After a long slew, identification of a faint target can be difficult or even impossible. An offset maneuver to your target from a relatively bright nearby star with accurate coordinates will generally be necessary. Possible reasons for using an offset maneuver are to observe objects which are too faint to be detected by the FES, save time in locating targets which are marginally detectable in the FES, observe precise locations in diffuse sources, observe targets with close companions, or locate some classes of moving targets.
For arc-second maneuver accuracy, the distance from the offset star to the target should be less than 0.25 degrees, ideally only a few arc minutes. A plot of offset maneuver accuracy as a function of total slew length is shown in Figure 3.2. If accurate 1950 right ascension and declination, corrected for proper motion, for both the target and offset star are used for calculating the offset maneuver, and if the offset slew distance is less than 15 arc minutes, then the slew should place the target within two arc seconds of the reference point. (The offset star is centered at the FES reference point before the offset slew.)
Figure 3.2: Blind offset maneuver accuracy. The total slew error in arc seconds is shown as a function of the offset distance in arc minutes. The data is taken from Arquilla (1990). The recommended maximum offset distance to insure a total slew error of no more than 2.0 arc seconds is 15 arc minutes.
At the normal telemetry rate of 20 Kbps, the limiting magnitude for a star in an FES image is 12.0. At a telemetry rate of 5 Kbps, the limiting magnitude is 13.5. Such an image with the default of 10.8 arc minutes in size requires over 3 minutes of integration. Special acquisition techniques are required for fainter objects.
One method is to command the FES, in track mode, to look at the reference point at the end of the offset maneuver. If the target falls within the FES's track pattern (12 arc seconds), a long integration (i.e. about one minute) is obtained of the target's position and brightness. Using this position the target is then centered at the reference point before maneuvering the target to the desired aperture with standard slews. This technique can be used for point sources down to approximately 14.0 magnitude. A second technique is to map out a one-arc-minute-square portion of sky around the reference point. The long integration at each pixel in this "postage stamp" FES image allows detection of objects as faint as 14.0 - 14.5 magnitude, depending on the sky background brightness. From the postage stamp, small corrections in telescope pointing can be commanded so that most of the light from the target falls on one FES pixel (8 arc seconds square) centered on the reference point. The target can then be maneuvered to the desired aperture by the standard slews. This technique is not as accurate as the FES tracking mode, (because of the low spatial resolution (±4 arc seconds) in FES images), however, it can be used for much fainter objects.
If the target is fainter than approximately 14.5 magnitude, it cannot be seen by the FES and the acquisition must be done blindly. Since the large aperture is only a 10 x 20 arc second oval (the small aperture is a 3 arc-second-diameter circle), positions accurate to one arc second or better, corrected for proper motion, are necessary for the offset star and target.