5.3 Observing Modes

The high sky background in the far infrared requires careful subtraction. That is achieved by chopping with SOFIA's secondary mirror and by nodding the telescope. The secondary will chop at 2Hz to efficiently remove the sky emission. To remove residual background not canceled by chopping, the telescope is nodded typically every 30s either to move the source to the other chop-beam or to an off-position. Since the instrument telescope communications and the telescope move take 10 s, a whole nod-cycle takes typically 80 s."

The following sections describe the possible observing modes. In the discussion of the overheads, $N$ is the number of map positions and ton is the on-source exposure time per map position. The main driver to choose the observing mode is to figure out possible chop configuration. However, the details like the exact chop throw and angle and other observing details do not need to be fixed until phase II of the proposal process. Information on how all the parameters for each mode has to be entered into SSPOT during phase II of the proposal process can be found on the Cycle 4, Phase II web page.

5.3.1 Observing Mode: Symmetric Chop

If possible this observing mode should be used, because the most efficient mode. This mode combines chopping symmetrically to the telescope's optical axis with a matched telescope nod to remove the residual telescope background. This mode is also known as nod-match-chop (NMC) mode (cf. FORCAST Sect. 7.3.1) or beam switching (BSW, cf. GREAT Sect. 9.1.1).

When observing using a symmetric chop, large chop amplitudes degrade the image quality due to the introduction of coma. This effect causes asymmetric smearing of the PSF in the direction of the chop. However, the effect is small (effect on SNR less than 10%) in the red channel for all chop throws and in the blue channel for total chop throws less then 5' and wavelengths longer than 63 μm.> For wavelengths shorter than 63 μm, we recommend total chop throws of less than 4'. Generally, it is recommended to use a chop as small as possible, but keep the FOV in the off-positions outside of any detectable emission.

The position angle of the chop can be specified relative to equatorial coordinates or telescope coordinates (e.g. horizontal). Keep in mind that the telescope nod matched to the chop creates two off-positions symmetric to the on-position (Figure 5-5, Left).

The total overhead in this mode is about 1.7N ton + 300 s, since the source is only observed during 50% of the observation and additional time is required for telescope moves, plus 300 s for the setup. This overhead estimate assumes that the on-source exposure time per map position ton is at least 30 s. If the on-source exposure time per map position ton is less than 10 s, the Bright Object Mode should be used. For values of ton in between, one needs to enter an alternate overhead in SPT. The total alternate overhead is N(ton + 20 s) + 300 s.

The geometry of chopping and nodding in the Symmetric Chop mode (left) and the Asymmetric mode (right)

Figure 5-5: The geometry of chopping and nodding in the Symmetric Chop mode (left) and the Asymmetric mode (right).

5.3.2 Observing Mode: Asymmetric Chop and Fast Asymmetric Chop

If the target's size or environment does not allow to use the Symmetric Chop Mode, one has to use the Asymmetric Chop Mode allowing larger chop throws at shorter wavelengths and is not creating symmetric off-positions around the source. The asymmetric chop keeps the on-beam on the optical axis. This results in an image unaffected by coma. Consequently, the off-beam is off-center by twice the amount compared to the symmetric chop with the same chop throw resulting in twice as much coma. But that is of no consequence as the off-beam should only see empty sky. The telescope is nodded to an off-position where the same chopped observation is executed to provide the residual background subtraction. Figure 5-4 illustrates this geometry. Note that this mode is similar to FORCAST's asymmetric chop-offset-nod (C2NC2) mode (see 5.3.1).

The total overhead in this mode is about 4.3N ton + 300 s, since the source is observed during 25% of the observation plus additional time for telescope moves and 300 s for the setup. This overhead estimate assumes that the on-source exposure time per map position ton is at least 15 s. For shorter values of ton, the Bright Object Mode should be used.

5.3.3 Observing Mode: Bright Object

For very bright objects, where the estimated on-source exposure time per map position is 10 s or less, the total observing time is dominated by telescope moves. The efficiency of mapping such bright objects can be improved by observing two map positions and one off-position per nod-cycle using an asymmetric chop. In this mode, the total overhead is 5 N ton + 300 s assuming an on-source exposure times per map position of about 5 s.

5.3.4 Observing Mode: Spectral Scan

This mode is offered on a shared risk basis. In contrast to the other observing modes, this experimental mode targets spectral features much wider than the bandwidth (see Sect. 5.2.1) 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. In Cycle 3, such an observation is being conducted. 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.

5.3.5 Spectral Dithering

Spectral dithering is always employed for self flat-fielding and increased redundancy. Spectral dithering implemented via a grating scan. The grating is moved in small steps, so that the spectrum moves over different pixels in the spectral dimension of the detector array.

The default pattern to cover the instantaneous bandwidth (BW, Sect. 5.2.1) is to move the grating 12 steps, each corresponding to half a spectral pixel width. This pattern results in a spectrum about 30% wider than the BW. The central 70% of the BW are observed during the whole observing time reaching the full SNR, while the remaining 15% on each side of the BW should reach on average 86% of the SNR. The SNR reached on the extra 30% should still be 46% on average based on the observing time for each part of the spectrum. For wider spectral coverage, the step size and number of steps of the grating scan will be adjusted by the instrument operators to achieve the desired spectral coverage. The steps will be evenly distributed over the nod-cycles.

5.3.6 Mapping

Mapping is supported by all of the three regular observing modes. It can be done on a rectangular grid with a user-defined spacing and extent. It is also possible to supply a list of mapping positions to achieve a map with a custom shape optimized to the source geometry. For both map types, a spacing of half a red array or 30'' might be a good choice, providing half pixel steps to achieve super-resolution with a good overlap for the red array and full coverage (but no overlap) for the blue array. Similarly, a spacing of 15'' yields super-resolution with a good overlap for the blue array and a very strong overlap for the red array. These details need to be specified only in Phase II of the proposal process (see the FIFI-LS Cycle 4, Phase II web page). In Phase I, the effective map area needs to be entered in SPT and the proposal should explain the suggested mapping strategy. The on-source integration time to be entered in SPT has to be the on-source integration time per raster point multiplied by the number of raster map points N ton.

If the source geometry allows the off-beam to be positioned symmetrically on both sides of the source, then one should use the much more efficient Symmetric Chop Mode for mapping. If that is not possible the Asymmetric Chop Mode has to be used. An asymmetric chop is also used in the bright object mode. Figure 5-6 illustrates mapping with an asymmetric chop. The off-beam (positions B1 to B3) covers an area while chopping that is the same size as the map itself. If this is undesirable, the map needs to be broken up into sub-maps with varying chop parameters to be specified in Phase II. The availability of guide stars might be another reason to break up a large map into sub-maps. In this case the sub-maps will be identified between Phase II and the actual observation by the support scientist in close collaboration with the guest investigator and the telescope operator.

When estimating the on-source integration time (Sect. 5.2.2), take into account the differing overlap of the red and blue FOV at the desired raster map spacing. The SNR entered into the calculation of ton is the SNR for a single raster map point. The final SNR for a point in the map should reach &radic(n) x SNR with n being the number of raster points from which a point is covered by the respective FOV. For example in Fig. 5-6, the area of the pixel in the middle is covered by 3 FOVs while 16 pixels are covered by 2 FOVs and the outer parts of the map are covered by 1 FOV.

The geometry of chopping and nodding while mapping using the asymmetric chop mode

Figure 5-6: The geometry of chopping and nodding while mapping using the asymmetric chop mode.