# 3.5 Line-of-Sight (LOS) Rewinds

In many respects, the design of the SOFIA telescope is similar to that of a typical altitude-azimuth telescope. One such similarity is that as the telescope tracks a target it must rotate in order for the sky to remain at the same orientation on the detectors. The range of free rotation by the telescope is limited, however, to ± ~3°. Hence, the telescope must periodically undergo a ''Line-of-Sight (LOS) rewind'', or de-rotation. The required frequency of LOS rewinds depends on rate of field rotation experienced by the target, which is a complex function of the position of the target in the sky relative to that of the aircraft heading. Since the relative target position is not known a priori , but is only determined during flight planning, it is not possible to determine precisely the field rotation rate until after the observations have been scheduled. However, one can estimate either the rate of field rotation using Figure 3-2 or the time it takes for a field of view to rotate by six degrees using Figure 3-3.

Figure 3-2: This plot shows the rate of change in the rotation angle (degrees/hour) as a function of target zenith angle and azimuth angle. The rates are calculated assuming an aircraft latitude of 37.415° N. The unvignetted range of zenith angles is shown in white.

Figure 3-3: This plot shows the time it takes for the field of view to rotate by 6 degrees as a function of target zenith angle and azimuth angle. The times are calculated assuming an aircraft latitude of 37.415° N. The unvignetted range of zenith angles is shown in white.

In order to use the plots effectively, one must first use the SOFIA Target Visibility Tool or similar visibility planning tool to determine the airplane heading ( AH ) and the target elevation ( El ) at the desired date of observation. Since the telescope is situated in the port side (i.e. the left side as seen by a person on board, facing the front of the aircraft), the target azimuth angle ( Az ) is calculated according to, Az = AH - 90° . The zenith angle ( ZA ) is calculated according to, ZA = 90° ‒ El . Once the ZA and Az of the target have been determined for a sample observation date, then the LOS plots provide an estimate of the rotation of field for the target.

For example, according to the SOFIA Target Visibility Tool, during the month of August, the W3 star forming region, is at an elevation that ranges from between about 20° and 60° throughout the evening. The aircraft heading would be ~130°. Hence, ZA = 70° ‒ 30° for an average of ~50° and Az ≅ 40° . On Figure 3-2, these coordinates correspond to a field rotation rate of approximately -25° per hour, or roughly 6 degrees every 15 minutes as indicated on Figure 3-3.

When using Figures 3-2 and 3-3 to estimate the rotation of field, it is important to bear in mind the associated caveats. First, the plots were calculated assuming that the aircraft position was at 37.415°, the latitude of NASA Ames. In practice, the latitude of the aircraft may deviate significantly from this. In addition, in the example, we made the estimates using the average ZA of the target, but in practice, neither the ZA at the time of observation nor the actual observation date will be known until flight planning is complete. This means that one should consider the estimates of the rotation of field as being only approximate. Finally, in flight, the LOS rewinds are performed after a field rotation of ~3° rather than the maximum possible range of 6°, in order to account for slight deviations in the airplane heading.

Special care must be taken when designing spectroscopic observations. GIs should bear in mind that the orientation of the slit on their targets will change with each LOS rewind. For point sources, this should not cause problems, but for extended sources, this means that after each rewind the slit will be sampling a slightly different region of the source. In addition, there is no way to choose the orientation of the slit on the target.