11.1.2 Aperture Response Corrections

The S$_{\lambda}^{-1}$ functions described above apply only to point source observations acquired through the large aperture. The small-aperture and trailed observing modes are known to have different relative responses as compared to large-aperture point spectra (e.g., Harris and Cassatella 1985, Bohlin 1986, Crenshaw and Park 1989). This effect has been ascribed to several sources, including vignetting effects of the entrance apertures in trailed and small-aperture spectra and spatial inhomogeneities in the UVC efficiency (Cassatella 1990). Application of the large-aperture point source calibration to spectra obtained either in trail mode or through the small aperture will introduce photometric errors of at least 5-7% in the regions near the detector edge and of 1-2% in regions where the camera response is flatter.

Calibration corrections for small-aperture observations and for large-aperture trailed observations have been derived and are applied by the processing system when necessary. IUE standard-star observations from the 1984-1985 epoch have been used to calculate the wavelength-dependent flux ratios of small- and large-aperture point sources, and of large-aperture point source and trail mode observations. Observations from the 1984-1985 epoch were chosen so as to match the large-aperture point source calibration epoch. Also, by limiting the range of time over which the observations were obtained, effects due to sensitivity degradation in the cameras are minimized.

Optimal exposure level observations of the four TD1 standard stars BD+28$^\circ$ 4211, HD 93521, HD 60753, and BD+75$^\circ$ 325 were used to compute the small-to-large aperture (S/L) ratios and the large-aperture trailed-to-point source (T/L) ratios. The effective exposure times of the trailed observations were determined using a value of 21.48 arcseconds for the SWP major-axis trail path length (Garhart 1992b), 22.55 arcseconds for the LWR major-axis trail path length, and 21.84 arcseconds for the LWP major-axis trail path length. The ratios of pairs of observations of the same object obtained through the small and large apertures, or in point and trail modes were averaged together to determine the mean S/L and T/L spectral ratios. Approximately 20 pairs of spectra were used to determine each of the two response ratios. The mean spectral ratios were resampled into the bin size of the appropriate inverse sensitivity function and a spline fit to the binned ratios was calculated. Tabulated values of the spline fits for each camera are listed in Tables 11.5-11.8.

Because centering errors in the small aperture can lead to large variations in the overall observed flux level for individual spectra, it is impossible to determine an absolute S/L ratio. Therefore, the average of S/L over all wavelengths is normalized to unity. As a result, the small-aperture fluxes are known in a relative sense but not in an absolute one. The relative small- and absolute large-aperture inverse sensitivities are related by

$$S_{\lambda}^{-1}(S) = S_{\lambda}^{-1}(L) / (S/L).$$

Investigators should be aware that absolute fluxes for small-aperture data are significantly less reliable than those of large-aperture data. For the ratio of trailed response to point sources in the large aperture, the absolute calibrations are related by

$$S_{\lambda}^{-1}(T) = S_{\lambda}^{-1}(L) / (T/L).$$

Only trailed large-aperture spectra are calibrated with the T/L ratio applied. Images processed as extended sources are calibrated as point source observations.

 

 

$$\lambda \lambda \lambda$$ S^-1a S^-1 S^-1

1850 93.736 2360 6.108 2870 2.823

1865 62.950 2375 5.855 2885 2.877

1880 45.372 2390 5.560 2900 2.925

1895 35.791 2405 5.221 2915 2.998

1910 30.233 2420 4.918 2930 3.135

1925 25.704 2435 4.699 2945 3.314

1940 21.886 2450 4.498 2960 3.414

1955 18.946 2465 4.221 2975 3.686

1970 16.467 2480 3.902 2990 3.911

1985 14.460 2495 3.892 3005 4.132

2000 13.095 2510 3.892 3020 4.465

2015 12.447 2525 3.650 3035 4.868

2030 12.234 2540 3.460 3050 5.326

2045 12.084 2555 3.222 3065 5.972

2060 11.730 2570 3.173 3080 6.614

2075 11.326 2585 3.162 3095 7.318

2090 11.123 2600 3.010 3110 8.075

2105 11.227 2615 2.904 3125 9.101

2120 11.355 2630 2.853 3140 10.060

2135 11.266 2645 2.801 3155 11.577

2150 11.174 2660 2.739 3170 13.041

2165 11.127 2675 2.673 3185 14.704

2180 11.095 2690 2.635 3200 16.832

2195 11.050 2705 2.666 3215 19.605

2210 10.963 2720 2.726 3230 23.110

2225 10.806 2735 2.633 3245 27.069

2240 10.533 2750 2.636 3260 31.107

2255 10.026 2765 2.618 3275 35.122

2270 9.261 2780 2.648 3290 40.094

2285 8.657 2795 2.759 3305 47.271

2300 8.456 2810 2.702 3320 57.623

2315 7.974 2825 2.705 3335 71.007

2330 7.225 2840 2.729 3350 87.001

2345 6.560 2855 2.769 6l^aInverse sensitivity in units of 10 ^-13 ergs cm^-2 Å^-1 FN^-1.

 

 

$$\lambda \lambda \lambda$$ S^-1a S^-1 S^-1

1850 150.815 2360 7.404 2870 2.962

1865 100.432 2375 7.082 2885 2.981

1880 67.875 2390 6.768 2900 3.059

1895 51.682 2405 6.422 2915 3.214

1910 43.664 2420 6.021 2930 3.408

1925 36.055 2435 5.564 2945 3.489

1940 29.671 2450 5.123 2960 3.571

1955 26.087 2465 4.791 2975 3.803

1970 21.773 2480 4.624 2990 4.141

1985 19.958 2495 4.545 3005 4.426

2000 18.914 2510 4.442 3020 4.668

2015 17.786 2525 4.236 3035 5.194

2030 16.714 2540 3.981 3050 5.378

2045 15.910 2555 3.765 3065 5.873

2060 15.509 2570 3.647 3080 6.553

2075 15.350 2585 3.569 3095 7.337

2090 15.195 2600 3.443 3110 8.238

2105 14.868 2615 3.227 3125 9.269

2120 14.438 2630 3.055 3140 10.442

2135 14.090 2645 3.045 3155 11.772

2150 14.099 2660 3.061 3170 13.527

2165 14.235 2675 2.951 3185 16.124

2180 14.129 2690 2.814 3200 18.375

2195 13.629 2705 2.907 3215 22.451

2210 13.475 2720 2.813 3230 25.421

2225 13.725 2735 2.654 3245 30.192

2240 12.470 2750 2.754 3260 38.728

2255 12.053 2765 2.812 3275 46.958

2270 11.348 2780 2.826 3290 51.385

2285 10.490 2795 2.871 3305 60.704

2300 10.036 2810 2.859 3320 70.023

2315 9.535 2825 2.857 3335 110.890

2330 8.595 2840 2.926 3350 151.761

2345 7.860 2855 2.959 6l^aInverse sensitivity in units of 10^-13 ergs cm^-2 Å^-1 FN^-1.

 

 

$$\lambda \lambda \lambda$$ S^-1a S^-1 S^-1

1850 149.533 2360 7.407 2870 2.929

1865 108.895 2375 7.025 2885 2.962

1880 71.052 2390 6.697 2900 3.014

1895 53.992 2405 6.369 2915 3.159

1910 45.698 2420 5.983 2930 3.390

1925 37.905 2435 5.513 2945 3.461

1940 31.051 2450 5.052 2960 3.573

1955 26.426 2465 4.723 2975 3.906

1970 22.927 2480 4.598 2990 4.149

1985 20.562 2495 4.540 3005 4.336

2000 19.567 2510 4.364 3020 4.620

2015 18.827 2525 4.085 3035 5.047

2030 16.966 2540 4.075 3050 5.382

2045 16.849 2555 3.633 3065 5.867

2060 16.174 2570 3.514 3080 6.509

2075 15.269 2585 3.543 3095 7.279

2090 15.296 2600 3.342 3110 8.172

2105 15.661 2615 3.133 3125 9.178

2120 14.869 2630 3.013 3140 10.290

2135 14.905 2645 2.987 3155 11.499

2150 14.822 2660 2.989 3170 12.880

2165 14.503 2675 2.917 3185 14.826

2180 14.337 2690 2.746 3200 17.627

2195 14.176 2705 2.794 3215 20.836

2210 13.233 2720 2.748 3230 23.865

2225 13.843 2735 2.653 3245 27.121

2240 12.606 2750 2.646 3260 33.308

2255 11.817 2765 2.672 3275 39.515

2270 11.317 2780 2.706 3290 43.614

2285 10.375 2795 2.745 3305 53.328

2300 10.228 2810 2.787 3320 63.042

2315 9.434 2825 2.829 3335 78.023

2330 8.562 2840 2.867 3350 93.004

2345 7.901 2855 2.899 6l^a Inverse sensitivity in units of 10^-13 ergs cm^-2 Å^-1 FN^-1.

 

 

$$\lambda \lambda \lambda$$ S^-1a S^-1 S^-1

1150 10.60 1440 1.491 1730 1.269

1160 7.032 1450 1.535 1740 1.226

1170 4.666 1460 1.568 1750 1.185

1180 3.204 1470 1.593 1760 1.149

1190 2.377 1480 1.616 1770 1.117

1200 1.921 1490 1.642 1780 1.089

1210 1.614 1500 1.677 1790 1.067

1220 1.396 1510 1.721 1800 1.049

1230 1.249 1520 1.766 1810 1.035

1240 1.153 1530 1.798 1820 1.025

1250 1.090 1540 1.808 1830 1.019

1260 1.045 1550 1.805 1840 1.017

1270 1.012 1560 1.798 1850 1.019

1280 0.991 1570 1.789 1860 1.025

1290 0.980 1580 1.778 1870 1.034

1300 0.977 1590 1.764 1880 1.046

1310 0.981 1600 1.745 1890 1.059

1320 0.992 1610 1.729 1900 1.071

1330 1.008 1620 1.706 1910 1.081

1340 1.030 1630 1.681 1920 1.086

1350 1.057 1640 1.652 1930 1.084

1360 1.089 1650 1.620 1940 1.073

1370 1.125 1660 1.584 1950 1.054

1380 1.166 1670 1.544 1960 1.038

1390 1.213 1680 1.501 1970 1.040

1400 1.265 1690 1.456 1980 1.075

1410 1.322 1700 1.409

1420 1.381 1710 1.362

1430 1.439 1720 1.315 6l^a Inverse sensitivity in units of 10^-12 ergs cm^-2 Å^-1 FN^-1.

 

 

$$\lambda \lambda \lambda$$ S/L T/L S/L T/L S/L T/L

1850 1.048 0.989 2360 1.024 1.003 2870 0.998 0.995

1865 1.026 0.989 2375 1.021 1.000 2885 0.997 0.995

1880 1.005 0.989 2390 1.017 0.996 2900 0.996 0.994

1895 0.989 0.989 2405 1.013 0.993 2915 0.994 0.994

1910 0.978 0.989 2420 1.009 0.990 2930 0.990 0.994

1925 0.973 0.988 2435 1.006 0.988 2945 0.987 0.994

1940 0.972 0.987 2450 1.004 0.987 2960 0.983 0.994

1955 0.974 0.986 2465 1.003 0.985 2975 0.979 0.995

1970 0.978 0.986 2480 1.002 0.985 2990 0.976 0.996

1985 0.983 0.985 2495 1.003 0.984 3005 0.973 0.997

2000 0.986 0.984 2510 1.004 0.984 3020 0.971 0.999

2015 0.988 0.984 2525 1.005 0.984 3035 0.970 1.001

2030 0.989 0.984 2540 1.007 0.985 3050 0.971 1.004

2045 0.987 0.984 2555 1.008 0.985 3065 0.973 1.007

2060 0.985 0.985 2570 1.009 0.986 3080 0.977 1.011

2075 0.983 0.987 2585 1.010 0.987 3095 0.982 1.016

2090 0.980 0.988 2600 1.010 0.989 3110 0.988 1.022

2105 0.978 0.991 2615 1.009 0.990 3125 0.995 1.029

2120 0.977 0.994 2630 1.008 0.991 3140 1.002 1.036

2135 0.976 0.998 2645 1.006 0.993 3155 1.008 1.045

2150 0.977 1.002 2660 1.004 0.994 3170 1.015 1.054

2165 0.980 1.006 2675 1.001 0.995 3185 1.021 1.064

2180 0.984 1.010 2690 0.999 0.996 3200 1.027 1.073

2195 0.990 1.013 2705 0.997 0.997 3215 1.031 1.080

2210 0.996 1.016 2720 0.995 0.998 3230 1.034 1.084

2225 1.003 1.019 2735 0.993 0.998 3245 1.037 1.085

2240 1.009 1.020 2750 0.992 0.998 3260 1.038 1.082

2255 1.016 1.021 2765 0.992 0.998 3275 1.040 1.074

2270 1.021 1.020 2780 0.992 0.998 3290 1.040 1.060

2285 1.025 1.019 2795 0.993 0.998 3305 1.040 1.039

2300 1.028 1.017 2810 0.995 0.997 3320 1.040 1.012

2315 1.029 1.014 2825 0.996 0.997 3335 1.039 0.980

2330 1.029 1.010 2840 0.997 0.996 3350 1.039 0.946

2345 1.027 1.007 2855 0.997 0.996

$$^\ast$$

 

 

$$\lambda \lambda \lambda$$ S/L T/L S/L T/L S/L T/L

1850 1.014 0.986 2360 0.997 0.986 2870 1.023 0.954

1865 1.014 0.998 2375 0.995 0.982 2885 1.027 0.956

1880 1.014 1.008 2390 0.994 0.977 2900 1.030 0.958

1895 1.014 1.012 2405 0.992 0.973 2915 1.032 0.961

1910 1.014 1.006 2420 0.991 0.968 2930 1.032 0.963

1925 0.980 0.992 2435 0.990 0.963 2945 1.032 0.966

1940 0.960 0.977 2450 0.991 0.959 2960 1.031 0.968

1955 0.951 0.970 2465 0.992 0.955 2975 1.029 0.971

1970 0.951 0.972 2480 0.995 0.952 2990 1.026 0.974

1985 0.955 0.979 2495 0.997 0.949 3005 1.023 0.976

2000 0.960 0.988 2510 1.001 0.947 3020 1.019 0.975

2015 0.963 0.993 2525 1.004 0.945 3035 1.014 0.969

2030 0.964 0.995 2540 1.008 0.943 3050 1.010 0.960

2045 0.963 0.995 2555 1.011 0.942 3065 1.005 0.950

2060 0.962 0.994 2570 1.013 0.941 3080 0.999 0.942

2075 0.959 0.992 2585 1.014 0.940 3095 0.993 0.949

2090 0.956 0.990 2600 1.014 0.939 3110 0.985 0.964

2105 0.953 0.988 2615 1.013 0.938 3125 0.975 0.975

2120 0.951 0.988 2630 1.011 0.938 3140 0.963 0.981

2135 0.949 0.989 2645 1.008 0.938 3155 0.949 0.984

2150 0.949 0.990 2660 1.004 0.938 3170 1.000 0.992

2165 0.951 0.992 2675 1.001 0.938 3185 1.000 0.998

2180 0.955 0.994 2690 0.997 0.938 3200 1.000 1.012

2195 0.959 0.996 2705 0.994 0.939 3215 1.000 1.044

2210 0.964 0.998 2720 0.991 0.939 3230 1.000 1.053

2225 0.970 1.000 2735 0.990 0.940 3245 1.000 1.022

2240 0.976 1.001 2750 0.990 0.941 3260 1.000 1.000

2255 0.982 1.002 2765 0.991 0.942 3275 1.000 1.000

2270 0.988 1.002 2780 0.994 0.943 3290 1.000 1.000

2285 0.992 1.001 2795 0.998 0.945 3305 1.000 1.000

2300 0.995 0.999 2810 1.002 0.946 3320 1.000 1.000

2315 0.997 0.997 2825 1.007 0.948 3335 1.000 1.000

2330 0.998 0.993 2840 1.013 0.950 3350 1.000 1.000

2345 0.998 0.990 2855 1.018 0.952

$$^\ast$$

 

 

$$\lambda \lambda \lambda$$ S/L T/L S/L T/L S/L T/L

1850 1.015 1.008 2360 0.997 0.978 2870 1.047 0.956

1865 1.015 1.015 2375 0.997 0.975 2885 1.052 0.957

1880 1.015 1.020 2390 0.997 0.972 2900 1.054 0.959

1895 1.015 1.019 2405 0.996 0.969 2915 1.053 0.961

1910 1.015 1.010 2420 0.996 0.966 2930 1.050 0.963

1925 0.971 0.995 2435 0.996 0.962 2945 1.044 0.965

1940 0.946 0.981 2450 0.997 0.958 2960 1.036 0.967

1955 0.935 0.974 2465 0.999 0.954 2975 1.026 0.979

1970 0.935 0.977 2480 1.002 0.951 2990 1.016 0.991

1985 0.941 0.985 2495 1.005 0.949 3005 1.004 1.000

2000 0.948 0.994 2510 1.008 0.948 3020 0.992 1.000

2015 0.952 0.999 2525 1.011 0.947 3035 0.981 1.000

2030 0.953 1.000 2540 1.014 0.946 3050 0.969 1.000

2045 0.953 0.999 2555 1.017 0.946 3065 0.959 1.000

2060 0.950 0.996 2570 1.018 0.946 3080 1.000 1.000

2075 0.947 0.992 2585 1.019 0.945 3095 1.000 1.000

2090 0.943 0.988 2600 1.018 0.945 3110 1.000 1.000

2105 0.939 0.985 2615 1.015 0.945 3125 1.000 1.000

2120 0.935 0.982 2630 1.011 0.945 3140 1.000 1.000

2135 0.933 0.981 2645 1.006 0.945 3155 1.000 1.000

2150 0.933 0.980 2660 1.001 0.945 3170 1.000 1.000

2165 0.934 0.980 2675 0.996 0.945 3185 1.000 1.000

2180 0.938 0.981 2690 0.991 0.945 3200 1.000 1.000

2195 0.943 0.981 2705 0.987 0.946 3215 1.000 1.000

2210 0.950 0.982 2720 0.985 0.946 3230 1.000 1.000

2225 0.957 0.983 2735 0.985 0.947 3245 1.000 1.000

2240 0.964 0.984 2750 0.986 0.947 3260 1.000 1.000

2255 0.972 0.985 2765 0.991 0.948 3275 1.000 1.000

2270 0.979 0.985 2780 0.997 0.949 3290 1.000 1.000

2285 0.985 0.985 2795 1.005 0.950 3305 1.000 1.000

2300 0.990 0.984 2810 1.014 0.951 3320 1.000 1.000

2315 0.994 0.983 2825 1.023 0.952 3335 1.000 1.000

2330 0.996 0.982 2840 1.032 0.953 3350 1.000 1.000

2345 0.997 0.980 2855 1.040 0.954

$$^\ast$$

 

 

$$\lambda \lambda \lambda$$ S/L T/L S/L T/L S/L T/L

1150 0.970 1.000 1440 1.020 0.981 1730 0.997 0.994

1160 0.974 0.995 1450 1.020 0.984 1740 0.996 0.995

1170 0.977 0.991 1460 1.020 0.986 1750 0.995 0.995

1180 0.981 0.986 1470 1.020 0.988 1760 0.993 0.995

1190 0.984 0.982 1480 1.020 0.990 1770 0.992 0.995

1200 0.987 0.977 1490 1.019 0.991 1780 0.991 0.996

1210 0.990 0.973 1500 1.019 0.992 1790 0.990 0.996

1220 0.992 0.969 1510 1.019 0.992 1800 0.988 0.996

1230 0.995 0.965 1520 1.018 0.992 1810 0.987 0.996

1240 0.997 0.962 1530 1.018 0.992 1820 0.986 0.997

1250 1.000 0.959 1540 1.017 0.993 1830 0.984 0.997

1260 1.002 0.956 1550 1.016 0.992 1840 0.983 0.997

1270 1.004 0.953 1560 1.016 0.992 1850 0.982 0.998

1280 1.006 0.951 1570 1.015 0.993 1860 0.981 0.998

1290 1.007 0.949 1580 1.014 0.993 1870 0.980 0.998

1300 1.009 0.947 1590 1.013 0.993 1880 0.979 0.998

1310 1.010 0.947 1600 1.012 0.993 1890 0.977 0.999

1320 1.012 0.946 1610 1.011 0.993 1900 0.976 0.999

1330 1.013 0.947 1620 1.010 0.993 1910 0.975 0.999

1340 1.014 0.948 1630 1.009 0.993 1920 0.974 1.000

1350 1.015 0.949 1640 1.008 0.993 1930 0.973 1.000

1360 1.016 0.952 1650 1.007 0.993 1940 0.972 1.000

1370 1.017 0.955 1660 1.006 0.993 1950 0.971 1.001

1380 1.018 0.958 1670 1.005 0.993 1960 0.971 1.001

1390 1.018 0.962 1680 1.004 0.993 1970 0.970 1.001

1400 1.019 0.965 1690 1.003 0.994 1980 0.969 1.002

1410 1.019 0.970 1700 1.001 0.994

1420 1.020 0.973 1710 1.000 0.994

1430 1.020 0.977 1720 0.999 0.994

$$^\ast$$