4.2 Planning Observations

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4.2   Planning Observations

FORCAST configuration and mode flowchart diagram

As is the case with ground based observations at mid-IR wavelengths, individual FORCAST exposures will be dominated by the sky and telescope background. Therefore chopping and nodding are essential for each observation. Selection of the observing mode and its parameters, including the distance and direction of chop and nod throws, depend on the details of the field of view around the target. The source(s) of interest may be surrounded by other IR-bright sources or may lie in a region of extended emission, which needs to be avoided to ensure proper background subtraction. Presented in this section is a discussion of how to best plan FORCAST observations in order to optimize the success of observations.

4.2.1   Imaging Observations

Proposers are strongly encouraged to familiarize themselves with the basics of techniques for performing background limited observations covered in Section 1.3. In brief, the imaging observation modes for FORCAST include the following:

NMC Mode
Nod Match Chop mode consists of a chop symmetric about the optical axis of the telescope with one of the two chop positions centered on the target. The nod throw is oriented 180° from the chop, i.e. anti-parallel, such that when the telescope nods, the source is located in the opposite chop position. The chop/nod subtraction results in two negative beams on either side of the positive beam, which is the sum of the source intensity in both nod positions and therefore has twice the intensity of either negative beam. This mode uses the standard ABBA nod cadence. An example of an observation taken in this mode is presented in the left panel of Figure 4-7.
 
NPC Mode
Similar to NMC, Nod Perpendicular to Chop mode also uses a chop that is symmetric about the optical axis, but in this case the nod is perpendicular to the chop. The final images produced using NPC show four sources arranged in a parallelogram with alternating positive and negative beams. Unlike NMC, each beam in NPC has the same relative intensity. This mode also uses the standard ABBA nod cadence. The right side of Figure 4-7 shows data obtained using NPC. This mode will not be supported in Cycle 8.
 
C2NC2 Mode
In Chop-Offset Nod mode, the chop throw is asymmetric, such that one chop position is centered on the optical axis (and the target) while the second (sky) position is off-axis. Rather than nodding, the telescope then slews to an offset position free of sources or significant background and the same chop pattern is repeated. Observations in C2NC2 mode follow a nod cadence of ABA and, by default, are dithered to remove correlated noise. 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.
 

Since only a single chop position out of a full chop/nod cycle is on source, NMC and NPC have a much greater efficiency than C2NC2. A sample mosaic demonstrating how a C2NC2 observation might be designed for a large, extended object is provided in Figure 4-8, and it is immediately apparent from the figure that C2NC2 has an efficiency of only ~20%. However, while mosaicking cna be performed for any of the available obseving modes, proposers should keep in mind that the effects of coma may compromise the image severely for fields requiring large chop amplitudes when chopping symmetrically (NMC or NPC modes). If the source has an angular extent large enough that multiple pointings are required, the central position of each FORCAST field must be specified, with due consideration of the desired overlap of the individual frames. For more on mosaicking, see Section 5.3 in the USPOT Manual.

Figure 4-7.

NMC and NPC mode illustration

Figure 4-7. Each source position (solid line) with its associated asymmetric chop position (dashed line) have matching colors. 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).

Figure 4-8.
Diagram of a sample mosaic demonstrating how a C2NC2 observation might be designed for a large extended object

Figure 4-8. Cartoon of a C2NC2 mode FORCAST observation with mosaic.

Once a proposal has been accepted, the proposer, in collaboration with the SMO instrument scientist, will specify the details of chopping and nodding for each observation using the SOFIA observation preparation tool (USPOT). Experienced proposers are encouraged to design their observations using USPOT before writing their proposals to prevent the loss of observing time that might occur if, during Phase II, the observations are discovered to be more challenging than expected.

Following are a few of the most important issues to consider when preparing a FORCAST Imaging proposal:

Check a Database
It is recommended that a near-IR or mid-IR database (e.g., 2MASS, Spitzer , WISE , MSX or IRAS) be checked to see if the target of interest is near other IR sources of emission. In the case of extended sources, where on-chip (i.e., on the detector array) chop and nod is not possible, it is necessary to pick areas free of IR emission for the chop and nod positions to get proper background subtracted images.

For Emissions Less than ~1.6 arcmin
If the IR emission from the region surrounding the source is restricted to a region smaller than half the FORCAST field of view (i.e. ∼1.6 arcmin), then the chop and nod can be done on-chip. Observations performed in NMC mode either on-chip or off-chip yield a S/N equal to or slightly better than that obtained in NPC mode. For additional discussion of this point, see the calculations of S/N for various FORCAST chop-nod scenarios provided here.

Chop Throw Constraints
When using a symmetric chop, chopping and nodding can be performed in any direction for chop throws less than 584 arcsec. When using an asymmetric chop, the maximum possible chop throw is 420 arcsec. However, some chop angles (as measured in the instrument reference frame) are not allowed for asymmetric chop throws between 250 arcsec and 420 arcsec. Since the orientation of the instrument relative to the sky will not be known until the flight plan is generated, Those requesting chop throws between 250–420 arcsec are required to specify a range of possible chop angles from which the instrument scientists can choose when the flight plan is finalized. 

Additionally, large chop amplitudes may degrade the image quality due to the introduction of coma. This effect causes asymmetric smearing of the PSF parallel to the direction of the chop at a level of 2 arcsec per 1 arcmin of chop amplitude.

For large, extended objects, it may not be possible to obtain clean background positions due to these limitations on the chop throw.

For Faint Targets
Currently, the longest nod dwell time (that is, the time spent in either the nod A or nod B position) for FORCAST is 30 sec in the SWC-only and dual channel configurations and up to 120 sec in the LWC-only configurations (depending on the filter). Run the exposure time estimator to determine if the object will be visible in a single A-B chop-subtracted, nod-subtracted pair, with an exposure time of 30 sec in each nod position. If the object is bright enough to be detectable with S/N greater than a few, it is recommended that dithering be used when observing in NMC or NPC mode. The dithering will mitigate the effects of bad pixels when the individual exposures are co-added.

If the object is not visible in a single A-B chop/nod-subtracted pair, with a nod dwell time of 30 sec in each nod position (60 sec integration), then dithering should not be used.

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4.2.1.1   Estimation of Exposure Times

The exposure times for FORCAST imaging observations should be estimated using the SOFIA Instrument Time Estimator (SITE). SITE can be used to calculate the signal-to-noise ratio (S/N) for a given total integration time, or to calculate the total integration time required to achieve a specified S/N. The total integration time used by SITE corresponds to the time actually spent integrating on-source without overheads. These integration times are used as input for USPOT, which will automatically calculate the necessary overheads. The format of the S/N values output by SITE depends on the source type. For Point Sources, the reported S/N is per resolution element, but for Extended Sources, it is the S/N per pixel.

For mosaic observations the total integration time required for a single field should be multiplied by the number of fields in the mosaic to obtain the total time, which is to be entered in USPOT.

An important consideration in planning observations is whether FORCAST should be used in single channel configuration, or in a dual channel configuration, since one gains the extra filter observation at the cost of lower system throughput in the individual bands. On the SITE form, a single channel configuration is specified by selecting the filter of interest for one channel and selecting None on the other channel in the Instrument properties section.

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4.2.2   Spectroscopic Observations

Proposers are strongly encouraged to familiarize themselves with the basics of techniques for performing background limited observations covered in Section 1.3. In brief, the spectroscopic observation modes for FORCAST include the following:

NMC Mode
As with FORCAST imaging observations, Nod Match Chop mode consists of a chop symmetric about the optical axis of the telescope with one of the two chop positions centered on the target. See Section 4.2.1 or Section 1.3.
 
NPC CAS and NPC NAS Modes
Grism Nod Perpendicular to Chop observations can be performed either in a Chop Along Slit mode (NPC CAS) or Nod Along Slit mode (NPC NAS). As with FORCAST imaging observations, FORCAST's grism NPC modes also impliment a chop that is symmetric about the optical axis–however, unlike in NMC mode, the nod is perpendicular to the chop. The final images produced using NPC CAS or NAS show two sources arranged along the slit with one positive and one negative beam. Unlike NMC, each beam in NPC has the same relative intensity.
 
NXCAC Mode
Nod not related to Chop with Asymmetric Chop mode is the grism version of C2NC2, i.e., an asymmetric chop with dithering along the slit. See Section 4.2.1 or Section 1.3.
 
Slitscan Mode
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.
 

During Cycle 8, grism spectroscopy with FORCAST will only be available in single channel, long-slit configurations (SWC and LWC). By default, observations will be set up using NMC aligned along the slit in Long Slit configurations and perpendicular to the slit in XD configuration. Due to the size of the PSF, neither chopping or nodding along the slit nor dithering are possible for high-resolution XD observations. For larger sources and for targets embedded in crowded fields it is advised to use C2NC2 mode.

The observing efficiency for FORCAST spectroscopic observations depends on a number of factors, including the observing mode, chop frequency and nod cadence, the detector frame rate, and LOS rewind cadence. The typical observing efficiency as measured from NMC and NPC observations is 50–75% of clock time. Work is ongoing to optimize the mode-dependent efficiency values. These efficiency estimates are built-in to USPOT and do not need to be specified.

It is important to note that due to the fixed position of the grisms/slits in the filter/aperture wheels, the orientation of the slit on the sky will be dependent on the flight plan and will not be able to be predetermined. Further, the slit orientation rotates on the sky with each telescope Line-of-Sight (LOS) rewind (Section 1.1). These limitations may be especially important to consider when proposing observations of extended objects.

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4.2.2.1   Estimation of Exposure Times

The exposure times for FORCAST Grism spectroscopic observations should be estimated using the SOFIA Instrument Time Estimator (SITE)SITE can be used to calculate the signal-to-noise ratio (S/N) for a given total integration time, to calculate the total integration time required to achieve a specified S/N, or to estimate the limiting flux for a desired S/N.

In either case, overheads should not be included, as USPOT calculates them independently.

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