, and (c) the calibration image and spectral image are of similar intensity. Although the standard production processing includes a temperature and time correction (amounting to a uniform shift) to a set of mean dispersion constants, there has been evidence for occasional differential shifts in the spectral format for which there is no correction or explanation. (The monitoring of the biweekly wavelength calibration images showed that such an event occurred for the LWR camera for several weeks in the fall of 1981). The possibility of such occurrences would suggest that Guest Observers who require the most accurate wavelength assignments should obtain their own calibration images and request special processing. It should be pointed out, however, that (a) fluctuating spacecraft temperatures will probably result in increased wavelength errors with either of the calibration procedures, (b) errors due to small-scale geometric distortions have been found to still exist with either procedure, (c) variations in image intensity cannot be corrected for, and (d) errors in centering the target in the aperture will still affect the wavelength accuracy. This last caution is particularly pertinent in the case of special calibrations since the cycling of the aperture mechanism required to perform a wavelength calibration has been known to introduce shifts of several pixels in the FES reference point used in target acquisition.

The special calibration procedure has become somewhat simplified in that separate TFLOOD exposures are no longer necessary. Reseau positions can now be directly measured from the WAVECAL images themselves (thereby eliminating the need for the separate TFLOOD).

Some Guest Observers have found it useful to obtain WAVECAL images with their normal spectral images and have them extracted using the standard production processing procedure. The wavelength errors measured on the WAVECAL image can then be used as a guide to correcting the other extracted spectral images.

Guest Observers who wish to use a special calibration should fill in the last two lines of the "Processing Specifications" portion of the observing script Section (1) indicating the image number of the special calibration to be used. Guest Observers should also be warned that special processing requests may cause some delay in the processing of their images.

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