3. Cycle 5

Cycle 5 observations will take place between 1 February, 2017 and 31 January 2018. The observations will take place during a series of Science Flight Campaigns, each of which will focus on a single instrument configuration, over the duration of the cycle. The campaigns will be interspersed with aircraft maintenance and instrument commissioning. Commissioning of the far infrared camera and polarimeter HAWC+ are ongoing in 2016 and 2017. These new capabilities are offered in Cycle 5, contingent on the success of commissioning activities. A single Southern Hemisphere observing series with up to two instruments is under consideration for the Cycle 5 time period, nominally in the summer of 2017.

For Cycle 5, the available instruments will be the mid-infrared high-resolution spectrograph EXES, the integral-field far-infrared spectrometer FIFI-LS, the near-IR camera FLITECAM, including its grism modes, the mid-infrared camera FORCAST, including its grism modes, the focal plane CCD imager FPI+, in science mode, the heterodyne spectrometer GREAT, including the seven-beam receiver array upGREAT, the high-speed optical photometer HIPO, and the far-IR camera and polarimeter HAWC+. The instrument capabilities, the available modes, and the resulting performance specifications of the telescope are described in later sections.

Proposals are being solicited for the Cycle 5 SOFIA flights by USRA on behalf of NASA.

3.1 Observing with SOFIA during Cycle 5

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-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 GI 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. GIs 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 GIs or designated CoIs of the proposals scheduled for that flight. Since there are a limited number of seats available on each flight, the choice of GIs 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 GIs 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 GI 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 GIs 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 GIs of approved proposals during the Phase II process.

3.2 Scheduling and Flight Planning

Scheduling and flight planning will be handled by the SMO staff and is not the responsibility of the GI. 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 East-bound with West-bound flight legs and South-bound with North-bound 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 3-1 below. Several more examples of flight plans flown during Basic Science can be found on the Information for Researchers Flight Plans web page.

Basic Science flight plan

Figure 3-1: 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.
  • GIs may propose 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.

GIs 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.

3.3 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 ''High Speed Slow Scan'' 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 1°x1°. It is expected to achieve a centroid precision of ~1'' for R = 11 or brighter stars. The FPI+ has an 8' diameter and is expected to provide a centroid precision of 0.05'' for R = 16 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.

3.4 Observing Moving Targets during Cycle 5

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. As of this writing, the non-sidereal tracking capabilities have been implemented, but have not been fully tested. Consequently, programs requesting non-sidereal tracking will inherently involve greater risk. Nevertheless, the SOFIA project will make every attempt to observe Solar System targets in highly ranked proposals.

Successful guiding on a moving target requires it to be bright at visible wavelengths, where the guider cameras operate. Our current estimate is that we will be able to guide on solar system targets with R ≤ 10 and that have a non-sidereal angular speed of 1'/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. For Cycle 4, the minimum solar elongation is 25 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.

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.

 

Rate of Change of Rotation Angle (deg/hr)

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.

 

Minutes to Rotate 6 Degrees

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.

3.6 Atmospheric Transmission

SOFIA operates at altitudes above 99% of the water vapor in the atmosphere. The average atmospheric transmission across the SOFIA bandpasses is about 80% at these altitudes. There are however a number of strong absorption features which, even at these altitudes, make the atmosphere nearly opaque. Broad band filters, such as those on FORCAST, account for the presence of such features. However, when using high-resolution tunable instruments such as GREAT and EXES, it is necessary to examine the atmospheric transmission at the wavelengths of interest in detail. This may be done using the web interface to the ATRAN program that was developed and kindly provided to the SOFIA program by Steve Lord. A plot of the atmospheric transmission seen by SOFIA in comparison to that achieved at Mauna Kea is shown in Figure 3-4 below.

In addition to its dependence on wavelength due to the presence of absorption features, the atmospheric transmission varies with latitude and with time of year, primarily due to differences in the amount of water vapor. It also exhibits variations on smaller time scales due to changes in the location of the tropopause. Full discussions of these issues may be found in Haas & Phister 1998 (PASP, 110, 339) and Horn & Becklin 2001 (PASP, 113, 997).

The variations in atmospheric water vapor could have a significant impact on some observations, particularly when using GREAT and EXES or grism modes with FLITECAM and FORCAST. For example, GREAT observations of a line situated on the shoulder of an atmospheric water feature could be strongly affected by water vapor variability. SITE allows the user to specify the water vapor overburden and adjusts the time estimates appropriately. The water vapor monitor has been installed and is currently undergoing testing, but may not be fully functional during Cycle 5.

 

Atmospheric transmission plot

Figure 3-4: This is a plot showing the atmospheric transmission for SOFIA (black) at an altitude of 41K feet and 7.3 μm of precipitable water vapor compared to Mauna Kea (red) at an altitude of 13.8K feet and 3.4 mm water vapor over the range of 1 ‒ 1000 μm. The transmission was calculated using the ATRAN code with a telescope zenith angle of 45°. and the data were smoothed to a resolution of R=2000.

3.7 Flux and Telluric Calibration

Flux and telluric calibration observations will be planned by the SMO staff and instrument scientists using standard stars, asteroids, and planets. Calibration plans will be specific to each instrument. In all cases, except EXES, the time used for calibration observations are considered part of the general overhead and will not be charged against the GI program time. GIs should look at the calibration details provided in each instrument chapter below for specific calibration plans.

The calibration accuracy will depend on the instrument and observation mode, but an accuracy of 20% is expected to be obtained, even in "Shared Risk" modes. Unless the proposer requires more accurate calibration, there is no need to request time for calibration or to choose calibration targets. If the GI requires more accurate calibration, it must be justified in the proposal, and the time required for the calibration observations should be included in the total time request. The only exception to this is EXES, for which all calibration observations must be specified in the proposal and will be charged to the science program time.