4. FORCAST

4.1 Specifications

4.1.1 Instrument Overview

The Faint Object infraRed CAmera for the SOFIA Telescope (FORCAST) is a dual-channel mid-infrared camera and spectrograph sensitive from 5–40 μm. Each channel consists of a 256x256 pixel array that yields a 3.4x3.2 arcmin instantaneous field-of-view with 0.768 arcsec pixels, after distortion correction. The Short Wavelength Channel (SWC) uses a Si:As blocked-impurity band (BIB) array optimized for λ < 25 μm, while the Long Wavelength Channel's (LWC) Si:Sb BIB array is optimized for λ > 25 μm. Observations can be made through either of the two channels individually or, by use of a dichroic mirror, with both channels simultaneously across most of the range. Spectroscopy is also possible using a suite of six grisms, which provide coverage from 5–40 μm with a low spectral resolution of R = λ/Δλ ~ 200.

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4.1.1.1 Design

The FORCAST instrument is composed of two cryogenically cooled cameras of functionally identical design. A schematic of the instrument layout is provided in Figure 4-1 below. Light from the SOFIA telescope enters the dewar through a 7.6 cm (3.0 in) caesium iodide (CsI) window and cold stop and is focused at the field stop, where a six position aperture wheel is located. The wheel holds the imaging aperture, the slits used for spectroscopy, and a collection of field masks for instrument characterization. The light then passes to the collimator mirror (an off-axis hyperboloid) before striking the first fold mirror, which redirects the light into the LHe-cooled portion of the cryostat.

The incoming beam then reaches a four-position slide, which includes an open position, a mirror, and two dichroics, one for normal use and the other as a spare. The open position passes the beam to a second fold mirror, which sends the beam to the LWC, while the mirror redirects the light to the SWC. The magnesium oxide (MgO) dichroic reflects light below 26 μm to the SWC and passes light from 26–40 μm to the LWC. The light then passes through a Lyot stop at which are located two filter wheels of six positions each, allowing combinations of up to 10 separate filters and grisms per channel. Well characterized, off-the-shelf filters can be used, since the filter wheel apertures have a standard 25 mm diameter (see Section 4.1.2.2).

Finally, the incoming beam enters the camera block and passes through the camera optics. These two-element catoptric systems are composed of an off-axis hyperboloid mirror and an off-axis ellipsoid mirror that focuses the light on the focal plane array. Also included is an insertable pupil viewer that images the Lyot stop on the arrays to facilitate alignment of the collimator mirror with the telescope optical axis and to allow characterization of the emissivity of both the sky and telescope.

Figure 4-1.

Figure 4-1. A schematic of the FORCAST layout.

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4.1.2 Performance

4.1.2.1 Camera Performance

The SWC and LWC arrays were selected to optimize performance across the 5–40 μm bandpass. Both arrays have a quantum efficiency (QE) greater than 25% over most of their spectral range. The cameras can be operated with variable frame rates and in either high or low capacitance modes (which are characterized by well depths of 1.8x107 and 1.9x106 e- respectively), depending upon the sky background and source brightness.

The best measured image quality (IQ) obtained by FORCAST on SOFIA is in the 7–11 μm range with a FWHM PSF (point spread function) of ~2.7 arcsec. This measured image quality in-flight is limited by telescope jitter arising from vibrations of the telescope itself (e.g., due to wind loading in the cavity), turbulence, and tracking accuracy. Presented in Figure 4-2 is a sample of FWHM IQ measurements made during a single observatory characterization flight during a Cycle 7 flight in comparison to the theoretical diffraction limit calculated for a 2.5 m primary with a 14% central obstruction combined with the FWHM telescope jitter, here assumed to be 2.08 arcsec (1.25 arcsec rms).

Figure 4-2.

Figure 4-2. Representative FWHM PSF for the FORCAST camera in select filters as measured during Cycle 7. Also shown are the diffraction limited FWHM (solid line; calculated for a 2.5-m primary with a 14% central obstruction) and the modeled IQ (dashed line), which includes shear layer seeing and 0.5 arcsec rms telescope jitter (corresponding to 0.83” FWHM).

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4.1.2.2 Filter Suite

Imaging with FORCAST can be performed in either a single channel or in both the SWC and LWC channels simultaneously (dual channel configuration). In a single channel configuration, any one of the available filters may be used. In the dual channel configuration, a dichroic is used to split the incoming beam, directing it to both the SWC and LWC.

The dichroic reduces the overall throughput. Table 4-1 shows the throughput with the dichroic in (dual channel configuration) relative to that for the single channel configuration. The degradation of the system throughput by the dichroic can have a significant effect on the instrument sensitivities as discussed in more detail in Section 4.1.2.3. In addition, since there are more short wavelength filters available than slots in the SWC filter wheels, some short wave filters will be housed in the LWC. The final filter wheel configuration for each observing cycle will be driven by proposal pressure. Depending on the final filter configuration, it is possible that not all LWC filters will be able to be used in the dual channel configuration due to the cutoff in the dichroic transmission at short wavelengths. The N' broadband filter (FOR_F112), FOR_F056, and FOR_Barr 3 are only offered in single-channel mode. Contact the helpdesk with questions.

Table 4-1: Dichroic Throughput

Bandpass

Throughput

Bandpass

Throughput

5-10 μm

60%

11-25μm

85%

25-40 μm

40%

The filters in the SWC are standard Optical Coating Laboratory, Inc. (OCLI) thin-film interference filters. These filters are stacked with blocking filters to prevent light leaks. The only exception is the 25.3 μm (FOR_F253) University of Reading filter, which is a custom double half-wave (three mesh) scattering filter stacked with a diamond scattering blocking filter to provide blue-light rejection. The 33.6 (FOR_F336), 34.8 (FOR_F347), and 37.1 μm (FOR_F371) filters in the LWC are LakeShore custom double half-wave (three mesh) scattering filters. The 31.5 μm filter is a Fabry-Perot Interferometer filter.

The central wavelengths, bandwidths, and the typical FWHM IQ in each of the filters are given in Table 4-2 below. The first column under the Imaging FWHM column (single channel) heading presents the best measured FWHM IQ in single channel configuration. These values are representative of the IQ observed since Cycle 3. The second column (dual channel) contains the average measured FWHM IQ in dual channel configuration. Not all of the filters have measured IQ values, but we expect that they will be comparable to those with measured values that are of similar λeffFigure 4-3 shows the filter transmission profiles (normalized to their peak transmission) over-plotted on an ATRAN model of the atmospheric transmission. The filter transmission curves (text tables) are available as a zip file or individually from Table 4-2.

Due to the limited number of slots available in the filter wheels, not all of the filters listed in Table 4-2 are available at any one time. For Cycle 10, there will be a nominal filter set (indicated by bold italic type in Table 4-2). Other filters can be requested for use, but a convincing scientific argument must be made to justify swapping out the existing filters. A filter swap during the cycle may be permitted dependent upon proposal pressure; more details are available in the Call for Proposals.

Table 4-2: Filter Characteristics

Channel

Filter

λeff (μm)

Δλ (μm)

Imaging FWHM (arcsec)

Profiles

Single Channel

Dual Channel

SWC

FOR_F054

5.4

0.16

b

txt file

FOR_F056

5.6

0.08

3.11+/-0.2

3.06+/-0.2

txt file

FOR_F064

6.4a

0.14

2.9

2.9

txt file

FOR_F066

6.6

0.24

2.9

3.1

txt file

FOR_F077

7.7

0.47

3.0

3.0

txt file

FOR_F088

8.8

0.41 

2.55+/-0.2

2.63+/-0.2

txt file

FOR_F111

11.1

0.95

2.8

2.9

txt file

FOR_F112c

11.2

2.7

2.41+/-0.2

2.71+/-0.25

txt file

FOR_F197

19.7

5.5

2.4

2.5

txt file

FOR_F253

25.3

1.86

2.3

2.1

txt file

LWC

FOR_F113

11.3

0.24

2.6

txt file

FOR_F118

11.8

0.74

2.6

txt file

FOR_F242

24.2

2.9

2.6

-

txt file

FOR_F315

31.5

5.7

2.8

2.8

txt file

FOR_F336

33.6

1.9

3.1

3.3

txt file

FOR_F348

34.8

3.8

3.1

3.0

txt file

FOR_F371

37.1

3.3

2.9

3.4

txt file

a Entries in bold are expected to be part of the default filter set for Cycle 10.

bIQ values for some filters have not been measured at this time, but it is expected that they will be similar to those of similar λeff with measured values.

cFOR_F112: This filter in dichroic mode has a ghost artifact at the ~2% level seen on bright targets.

Figure 4-3.

Figure 4-3. FORCAST filter transmission profiles along with an ATRAN model of the atmospheric transmission across the FORCAST band (assuming a zenith angle of 45 degrees and 7 μm of precipitable water vapor). For clarity, the filter profiles have been normalized to their peak transmission. SWC filters alternate between green and blue, while LWC filters alternate between red and yellow.

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4.1.2.3 Imaging Sensitivities

The FORCAST imaging sensitivities for a continuum point source for each filter are presented in Table 4-3 and Figure 4-4. The Minimum Detectable Continuum Flux (MDCF; 80% enclosed energy) in mJy needed to obtain a S/N = 4 in 900 seconds of on-source integration time is plotted versus wavelength. The MDCF scales roughly as (S/N) / √(t) where t = on-source exposure time. The horizontal bars indicate the effective bandpass at each wavelength. At the shorter wavelengths the bandpass is sometimes narrower than the symbol size.

Atmospheric transmission will affect sensitivity, depending on water vapor overburden. The sensitivity is also affected by telescope emissivity, estimated to be 15% for Figure 4-4.

Observations with FORCAST are performed using standard IR chop-nod techniques. Chop/nod amplitudes can be chosen such as that they are small enough to leave the source on the array in each position or large enough that the source is positioned off the chip for one of the chop positions. Calculations of S/N for various chop-nod scenarios are provided here.

Table 4-3: Filter Sensitivities

Filter

Channel

Single Chan. MDCF (mJy)

Dual Chan. MDCF (mJy)

Filter

Channel

Single Chan. MDCF (mJy)

Dual Chan. MDCF (mJy)

FOR_F054a

SWC

41.5

575

FOR_F056b

SWC

58.1

225

FOR_F064

SWC

39.5

54.7

FOR_F066

SWC

58.8

77.7

FOR_F077

SWC

49.5

59.8

FOR_F088

LWC

64.5

68.0

FOR_F111

SWC

79.4

74.5

FOR_F112

SWC

51.2

62.6

FOR_F113

LWC

173

-

FOR_F118

LWC

108

-

FOR_F197

SWC

59.7

60.1

FOR_F242

LWC

75.7

-

FOR_F253

SWC

115

128

FOR_F315

LWC

90.3

123

FOR_F336

LWC

181

266

FOR_F348

LWC

137

195

FOR_F371

LWC

189

290

a MDCF values shown are those measured up to Cycle 9 data.

b No observations were available to test the theoretical MDCF values.

Figure 4-4.

Figure 4-4. Cycle 10 continuum point source sensitivities for single and dual channel modes. Values are for S/N = 4 in 900 s under nominal conditions. Investigators are encouraged to use the  for their calculations.

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4.1.2.4 Grisms

The suite of four grisms available for FORCAST provide low to medium resolution coverage throughout most of the range from 5–40 μm (see Table 4-4). The grisms are situated in the four filter wheels: two in each SWC wheel and one in each LWC wheel. The arrangement is chosen to minimize the impact on the imaging capabilities of the instrument. The grisms are blazed, diffraction gratings used in transmission and stacked with blocking filters to prevent order contamination. A summary of the grism properties is provided in Table 4-4

Grisms FOR_G063, FOR_G227 and FOR_G329 provided by the University of Texas at Austin, are made of silicon to take advantage of its high index of refraction, which allows optimum spectral resolution. However, these grisms suffer from various absorption artifacts precluding their use in the 8–17 μm window. Coverage in this region is provided by the FOR_G111 grism, constructed of KRS-5 (thallium bromoiodide) by Carl-Zeiss (Jena, Germany). These latter two grisms have a lower spectral resolution due to the lower index of refraction of the KRS-5 material.

Two long slits have been designed for FORCAST: 2.4 x 191 arcsec and 4.7 x 191 arcsec. The narrow slits yield higher resolution data. All of the slits are located in the aperture wheel of the instrument.

Although grisms are available in both cameras, during Cycle 10 grism spectroscopy will be available only in single channel configuration.

It is important to note that due to the fixed position of the slits in the aperture wheels, the lack of a field de-rotator, and the fact that SOFIA behaves in many respects as an Alt-Az telescope, the orientation of the slit on the sky will be dependent on the flight plan and will not be able to be predetermined. Furthermore, the slit orientation rotates on the sky with each telescope Line-of-Sight (LOS) rewind (see Section 1.2.4). These limitations may be especially important to consider when proposing observations of extended objects.

Table 4-4: Grism Characteristics

Channel

Grism

Material

Groove Sep. (μm)

Prism Angle (°)

Order

Coverage (μm)

R (λ/Δλ)

Wide Slit

4.7” x 191”

R (λ/Δλ)

Narrow Slit 2.4” x 191”

Channel

Grism

Material

Groove Sep. (μm)

Prism Angle (°)

Order

Coverage (μm)

R (λ/Δλ)

Wide Slit

4.7” x 191”

R (λ/Δλ)

Narrow Slit 2.4” x 191”

SWC

FOR_G063

Si

25

6.16

1

4.9 - 8.0

120c

180

FOR_G111

KRS-5

32

15.2

1

8.4 - 13.7

130c

260

LWC

FOR_G227

Si

87

6.16

1

17.6 - 27.7

110

120

FOR_G329

Si

142

11.07

2

28.7 - 37.1

160

 

 

b Not available during Cycle 10

c The resolution of the long, narrow-slit modes is dependent on (and varies slightly with) the in-flight IQ

d Only available with the 2.4'' x 11.2'' slit

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4.1.2.5 Spectroscopic Sensitivity

Tables 4-5 and 4-6 provide samples of the MDCF and Minimum Detectable Line Flux (MDLF) calculated at three different wavelengths across each grism bandpass for each of FORCAST's spectroscopic configurations. The data are provided for point sources only. The MDCF and MDLF estimates are for the raw integration time of 900 seconds and do not include observing overheads, but do account for a two-position chop (perpendicular to the slit).

Figure 4-5 presents the continuum point source sensitivities for the FORCAST grisms. The plots are the MDCF in Jy needed for a S/N of 4 in 900 seconds at a water vapor overburden of 7 μm, an altitude of 41K feet, and a zenith angle of 60°. The rapid variations with λ are due to discrete atmospheric absorption features (as computed by ATRAN).

To determine the required integration time necessary to achieve a desired S/N ratio for a given source flux, the SOFIA Instrument Time Estimator (SITE) should be used. SITE also allows for calculation of the limiting flux for a given integration time and required S/N. Since FORCAST observations are background limited, the values given in Tables 4-5 and 4-6 and Figure 4-5 can be used to make an estimate of the required integration time using Equation 4-1:

(Eq. 4-1)

where [S/N]req is the desired signal-to-noise ratio, Fsrc is the continuum flux of the target, texp is the exposure time on source (without taking into consideration observational overheads), and the MDCF is taken from the tables for the point-source sensitivities or estimated from the figures. For emission lines, simply use the line flux for Fsrc and use the MDLF value instead of the MDCF. However, these tables may not contain the most recent or best determined sensitivity values and therefore the on-line calculator results should be used in the actual proposal.

Table 4-5: Long Slit Point Source Sensitivities

Grism

 λ (μm)

R = (λ/Δλ)

MDCF (mJy)

MDLF (W m-2)

R= (λ/Δλ)

MDCF (mJy)

MDLF (W m-2)

Grism

 λ (μm)

R = (λ/Δλ)

MDCF (mJy)

MDLF (W m-2)

R= (λ/Δλ)

MDCF (mJy)

MDLF (W m-2)

 

4.7'' Slit

2.4'' Slit

FOR_G063

5.1

120

79

2.3E-16

180

98

2.9E-16

FOR_G063

6.4

120

219

5.2E-16

180

268

6.3E-16

FOR_G063

7.7

120

496

5.2E-16

180

724

6.3E-16

FOR_G111

8.6

130

419

4.9E-16

300

532

6.2E-16

FOR_G111

11.0

130

449

4.1E-16

300

575

5.2E-16

FOR_G111

13.2

130

593

4.5E-16

300

764

5.8E-16

FOR_G227

17.8

110

715

8.6E-16

140

936

1.1E-15

FOR_G227

22.8

110

834

7.9E-16

140

989

9.3E-16

FOR_G227

27.2

110

1979

1.6E-15

140

2586

2.0E-15

FOR_G329

28.9

160

1365

6.5E-16

 

 

 

FOR_G329

34.1

160

1408

5.6E-16

 

 

 

FOR_G329

37.0

160

1763

5.6E-16

 

 

 

 

 

Figure 4-5.

Figure 4-5. Cycle 7 grism continuum point source sensitivities for both wide and narrow long slits overlaid on an atmospheric transmission model (light blue). Values are for S/N = 4 in 900 s under nominal conditions.

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4.2 Planning Observations

As is the case with ground based observations at mid-IR wavelengths, individual FORCAST exposures will be dominated by the sky and telescope background. Therefore chopping and nodding are essential for each observation. Selection of the observing mode and its parameters, including the distance and direction of chop and nod throws, depend on the details of the field of view around the target. The source(s) of interest may be surrounded by other IR-bright sources or may lie in a region of extended emission, which needs to be avoided to ensure proper background subtraction. Presented in this section is a discussion of how to best plan FORCAST observations in order to optimize the success of observations.

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4.2.1 Imaging Observations

Proposers are strongly encouraged to familiarize themselves with the basics of techniques for performing background limited observations covered in Section 1.3. In brief, the imaging observation modes for FORCAST include the following:

NMC Mode
Nod Match Chop mode consists of a chop 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 4-7.

C2NC2 Mode
In Chop-Offset Nod 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. 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.

Since only a single chop position out of a full chop/nod cycle is on source, NMC have a much greater efficiency than C2NC2. A sample mosaic demonstrating how a C2NC2 observation might be designed for a large, extended object is provided in Figure 4-8, and it is immediately apparent from the figure that C2NC2 has an efficiency of only ~20%. However, while mosaicking can be performed for any of the available obseving modes, proposers should keep in mind that the effects of coma may compromise the image severely for fields equiring large chop amplitudes (greater than 120") when chopping symmetrically (NMC mode). The use of C2NC2 mode is required for chop amplitudes greater than 120". If the source has an angular extent large enough that multiple pointings are required, the central position of each FORCAST field must be specified, with due consideration of the desired overlap of the individual frames. For more on mosaicking, see Section 5.3 in the USPOT Manual.

Figure 4-7.

Figure 4-7. The NMC mode resulting raw image.

 

Figure 4-8.

Figure 4-8. 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).

 

Once a proposal has been accepted, the proposer, in collaboration with the SMO instrument scientist, will specify the details of chopping and nodding for each observation using the SOFIA observation preparation tool (USPOT). Experienced proposers are encouraged to design their observations using USPOT 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. If there are any time constraints on targets, please specify them in the “Special Instructions” section of the proposal. If the proposer is not certain which mode to use, please contact the helpdesk (sofia_help@sofia.usra.edu)

Following are a few of the most important issues to consider when preparing a FORCAST Imaging proposal:

Check a Database
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 (i.e., on the detector array) 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.

For Emissions Less than ~1.6 arcmin
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 arcmin), then the chop and nod can be done on-chip. For additional discussion of this point, see the calculations of S/N for various FORCAST chop-nod scenarios provided here.

Dithering

For all imaging observations the user is strongly encouraged to use five dither positions. The total on-source time to achieve S/N can be split between the number of dithers selected and a minimum of 10 seconds per dither position is required. Dithering ensures high quality images to correct for background, artifacts on the array, etc.

Chop Throw Constraints
When using a symmetric chop, chopping and nodding can be performed in any direction for chop throws less than 584 arcsec. When using an asymmetric chop, the maximum possible chop throw is 420 arcsec. However, some chop angles (as measured in the instrument reference frame) are not allowed for asymmetric chop throws between 250 arcsec and 420 arcsec. Since the orientation of the instrument relative to the sky will not be known until the flight plan is generated, Those requesting chop throws between 250–420 arcsec are required to specify a range of possible chop angles from which the instrument scientists can choose when the flight plan is finalized. 

Additionally, 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 arcsec per 1 arcmin of chop amplitude.

For large, extended objects, it may not be possible to obtain clean background positions due to these limitations on the chop throw.

For 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 configurations and up to 120 sec in the LWC-only configurations (depending on the filter). 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 NMC mode. The dithering will mitigate the effects of bad pixels when the individual exposures are co-added.

 

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4.2.1.1 Estimation of Exposure Times

The exposure times for FORCAST imaging observations should be estimated using the SOFIA Instrument Time Estimator (SITE)SITE can be used to calculate the signal-to-noise ratio (S/N) for a given total integration time, or to calculate the total integration time required to achieve a specified S/N. The total integration time used by SITE corresponds to the time actually spent integrating on-source without overheads. These integration times are used as input for USPOT, which will automatically calculate the necessary overheads. The format of the S/N values output by SITE depends on the source type. For Point Sources, the reported S/N is per resolution element, but for Extended Sources, it is the S/N per pixel.

For mosaic observations the total integration time required for a single field should be multiplied by the number of fields in the mosaic to obtain the total time, which is to be entered in USPOT.

An important consideration in planning observations is whether FORCAST should be used in single channel configuration, or in a dual channel configuration, since one gains the extra filter observation at the cost of lower system throughput in the individual bands. On the SITE form, a single channel configuration is specified by selecting the filter of interest for one channel and selecting None on the other channel in the Instrument properties section.

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4.2.2 Spectroscopic Observations

Proposers are strongly encouraged to familiarize themselves with the basics of techniques for performing background limited observations covered in Section 1.3. In brief, the spectroscopic observation modes for FORCAST include the following:

NMC Mode
As with FORCAST imaging observations, Nod Match Chop mode consists of a chop symmetric about the optical axis of the telescope with one of the two chop positions centered on the target. See Section 4.2.1 or Section 1.3.

NXCAC Mode
Nod not related to Chop with Asymmetric Chop mode is the grism version of C2NC2, i.e., an asymmetric chop with dithering along the slit. See Section 4.2.1 or Section 1.3.

Slitscan Mode
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.

During Cycle 10, grism spectroscopy with FORCAST will only be available in single channel, long-slit configurations (SWC and LWC). By default, observations will be set up using NMC aligned along the slit in Long Slit configurations. For larger sources and for targets embedded in crowded fields it is advised to use C2NC2 mode.

The observing efficiency for FORCAST spectroscopic observations depends on a number of factors, including the observing mode, chop frequency and nod cadence, the detector frame rate, and LOS rewind cadence. The typical observing efficiency as measured from NMC observations is 35–45% of clock time. Work is ongoing to optimize the mode-dependent efficiency values. These efficiency estimates are built-in to USPOT and do not need to be specified.

It is important to note that due to the fixed position of the grisms/slits in the filter/aperture wheels, the orientation of the slit on the sky will be dependent on the flight plan and will not be able to be predetermined. Further, the slit orientation rotates on the sky with each telescope Line-of-Sight (LOS) rewind (Section 1.1). These limitations may be especially important to consider when proposing observations of extended objects.

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4.2.2.1 Estimation of Exposure Times

The exposure times for FORCAST Grism spectroscopic observations should be estimated using the SOFIA Instrument Time Estimator (SITE)SITE can be used to calculate the signal-to-noise ratio (S/N) for a given on-source exposure time, to calculate the total integration time required to achieve a specified S/N, or to estimate the limiting flux for a desired S/N.

In either case, overheads should not be included, as USPOT calculates them independently. From Cycle 10 onwards, SITE will be updated so that the mode (C2NC2, NMC, etc...) will be selectable for a better estimate of the observing time required

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