1.2 Observing on an Aircraft


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1.2   Observing on an Aircraft

The duration of each SOFIA flight is expected to be between 9–10 hours, 7–8 hours of which will be available for observing at altitudes of 37,000–45,000 feet. FPI+ is always available. Among the other instruments, only one will be installed on the telescope at any time, with the exception of the FLIPO configuration (FLITECAM + HIPO). The SMO director will determine the total number of flights dedicated to each instrument, after consideration of the number of TAC (Time Allocation Committee) approved proposals for each.

Proposals should request observing time in units of hours. Once a proposal has been approved, the first stage is complete and the proposer is then expected to carry out the detailed planning of their observations in consultation with a support scientist or, for PI instruments, with the instrument team. This second stage of observation planning is known as Phase II. Proposers of successful proprosals will be informed who their SMO support scientists are and how to contact them.

On each SOFIA flight, there will be one or more seats available for PIs or designated Co-Investigators (CoIs) of the proposals scheduled for that flight. Since there are a limited number of seats available on each flight, the choice of proposers given the opportunity to fly on SOFIA will be made by the SMO director according to a number of considerations, including the complexity of the observations to be performed, the duration of science observations for each program on the flight, and the proposal rank.

The observations will be carried out either by members of the instrument team along with SOFIA personnel, or solely by SOFIA personnel. The proposers on board SOFIA will participate in the observing, and monitor the data as it is received, but will have limited decision making abilities. For example, the proposer will be allowed to make real-time changes to exposure times for different filters or channels. However, changing targets or any modifications that alter the durations of flight-legs will not be allowed.

Those PIs or CoIs chosen to fly aboard SOFIA will be required to complete a flight participation form, a medical release form, and documentation related to badging. In addition, they will be required to participate in an Egress Training course prior to being allowed on board the aircraft. Full details will be provided to proposers of approved proposals during the Phase II process.

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1.2.1   Scheduling and Flight Planning

Scheduling and flight planning will be handled by the SMO staff and is not the responsibility of the proposer. However, an understanding of the flight planning process and the restrictions inherent to airborne astronomy may be useful in preparing a successful proposal.

The most distinctive aspect of SOFIA flight planning is the interdependency of the targets observed in a flight. Because the azimuthal pointing is controlled primarily by the aircraft heading and because, in normal operations, the take-off and landing air fields are the same, efficient flight plans must generally balance eastbound with westbound flight legs and southbound with northbound legs. This also means that for any flight only a limited fraction of the observing can be performed in a given region of the sky. An example of a flight plan flown during Basic Science in May 2011 is shown in Figure 1.2-1 below. More examples of flight plans can be found on the webpages for earlier cycles. 

Figure 1.2-1.
flight plan example

Figure 1-1. This is a sample flight plan flown in May 2011 during Basic Science. The take-off and landing were both from Palmdale, CA. Each leg is labeled with a time stamp and observing target when appropriate. Flight legs shown in black were ''dead legs'' during which no target was observed. The orange and yellow outlines indicate airspace with varying degree of restrictions which add to the complexity of designing efficient flight plans.

For the proposer this leads to several considerations:

  • A strong scientific case must be made for observations with rigid time constraints or strict cadences in order to justify the restrictions they will impose on flight planning.
  • Because the sky distribution of targets typically proposed for SOFIA observations (centered on the Galactic plane and certain regions of star formation, including Orion) is highly inhomogeneous, targets in areas that complement these high-target-density regions will allow more efficient flight planning and will likely have a higher chance ‒ for a given scientific rating ‒ to be scheduled. Consequently, it may be advantageous for those who can choose between targets from a large source pool for their SOFIA proposals and for those who plan to submit survey proposals to emphasize sources from complementary regions.
  • For example, objects that complement the potentially popular Orion molecular clouds include circumpolar targets or targets north of about 40° with a right ascension in a roughly 6 to 8 hour wide window centered about 6 hours before or after the right ascension of Orion.
  • The maximum length of flight legs will be determined by the need for efficient flight plans as well as the typical requirement that SOFIA take-off and land in Palmdale, California. In most cases, the longest possible observing leg on a given target is ~ 4 hours. Therefore, observations of targets requiring long integrations may have to be done over multiple flights and flight legs.
  • Proposals may be submitted for observations for which the flight does not originate or end in Palmdale, CA, for example, in order to conduct observations under time constraints that require a specific flight path or that require a single flight leg in excess of ~ 4 hours. Such proposals would be equivalent to a deployment and due to resource requirements and the impact that this would have on flight planning, the scientific justification must be strong. The final decision on whether to allow programs with such a high impact on scheduling and flight planning will be made at the Director's discretion.

Proposers are encouraged to review the Flight Planning presentation delivered by Dr. Randolf Klein at the SOFIA User's Workshop in November, 2011. The full list of presentations can be found on the SOFIA web site. In addition, a much more detailed discussion of target scheduling and flight planning can be found in the Observation Scheduling and Flight Planning White Paper.

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1.2.2   Acquisition and Guiding

SOFIA has three optical cameras for acquisition, guiding, and tracking. The Wide Field Imager (WFI) and Fine Field Imager (FFI) are mounted on the telescope head ring. The upgraded Focal Plane Imager (FPI+) images the focal plane of the telescope via a dichroic and a tertiary mirror. All three imagers use 1024x1024 pixel, frame-transfer CCD cameras.

The WFI has a 6°x6° field of view, and is expected to achieve a centroid precision of ~8'' for stars brighter than R = 9. The field of view of the FFI is 70 x 70 arcmin2. It is expected to achieve a centroid precision of ~1 arcsec for R = 11 or brighter stars. The FPI+ has an ~8 arcmin diameter field of view and is expected to provide a centroid precision of 0.05 arcsec for R = 16 (no chopping) and R = 14 (chopping) or brighter stars.

Most observers do not need to select guide stars as they will be chosen by the SMO staff. However proposers should be aware that the guiding cannot be done on IR sources unless they are optically bright.

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1.2.3   Observing Moving Targets

Once SOFIA achieves its nominal operating capabilities, it will be able to observe solar system targets by (i) guiding on the object itself, (ii) offset guiding from field stars, or (iii) predictive tracking based on accurate ephemerides.

Successful guiding on a moving target requires it to be bright at visible wavelengths, where the guider cameras operate. We are typically able to guide on solar system targets with R ≤ 10 and that have a non-sidereal angular speed of 1 arcsed/s or less. The minimum acceptable solar elongation for a target is limited by the lower elevation limit of the telescope and the rule that no observations can be acquired before sunset or after sunrise. The minimum solar elongation is roughly 24 degrees.  

Identification of solar system targets will be done manually by the Telescope Operator by inspecting images obtained with the FPI. The ephemerides of the proposed target must be accurate enough to allow for unambiguous identification. While the required accuracy could vary somewhat based on the complexity of the background star field, it should in general be better than about 30/arcsec.

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1.2.4   Line-of-Sight (LOS) Rewinds

The SOFIA telescope mounting is similar to that of a typical altitude-azimuth telescope. One such similarity is that while tracking a target, the image rotates within the field of view. However, the SOFIA telescope is also similar to an on-orbit gyro-stabilized telescope, with a third control axis along the line of sight (LOS). So the sky image in the focal plane does not change orientation until the telescope approaches an LOS limit.  Then the telescope must be slewed about the LOS axis to at least mid-range, or more typically to near the opposite limit.  Each of these "LOS rewinds" interrupts observing for ~10 to 15 seconds and may have to occur several times during an observing leg. The range of LOS rotation is limited to only ± ~3°, and the frequency of LOS rewinds depends on the rate of field rotation. This in turn depends on the target’s current azimuth and elevation, and weakly on the aircraft latitude. This is similar to the field rotation that occurs at ground-based altazimuth telescopes, but the rate differs due to the aircraft ground speed. Each target’s azimuth and elevation are unknown until the observation is scheduled into a flight plan, and therefore the field rotation angles and rotation rate are not available until then. The overall character of the airborne field rotation rate in the observable sky above the aircraft is shown in Figure 1.2-2. The corresponding maximum time between LOS rewinds is shown in Figure 1.2-3.

Figure 1.2-1.
Rate of Change of Rotation Angle plot

Figure 1-2. This plot shows the rate of change in the rotation angle (degrees/hour) as a function of target elevation and azimuth. The rates are calculated assuming an aircraft latitude of 37° N. The observable range of elevation angles is shown in white.

Figure 1.2-3.
Minutes to Rotate 6 Degrees diagram

Figure 1-3. This plot shows the time it takes for the field of view to rotate by 6 degrees as a function of target elevation and azimuth. The times are calculated assuming an aircraft latitude of 37° N. The observable range of elevation angles is shown in white.

For the majority of SOFIA flights that originate in Palmdale, Figures 1.2-2 or 1.2-3 can be used to anticipate what may occur in this regard. Targets at high northern declinations require eastward headings, and may require quite frequent LOS rewinds. Targets near the celestial equator are likely to have very little or no field rotation and may not need any LOS rewind, even during a long observing leg. 

For example, during the summer months the W3 star forming region rises in the northeast while it is in the observable elevation range (20° to 60°). On Figure 1.2-2, this indicates field rotation rates of about -25° to -35° per hour, or roughly 6 degrees every 15 minutes as indicated on Figure 1.2-3.

When using Figures 1.2-2 and 1.2-3 to estimate the rotation of field, it is important to bear in mind some associated caveats. In practice the time between LOS rewinds is often a little shorter due to the need for some margin near the limits, especially if there is any turbulence. The plotted rates were calculated for latitude North 37° and the rates are weakly dependent on latitude. Even on local flights from Palmdale, SOFIA may make observations in the latitude range North 20° to North 55°.

Special care must be taken when designing spectroscopic observations of extended regions. Proposers 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. However, once the likely range of rotation angle values is known, the orientation of a spectrograph slit (e.g. in EXES) on a region can be anticipated. 

Two of the Science Instruments, FIFI-LS and GREAT, use a K-mirror to rotate the telescope FIR image before it arrives at the detectors. This is slaved to the onboard real-time rotation angle, so that during an observing leg the observed orientation of the FIR image is held constant. 

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