4.2 Planning EXES Observations

EXES commissioning observations were performed in April 2014 and February/March 2015. The analysis of these data is ongoing. The Low configuration will be offered in shared-risk mode.

The proposer needs to supply the central wavelength, the spectroscopic configuration, the slit width, and the observing mode for each observation (Table 4). These parameters define the default instrument set-up. Each central wavelength specified should count as a separate observation. In addition, the proposer should estimate the clock time necessary to reach the desired S/N.

The minimum detectable point source flux as a function of wavelength for the Medium (top) and Low (bottom) configuration

Figure 4-2: The minimum detectable point source flux as a function of wavelength for the Medium (top) and Low (bottom) configuration, assuming the conditions mentioned at the end of Section 4.1.4. The vertical dotted lines show the boundaries between the slit widths used (1.4", 1.9", 2.4", 3.2").

The calculation may be based on Figure 4-1 or 4-2 for point sources and on Figure 4-3 and 4-4 for extended sources, noting that the minimum detectable flux ∝ (S/N) / √(texp).However, it is recommended that the online ETC at http://irastro.physics.ucdavis.edu/exes/etc/ is checked as well for the latest updates.

The minimum detectable extended source flux as a function of wavelength for the High_Medium and High_Low configurations (using off-slit nodding).

Figure 4-3: The minimum detectable extended source flux as a function of wavelength for the High_Medium and High_Low configurations (using off-slit nodding).

EXES operates in a wavelength region, parts of which are accessible from ground based telescopes. Proposers should carefully check the atmospheric transmission (using ATRAN, for example) and make sure that the observations require, or would greatly benefit from, using SOFIA. The proposer should take into account the Doppler shift of the target(s) due to their motion relative to the Local Standard of Rest, and to the Earth. If proposers find that the atmospheric transmission at the wavelength of interest is lower than the local median (calculated over a range ± 0.0125 μm), then more time will be required to reach the desired S/N. Higher transmission would imply shorter required times. In general the, S/N scales as transmission/√((1 - transmission) + 0.3). Note that the online ETC includes the impact of the atmosphere at precise wavelength of interest and so may differ from the Figures. The ETC provides the clock-time required to achieve the desired S/N per resolution element on a continuum object at the specific wavelength of interest and then indicates what the expected S/N should be for the entire setting.

Proposers should specify the slit width, which sets the resolving power for each configuration (Table 4). Note that the narrowest slit (1.4") is only effective below 12 μm (above this wavelength no gain in resolving power is achieved, while flux is lost with respect to the wider slits). Similarly, the 1.9" slit can only be used below 16 μm, and the 2.4" slit below 21 μm.

The minimum detectable extended source flux as a function of wavelength for Medium (top) and Low (bottom) configurations

Figure 4-4: The minimum detectable extended source flux as a function of wavelength for Medium (top) and Low (bottom) configurations, assuming the conditions mentioned at the end of Section 4.1.4. The vertical dotted lines show the boundaries between the slit widths used (1.4", 1.9", 2.4", 3.2").

In configurations using the medium resolution grating (Medium and High_Medium), the single setting spectral coverage ranges between 0.03 μm at the shortest wavelengths, and 0.3 μm at longer wavelengths (Fig. 4-5). For the low resolution grating (the Low and High_Low configurations) this is 0.2-0.8 μm. Note that while High_Low and High_Medium have the same spectral resolution, the larger wavelength coverage of High_Low comes at the expense of a smaller slit length, which is illustrated in Figure 4-6.

The single setting spectral coverage as a function of wavelength

Figure 4-5: The single setting spectral coverage as a function of wavelength. Note that these values are the same for the High_Medium and Medium configurations, and for the High_Low and Low configurations.

Proposers should choose a single line of interest for each observation. Fine tuning of the bandpass to observe lines at the extreme edges of a single setting should be done in consultation with the EXES team to see if existing data indicates such tuning is possible.

The slit orientation on the sky depends upon the time when the target is observed, and therefore the position angle cannot be specified.

Comparison of raw 2D spectra of EXES in the High_Med (left) and High_Low (right) configurations obtained during commissioning flights.

Figure 4-6: Comparison of raw 2D spectra of EXES in the High_Med (left) and High_Low (right) configurations obtained during commissioning flights. The spectra are not nod-subtracted, highlighting the sky emission lines (dark). This comparison shows that, while the High_Med and High_Low configurations have the same spectral resolution, High_Low has a much larger spectral coverage at the expensive of a shorter slit to be able to pack more orders on the array. In contrast, the Low and Medium configurations (not shown) have the same spectral coverage as High_Low and High_Medium, respectively, but a lower spectral resolution without cross dispersion. The approximate start and end wavelengths are indicated in red.

EXES will not use the secondary for chopping in any of its observations. There will be two scientific modes - Nod and Map mode.

Nod mode: In this mode, the telescope is moved to a new position in order to remove the sky background. For point sources observed with a sufficiently long slit, the telescope is moved such that the object remains on the slit. For sources larger than about a quarter of the slit length, the telescope is moved such that the object is not on the slit. The time between telescope motions will depend on the sky variation, the telescope settling time, and the integration time. The goal is to maximize the signal-to-noise per clock time. For observations of point sources, the detectable flux plots (Figs. 4-1, 4-2) and the ETC include assumptions regarding whether nodding off the slit is required due to short slit lengths. For nodded observations of extended objects, proposers should contact the EXES team to check if nodding on the slit is possible. If not, the observing time required should be doubled. Unless specific nod parameters are requested for such observations, the instrument team will define the nod amplitude, direction and frequency. The sensitivities for extended source observations shown in Figures 4-3 and 4-4 assume that the source is nodded off-slit and take into account the variation in spatial resolution with wavelength. The atmospheric and overhead factors for nodding are included. If the source is small relative to the slit length, then it may be possible to nod along the slit. In this case, the source brightness given in the figures is for a SNR of 10 in 450 seconds.

Map mode: In this mode, the telescope is moved sequentially such that a series of positions along a straight line on the sky (a "stripe") are observed to create a map. The sky background is taken from the first positions and, depending on the size of the map, from the last positions. In general, we anticipate the telescope motions to be half the slit width to create a well-sampled map.

Proposers should specify the number of steps in a map and the step size. Map steps are generally assumed to be perpendicular to the slit. The first three positions for taking data in a map must be blank sky. These could be the first three positions of the map or at a separate sky offset position specified by the user. It is recommended that additional blank sky positions are observed at the end of the map on the other side of the object. For all maps, the instrument software returns to the sky offset position for three final sky observations at the end of the observation.

Proposers should specify the required clock time based on the flux limit desired, using the values in Figures 4-3 and 4-4, including any assumptions regarding binning of map positions to yield the final required SNR. The SNR for a single map position can be estimated by assuming that the required time is similar to that for nodding an extended object on slit, i.e. 10σ in 450 seconds for a given source brightness. If any spatial binning is required -- at least a 2-step sum is recommended -- then the SNR will improve by the square root of the number of steps in the sum. The online ETC allows the user to specify the number of steps and bins according to the predicted image quality in producing a clock-time estimate.

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