1. Introduction

1.1 SOFIA and Its Instruments

1.1   SOFIA and Its Instruments

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a telescope with an effective diameter of 2.5 meters, carried aboard a Boeing 747-SP aircraft. It is the successor to the smaller Kuiper Airborne Observatory, which was operated by NASA from 1974 to 1996. The observing altitudes for SOFIA are between 37,000 and 45,000 feet, above 99% of the water vapor in the Earth's atmosphere. The telescope was designed to allow imaging and spectroscopic observations from 0.3 to 1600 μm, with specific capabilities dependent on an evolving science instrument suite, making it one of the premier facilities for astronomical observations at infrared and sub-millimeter wavelengths. The present instrument suite provides coverage from 0.3–612 μm with additional capabilities currently in development.

The telescope for SOFIA was supplied by the DLR as the major part of the German contribution to the observatory. It is a bent Cassegrain with 2.7 m parabolic primary mirror (2.5 m effective aperture) and a 0.35 m diameter hyperbolic chopping secondary mirror. The telescope is mounted in an open cavity in the aft section of the aircraft and views the sky through a port-side door. The telescope is articulated by magnetic torque motors around a spherical bearing through which the Nasmyth beam passes. The unvignetted elevation range of the telescope is 20°–60°. The cross-elevation travel is only ± 3° and, therefore, most of the azimuthal telescope movement required for tracking is provided by steering the airplane. Thus, the flight plan is determined by the list of targets to be observed during each flight.

The telescope feeds two f/19.6 Nasmyth foci, an IR focus for the science instruments and a visible light focus for guiding, using a dichroic and an aluminum flat. The secondary mirror is designed to chop at amplitudes of up to ± 5 arcmin at a frequency ≤ 10 Hz and up to ± 10 arcmin at a rate of ≤ 2 Hz. The visible beam is fed into the Focal Plane Imager (FPI+), which is an optical focal plane guiding camera. Independent of the FPI are two other optical imaging and guiding cameras, the Wide Field Imager (WFI) and the Fine Field Imager (FFI), both of which are installed on the front ring of the telescope.

Eight instruments, covering a wide range of wavelengths and resolving powers as shown in Figure 1.1-1, are available for use on SOFIA. Three of the instruments are Facility-class Science Instruments (FSIs), which will be maintained and operated by the Science Mission Operations (SMO) staff.

FIFI-LS: Far Infrared Field-Imaging Line Spectrometer
An integral-field far-infrared spectrometer
 
FLITECAM: First Light Infrared Test Experiment Camera
A near-IR camera (including its grism modes) (FSI)
 
FORCAST: Faint Object InfraRed Camera for the SOFIA Telescope
A focal plane CCD imagerd-IR camera (including its grism modes) (FSI)
 
FPI+: Focal Plane Imager
A focal plane CCD imager
 
HAWC+: High-resolution Airborne Wideband Camera + Polarimeter
A far-IR camera and polarimeter (FSI)
 
Two instruments are Principal Investigator-class Science Instruments (PSIs), which will be maintained and operated by the Instrument Principal Investigator (PI) teams.
 
EXES: Echelon-Cross-Echelle Spectrograph
A mid-infrared high-resolution spectrograph (PSI)
 
GREAT: German Receiver for Astronomy at Terahertz Frequencies
A heterodyne spectrometer, including the seven-beam receiver array upGREAT (PSI)
 
One instrument is a Special Purpose, Principal Investigator-class Science Instrument (SSI).
 
HIPO: High-speed Imaging Photometer for Occultations
A high-speed optical photometer
 
Figure 1.1-1.

SOFIA instruments resolving power

Figure 1.1-1. The resolving power ranges of the SOFIA instrument suite.

The instrument capabilities, the available modes, and the resulting performance specifications of the telescope are described in later sections. For the purpose of this document, configuration refers to the setup of the telescope and instrument whereas mode refers to observational techniques employed during operations. Common combinations of configurations and modes are represented as selectable options within the Unified SOFIA Proposal and observation Tool (USPOT) via individual Astronomical Observation Templates (AOTs). Please note that this naming convention may not necessarily be employed uniformly in external resources for the instruments, i.e. in websites or documentation not managed by the SOFIA team.

Most of the observing time on SOFIA is open to the international astronomical community via Guest Observer (GO) proposal calls, which are issued on a yearly basis. The first of these proposal calls was for Early Science, for which observations were obtained in a series of flights from May–July, September, and November 2011. The first open call for proposals, Cycle 1, covered the period from late 2012 to the end of 2013. The current proposal call is for Cycle 6 observations, which will take place between 1 February, 2018 and 31 January, 2019. Proposals are being solicited for the Cycle 6 SOFIA flights by USRA on behalf of NASA. 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. A single Southern Hemisphere observing series with up to two instruments is under consideration for the Cycle 6 time period, nominally in the months of July and August, during the southern winter.

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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1.3 Performing Background Limited Observations

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1.3   Performing Background Limited Observations

Because the sky is so bright in the infrared (IR) relative to astronomical sources, the way in which observations are made in the IR is considerably different from the (more familiar) way they are made in the optical. Any raw image or spectrum of a region in the IR is overwhelmed by this sky background emission. The situation is similar to trying to observe in the optical during the day. The bright daylight sky swamps the detector and makes it impossible to see astronomical sources in the raw images.

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, can make the atmosphere 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 EXES, FIFI-LS, and GREAT, it is necessary to examine the atmospheric transmission at the wavelengths of interest in detail. This may be done using the more general web interface to the ATRAN program that was developed and provided to the SOFIA program by Steve Lord, or through the more instrument specific SOFIA Instrument Time Calculator (SITE). A plot of the atmospheric transmission seen by SOFIA in comparison to that achieved at Mauna Kea is shown in Figure 1.3-1 below.

Figure 1.3-1.

SOFIA atmospheric transmission plot

Figure 1.3-1. 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.

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 EXES, FIFI-LS, and GREAT 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 6.

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1.3.1   Chopping and Nodding

In order to remove the background from the IR image and detect the faint astronomical sources, observations of another region (free of sources) are made and the two images are subtracted. However, the IR is highly variable, both spatially and—more importantly—temporally. It would take far too long (on the order of seconds) to reposition a large telescope to observe this sky background region: by the time the telescope had moved and settled at the new location, the sky background level would have changed so much that the subtraction of the two images would be useless. In order to avoid this problem, the secondary mirror (which is considerably smaller than the primary mirror) of the telescope is tilted, rather than moving the entire telescope. This allows observers to look at two different sky positions very quickly (on the order of a few to 10 times per second) by tilting the secondary. Tilting the secondary between two positions is known as chopping.

Unfortunately, moving the secondary mirror causes the telescope to be slightly misaligned, which introduces optical distortions in the images—notably, the optical aberration known as coma and additional background emission from the telescope that is considerably smaller than the sky emission but present nonetheless. The additional telescopic background mentioned can be removed by moving the entire telescope to a new position and then chopping the secondary again between two positions. (Subtracting the two chop images at this new telescope position will remove the sky emission but leave the additional telescopic background due to the misalignment; subtracting the result from the chop‐subtracted image at the first telescope position will then remove the background.)

Since the process of moving to a new position is needed to remove the additional background from the telescope, not the sky, it can be done on a much longer timescale. (The variation in the telescopic backgrounds occurs on timescales on the order of tens of sec to minutes, much slower than that the variation in the sky emission.) This movement of the entire telescope, on a much longer timescale than chopping, is known as nodding. The two nod positions are usually referred to as nod A and nod B. The distance between the two nod positions is known as the nod throw.

The chop-­subtracted images at nod position B are then subtracted from the chop-­subtracted images at nod position A. The result will be an image of the region, without the sky background emission or the additional emission resulting from tilting the secondary during the chopping process. The sequence of chopping in one telescope position, nodding, and chopping again in a second position is known as a chop/nod cycle.

Again, because the IR sky is so bright, deep images of a region cannot be obtained (as they are in the optical) by simply observing the region for a long time with the detector collecting photons (integrating) continuously. As stated above, the observations require chopping and nodding at fairly frequent intervals. Hence deep observations are made by effectively stacking a series of chop/nod images. Furthermore, IR detectors are not perfect, and often have bad pixels or flaws. In order to avoid these defects on the arrays, and prevent them from marring the final images, observers employ a technique known as dithering. Dithering entails moving the position of the telescope slightly with respect to the center of the region observed each time a new chop/nod cycle is begun, or after several chop/nod cycles. When the images are processed, the observed region will appear in a slightly different place on the detector. This means that the bad pixels do not appear in the same place relative to the observed region. The individual images can then be registered and averaged or medianed, a process that will eliminate (in theory) the bad pixels from the final image.

Many of the instruments onboard SOFIA impliment chopping and/or nodding techniques in order to minimize the contribution of background noise in observations; Table 1.3-1 provides the nomenclature between some of the SOFIA instruments with similar chopping and nodding techniques. Depending on the instrument and the required exposure time and resolution for the object being observed, other methods of optimization may be more beneficial to the observation (Section 1.3.2).

Table 1.3-1.

Inter-Instrumentation Mode Translations
Technique EXES FIFI-LS FLITECAM FORCAST GREAT3 HAWC+
Symmetric Chopping1   Symmetric Chop   NMC Beam Switching NMC
      NPC    
Asymmetric Chopping1   Bright Object        
  Asymmetric Chop   C2NC2    
      NXCAC    
Nodding Only Nod On Slit   Nod Along Slit   Total Power  
Nod Off Slit   Nod Off Slit    
Dithering Only Map   Stare Slitscan    
Continuous2           OTFMAP
1Modes listed under Chopping may or may not also nod and/or dither.
2Modes listed under Continuous are non-discrete methods of observation for eliminating background noise and are discussed in Section 1.3.2.
3GREAT impliments chopping and nodding techniques, but is also limited by the temperature of the receiver. See Section 7.1.2.1 for calculations.
 

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1.3.1.1   Symmetric and Asymmetric Chopping Techniques

Chopping can be done either symmetrically or asymmetrically. The distance between the two chop positions is known as the chop throw.

Symmetric chopping means that the secondary mirror is tilted symmetrically about the telescope optical axis (also known as the boresight) in the two chop positions. Variations of symmetric chopping techniques include the general C2 and C2N techniques, with variations known as NMC (Nod Match Chop) and NPC (Nod Perp Chop). Symmetric chopping modes use the standard ABBA nod cadence, as described in Section 1.3.1.1.

Asymmetric chopping means that the secondary is aligned with the telescope boresight in one position, but is tilted away from the boresight in the chop position. A variation of the basic asymmetric chop mode is C2NC2. Asymmetric chopping modes use an ABA nod cadence, as described in Section 1.3.1.1b

Symmetric Chop and Asymmetric chop diagram

Figure 1.3-2.

 

 

 

AB and ABBA nod cadence

Figure 1.3-3. A simple AB nod cadence involves (1) an exposure at position A, (2) a move to position B, (3) an exposure at position B, and (4) a move back to position A. The ABBA nod cadence: (1) an exposure at position A, (2) a move to position B, (3) two exposures at position B, (4) move back to position A, and (5) a final exposure at position A.

 

1.3.1.1a   Symmetric Chopping Variations

The shared characteristic of symmetric chopping methods is the symmetric chop about the optical axis of the telescope. NMC and NPC differ in their nodding techniques. Note from Table 1.3-1 that NMC is referred to as Asymmetric Chop for FIFI-LS, Beam Switching for GREAT, and NMC for both HAWC+ and FORCAST.

In NMC, the telescope is pointed at a position half of the chop throw distance away from the object to be observed, and the secondary chops between two positions, one of which is centered on the object. The nod throw has the same magnitude as the chop throw (hence the name Nod_Match_Chop), and is in a direction exactly 180 degrees from that of the chop direction i.e. anti-parallel, such that when the telescope nods, the source is located in the opposite chop position. This mode uses the standard ABBA nod cadence. The final image generated by subtracting the images obtained for the two chop positions at nod A and those at nod B and then subtracting the results will produce three images of the star, one positive and two negative, with the positive being twice as bright as the negatives. 

In the case of NPC, the nod is perpendicular to the chop. The telescope is offset by half the nod throw from the target in a direction perpendicular to the chop direction, and the secondary chops between two positions. The nod throw usually (but not necessarily) has the same magnitude as the chop but is in a direction perpendicular to the chop direction. This mode also uses the standard ABBA nod cadence. The final image is generated by subtracting the images obtained for the two chop positions at nod A and those at nod B and then subtracting the results; it will therefore have four images of the star in a rectangular pattern, with the image values alternating positive and negative. Unlike NMC, each beam in NPC has the same relative intensity.

Nod match chop and nod perpendicular to chop diagram

Figure 1.3-4. NMC vs NPC

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1.3.1.1b   Asymmetric Chopping Variations

In C2NC2 (known as Asymmetric Chop in FIFI-LS observations), the telescope is first pointed at the target (position A). In this first position, the secondary is aligned with the optical axis (or boresight) for one observation and then is tilted some amount (often 180–480 arcseconds) for the second (asymmetrically chopped) observation. This is an asymmetric C2 mode observation. The telescope is then slewed some (usually large) amount away from the target to a sky region without sources (position B), and the asymmetric chop pattern is repeated. C2NC2 observations are taken as a series of 8 (C2) files in the sequence A B A A B A A B, i.e. an ABA nod cadence with dithering to remove correlated noise. Again, the time between slews is typically 30 sec. This mode is particularly useful for large extended objects, smaller objects that are situated within crowded fields, or regions of diffuse emission with only limited sky positions suitable for background removal.

asymmetric chop with offset nod diagram

Figure 1.3-5.

Figure 1.3-6 demonstrates how a C2NC2 observation might be designed for a large, extended object. It is immediately apparent from the figure, that C2NC2 has an efficiency of only ~20%. This is a much lower efficiency than either symmetric chopping variations since only a single chop position out of a full chop/nod cycle is on source. This should be taken into consideration when designing science proposals.

diagram of a sample C2NC2 observation design for a large, extended object

Figure 1.3-6. A sample C2NC2 observation design for a large, extended object. Each source position (solid line) and its associated asymmetric chop position (dashed line) have matching colors. Each source position has an independent chop configuration, the parameters for which are given in the dashed line boxes (chop angle in degrees and chop throw in arcseconds). After a full chop cycle at each position, the telescope is slewed to a location off of the source, shown in black and labeled with the coordinates (600, 600). The chop throw and angle at that position is the same as it is for the source position to which it is referenced (not shown in the figure).

The Asymmetric Chop mode of FIFI-LS typically impliments an ABBA nod sequence, but is otherwise equivalent to C2NC2. NXCAC (Nod not related to Chop with Asymmetric Chop) mode is the FORCAST grism version of C2NC2.

Bright Object mode for FIFI-LS is also an asymmetric chopping mode, using observations of two map positions and one off-position per nod-cycle through an asymmetric chop. This technique is utilized to improve the efficiency of mapping bright objects, where the total observing time is dominated by telescope movements.

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1.3.2   Alternatives to Chop-Nod Cycles

As chopping and nodding require a considerable amount of an observation's awarded time spent off-source, the amount of exposure time observing on the target is limited and subject to large observational overheads waiting for the telescope and/or secondary mirror assembly to complete chop/nod/dither movements. An alternative is to impliment continuous scanning techniques. Of the instruments currently offered on SOFIA, only GREAT, HAWC+, and FPI+ offer continuous scanning methods of observation. 

Nodding Only Modes 

GREAT 
Total Power - The telescope alternates between the target and a nearby reference position that is free of emission, using ON–OFF source cycles (typically spending ≤ 30 seconds on source). This mode is used when observing an extended source or a crowded region.
 
EXES
Nod On Slit - The telescope is nodded along the slit at distances that keep the target within the slit length. The nod throw is half the slit length. This is the most efficient EXES mode for point sources.
 
Nod Off Slit - The telescope is nodded such as that the object is not on the slit. This is used for extended sources or when the PSF is larger than four times the slit length.
 
Dithering Only Modes 
 
EXES
Map - The telescope is moved perpendicular to the slit while EXES takes spectra on a grid of telescope positions, which are always one dimensional stripes. 
 
FLITECAM
Stare - STARE mode observations involve a single telescope pointing centered on the source with dither pattern relative to it to facilitate background subtraction and image calibration. Dithering can be performed by selecting from pre-programmed dither patterns, or by defining a custom pattern. Observational experience has shown that STARE observations without dithers are particularly challenging with FLITECAM due to thermal background issues. Therefore, we recommend using dithers with at least 9 points, particularly longward of 2.2 μm.
 
FORCAST
Slitscan - In Slitscan mode, the slit is moved across a target in discrete steps using dithers perpendicular to the slit axis to yield a spectroscopic map of an entire area of sky.
 
Continuous Scans
 
HAWC+
On The Fly Mapping (OTFMAP) - In this case, scan rates must reach (~2 Hz) x (HAWC+ beam width) in order to remove the source from the atmospheric background—implying rates of ~10–80 arcseconds per second depending on the bandpass. HAWC+ offers two scan types for OTFMAP scan patterns: Lissajous and Box. In Lissajous observations, the telescope follows a parametric curvature at a non-repeating period to eventually cover the scan amplitude. In contrast, Box scans drive the telescope in a linear fashion at a specified rate in one direction for a given length and then moved perpendicularly before scanning in the reverse direction—similar to how one would mow a very large lawn. Lissajous scans are recommended for soucres smaller than the HAWC+ FOV at a given bandpass, while Box scans are more efficient at mapping large areas several times the FOV. 
 
Note: GREAT also has an On the Fly Astronomical Observation Template (AOT), but is not included in this section because GREAT observations are performed in either Total Power or Beam Switching modes.
 
Other Scans
 
FIFI-LS
Spectral Scan - This mode is implimented to target spectral features much wider than the bandwidth like solid state features. The problem is a good atmospheric calibration over the whole observed wavelength range. The spectrum has to pieced together from many different exposures. The best way to take such data and how to reduce it is still being investigated. If this observing mode is considered, please contact the instrument scientist via the SOFIA Help-Desk.