The IUE scientific instrument does not have an exposure meter, so an accurate estimate of the exposure time is necessary. Several methods of estimating exposure times are available. Details for scaling existing information to arrive at an accurate exposure time are given below. If the object has been previously observed with IUE, you can usually scale the exposure times to the appropriate dispersion, aperture, and desired DN level. If the object has been observed previously by another UV satellite, such as TD-1 or OAO-2, you can first scale the fluxes to the IUE flux scale and then to the correct dispersion, aperture, and desired DN level. If the object has not been previously observed in the UV, you can estimate the exposure time from similar objects observed by IUE which are listed in the IUE Merged Log. Models may also be used to predict the expected UV fluxes and thus the exposure times. Be sure to include the effects of extinction in your calculations since UV extinction is much larger and more variable than that in the visual or infrared (e.g. Savage and Mathis 1979; Bless and Savage 1972). See Table 3.3 for the average normalized interstellar extinction as a function of wavelength in the ultraviolet region.
| Wavelength(Å) | E( -V)/E(B-V) | Wavelength(Å) | E(-V)/E(B-V) |
| 1000 | 11.30 | 1900 | 4.90 |
| 1050 | 9.80 | 2000 | 5.52 |
| 1110 | 8.45 | 2100 | 6.23 |
| 1180 | 7.45 | 2190 | 6.57 |
| 1250 | 6.55 | 2300 | 5.77 |
| 1390 | 5.39 | 2400 | 4.90 |
| 1490 | 5.05 | 2500 | 4.19 |
| 1600 | 5.02 | 2740 | 3.10 |
| 1700 | 4.77 | 3440 | 1.80 |
| 1800 | 4.65 | 4000 | 1.30 |
| Values taken from Savage and Mathis (1979). | |||
The specified exposure time is quantized in units of 0.4096 seconds by the ground computer for execution by the OBC. The OBC times the exposure for the correct number of counts. For very short exposures, exposure time quantization must be taken into account when estimating correct exposure times. For example, a commanded exposure time of one second corresponds to two counts, or 0.8192 seconds. A second-order effect is the camera response time. The net rise and fall time of the detector's high voltage is 0.128±0.006 seconds for each exposure (LWR Schiffer 1980b; SWP, LWP Gonzalez-Riestra 1990). Therefore, a single count exposure (0.4096 seconds) has an effective exposure time of 0.281 seconds. See Oliverson (1991) for more details.
There are a number of steps involved in scaling published UV flux or IUE Merged Log exposure data into an estimated exposure time for a particular camera, aperture, and dispersion. Please read carefully all applicable sub-sections.