5.3 FORCAST

v8.1.0

Table of Contents

Return to the Table of Contents for this section at any time by selecting Return to Table of Contents. Users may also navigate through the entire USPOT Manual by using the complete Table of Contents menu to the right.

5.3   FORCAST AOR Fields

FORCAST specific instructions and reminders of general issues are given in the following topics below. It is necessary to read the FORCAST chapter of the Observer's Handbook before preparing detailed FORCAST observations in USPOT. Astronomical Observation Requests (AORs) should be created as described in Chapter 3.

The USPOT Observation drop-down menu lists the four Astronomical Observing Templates (AOTs) available for FORCAST: Imaging, Grism, Grism XD, and Acquisition. Refer to the Observer's Handbook for a complete description of available combinations of configurations and modes for FORCAST.

The USPOT FORCAST Main AOR Window contains several frames: Chop / Nod, Dither Offset, Dither Patt..., and Exposure Set-Up (the latter of which applies only to Acquisition AORs). Figure 5.3-1 shows an example of the Main AOR Window of a FORCAST AOT. The instrument-specific fields are discussed in detail in this chapter. Contact the Help-Desk with any questions.

Figure 5.3-1
Main AOR Window of a FORCAST AOT

Figure 5.3-1. An example of a FORCAST AOT Main AOR Window, using the FORCAST Imaging AOT.

FORCAST support scientists will be completing the Phase II portion of AORS, in addition to reviewing Phase I entries, and uploading these new drafts of the AORs into the DCS system for proposers to review. The proposers will be notified by their support scientists when their AORs are available for review. Proposers should work directly with their support scientists to make necessary changes to their AORs according to the information provided here. Contact the Help-Desk with any questions.

In order to ensure that the time requested on the proposal accurately reflects the time needed, consider the following checklist:
Note: The Observer's Handbook links below point to the latest version of the Observer's Handbook—currently Cycle 8. Be sure you are using the version of the Observer's Handbook that corresponds to your observing cycle. The documentation for all cycles can be found on the Proposal Documents webpage.

  • Utilize WISE, Spitzer, MSX, or IRAS images to ensure chopping and nodding configurations are set up properly (i.e., that observations fall onto a clean sky).
  • In order for the source to fit within the field of view of FORCAST given any rotation of field, the object must not be any bigger than 3.2 arcmin across in any dimension. If it does, observations must chop far enough such as that they are chopping off of the source--otherwise, a mosaic strategy must be specified. (See Section 5.3.1.2b.)
  • An Observing Priority must be set up if some targets are more important to observe than others. Of particular importance is ensuring that all AORs for a particular target have the same Observing Priority. Additionally, inidividual AORs for each target must also be prioritize by specifiying the preferred observation Order (Section 5.3.3.1b).
  • Provide Comments for non-standard observations or special requests (Section 5.3.3.2).
  • Proposers using C2NC2 or NXCAC mode with a Chop Throw >250 arcsec must specify as large a range of chop angles as possible.
  • For the imaging or spectroscopy of extended and bright sources, dithers to mitigate array artifacts must be set up.
  • For imaging mosaics of extended objects larger than the FORCAST field of view, the mosaic offsets must be set up such as that there is adequate overlap to ensure that there are no gaps in coverage regardless of the sky orientation at the time of observation. (See Section 5.3.1.2b.)

Other help:

 

Tables 5.3-1, 5.3-2, and 5.3-3 list the required fields for Phase I and Phase II for each available FORCAST AOT. Conditional fields (i.e., fields not editable unless certain parameters are specified) are denoted with a footnote, with a reference to the required field to activate the conditional field. Fields that are not listed in these tables fall under one of three categories: fields not directly editable in USPOT (but may be affected by updating other fields, which are required; for more information on how particular fields may be related, refer to the corresponding sections within the Observer's Handbook—denoted in Tables 5.3-1, 5.3-2 and 5.3-3 by OH followed by the appropriate section number), fields inteded for use only by SOFIA Support Scientists only, or optional fields. 

 
Table 5.3-1.

Imaging Phase I Required Fields
All Observer's Handbook (OH) Reference links in the table below point to the latest version of the Observer's Handbook—currently Cycle 8. Be sure you are using the version of the Observer's Handbook that corresponds to your observing cycle. The documentation for all cycles can be found on the Proposal Documents webpage.

Field Location Field Reference
Main AOR Window Specify Target § 3.4
Exposure Time SITEOH § 4.2.1.1
Config SWC LWC OH § 4.1.2.4
Chop / Nod Frame  Chop/Nod Style OH § 4.2.1
Observing Condition & Acquisition / Tracking Window Is Time Critical OH § 4.2

Imaging Phase II Required Fields

Field Location Field Reference
Main AOR Window Min Contiguous Exp Time § 5.3.1.1b
Dither Patt... § 5.3.1.1a§ 5.3.1.2
Dither Offset Frame 1Dither Coordinate § 5.3.1.2a
1DitherOffset 1ExpTimePerDither § 5.3.1.1a
Chop / Nod Frame  Chop Throw § 5.3.1.2
2Chop Angle Coordinate § 5.3.1.2a
Chop Angle § 5.3.1.2
Observing Condition & Acquisition / Tracking Window Target Priority § 3.4
 
Table 5.3-2.

Grism Phase I Required Fields
All Observer's Handbook (OH) Reference links in the table below point to the latest version of the Observer's Handbook—currently Cycle 8. Be sure you are using the version of the Observer's Handbook that corresponds to your observing cycle. The documentation for all cycles can be found on the Proposal Documents webpage.

Field Location Field Reference
Main AOR Window Specify Target § 3.4
Exposure Time Grism ETCOH § 4.2.2.1
Instrument Configuration OH § 4.1.2.4
Slit SWC LWC
Chop/Nod Style § 5.3.1.2§ 5.3.2.2
Observing Condition & Acquisition / Tracking Window Target Priority § 3.4
Is Time Critical OH § 4.2

Grism Phase II Required Fields

Field Location Field Reference
Main AOR Window Min Contiguous Exp Time § 5.3.2.1b
Dither Patt... § 5.3.2.1a§ 5.3.2.2d
1Dither Coordinate 1DitherOffset § 5.3.1.2a
1ExpTimePerDither § 5.3.2.1a
Chop Throw § 5.3.2.2
2Chop Angle Coordinate § 5.3.1.2a
Chop Angle § 5.3.2.2
Observing Condition & Acquisition / Tracking Window Target Priority § 3.4
 
Table 5.3-3.

Acquisition Phase I Required Fields
All Observer's Handbook (OH) Reference links in the table below point to the latest version of the Observer's Handbook—currently Cycle 8. Be sure you are using the version of the Observer's Handbook that corresponds to your observing cycle. The documentation for all cycles can be found on the Proposal Documents webpage.

Field Location Field Reference
Main AOR Window Specify Target § 3.4
Config SWC LWC SLIT OH § 4.1.2.4
Exposure Time SITEOH § 4.2.1
Chop/Nod Style OH § 4.2.1
Observing Condition & Acquisition / Tracking Window Is Time Critical OH § 4.2

Acquisition Phase II Required Fields

Field Location Field Reference
Main AOR Window Chop Throw § 5.3.2.2
2Chop Angle Coordinate § 5.3.1.2a
Chop Angle § 5.3.2.2
Observing Condition & Acquisition / Tracking Window Target Priority § 3.4.1

 

Grism XD Phase I Required Fields

All Observer's Handbook (OH) Reference links in the table below point to the latest version of the Observer's Handbook—currently Cycle 8. Be sure you are using the version of the Observer's Handbook that corresponds to your observing cycle. The documentation for all cycles can be found on the Proposal Documents webpage.

Field Location Field Reference
Main AOR Window Exposure Time (sec) SITEOH § 4.2.1
SW Frame Grism  
Slit Frame Slit  
Chop/Nod Frame Chop/Nod Style OH § 4.2.1
Observing Condition & Acquisition / Tracking Window Is Time Critical OH § 4.2
 
1For Ditther Patt... = any option other than None
2For Chop/Nod Style = Nod Match Chop
Return to:   Table 5.3-1   |   Table 5.3-2   |   Table 5.3-3
 

Return to Table of Contents

5.3.1   Imaging AOR Fields

5.3.1.1   Instrument Parameters

5.3.1.1a   Exposure Time

The desired total on-source integration time should be determined by using the SITE on-line calculator and entered into the Exposure Time field. This value does not include overheads.

Selecting the Observation Est... button on the bottom of the main AOR panel will launch an information window that gives the total requested exposure time, overhead, and duration of the observation. One can therefore check the observation efficiency by configuring the observations and pressing this button. For example, if the AOR is set up in NMC (Nod Match Chop Chop/Nod Style) to have a 3-point dither pattern with an exposure time of 60 s per dither position and one cycle of dithers, the resultant total exposure time will be 180 s and the duration of the observation will be 371 s (including estimates for line-of-sight (LOS) rewinds). If the AOR is set up in NMC mode with a 9-point dither pattern, 10 s exposure time per dither position, and two cycles, the resultant total on-source exposure time is still 180 s, but the observation will take 458 s (about 25% longer). Since proposers are awarded time in duration, not exposure time, it is in their best interest to figure out how to use that time most efficiently. Note too that when one specifies multiple Cycles, the Exposure Time field only displays the on-source time for a single cycle. The total on-source exposure time is displayed in the window accesed via the Observation Est... button at the bottom of the AOR editing window.

If dithering only once through the pattern, the desired total on-source integration time must be divided by the number of dithers and this value entered in the ExpTimePerDither field in the Dither Offset frame. This will automatically update the Exposure Time field with the total on-source time. Likewise, if the dither pattern will be repeated multiple times, the desired total on-source integration time must be divided by the number of dithers plus the number of times the pattern is to be repeated. The number of times the dither pattern is to be repeated should then be specified in the Cycles field. USPOT calculates the actual duration of the observations based upon the chop-nod mode selected, exposure time, number of dithers, and number of cycles.

Return to Table of Contents

5.3.1.1b   Min Contiguous Exposure Time Field

In some cases, it is necessary to split an observation among multiple flight legs. The Min Contiguous Exp Time field should be used to provide flight planners with information on the minimum on-source exposure time that can be scheduled for a single flight leg to be scientifically useful. For example, if a program has been awarded 2 hours of time for imaging a faint target, it may be necessary to divide the observation over multiple flight legs. If the source is faint enough to require at least 45 minutes of on-source time in order to be able to accurately coadd the data from multiple flight legs, then this should be entered into the Min Contiguous Exp Time field.

Return to Table of Contents

5.3.1.1c   Filter Selection

The appropriate instrument configuration (the Config field) and filters (the SWC and LWC fields) must then be selected. All of the available FORCAST filters are listed, but there are some important considerations that must be made. First, the throughput with the 5.4 and 5.6 μm filters in Dual Channel configuration (Config = IMAGING_DUAL) is very poor. Thus, proposers are discouraged from using these filters in Dual Channel configuration unless they are observing a blue source. Second, though all of the filters available for use in FORCAST are listed, only 12 are available in the instrument during any single flight series. Filters that are not included in the non-standard filter set may not be available. If a program that includes non-standard filters has been awarded time, contact the Instrument Scientist directly to determine if the non-standard filters will be available during the period when the program is scheduled. If not, then alternative filters must be used. The nominal filter set for SWC includes 5.6, 6.4, 7.7, 8.8, 11.1, N' (broadband), 19.7, and 25.3 μm. The supplemental filter set includes 5.4, 6.6, 11.3, 11.8 μm. For LWC, the default filter set includes 31.5, 33.6, 34.8, and 37.1 μm, while the supplemental set includes the 24.2 and 25.4 μm filters.

Return to Table of Contents

5.3.1.2   Chop / Nod Style

The available selections for the Chop / Nod Style field and their related parameters are discussed below. For observations dealing with extended objects larger than the FORCAST FOV, see Section 5.3.1.2b.

Return to Table of Contents

5.3.1.2a   Nod Match Chop

The Observer's Handbook links below point to the latest version of the Observer's Handbook—currently Cycle 8. Be sure you are using the version of the Observer's Handbook that corresponds to your observing cycle. The documentation for all cycles can be found on the Proposal Documents webpage.

The majority of sources imaged with FORCAST will be relatively compact (source diameters <120 arcsec) and lie in areas of the sky free from contaminating extended emission. For such observations, selecting the Nod Match Chop (NMC) in the Chop/Nod Style field is recommended. Because coma will increase with larger chop throw (1 arcsec of coma for every 60 arcsec of chop throw in NMC), the Chop Throw values should be configured to be as small as possible but large enough such as that it is obvious that chopping will occur off-source and onto clean sky. If there are clear chop reference areas all around the science source, then NMC mode should be configured with a chop angle of 30 degrees (which mitigates problems due to array artifacts) in the Array Chop Angle Coordinate field.

If the source is surrounded by extended emission, the Chop Throw and Chop Angle in the Sky Chop Angle Coordinate field must be configured to avoid contamination in the chop reference fields. This is discussed in more detail and with examples in Section 5.3.1.2b. This is also where you will find a discussion of using the C2NC2 Chop/Nod Style for imaging very large and extended objects.

Section 5.3.1.3 discusses the general rules of thumb for dithering with FORCAST.

When using the default Nod Match Chop selection for a compact or faint source, it is recommended to not employ dithers in the observation and, consequently, to leave the Dither Pattern field set to None. For observations that do not require dithering, the on-source integration time from SITE can be used in the Exposure Time field. For observations that do require dithering (such as an extended source observed using the default Nod Match Chop selection) there are two possible approaches: stepping through each position in the dither pattern once until the pattern is complete or looping through the dither pattern multiple times until the total on-source time is achieved. For NMC mode, it is better to only loop through the dither pattern once as this minimizes the overheads associated with the observation and maximizes the observing efficiency. In either case, the desired pattern must be selected in the Dither Patt... frame of the AOR editing window. This will update the dither offset parameters to the default values for a pattern with the selected number of positions. One can also define the dither offsets and whether to do these offsets in RA and Dec (Dither Coordinate = Sky) or in x and y pixel coordinates (Dither Coordinate = Array).

Return to Table of Contents

5.3.1.2b   C2NC2

Observations with C2NC2 chosen for the Chop/Nod Style field must be dithered. However, the dither parameters cannot be specified in USPOT. Instead, the support scientist will determine the best dither pattern and exposure time per dither position once the observations have been flight planned and the LOS rewind cadence is known. Only the total on-source exposure time required in the Exposure Time field is necessary. USPOT then provides an estimate for the total exposure time based upon a nominal rewind cadence and observing efficiency. 

Extended Objects Larger than FORCAST FOV

In some cases, the target of interest will be quite large. Chopping beyond about 180 arcsec, the imaged sources will have significant coma (>3 arcsec). The C2NC2 observing mode can be advantageous to employ in these cases. The C2NC2 Chop/Nod Style selection allows for chop throws of up to 420 arcsec and yields images with no coma. However, these observations are much less efficient (almost 2 times longer duration than C2N modes), so if a proposer was awarded time for observations under the assumption that Nod Match Chop or Nod Perp Chop Chop/Nod Style would be used, but then found during Phase II that C2NC2 is required, then fewer filters would have to be used or less targets observed to still fit the C2NC2 observations within the awarded time.

Imaging an area much larger than the FORCAST FOV can be accomplished by creating a mosaic. Any mosaic with both dimensions larger than the FORCAST field of view should also be set up in C2NC2 Chop/Nod Style (given the constraints on image quality due to coma when using large chop throws in Nod Match Chop Chop/Nod Style). See Figure 5.3-2 for an example. At present, there is no mapping mode available to create a mosaic automatically. Instead, the the RA and Dec coordinates for each position of the mosaic must be manually specified and enterered as independent targets (and, consequently, independent AORs). However, the proposer cannot control the field orientation on the sky. This is determined by the object's position in the sky during the observing leg (i.e., by the flight plan) and this is not known until the specific observing leg is planned by the flight planners. The positions should be specified close enough to one another that they will overlap for any field orientation, allowing them to be combined into an uninterrupted map in the post-processing stages. This can be tested by changing the Example Rotation Angle in the AORs and reloading the AORs into the visualizer.

Figure 5.3-2.
Illustration showing imaging an area much larger than the FORCAST FOV can be accomplished by creating a mosaic

Figure 5.3-2. An example of a mosaic in C2NC2, with the field of interest mapped by the red boxes. The Chop Angle is defined as 110 degrees and is shown in green for each position; the nod B fields are not shown. The C2NC2 mosaic observations have been defined so that the target fields to be mosaicked are barely overlapping, thus maximizing the area sampled (Left). However, if the actual sky orientation is 45 degrees different from this in flight, the GI may end up with a mosaic with large coverage gaps (Right).

In order to freeze the target field on the detector, the telescope must rotate about its optical axis during an observation (neither SOFIA nor FORCAST have image rotators). Because of the construction of the telescope, there is a limit to the amount the telescope can rotate of +/- 3 degrees. Once the telescope reaches this limit, the observation must be stopped and the rotation angle of the telescope is reset (a.k.a., an LOS rewind). Then the source is required and a new observation is started. The speed of the field rotation is set by the location of the object in the sky and the location of the aircraft. Because of the rotation limit, long integrations or observations of sources in parts of the sky that rotate quickly, may need to be broken up into several observations, each of which will have a different rotation angle.

Therefore, for long integrations or for sources in parts of the sky that rotate rapidly, different elements of a mosaic may sample the source at different position angles. An example of this with a mosaic of W3 is shown in Figure 5.3-2. As in the previous example, the Chop Angle and Chop Throw were both kept the same for each element of the mosaic since there was adequate emission free space available near the science target, but the same nod position was used for each pointing (purple and blue boxes). If one needs to use different chop angles or throws for some positions, then the background nod positions will not align as in the example, but should be checked to ensure they are sampling empty sky.

In any case, assume that the left image in Figure 5.3-3 is the way the proposer has originally set up the observations. Note that, given the small overlap of these fields, it is likely the problem of not fully sampling the area will occur at certain field orientations. Assume that 10 minutes per position is needed to integrate down to the level required by the science and that, for this source, the sky is rotating rapidly enough that the telescope needs to perform an LOS rewind after every 10 minutes. In that case there would be an LOS rewind after each element is sampled in the mosaic. Assuming that this rotation is the maximum 3 degrees, the final sampling of the field will look more as is depicted in the right image of Figure 5.3-2. This again demonstrates why it is important for the proposer to ensure their mosaic fields will overlap enough that field rotation will not be a problem.

Figure 5.3-3.
Illustration showing FOV

Figure 5.3-3. A mosaic of W3 in C2NC2, with the field of interest mapped by the red boxes. The Chop Angle is defined as 110 degrees and is shown in green for each position; the nod B fields are not shown. Left, The original set-up of the observation. Right, the adjusted set-up with LOS rweinds after each element is sampled in the mosiac. In this example, each field has rotated an additional 3 degrees from the last one, the final field (upper left red box) has rotated a total of 12 degrees from the first field of the mosaic (lower right red box).

Return to Table of Contents

5.3.1.3 Dithering

The Observer's Handbook links below point to the latest version of the Observer's Handbook—currently Cycle 8. Be sure you are using the version of the Observer's Handbook that corresponds to your observing cycle. The documentation for all cycles can be found on the Proposal Documents webpage.

The FORCAST array is relatively clean cosmetically for most observations. For very bright extended sources, there can be some array artifacts present. Removal of some of these effects can be accomplished with dithering. Dithering refers to small movements (of the order of 10–15 arcsec) of the telescope which place the imaged object at different locations on the array. Typically one dithers 3 or 5 times. Shifting and then median combining these images can remove any patterns that are positionally dependent on the array. For faint sources (those not immediately detectable in a few minutes), it is best if no dithering is performed. Be aware, however, that dithering results in additional overheads that can be significant for short observations or observations with a large number of dither positions (see the example given in Section 5.3.1.1.a). These additional overheads are included in USPOT observation time estimates. Instructions for how to set up dithers is straight forward and given in Section 3.2.1. An example of what a 5-element dither with 10 arcsec offsets looks like can be seen in Figure 5.3-4. Here the field of interest (NodA-Chop1) with the 4 dither offsets are being visualized without the reference fields to show the dither offsets more clearly.

Figure 5.3-4.
Screenshot of planning tool showing 5-element dither with 10 arcsec offsets

Figure 5.3-4.

Return to Table of Contents

5.3.2   Spectroscopic Observations: Acquisition and Grism AOR Fields

For each target (or observations of a target over multiple flights) a pair of Acquisition AORs must be established: the first with the Slit field set to None and the second with the Slit field set to FOR_LS47.

Since it is important for the acquisition image configuration to be the same as that of the science observations, one can select an AOR that has the appropriate configuration and import that into the acquisition AOR. This is done by selecting the desired AOR template from the Import Config from AORID dropdown menu within the FORCAST Acquisition AOR editing window. Only one set of acquisition AORs per target per slit used must be created. The recommended filter for acquisition is F111. However, one may choose to instead match the acquisition filter to the grism being used as follows: F077 for G063; F111 for G111; F197 for G227; and F315 for G329. If necessary, other filters may be used for acquisition after discussion with the Support Scientist. An estimation of the integration time necessary to achieve a S/N ≥ 5 in that filter should then be performed. It is assumed that for most spectroscopic targets, this will be on the order of 10–60 seconds. Every acquisition will add 5 minutes of duration to your program.

Return to Table of Contents

5.3.2.1   Instrument Parameters

5.3.2.1a   Exposure Time

The desired on-source integration time should be determined by using the FORCAST Grism Observation Calculator and is entered into the Exposure Time field. This value does not include overheads.

Though possible to dither along the slit in spectroscopy observations, it is recommended to only do so for bright sources. In this case, there are two possible approaches to conducting the observations: one can step through each position in the dither pattern once until the pattern is complete, or the dither pattern can be looped through multiple times until the total on-source time is achieved. In general, it is better to only loop through the dither pattern once as this minimizes the overheads associated with the observation (and maximizes the observing efficiency). However, in some cases it may be desirable to loop through the dither pattern. This can be specified with the Cycles field. The total on-source time will be the value entered into the Exposure Time field multiplied by the number of Cycles. Note that at this time, USPOT does not properly account for the additional overheads associated with looping through a dither pattern, and therefore this option should be disussed with the support scientist if required.

Return to Table of Contents

5.3.2.1b   Min Contiguous Exp Time Field

In some cases, it is necessary to split an observation among multiple flight legs. The Min Contiguous Exp Time field should be used to provide flight planners with information on the minimum amount of time that can be scheduled for a single flight leg to be scientifically useful. For example, if a program has been awarded 2 hours of time for imaging a faint target, it may be necessary to divide the observation over multiple flight legs. If the source is faint enough to require at least 15 minutes on-source in order to be able to accurately coadd the data from multiple flight legs, then this should be entered into the Min Contiguous Exp Time field.

Return to Table of Contents

5.3.2.1c   IR Source Type

When pipeline processing FORCAST spectroscopic data, it is useful to know whether or not the source is extended or point-like in the IR. Setting the IR Source Type to Point Source, Extended Source, or Unknown ensures that the proper extraction routines are used during processing of the science data.

Return to Table of Contents

5.3.2.1d   Instrument Configuration, Grisms, and Slit Fields

Finally, the appropriate Instrument Configuration, Grism (SWC and LWC), and Slit must be selected for the AOR. The options in the Instrument Configuration field includes both single channel configurations: GRISM_SWC and GRISM_LWC.

Return to Table of Contents

5.3.2.2   Chop/Nod Style

The available selections for the Chop / Nod Style field and their related parameters are discussed below. For observations dealing with extended objects larger than the FORCAST FOV, see Section 5.3.2.2b.

Return to Table of Contents

5.3.2.2a   Nod Match Chop

As is the case for imaging observations, the majority of spectroscopic observations with FORCAST will involve objects that are relatively compact (source diameters <120 arcsec) and lie in areas of the sky free from contaminating extended emission. For such observations, using the Nod Match Chop (NMC) Chop/Nod Style is recommended. Because coma will increase with larger chop throw (about 1 arcsec of coma for every 60 arcsec of chop throw in NMC mode), chop throws should be configured to be as small as possible but large enough that they know they will be chopping off of their source and onto clean sky. If there are clear chop reference areas all around the science source, it is recommended that the NMC mode be used and configured with a Chop Angle of 30 degrees and in the Array Chop Angle Coordinate system for long-slit spectroscopic observations.

Figure 5.3-5 shows a long-slit spectroscopic observation using a NMC set up is shown on the bright elongated object at the center of W3. This chop/nod set-up is configured in Sky coordinates so that the chop/nod reference positions avoid the bright nearby emission to the north of this science target. However, there still appears to be extended diffuse emission contained in the reference slit positions (as seen in the MSX image). If the observations are short, the extended diffuse emission contained in the reference slit positions may not be a problem. The proposer must confirm that this emission is below FORCAST's detection level given the exposure time of the proposed observations and the wavelength dependence of the extended emission. However, if the proposer wants to perform deep observations of the region or is unsure if the extended emission in the reference fields will be a problem at the particular wavelengths to be observed, then he/she should try to chop and nod farther away. However, beyond about 180 arcsec, the chopped sources will have significant coma (>3 arcsec). Though image quality is less of an issue for spectroscopy than imaging, a large coma will decrease the effective brightness of the source by spreading out the flux, thus decreasing the expected S/N of a spectrum in a given amount of exposure time.

If very large throws are needed to reach clean sky, observations should be configured using NXCAC mode. Please note that these observations are much less efficient (about 3.5 times longer duration than C2N modes). Therefore, proposers who were awarded time for observations under the assumption that NMC mode would be used but find, during Phase II, that NXCAC will be required, will have to observe with fewer grisms or observe fewer targets to still fit the NXCAC observations within the awarded time.

Figure 5.3-5.
Illustration showing a long-slit spectroscopic observation using a NMC set up

Figure 5.3-5. A long-slit spectroscopic observation using a NMC set up is shown on the bright elongated object at the center of W3. The long red rectangle shows the long slit centered on the source to be observed, while the long green rectangle and long blue rectangle show the chop/nod reference slit positions.

Return to Table of Contents

5.3.2.2b   NXCAC

If very large throws are needed to reach clean sky, the observations should be configured using NXCAC mode. The NXCAC (Nod Unrelated to Chop/Asymmetric Chop) Chop/Nod Style is analogous to C2NC2 Chop/Nod Style for imaging in that it is less efficient (about 3.5 times longer duration than C2N modes) but allows for large throws without negatively affecting image quality. Proposers who were awarded time for observations under the assumption that NMC mode would be used but find, during Phase II, that NXCAC will be required, will have to observe with fewer grisms or observe fewer targets to still fit the NXCAC observations within the awarded time.

If the source is surrounded by or has nearby extended emission, the Chop Throw and Chop Angle must be configured in the Sky Chop Angle Coordinate system to avoid contamination in the chop reference fields. An example is shown in the left image of Figure 5.3-6, which allows chop throws up to 8 arcminutes and nod throws up to 10 degrees. In this asymmetric chop mode, sources will have no coma and one can sample clean sky relatively far from sources. If one wishes to perform very deep spectroscopic observations on the source of interest (long red rectangle), the chop should be configured to be far away and at an angle that will get the chop reference slit positions on much cleaner sky (long green rectangle). The nod position is then chosen to be far enough away that there is no chance of background emission in those slit positions (long blue rectangle and long purple rectangle).

This set-up will work up to a chop throw of 250 arcsec. If chops larger than this are still necessary to reach a chop reference field of fainter background emission, then the NXCAC AOR setup must be configured with a preferred chop angle and a range of other possible chop angles in the likelihood that the preferred angle cannot be used (due to hardware limits involving the sky rotation angle at the time of observation) specifiied in the Chop Angle Range Set Up diaglog window via the Set Chop Angle Ranges button. The right image in Figure 5.3-6 gives an example of this setup for W3. Notice these angles encompass the cleanest sky areas in the MSX image for this region, and thus are likely to be suitable for chop reference slit positions for very long integrations. At the time of observing, a chop angle will be chosen from these ranges if the preferred angle is not possible. It is in the PI's best interest to specify as large a range (or ranges) of chop angles as possible.

Figure 5.3-6.
Illustration showing how Chop Throw and Chop Angle must be configured in the Sky Chop Angle Coordinate system

Figure 5.3-6. Left, An example of observations using NXCAC mode in the Sky Chop Angle Coordinate system. Right, An example of NXCAC observations, where the preferred chop angle of 120 degrees is shown as the long green rectangle and two alternate ranges are shown as the orange-to-cyan lines, one from 70–170 degrees and the other from 225–270 degrees.

Dithering observations along the slit may be advisable; Section 5.3.2.3 discusses the general rules of thumb for using dithers with FORCAST.

Return to Table of Contents

5.3.2.2c   Nod Perp Chop

Nod-Perpendicular-to-Chop (NPC; Nod Perp Chop CAS and Nod Perp Chop NAS) Chop/Nod Styles are offered in USPOT for the long-slit spectroscopy. Experienced IR observers wishing to use this mode must submit an appropriate justification for using this mode to be discussed this with the SOFIA Support Scientist. Two special NPC setups are available: Nodding Along the Slit (Nod Perp Chop NAS) or Chopping Along the Slit (Nod Perp Chop CAS). As is the case with imaging mode, and contrary to popular belief, there is no sensitivity advantage in using one of these NPC modes over NMC mode, as both modes yield the same S/N in the same exposure time (as discussed in Signal-to-Noise as a Function of Chopping and Nodding and Signal‐to‐Noise Estimates for Various Chop/Nod Techniques with FORCAST).

Return to Table of Contents

5.3.2.2d   SLITSCAN

It is also possible to perform slit scan observations of extended sources with FORCAST grisms, wherein spectra are acquired at a number of positions across the source as defined by the PI. This mode is defined by specifying a set of dithers that offset the telescope perpendicular to the slit. Once SLITSCAN mode has been chosen, the default five position Dither Pattern is loaded into the dither panel. The default scan pattern is defined with the spacing between each consecutive slit position equal and with the scan pattern centered on the given source position. So, for example, if the proposer wants the slit scan to include five slit positions with overlapping coverage, then they might set the dither pattern as shown in Figure 5.3-7. Since the wide long slit is 4.7 arcsec wide, setting a dither offset of 4 arcsec will result in a slit overlap between consecutive slit positions. The number of positions in the scan cannot be explictly set—instead, the number of discrete slit positions is determined by the user specified Dither Offset and Scan Size, i.e. the total length of the scan. The number of slit positions then is the scan size divided by the dither offset, rounded up, plus 1. Since the slit scan is performed perpendicular to the slit, this mode can only be performed in array coordinates. This means that the actual orientation of the scan on the sky will not be known until after flight planning.

Figure 5.3-7.
Screenshot showing example dither pattern

Figure 5.3-7

Return to Table of Contents

5.3.2.3   Dithering

The FORCAST array is relatively clean cosmetically for most observations. For spectroscopic observations, a spectrum may lie across bad or hot pixels and could be misinterpreted in the extracted spectrum as a line. To mitigate this, we set up our calibration stars on the same pixel as the science targets, which means that the science spectra are dispersed over the same pixels of the array as the calibration spectra. Any pixel-to-pixel variation should therefore be removed when the science data are divided by the calibration data. For extended sources, the removal of such cosmetic issues can be accomplished with dithering. Dithering refers to small movements (of the order of 3–15 arcsec) of the telescope which place the imaged object at different locations along the slit and, therefore, place the spectra at different positions on the array. In spectroscopy, typically one dithers 3 or 5 times along the slit direction only. Shifting and then median combining these images can remove any patterns that are positionally dependent on the array. For faint sources (those not immediately detectable in a few minutes), it is best if no dithering is performed. If you are imaging a bright source and wish to dither, any of the spectroscopic observation modes discussed (NMC or NXCAC) can be performed with dithers along the slit included. Instructions for how to set up dithers is straight forward and given in the Observer's Handbook. Proposers who have a strong justification for dithering in this mode should discuss this with their support scientist. For observations in C2NC2 imaging mode, dithering is required but should not be set up by investigators during Phase I proposal submission. An appropriate C2NC2 dithering pattern will be chosen by the support scientist during Phase II.

Return to Table of Contents

5.3.3   General AOR Fields

5.3.3.1 Observation Condition & Acquisition/Tracking Window

5.3.3.1a   Visual Magnitude

It is important to check the visual magnitude of each science target. If the target is brighter than 14th magnitude and point-like (i.e., compact), then the magnitude and the reference wavelength (band V, B, or R) should be provided in the Observing Condition & Aquisition Tracking window. In this case, the telescope can guide on the source directly, which greatly decreases observation set-up overhead and translates to more time taking science data on the target during a scheduled flight leg. If the object is fainter than this, Invisible must be selected from the Visible Wavelength pull-down (any value in the Visible Magnitude field will then be ignored).

It is also a good idea to fill in the IR flux of the source at a reference wavlength close to the one being observed. This helps the observer to assess whether or not the observations are proceeding as expected.

Return to Table of Contents

5.3.3.1b   Observation Order

The most important parameter to set in terms of observation prioritization is given by the Observation Order parameter. This field allows the priority of their observations of a single target within an observing leg to be numerically listed. The length of an observing leg is rigidly defined during flight planning and cannot be extended in flight. If, for instance, acquisition takes an unusually long time or if there are any hardware/software failures during the observing leg for a particular target, it may be that not enough time will remain for all of the scheduled observations of the target to be performed. For this reason, observations should be prioritized.

If achieving the full exposure time in some filters or grisms over others is most important to a program, then the filters must be ordered serially (i.e. the first observation to be executed will be Order = 1, the second will be Order = 2, and so on). In this way, if time is cut short on an observing leg, all of the time in the observations with the lowest Order values will be achieved, but observations with the highest Order values may get little or no time. However, if it is better to get some time in all filters/grisms and any time lost is taken from all observations equally, then the observations of a single AOR should be separated into multiple, smaller AORs and prioritized using the Order parameter so that essentially all filters throughout the leg will be looped through.

Here is a simple FORCAST example that shows how breaking up AORs of a single target would be advantageous:

A proposer wants to observe Jupiter for 40 m with 20 m in the 37.1 μm filter and 20 m in the 7.7 μm filter. If the proposer gave the 37.1 μm filter Order = 1 and the 7.7 μm filter Order = 2, and a 40 m leg is scheduled but an in-flight computer malfunction leads to a loss of 20 m on the leg, then the proposer will end up with a 20 m observation of Jupiter in the 37.1 μm filter, but no 7.7 μm observation. If, instead, the proposer splits each 20 m AOR into two 10 m AORs and gave the first 37.1 μm filter Order = 1, the first 7.7 μm filter Order = 2, the second 37.1 μm filter Order = 3, and the second 7.7 μm filter Order = 4, this would have the effect of looping twice through the filters with half the time in each visit. Therefore, during a 40 m leg with a 20 m time loss, the proposer would end up with about 10 m in both filters (or ~70% the S/N of 20m observations; however note that there is also some small additional loss of efficiency due to filters changes).

Return to Table of Contents

5.3.3.1c   Requested WV Overburden

Another common concern is whether extra sensitivity is needed for a particularly challenging observation. Generally, the higher the aircraft flies, the lower the water vapor overburden and the greater the sensitivity of FORCAST (especially at wavelengths >25 microns). If a target is particularly faint, the Requested WV Overburden may be changed from Nominal, to Low or Very Low. Though there is no guarantee that the observations will be taken under low water vapor conditions, a best effort will be made to accommodate such requests by scheduling the observations at the highest altitudes within the limitations of the flight plan.

Return to Table of Contents

5.3.3.2 Comments

It is very likely that most or all of the PI's observations will be performed without them on board the aircraft. Therefore it is vitally important that any special requests or procedures be conveyed through the comment tool. These comments will be read by the observers in flight and will also be viewed by flight planners. Therefore any comments to either of these groups should be written in the text field of this pop-up window (see example in Figure 5.3-8). These comments will be reviewed by your support scientist during the Phase II process to ensure that they are thorough, clear, and understood.

Figure 5.3-8.
Screenshot of comment box

Figure 5.3-8.

Return to Table of Contents

Download the PDF Version

Share This Page