2.2 Planning Observations


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2.2   Planning Observations

EXES configuration and mode flow chart diagram

All EXES configurations and modes are released for observations. The observing modes for EXES are summarized as follows:

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.
The telescope is moved perpendicular to the slit while EXES takes spectra on a grid of telescope positions, which are always one dimensional stripes. 
Observers are always encouraged to contact the instrument team for the latest performance results—especially for the Low configuration, which suffers from saturation effects. There are potential work-arounds that reduce the instrument sensitivity and may require extra overheads to prepare for and recover from low resolution observations. Proposers interested in low resolution observations should contact the instrument team ready to discuss their goals and hear about possible options.

The proposer needs to supply the central wavelength, the spectroscopic configuration, the slit width, and the observing mode for each observation (Table 2-1). 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.

Figure 2-2.
Minimum detectable point source flux

The calculation may be based on Figure 2-1 or 2-2 for point sources and on Figure 2-3 and 2-4 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.

Figure 2-3.
Minimum detectable extended source flux

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 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 above. 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 2-1). Note that the narrowest slit (1.4 arcsec) 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 arcsec slit can only be used below 16 μm, and the 2.4 arcsec slit below 21 μm.

Figure 2-4.

In configurations using the medium resolution grating (Medium and HIGH_MED), the single setting spectral coverage ranges between 0.03 μm at the shortest wavelengths, and 0.3 μm at longer wavelengths (Figure 2-5). 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_MED 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 2-6.

Figure 2-5.

plot showing the single setting spectral coverage as a function of wavelength

Figure 2-5. The single setting spectral coverage as a function of wavelength. Note that these values are the same for the HIGH_MED 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.

Figure 2-6.

comparison of spectra in HIGH_MED configuration and HIGH_LOW configuration

Figure 2-6. 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_MED, 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. Rather, only nodding and mapping modes will be implimented.

In a nod mode (either Nod On Slit or Nod Off Slit), 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 (Nod On Slit mode). For sources larger than about a quarter of the slit length, the telescope is moved such that the object is not on the slit (Nod Off Slit mode). 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 (Figures 2-1 and 2-2) 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 2-3 and 2-4 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.

In Map mode, the telescope is moved sequentially such that a series of positions along straight lines on the sky (i.e. along stripes) 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 1-3 and 1-4, 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 two-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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2.2.1   Wavelength Calibration

Wavelength calibration with EXES will be performed by applying the grating equation to atmospheric lines observed in the source spectra. As long as there is a single telluric feature in the bandpass with depth of at least 5%, the wavelength calibration is expected to be accurate to ≅ 0.5 km s-1.

If atmospheric models show no telluric features within the EXES instantaneous band pass for a given observation, then obtaining a good wavelength solution will require a few additional steps. Note that the absence of telluric features from SOFIA suggests the observation may be better done from the ground. First, the order sorting filter (OSF) is rotated so that a different order from the echelle is observed that includes a suitable telluric line. The grating equation can then be applied, providing wavelength calibration accuracy down to ≈ 1 km s-1. The process of rotating the OSF, observing blank sky, and rotating the OSF back to the original orientation should take less than 5 minutes of additional time. A demonstration of this procedure can be found in Harper et al. (2009, ApJ, 701, 1464).

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2.2.2   Flux Calibration and Atmospheric Line Correction

For every EXES science observation, the EXES temperature-controlled black body source and a nearby blank sky field will be observed. From these, a calibration spectrum will be constructed that will correct for response variations and provide flux calibration after division over the science observations. In principle, division by the calibration spectrum would also correct for telluric absorption lines (see Lacy et al., PASP volume 114, issue 792, p. 153), but this is presently not the case because of the large difference between the blackbody and sky temperatures. The flux calibration is expected to be better than 20% but the true accuracy is currently uncertain. Experiments focused on line profile information and those that can normalize the continuum level (or use past observations for setting the continuum) will likely be more successful. Projects requesting a telluric calibration object, in particular those observing lines near strong telluric features or those observing relatively broad lines, will need to include the observation time required in their proposal. Because of the difficulty of scheduling a given telluric calibrator with the science target in a given flight, the specific calibrator will need to be chosen at the time of flight planning in consultation between the program proposer, the instrument PI, and the SMO support scientist. For wavelengths below 8–10 μm this will most likely be a hot, bright star (e.g., Vega or Sirius) and at longer wavelengths an asteroid. Galilean moons will also be considered, provided they are well separated from Jupiter. For the proposal, a separate observation entry should be entered via USPOT with name Cal_target where target is the name of the associate science target (e.g., IRC+10216 and Cal_IRC+10216) and given the coordinates RA:00:00:00, Dec:+00:00:00. The observing time for such a telluric standard observation will depend on the instrument configuration and wavelength observed as well as on the signal-to-noise level needed. Proposers must use the EXES ETC to estimate this, assuming a continuum brightness of 100 Jy below 10 μm and 150 Jy above 10 μm for the HIGH_MED and HIGH_LOW configurations. For the Medium and Low configurations the proposer should assume at all wavelengths a brightness of 50 Jy and 25 Jy, respectively. Proposers are urged to limit the EXES clock times on the telluric standard at a given wavelength and instrument configuration to less than about 30 minutes. Further improvement of the removal of telluric absorption features may be achieved by employing models of the Earth's atmospheric transmission.

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2.2.3   Overheads

The treatment of overheads for EXES differs from that of most other SOFIA instruments: instead of on-source times, users are required to specify wall clock times (i.e. total elapsed time) in USPOT, which is the on-source time plus all overheads except those related to acquisition and instrument set-up. The overheads include time on empty sky in the off-slit nodding and mapping modes as well as read-out and other telescope and instrument inefficiencies. The ETC and the figures in this manual give the clock times needed. The figures in this manual only give clock times. USPOT will add an additional 15 minutes for peak-up, wavelength optimization, flux calibration, and flat field overheads in the HIGH_MED, Medium, and Low configurations and 20 minutes in HIGH_LOW. In all configurations, an extra 3 minutes of peak-up time is needed for the narrow (1.4 arcsec) slit. If no peak-up is necessary (i.e., after a wavelength change on the same target, if the source is extended, or if the continuum emission is too weak), the overheads can be reduced by using the No Peak-Up option in USPOT. Overheads can also be reduced if multiple sky positions are observed in the same wavelength setting. In this case, click the No Wavelength Setup button in USPOT. Note that the time on a given target on a single flight is limited to 90–180 minutes, so full overheads may be needed again once the sum of AOR times exceeds 90 minutes. Conversely, if a single observation takes more than 90 minutes it may need to be split into multiple AORs, each with full overheads. Please consult the EXES and SOFIA staff in these cases.

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2.2.4   As a Principle Investigator Instrument

EXES is a Principal Investigator (PI) class instrument. The proposer is encouraged (but not required) to contact the Instrument PI before preparing or submitting the proposal, since the PI has the most up to date information about the instrument capabilities. However, it is recommended that they do so, in order to get the most up to date information about instrument capabilities. The data collection and reduction will be done by the instrument team, and it is expected that data analysis and preparation of the results for publication will be done by the proposers in collaboration with the instrument team.

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