4. FORCAST

4.1 Specifications

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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 optmized 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. The instrument has space for cross dispersing grisms allowing for high resolution cross-dispersed (XD) spectroscopy at R ~ 800–1200 in the 5–14 μm range. The availability of the XD configuration during a given cycle is published in the Call-for-Proposals.

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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.
FORCAST instrument schematic diagram

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 the winter of 2010 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.
FORCAST resolution plot

Figure 4-2. Representative FWHM Image Quality for the FORCAST camera in select filters as measured during Cycle 4. 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 1.25 arcsec rms telescope jitter.

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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.

Table 4-1: Dichroic 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 25.4 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), 34.7 (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 λeff. Figure 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 7, there will be a nominal filter set (indicated by bold italic type in Table 4-2). Other filters can be requesed 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 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  - -
FOR_F111 11.1 0.95 2.8 2.9 txt file
FOR_F112 11.2 2.7 - - -
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 -
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 Entires in blue are expected to be part of the default filter set for Cycle 7.
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.
 
Figure 4-3.
FORCAST filter transmission profiles plot

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 = net integration 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. For background-limited observations, as is the case with FORCAST on SOFIA, chopping and nodding off-chip in nod-match-chop (NMC; see Section 4.2.1) will generally result in the same signal to noise (S/N) as chopping and nodding on-chip in nod-perp-chop (NPC; see Section 4.2.1). 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)
FOR_F054a SWC 41.5 575
FOR_F056b SWC 48.9 225
FOR_F064 SWC 52.5 59.5
FOR_F066 SWC 58.8 77.7
FOR_F077 SWC 54.7 66.6
FOR_F088a LWC 63.0 65.7
FOR_F111a SWC 90.2 97.1
FOR_F112a SWC 50.0 63.4
FOR_F113a LWC 206 -
FOR_F118a LWC 123 -
FOR_F197 SWC 73.5 77.4
FOR_F242 LWC 97.9 -
FOR_F253 SWC 150 158
FOR_F315 LWC 126 171
FOR_F336 LWC 265 380
FOR_F348 LWC 180 257
FOR_F371 LWC 245 376
a MDCF values shown are those measured from Cycle 4 data.
b No observations were available to test the theoretical MDCF values.
 
Figure 4-4.
FORCAST sensitivity plot

Figure 4-4. Cycle 7 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 SOFIA Integration Time Calculator (SITE) for their calculations.

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4.1.2.4   Grisms

The suite of 6 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. Note that during Cycle 7, the cross-dispersed XD configuration will not be available. The information on XD configuration below is for informational purposes, only.

Grisms FOR_G063, FOR_G227, FOR_G329, and the FOR_XG063 dispersing grism 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 (and its cross-dispersing counterpart), 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.

Three slits have been designed for FORCAST: two long slits (2.4 x 191 arcsec, 4.7 x 191 arcsec) and a short slit (2.4 x 11.2 arcsec). The narrow slits yield higher resolution data. All of the slits are located in the aperture wheel of the instrument. The cross-dispersed spectra are obtained by using the short slit and passing the beam first through the low-resolution grism (either FOR_G063 or FOR_G111), followed by a disperser.

Although grisms are available in both cameras, during Cycle 7 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)a
SWC FOR_G063 Si 25 6.16 1 4.9 - 8.0 120c/180
FOR_XG063b Si 87 32.6 15 - 23 4.9 - 8.0 1170d
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 - 37.1 160/170b
a For the 4.7'' x 191'' and the 2.4'' x 191'' slits, respectively
b Not available during Cycle 7
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 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 FORCAST online grism exposure time calculator should be used. The on-line calculator 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) [S/N]req4=FsrctexpMDCF900

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)
  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 220a 1899 9.0E-16
FOR_G329 34.1 160 1408 5.6E-16 220a 1994 8.0E-16
FOR_G329 37.0 160 1763 5.6E-16 220a 2439 8.0E-16
a The 2.4 arcsec long slit mode for G329 will not be available during Cycle 7.

 

Table 4-6: Cross-Dispersed Point Source Sensitivitiesa

 Grism λ (μm) R = (λ/Δλ) MDCF (mJy) MDLF (W m-2)
    2.4'' x 11.2'' Slit
FOR_XG063 5.1 1200 238 1.2E-16
FOR_XG063 6.4 1200 703 2.8E-16
FOR_XG063 7.7 1200 918 3.0E-16
a XD configurations are not available during Cycle 7.
 
Figure 4-5.
FORCAST grism sensitivities plot

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

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4.2   Planning Observations

FORCAST configuration and mode flowchart diagram

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.

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.
 
NPC Mode
Similar to NMC, Nod Perpendicular to Chop mode also uses a chop that is symmetric about the optical axis, but in this case the nod is perpendicular to the chop. The final images produced using NPC show four sources arranged in a parallelogram with alternating positive and negative beams. Unlike NMC, each beam in NPC has the same relative intensity. This mode also uses the standard ABBA nod cadence. The right side of Figure 4-7 shows data obtained using NPC. This mode will not be supported in Cycle 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 and NPC 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 cna 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 requiring large chop amplitudes when chopping symmetrically (NMC or NPC modes). 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 5.3.1.2b in the USPOT Manual.

Figure 4-7.

NMC and NPC mode illustration

Figure 4-7. 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).

Figure 4-8.
Diagram of a sample mosaic demonstrating how a C2NC2 observation might be designed for a large extended object

Figure 4-8. Cartoon of a C2NC2 mode FORCAST observation with mosaic.

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.

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. Observations performed in NMC mode either on-chip or off-chip yield a S/N equal to or slightly better than that obtained in NPC mode. For additional discussion of this point, see the calculations of S/N for various FORCAST chop-nod scenarios provided here.

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 or NPC mode. The dithering will mitigate the effects of bad pixels when the individual exposures are co-added.

If the object is not visible in a single A-B chop/nod-subtracted pair, with a nod dwell time of 30 sec in each nod position (60 sec integration), then dithering should not be used.

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4.2.1.1   Estimation of Exposure Times

The exposure times for FORCAST imaging observations should be estimated using the on-line exposure time calculator, 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.
 
NPC CAS and NPC NAS Modes
Grism Nod Perpendicular to Chop observations can be performed either in a Chop Along Slit mode (NPC CAS) or Nod Along Slit mode (NPC NAS). As with FORCAST imaging observations, FORCAST's grism NPC modes also impliment a chop that is symmetric about the optical axis–however, unlike in NMC mode, the nod is perpendicular to the chop. The final images produced using NPC CAS or NAS show two sources arranged along the slit with one positive and one negative beam. Unlike NMC, each beam in NPC has the same relative intensity.
 
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 7, 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 and perpendicular to the slit in XD configuration. Due to the size of the PSF, neither chopping or nodding along the slit nor dithering are possible for high-resolution XD observations. 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 and NPC observations is 50–75% 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 online FORCAST Grism Exposure Time Calculator tool. This calculator can be used to calculate the signal-to-noise ratio (S/N) for a given total integration 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.

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