1.3 Performing Background Limited Observations

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1.3   Performing Background Limited Observations

Because the sky is so bright in the infrared (IR) relative to astronomical sources, the way in which observations are made in the IR is considerably different from the (more familiar) way they are made in the optical. Any raw image or spectrum of a region in the IR is overwhelmed by this sky background emission. The situation is similar to trying to observe in the optical during the day. The bright daylight sky swamps the detector and makes it impossible to see astronomical sources in the raw images.

SOFIA operates at altitudes above 99% of the water vapor in the atmosphere. The average atmospheric transmission across the SOFIA bandpasses is about 80% at these altitudes. There are however a number of strong absorption features which, even at these altitudes, can make the atmosphere opaque. Broad band filters, such as those on FORCAST, account for the presence of such features. However, when using high-resolution tunable instruments such as EXES, FIFI-LS, and GREAT, it is necessary to examine the atmospheric transmission at the wavelengths of interest in detail. This may be done using the more general web interface to the ATRAN program that was developed and provided to the SOFIA program by Steve Lord, or through the more instrument specific SOFIA Instrument Time Calculator (SITE). A plot of the atmospheric transmission seen by SOFIA in comparison to that achieved at Mauna Kea is shown in Figure 1.3-1 below.

Figure 1.3-1.

Atmospheric transmission of SOFIA and Mauna Kea plot

Figure 1.3-1. This is a plot showing the atmospheric transmission for SOFIA (black) at an altitude of 41K feet and 7.3 μm of precipitable water vapor compared to Mauna Kea (red) at an altitude of 13.8K feet and 3.4 mm water vapor over the range of 1 ‒ 1000 μm. The transmission was calculated using the ATRAN code with a telescope zenith angle of 45°. and the data were smoothed to a resolution of R=2000.

In addition to its dependence on wavelength due to the presence of absorption features, the atmospheric transmission varies with latitude and with time of year, primarily due to differences in the amount of water vapor. It also exhibits variations on smaller time scales due to changes in the location of the tropopause. Full discussions of these issues may be found in Haas & Phister 1998 (PASP, 110, 339) and Horn & Becklin 2001 (PASP, 113, 997).

The variations in atmospheric water vapor could have a significant impact on some observations, particularly when using EXES, FIFI-LS, and GREAT or grism modes with FORCAST. For example, GREAT observations of a line situated on the shoulder of an atmospheric water feature could be strongly affected by water vapor variability. SITE allows the user to specify the water vapor overburden and adjusts the time estimates appropriately. The water vapor monitor has been installed and is currently undergoing testing, but may not be fully functional during Cycle 8.

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1.3.1   Chopping and Nodding

In order to remove the background from the IR image and detect the faint astronomical sources, observations of another region (free of sources) are made and the two images are subtracted. However, the IR is highly variable, both spatially and—more importantly—temporally. It would take far too long (on the order of seconds) to reposition a large telescope to observe this sky background region: by the time the telescope had moved and settled at the new location, the sky background level would have changed so much that the subtraction of the two images would be useless. In order to avoid this problem, the secondary mirror (which is considerably smaller than the primary mirror) of the telescope is tilted, rather than moving the entire telescope. This allows observers to look at two different sky positions very quickly (on the order of a few to 10 times per second) by tilting the secondary. Tilting the secondary between two positions is known as chopping.

Unfortunately, moving the secondary mirror causes the telescope to be slightly misaligned, which introduces optical distortions in the images—notably, the optical aberration known as coma and additional background emission from the telescope that is considerably smaller than the sky emission but present nonetheless. The additional telescopic background mentioned can be removed by moving the entire telescope to a new position and then chopping the secondary again between two positions. (Subtracting the two chop images at this new telescope position will remove the sky emission but leave the additional telescopic background due to the misalignment; subtracting the result from the chop‐subtracted image at the first telescope position will then remove the background.)

Since the process of moving to a new position is needed to remove the additional background from the telescope, not the sky, it can be done on a much longer timescale. (The variation in the telescopic backgrounds occurs on timescales on the order of tens of sec to minutes, much slower than that the variation in the sky emission.) This movement of the entire telescope, on a much longer timescale than chopping, is known as nodding. The two nod positions are usually referred to as nod A and nod B. The distance between the two nod positions is known as the nod throw.

The chop-­subtracted images at nod position B are then subtracted from the chop-­subtracted images at nod position A. The result will be an image of the region, without the sky background emission or the additional emission resulting from tilting the secondary during the chopping process. The sequence of chopping in one telescope position, nodding, and chopping again in a second position is known as a chop/nod cycle.

Again, because the IR sky is so bright, deep images of a region cannot be obtained (as they are in the optical) by simply observing the region for a long time with the detector collecting photons (integrating) continuously. As stated above, the observations require chopping and nodding at fairly frequent intervals. Hence deep observations are made by effectively stacking a series of chop/nod images. Furthermore, IR detectors are not perfect, and often have bad pixels or flaws. In order to avoid these defects on the arrays, and prevent them from marring the final images, observers employ a technique known as dithering. Dithering entails moving the position of the telescope slightly with respect to the center of the region observed each time a new chop/nod cycle is begun, or after several chop/nod cycles. When the images are processed, the observed region will appear in a slightly different place on the detector. This means that the bad pixels do not appear in the same place relative to the observed region. The individual images can then be registered and averaged or medianed, a process that will eliminate (in theory) the bad pixels from the final image.

Many of the instruments onboard SOFIA impliment chopping and/or nodding techniques in order to minimize the contribution of background noise in observations; Table 1.3-1 provides the nomenclature between some of the SOFIA instruments with similar chopping and nodding techniques. Depending on the instrument and the required exposure time and resolution for the object being observed, other methods of optimization may be more beneficial to the observation (Section 1.3.2).

Table 1.3-1: Inter-Instrumentation Mode Translations

Technique EXES FIFI-LS FORCAST GREAT3 HAWC+
Symmetric Chopping1   Symmetric Chop NMC Beam Switching NMC
    NPC    
Asymmetric Chopping1   Bright Object      
  Asymmetric Chop C2NC2    
    NXCAC    
Nodding Only Nod On Slit     Total Power  
Nod Off Slit      
Dithering Only Map   Slitscan    
Continuous2         OTFMAP
1Modes listed under Chopping may or may not also nod and/or dither.
2Modes listed under Continuous are non-discrete methods of observation for eliminating background noise and are discussed in Section 1.3.2.
3GREAT impliments chopping and nodding techniques, but is also limited by the temperature of the receiver. See Section 7.1.2.1 for calculations.
 

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1.3.1.1   Symmetric and Asymmetric Chopping Techniques

Chopping can be done either symmetrically or asymmetrically. The distance between the two chop positions is known as the chop throw.

Symmetric chopping implies that the secondary mirror is tilted symmetrically about the telescope optical axis (also known as the boresight) in the two chop positions. Variations of symmetric chopping techniques include the general C2 and C2N techniques, with variations known as NMC (Nod Match Chop) and NPC (Nod Perp Chop). Symmetric chopping modes use the standard ABBA nod cadence, as described in Section 1.3.1.1a below.

Asymmetric chopping means that the secondary is aligned with the telescope boresight in one position, but is tilted away from the boresight in the chop position. A variation of the basic asymmetric chop mode is C2NC2. Asymmetric chopping modes use an ABA nod cadence, as described in Section 1.3.1.1b

Symmetric Chop and Asymmetric Chop diagram

Figure 1.3-2.

 

 

 

diagram of FIFI-LS chop geometry

Figure 1.3-3. A simple AB nod cadence involves (1) an exposure at position A, (2) a move to position B, (3) an exposure at position B, and (4) a move back to position A. The ABBA nod cadence: (1) an exposure at position A, (2) a move to position B, (3) two exposures at position B, (4) move back to position A, and (5) a final exposure at position A.

 

1.3.1.1a   Symmetric Chopping Variations

The shared characteristic of symmetric chopping methods is the symmetric chop about the optical axis of the telescope. NMC and NPC differ in their nodding techniques. Note from Table 1.3-1 that NMC is referred to as Asymmetric Chop for FIFI-LS, Beam Switching for GREAT, and NMC for both HAWC+ and FORCAST.

The Nod Match Chop observing pattern is defined by the two alternating positions of the telescope: Nod A and Nod B.

The telescope begins with its boresight centered at a position, Nod A. The secondary mirror then alternates (chops) between a position on the target (position Chop A) and a position equidistant from and on the opposite side of Nod A (position Chop B). The distance between Chop A and Chop B is called the chop throw. The chop angle is the direction between Chop A and Chop B from east-of-north in sky coordinates (or counterclockwise from the top of the array in array coordinates).

The telescope then shifts to center its boresight on a new position (Nod B). The distance between Nod A and Nod B is called the nod throw, and it is equivalent to the chop throw—hence the name Nod Match Chop. The nod throw is always 180 degrees from the chop angle.

The process that occurs at position Nod B is similar to that which occurred at Nod A: the telescope’s secondary mirror alternates between a position centered on the target (Chop A’, which is equivalent to Chop A) and a position equidistant from Nod A on the opposite side of Nod A (position Chop B).

The observations occur in a standard ABBA nod sequence. The final image generated by subtracting the images obtained for the two chop positions at nod A and those at nod B and then subtracting the results will produce three images of the star, one positive and two negative, with the positive being twice as bright as the negatives.

In the case of NPC, the nod is perpendicular to the chop. The telescope is offset by half the nod throw from the target in a direction perpendicular to the chop direction, and the secondary chops between two positions. The nod throw usually (but not necessarily) has the same magnitude as the chop but is in a direction perpendicular to the chop direction. This mode also uses the standard ABBA nod cadence. The final image is generated by subtracting the images obtained for the two chop positions at nod A and those at nod B and then subtracting the results; it will therefore have four images of the star in a rectangular pattern, with the image values alternating positive and negative. Unlike NMC, each beam in NPC has the same relative intensity.

Nod Match Chop and Nod Perpendicular to Chop diagram

Figure 1.3-4. NMC vs NPC

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1.3.1.1b   Asymmetric Chopping Variations

In C2NC2 (known as Asymmetric Chop in FIFI-LS observations), the telescope is first pointed at the target (position A). In this first position, the secondary is aligned with the optical axis (or boresight) for one observation and then is tilted some amount (often 180–480 arcseconds) for the second (asymmetrically chopped) observation. This is an asymmetric C2 mode observation. The telescope is then slewed some (usually large) amount away from the target to a sky region without sources (position B), and the asymmetric chop pattern is repeated. C2NC2 observations are taken as a series of 8 (C2) files in the sequence A B A A B A A B, i.e. an ABA nod cadence with dithering to remove correlated noise. Again, the time between slews is typically 30 sec. 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.

Asymmetric Chop with Offset Nod diagram

Figure 1.3-5.

Figure 1.3-6 demonstrates how a C2NC2 observation might be designed for a large, extended object. It is immediately apparent from the figure, that C2NC2 has an efficiency of only ~20%. This is a much lower efficiency than either symmetric chopping variations 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.

Diagram showing how a C2NC2 observation might be designed for a large extended object

Figure 1.3-6. A sample C2NC2 observation design for a large, extended object. Each source position (solid line) and its associated asymmetric chop position (dashed line) have matching colors. Each source position has an independent chop configuration, the parameters for which are given in the dashed line boxes (chop angle in degrees and chop throw in arcseconds). 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).

The Asymmetric Chop mode of FIFI-LS typically impliments an ABBA nod sequence, but is otherwise equivalent to C2NC2. NXCAC (Nod not related to Chop with Asymmetric Chop) mode is the FORCAST grism version of C2NC2.

Bright Object mode for FIFI-LS is also an asymmetric chopping mode, using observations of two map positions and one off-position per nod-cycle through an asymmetric chop. This technique is utilized to improve the efficiency of mapping bright objects, where the total observing time is dominated by telescope movements.

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1.3.2   Alternatives to Chop-Nod Cycles

As chopping and nodding require a considerable amount of an observation's awarded time spent off-source, the amount of exposure time observing on the target is limited and subject to large observational overheads waiting for the telescope and/or secondary mirror assembly to complete chop/nod/dither movements. An alternative is to impliment continuous scanning techniques. Of the instruments currently offered on SOFIA, only GREAT, HAWC+, and FPI+ offer continuous scanning methods of observation. 

Nodding Only Modes 

GREAT 
Total Power - The telescope alternates between the target and a nearby reference position that is free of emission, using ON–OFF source cycles (typically spending ≤ 30 seconds on source). This mode is used when observing an extended source or a crowded region.
 
EXES
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.
 
Dithering Only Modes 
 
EXES
Map - The telescope is moved perpendicular to the slit while EXES takes spectra on a grid of telescope positions, which are always one dimensional stripes. 
 
FORCAST
Slitscan - 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.
 
Continuous Scans
 
HAWC+
On The Fly Mapping (OTFMAP) - In this case, scan rates must reach (~2 Hz) x (HAWC+ beam width) in order to remove the source from the atmospheric background—implying rates of ~10–80 arcseconds per second depending on the bandpass. HAWC+ offers two scan types for OTFMAP scan patterns: Lissajous and Box. In Lissajous observations, the telescope follows a parametric curvature at a non-repeating period to eventually cover the scan amplitude. In contrast, Box scans drive the telescope in a linear fashion at a specified rate in one direction for a given length and then moved perpendicularly before scanning in the reverse direction—similar to how one would mow a very large lawn. Lissajous scans are recommended for soucres smaller than the HAWC+ FOV at a given bandpass, while Box scans are more efficient at mapping large areas several times the FOV. 
 
Note: GREAT also has an On the Fly Astronomical Observation Template (AOT), but is not included in this section because GREAT observations are performed in either Total Power or Beam Switching modes.
 
Other Scans
 
FIFI-LS
Spectral Scan - This mode is implimented to target spectral features much wider than the bandwidth 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. 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.
 
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