A variety of operational problems can make the observation of a given object difficult or even impossible. A number of these can be anticipated, and frequently avoided with advance planning of your observing run.
Earthlight or moonlight - Bright scattered earthlight or moonlight can make the acquisition of a target impossible. The Earth or Moon will have to move away from the target before it can be acquired by IUE. This could mean a loss of one to three hours for the Earth, and 30 to 60 minutes for the Moon. Earthlight or moonlight can also be a problem if it approaches the target during an exposure. If the FES is being used for offset guiding, earthlight could confuse the tracker and cause the spacecraft to track on the earthlight rather than the guide star! This could result in considerable loss of observing time and potentially endanger the safety of the spacecraft.
Loss of Telemetry - There are four S-band antennas on the IUE spacecraft which transmit science and engineering data to the ground. Two are located on the bottom of the spacecraft and two on the sides parallel to the solar arrays. There is no antenna which efficiently transmits in the direction of the telescope pointing. Consequently, when the Earth is within 30 degrees of the telescope pointing the received signal strength may be very poor. A weak signal can slow down acquisitions and result in permanent loss of image data if the signal is lost while reading a camera. In a weak-signal situation the spacecraft might have to be slewed away from the target to another position with more favorable antenna orientation before an image could be read.
Accurate coordinates - It is essential that accurate coordinates for epoch 1950 be provided for each target. Inaccurate coordinates can result in loss of spacecraft attitude reference, requiring up to several hours to recover attitude. For blind offsets and faint targets the coordinates for the target and offset star should be corrected for proper motion to the current date. Failure to do so has resulted in wasted hours observing the wrong target or blank sky.
Double- and Multiple-Star Systems - Close companions (within 24 arc seconds for targets fainter than about 5th magnitude and 44 arcseconds for targets brighter than 5th magnitude) may confuse the FES, preventing accurate placement of the light from the target in the aperture. Targets in crowded fields, such as multiple-star systems and open clusters, have been observed using the blind offset technique for faint targets as discussed above. Accurate positions for both target and offset star are essential.
Extended Sources - The FES will normally track the center of light of a source with some central concentration, including most comets, globular clusters, and galaxies. Offsets can be generated if the target is not at the center of light. Sources lacking central concentration must be observed as blind offsets as described in Section 3.6 above.