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7.2.1 Chopping and Nodding in Imaging Mode
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FORCAST can use three different imaging observation modes including: (1) symmetric nod-match-chop (NMC), (2) symmetric nod-perp-chop (NPC), and (3) asymmetric chop-offset-nod (C2NC2). NMC and NPC are both variations of the standard two-position chop with nod (C2N) IR observing mode. NMC is the default C2N observing mode. A brief description of each observing mode is provided below, but users are strongly encouraged to read the more complete description provided in the FORCAST Observing Modes document.
- NMC: In NMC mode, the chop is 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 7-7.
- NPC: NPC 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 7-7 shows data obtained using NPC.
- C2NC2: The third supported observing mode is asymmetric chop with offset nod (C2NC2). 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. In this 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.
Figure 7-8 is a cartoon demonstrating how a C2NC2 observation might be designed for a large, extended object. 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). It is immediately apparent from the figure, that C2NC2 has an efficiency of only ~20%. This is a much lower efficiency than either NMC or NPC 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.
Once a proposal has been accepted, the GI, in collaboration with the SMO instrument scientist, will specify the details of chopping and nodding for each observation using the SOFIA observation preparation tool (SSPOT). Experienced GIs are encouraged to design their observations using SSPOT 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 that GIs should consider when preparing a proposal.
- 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 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.
- 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′ ) 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.
- The chop throw is defined as the distance between the two chop positions in either symmetric or asymmetric chopping modes. When using a symmetric chop, e.g., in C2N mode, chopping and nodding can be performed in any direction for chop throws less than 584′′ .
- When observing in one of the symmetric chop modes, 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′′ per 1′ of chop amplitude.
- When using an asymmetric chop, the maximum possible chop throw is 420''. However, some chop angles (as measured in the instrument reference frame) are not allowed for asymmetric chop throws between 250'' and 420''. Since the orientation of the instrument relative to the sky will not be known until the flight plan is generated, GIs requesting chop throws between 250 - 420'' are required to specify a range of possible chop angles from which the instrument scientists can choose when the flight plan is finalized.
- For large, extended objects, it may not be possible to obtain clean background positions due to these limitations on the chop throw.
There are some additional considerations for observations of 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 modes and up to 120 sec in the LWC-only modes (depending on the filter). Proposers should 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 C2N 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-subtracted, nod-subtracted pair, with an exposure time of 30 sec in each nod position, dithering should NOT be used." --> "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.