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3.2. Zero-Points Errors between Large and Small Apertures

  Each of the three IUE cameras included a pair of science apertures in the telescope plane, a small (``SMAP") and large (``LGAP") aperture. The SMAP was used primarily during the first few years of the mission because of concerns about the wavelength stability for large-aperture observations. Thus, acquisition/guiding errors might result in light losses, but they would not so seriously affect wavelength accuracy as they would for the LGAP. With experience in acquiring objects in the Fine Error Sensor (FES), the IUE Project realized that better wavelength precisions could be obtained than initially anticipated, and observations through the large aperture eventually became routine. A related operational issue was the different centroiding modes in which the FES was used to acquire targets prior to guiding. In particular, the so-called ``underlap" and ``overlap" modes were to acquire bright and faint stars, respectively. Although the underlap mode in principle causes an offset in the target position, pains were taken to correct for this offset. To optimize these complimentary acquisition strategies, the Project settled on a suitable magnitude demarcation of mv = 4.5. Thus, the different acquisition strategies can lead potentially to differences in the wavelength scale between bright and faint stars.

To evaluate aperture-to-aperture differences, we proceeded to evaluate shifts from cross-correlations of similar spectra. We chose NEWSIPS-processed spectra from several well observed OB and white dwarf stars. Table 1 gives the numbers of IUE high-dispersion echellograms used for the analysis of the SWP camera data. As before, these data were cross-correlated for echelle orders m = 70-119. The zero-point errors are expressed in velocity units, a represention that we justify in $\S$4.1.. Table 1 shows that both the mean differences, expressed as RVLGAP - RVSMAP velocities, are -0.8 km s-1 for six bright stars and -0.7 km s-1 for six white dwarfs. The mean shift for any one of these stars is accurate to about ± 2 km s-1 or better. Thus, there are no noticeable systematic differences for shifts between the two apertures and between the over-/underlap modes guiding modes.

Table 1:   Large Minus Small Aperture Zero-Point Differences (km s -1)
SWP Camera:            
Bright Stars: 10 Lac $\tau$ Sco $\zeta$ Oph $\eta$ UMa $\lambda$ Lep $\zeta$ Cas
LGAP - SMAP 0.4 -3.5 -0.6 +0.2 +0.2 -1.7
No. LGAP: 145 74 59 48 40 54
No. SMAP: 6 33 15 12 10 12
Faint Stars: RR Tel Sirius B BD+75o325 BD+28o4211 HD93521 HD60753
LGAP - SMAP: -2.5 -2.3 -4.7 2.4 2.1 0.7
No. LGAP: 46 3 86 76 99 82
No. SMAP: 5 1 5 2 14 1
LWP Camera: $\tau$ Sco $\eta$ UMa HD93521      
LGAP - SMAP -2.5 -1.1 0.1      
No. LGAP: 75 156 20      
No. SMAP: 15 4 2      
LWR Camera: 10 Lac $\tau$ Sco $\zeta$ Oph RR Tel    
LGAP - SMAP 1.9 -1.0 -4.3 +4.3    
No. LGAP: 3 24 11 15    
No. SMAP: 4 14 7 3    

Table 1 lists similar results for three stars for the LWP camera and four stars for the LWR. The mean LGAP - SMAP difference for the two cameras are -1.2 ± 3 and +0.2 ± 3 km s-1, respectively. The quoted errors are sums (added in quadrature) of both LGAP - SMAP differences and zero-points of individual SMAP observations. From this work it is clear that no major systematic aperture-to-aperture differences are detectable for any of the three cameras.

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