This article discusses a high contrast imaging science use case using MIRI and NIRCAM coronagraphy.


Introduction

In planning any JWST coronagraphic observation, the following steps should be followed:

  1. Choose which instrument(s), filter(s) and coronagraph(s) based on the observer's science goals, and to some extent on the contrast and separation of the science target (e.g., disk, low-mass companion). When assessing feasibility, observers may need to use models to extrapolate shorter wavelength measurements to 2–25 µm, for instance to determine companion contrasts at NIRCam or MIRI wavelengths from far infrared or submillimeter wavelengths—e.g., from Herschel, Spitzer, IRAC, etc. Additionally, the user will want to use the JWST Coronagraph Visibility Tool (CVT) to examine the target's visibility and available position angles versus time.

  2. Select a PSF subtraction strategy. The recommended standard sequence is 2 rolls on a science target, followed by a PSF calibrator, all observed back-to-back in sequence.
  3. Calculate the necessary exposure times using the JWST Exposure Time Calculator (ETC) coronagraphic capabilities. You may want to upload custom spectral models for cool substellar atmospheres. 
  4. Select a PSF reference star (See Selecting Suitable PSF Reference Stars for JWST High-Contrast Imaging). This should be based on proximity, brightness, spectral type match, and avoidance of binaries. 
  5. Set up observations in the Astronomer's Proposal Tool (APT); assess overheads and total program time; iterate as desired. It is important to consider significant overheads: slews and rolls, guide star acquisitions, target acquisitions, etc.

 


Coronagraphy of the debris disk archetype Beta Pictoris

Science goals

In this science use case we will observe the debris disk around Beta Pictoris (Beta Pic) using both the NIRCam and MIRI coronagraphs to cover a range of 1.823 µm. One of the brightest and largest disks on the sky, Beta Pictoris is famous as the first circumstellar disk to be spatially resolved. While generations of observations have delivered insights into the physical processes that shape the disk, composition of its constituent particles and complex structure, it remains a compelling target for detailed investigations at the unprecedented sensitivities capable with JWST. NIRCAM's 1.824.44 µm filters are sensitive to the presence of water, CO ices and organic tholins, which will allow for characterization of the disk composition and spatial variations. MIRI 15 and 23 µm coronagraphy will probe the warm inner asteroid belt analogue and the cooler outer main disk, respectively. In addition to revealing the disk in detail, deep NIRCam F444W observations will achieve extraordinarily low detection limits to search for unknown wide-separation planetary companions, reaching well below the mass of Saturn. Note, the known planet Beta Pic b will be at a small projected angular separation from the star at this time, after its near transit in front of the star in 2017. Observing the planet is not a goal of this program. 

The specific goals of this program are to:

  • Characterize the archetypal debris disk around Beta Pictoris with deep imaging in multiple filters across JWST's entire wavelength range.
  • Measure the disk structure, composition and interactions with planets.
  • Test for the presence of water and CO2 ices and organic tholins (like on Titan)
  • Measure color variations and asymmetries across the disk.
  • Probe the thermal emission from both the warm inner belt and outer cooler main disk.
  • Obtain a comprehensive legacy dataset on this target, for analysis alongside similar data and/or on other debris disks studied with NIRCam and MIRI.

Observing strategy

We will use NIRCam's 6 medium band filters for probing the disk composition: two of which, 1.82 and 2.10, are in the short wavelength (SW) channel; four of which, 2.5, 3.0, 3.3 and 4.1, are in the long wavelength (LW) channel. We will use the round coronagraphs to obtain full azimuthal coverage (i.e. the MASK210R in 2 filters and MASK335R in 4 filters). For MIRI observations, we will observe with the 1550 FQPM and 2300 Lyot masks.

We expect to be most sensitive to the disk at separations <5", where stellar PSF residuals will be significant. Thus, we will employ the standard coronagraphic observing sequence for every instrument/mask/filter combination observed: an initial observation oriented at a desired nominal aperture position angle, followed by a second observation with an aperture PA ~10° relative to the first observation, followed by observation of the PSF reference star. All of our observations will be linked as a non-interruptible sequence. Because Beta Pictoris is one of the brightest disks in the sky, we will not require the use of small grid dithers. 

Choosing a PSF reference star for Beta Pic

Our science target, Beta Pic, is a spectral type A6 star with a K magnitude of 3.48 and celestial co-ordinates of 05 47 -51 03. Following the guidelines provided in the "Selecting Suitable PSF Reference Stars for JWST High-Contrast Imaging" article, we are looking for a PSF reference calibrator that is:

  • Relatively nearby (to ensure scheduability and minimal slew time – which in turn minimizes thermal changes to the telescope and thus changes to the PSF)
  • Closely matched in spectral type (less critical at MIRI wavelengths, because the FQPM filters are relatively narrow, but has a stronger impact for NIRCam as the wavelengths shorten and as the spectral bandwidth widens)
  • Close in magnitude (allowing shorter exposure time on PSF star – especially helpful given our choice to use SGDs)
  • Is non-binary (and so will appear optically single at JWST resolution)

A useful tool to aid in the selection of a PSF reference star is the Jean-Marie Mariotti Center (JMMC)'s SearchCal, a GUI that allows to select suitable, non-resolved calibrators using a number of search criteria. SearchCal dynamically finds calibrators near science objects by querying CDS3 hosted catalogs according to observational parameters.


Figure 1. Search for a suitable PSF-reference target for Beta Pictoris with SearchCal

For more details, see Selecting Suitable PSF Reference Stars for JWST High-Contrast Imaging.


Using SearchCal, in combination with a number of other planning tools, we find Alpha Pictoris will serve as a good reference. It is located ~19° from the target star (at 06 48 11.4516; -61 56 28.8060) and has an overlapping visibility window. The Vizier Photometry viewer indicates no obvious IR excess around Alpha Pic and it has been previously used in HST observations as a PSF calibrator for Beta Pic at shorter wavelengths. The difference in K mag between Alpha Pic (K=2.57) and Beta Pic (K=3.48) is ~0.9. Furthermore, Alpha Pic (A8V) is fairly close in spectral type to Beta Pic (A6V), which provides a more than adequate color match (given previous estimates of the impact of color mismatch on NIRCam coronagraphy contrast) to minimize chromatic differences between the reference PSF and target PSF. 


 

Observing description

We find there are two strategies in which to schedule our set of observations. In the first approach, the observations made in each instrument are scheduled together in a non-interruptible sequence, at the orient in which the spatial coverage of the disk is maximized. Alternatively, by making a slight sacrifice in orientation and requiring that the NIRCam and MIRI observations be scheduled together in one long non-interruptible sequence (with the PSF reference observations placed between them) it is possible to save on overheads. However, this second approach puts very tight restrictions on the scheduability of the observations, resulting in a single 2 day scheduling window. This is potentially problematic: if any observation failures were to occur that were not caused by the program itself (e.g., caused by an instrument or telescope operational problem, malfunction or safing event), there would not be another opportunity in which to repeat this sequence of observations within that observing cycle -- the program would need to be deferred for at least another year. Thus, we will go with the first scheduling approach. This increases the schedulability of the observations from ~2 days per year to ~2 weeks and allows us slightly more ideal instrument orientations, at the expense of slightly longer overheads.  We note that if there is a TA issue or something that prevents scheduling of the observations, there are other possible orientations that are availabel other than those provided. 

The order in which the two sets of observations will be scheduled is as follows (NB: times specified are according to APT 25.4.2):

 

Table 1. Beta Pictoris observation techniques with NIRCam and MIRI coronagraphs

Observing Beta Pictoris with the NIRCam coronagraphs

Observing Beta Pictoris with the MIRI Coronagraphs:

  1. Slew to PSF reference star
    (1800s)
  2. Observe PSF reference star with NIRCAM LW filters
    (3616s science exposure, 6590s total time)
  3. Observe PSF reference star with NIRCAM LW filters
    (1676s science exposure, 4152s total time)
  4. Slew to science target 
    (976s)
  5. Observe science target with NIRCAM SW filters
    (3014s science exposure, 6082s total time)
  6. Roll Observatory ~12° 
    (925s)
  7. Observe science target with NIRCAM SW filters
    (3014s science exposure, 6074s total time)
  8. Observe science target with NIRCAM LW filters
    (6967s science exposure, 10416s total time)
  9. Roll Observatory ~12° 
    (925s)
  10. Observe science target with NIRCAM LW filters
    (6967s science exposure, 10505s total time)

  1. Slew to science target 
    (318s)
  2. Observe science target with MIRI F2300C
    (1782s science exposure, 3430s total time)
  3. Observe science target with MIRI F1550C 
    (1319s science exposure, 2824s total time)
  4. Roll observatory ~12° 
    (925s)
  5. Observe science target with MIRI F1550C
    (1319s science exposure, 2775s total time)
  6. Observe science target with MIRI F2300C
    (1782s science exposure, 3445s total time)
  7. Slew to PSF reference star 
    (977s)
  8. Observe PSF reference star with MIRI F2300C
    (972s science exposure, 2596s total time) 
  9. Observe PSF reference star with MIRI F1550C
    (720s science exposure, 2129s total time)

This order was chosen in order to minimize overheads while also minimizing time between reference PSF observations and science target observations. All NIRCam and MIRI observations are grouped separately by a "Sequence Observations, Non-Interruptible" special requirement (aka "Seq Non-Int SR") to ensure the PSF calibrator is observed close in time to the science target. 

Note that the total time is dominated by overheads: slews, target acquisition procedures etc. As such, the program contains 9.21 hours of science time and 18.84 hours of total charged time (including all overheads and observatory indirects), according to APT 25.4.2's Smart Accounting tool

There are three observatory tools that you will want to utilize for coronagraphic observational planning, these are the: JWST Coronagraph Visibility Tool (CVT), JWST Exposure Time Calculator (ETC) and JWST Astronomer's Proposal Tool (APT). The CVT and ETC play crucial roles in the planning of planning MIRI and NIRCam coronagraphic programs prior to entering targets and observations into APT—APT is the final arbiter of schedulabilityWe refer the reader to the JWST Coronagraphic Observation Planning page for the procedures and strategies that should be followed when planning coronagraphic observations with JWST.

 


Using the Coronagraph Visibility Tool

For our science case, we will utilize the Coronagraph Visibility Tool (CVT) to assess the target visibilities and their available position angles (PAs) versus time relative to the MIRI and NIRCam coronagraphic masks—particularly relative to instrumental structures such as occulting bars in NIRCam or boundaries in the MIRI 4-quadrant phase mask (4QPM) coronagraphs. 

We can resolve Beta Pic using SIMBAD (CVT step-by-step guide). The disk's midplane is at a PA of 32° and has a separation and outer radius of around 0.4" and 13.4", respectively. Thus, we specify companions at a PA of 32 and separations of 0.4" and 13.3", to get an indication of the extent of the disk on the selected instrument and mask. We also define a another companion at a separation of 13.4" diametrically opposed to the first, to act as a proxy for the disk structure's position angle versus time. 


Figure 1. CVT output for Beta Pictoris

 


Because our observations are split into two separate groups/ sequences, we can chose slightly different orientations for the MIRI and NIRCam observations. Our aim is to determine orients that maximize the spatial coverage of the disk, but that are not coincident with any obscurations in the coronagraphic fields of view.

For the NIRCam observations, because the disk is edge-on, we will chose an orientation that places the disk midplane near the diagonal of the NIRCam coronagraph subarray to maximize spatial coverage, but avoid the ND spots. We find that the ideal orientation would be at an aperture position angle (APA) of ~350°, and other orientations would sacrifice some of the science. Consequently, we determine that an APA range of 345°–360° will be suitable for our NIRCam observations.

For MIRI, we will orient the disk at an angle of ~45° from the 4QPM axes, which corresponds to an APA of ~345°. Note that this orientation also avoids the two supporting struts in the mounting bracket of the Lyot coronagraph, which themselves block light in the FOV. We will define a corresponding APA range of 340°–355°.

We note that while we plan to define these specific PA range requirements, they are not completely rigid. In the event of a TA issue or other scheduling problem, we could consider alternative orientations. For the MIRI observations in particular, we only require the disk midplane is oriented near ~45 deg from the 4QPM axes-- there are several orientations acceptable, but we can only program one into APT. For NIRCam, the orientation we request is ideal however alternative orientations could be considered if absolutely necessary. 

Here's a summary of the corresponding observing restrictions that have determined for our program, using the CVT:

  1. We will define an APA Range of 345° to 360° for our NIRCam observations, specifically placing the disk near the diagonal of the coronagraphic subarray, maximizing spatial coverage but avoiding the ND spots. 
  2. We will define an APA Range of 340° to 355° for the MIRI observations, placing the disk midplane ~45° from the 4QPM axes. 
  3. The selected orientation of our MIRI observations places the disk in quadrants 2 and 4 of the 4QPM; thus we will specify that TA is performed in quadrant 1.
  4. We will adopt a relative roll angle between observations of 10° to 14°.

 


Using the Exposure Time Calculator

For our ETC calculations, we must begin by defining a scene in which to associate our sources within the Scenes and Sources page: we will create a scene consisting of the Beta Pic target star and the Alpha Pic reference star (both point sources with their spectra modeled using Phoenix stellar models of the appropriate temperatures) and model the disk by placing four extended sources at a few arcseconds from the star, each normalized to a different surface brightness, in order to represent the disk at different circumstellar distances. 

We will emulate Alpha and Beta Pic using A5V model Phoenix spectrums (with effective temperature of 8250 and log g of 4.0) renormalized to vega magnitudes of K = 2.57 and 3.48 in the Johnson filter, respectively. We will position Beta Pic and Alpha Pic at the center of the scene.

For our NIRCam observations we shall create separate scenes for both our LW and SW observations. We will normalize the surface brightness to 1.82 µm and adopt the A5V 8250 4.0 Pheonix model stellar spectrum as a reasonable assumption for the disk. The 1.82 µm surface brightnesses are calculated by extrapolating the STIS surface brightness vs. separation curves shown in Ballering et al. (2016). The four extended sources (with semi-major and minor axes of 0.5") that will be used as a proxy for the disk will be placed at equidistant offsets from Beta Pic such that the 1.82 µm surface brightness of the sources are 10, 1, 0.1 and 0.01 mJy per arcsec2 clockwise from the source positioned in the top right-hand corner of the scene. For the SW NIRCam observations, this separation should be 1", such that the 10, 1, 0.1 and 0.01 mJy/arcsec2 sources are located at X and Y co-ordinates of (1, 1), (1, -1), (-1, -1) and (-1, 1) arcseconds, respectively. For the LW observations, these offset co-ordinates will be (2.5, 2.5), (2.5, -2.5), (-2.5, -2.5) and (-2.5, 2.5) arcseconds from Beta Pic, respectively.

 


Figure 2. Scene sketches of scenes defined for our NIRCam LW and SW observations

 

 



For the MIRI observations, we will create a different scene for the Lyot and 4QPM observations. In each scene, we will estimate the surface brightness using the 24 µm surface brightness maps shown in Ballering et al. (2016). Four our Lyot observations, we create three flat continuum extended sources, one placed at X and Y offsets of 3.5" and 3.5"; another at 3.5" and -3.5"; and a third at -3.5" and -3.5", with 24 µm surface brightnesses of 60, 10 and 1 mJy/arcsec2, respectively. For the 4QPM scene, we instead normalize the spectra to have a flux densities of 60, 10 and 1 mJy at 15 μm.

 


Figure 3. Scene sketches of the scenes defined for our MIRI Lyot and F1550C 4QPM observations

 

 



Target Acquisition 

We now determine the target acquisition exposure parameters using the ETC. The JWST ETC MIRI Target Acquisition and JWST ETC NIRCam Target Acquisition articles may be helpful when setting up these calculations. We must determine the exposure time required to obtain a sufficient signal-to-noise for the TA procedure to achieve the desired centroid accuracy. In order to achieve the centroid accuracy requirements for NIRCam and MIRI target acquisition, the minimum required integrated (within the extraction aperture) signal-to-noise ratio (SNR) must be ≥ 30. Saturation can also affect the accuracy of the centroiding procedure, and should be avoided. Our aim is to find an exposure configuration for each instrument that strikes a balance between a high SNR and low exposure time (MIRI & NIRCam coronagraphic best practices articles pending). Because both the target and reference star are bright, we require the use of neutral density (ND) filters to prevent the star from saturating the detector during the subarray exposures used for the target locates.

As we discovered from our previous work in the CVT, the orientation selected for our observations places the disk midplane in MIRI quadrants 2 and 4. Because Beta Pic is edge-on, we will perform target acquisition (TA) in the quadrants that the disk does not reside in. As we do not expect to perform science in the TA quadrants, we do not require a 2nd exposure with the MIRI observations to treat persistence. Thus we will perform TA in quadrant 1. 

Note that NIRCam can take multiple filters on the same coronagraph after one target acquisition; whereas MIRI needs t switch coronagraph and do TA for every filter change.

 For the NIRCam 335R observations with RAPID, the ETC indicates that NGROUPS > 9 is required to meet the ETC's recommended threshold of 30 for the extracted SNR. Because TA exposures are so short compared to the rest of the program (low cost), we might as well do a little better than this, so we chose NGROUPS = 33 which gives an extracted SNR of 278.15 and no saturation. For NIRCam 210R observations, the ETC indicates NGROUPS > 3 with RAPID to achieve an extracted SNR > 30. We chose NGROUPS = 17 which gives an extracted SNR = 383.3 with no saturation issues.

 For MIRI 1550 observations, the ETC indicates that NGROUPS > 3 with FAST is sufficient to achieve an extracted SNR > 30. As such, we set NGROUPS=9 which gives an extracted SNR = 324.34 and no saturation issues. For the MIRI 2300 observations, the ETC indicates NGROUPS > 5 with FAST is required to achieved an extracted SNR > 30. We opt for NGROUPS = 45; this gives an extracted SNR of 378.48 and returns no saturation issues.

Because the reference PSF target is only a factor of 2 brighter, and we have plenty of dynamic range with TA left, we adopt the same exposure settings for the reference PSF target. 

 


Figure 4. The SNR per pixel over the 2D scene for our MIRI (LEFT) and NIRCam (RIGHT) TA calculations on Beta Pic, produced by the ETC

 



Coronagraphy

For our coronagraphic observations we arrived at the following exposure settings. Because we are interested in the disk at a range of separations, including regions where the stellar residuals dominate, we have a large dynamic range to consider and must avoid saturation for precise PSF subtraction. Thus we will avoid full saturation of the target star's speckles. We will allow a few dozen pixels to be partially saturated, under the assumption that the ramps for these pixels can be recovered by the pipeline.

We note that because the target star is very bright (Kmag = 3.5), even when reading out the coronagraphic subarray in rapid, a small number of pixels may fully saturate during the NIRCam 210R observations. We should observe the disk at a SNR of ~3 per pix or more interior to ~5". However by binning pixels, we may be able to detect the disk at ~10", near the outer edge of the subarrays. 

Our calculations used to estimate the NIRCam observations of Beta Pictoris are entered into the ETC as follows (in the form Coronagraph, Filter, Subarray, Readout pattern, Groups per integration, Integrations per exposure):

  1. MASK335R, F250M, SUB320, BRIGHT2, NGROUPS = 10, NINTS = 80 
  2. MASK335R, F300M, SUB320, BRIGHT2, NGROUPS = 10, NINTS = 80
  3. MASK335R, F335M, SUB320, SHALLOW4, NGROUPS = 10, NINTS = 35
  4. MASK335R, F444W, SUB320, BRIGHT2, NGROUPS = 10, NINTS = 80 
  5. MASK210R, F182M, SUB640, RAPID, NGROUPS = 4, NINTS = 90 
  6. MASK210R, F210M, SUB640, RAPID, NGROUPS = 4, NINTS = 90

Here equations 1-4 are used to determine our exposure configurations for the NIRCam LW observations of Beta Pic (i.e. observations 12 & 13) and equations 5 & 6 for the NIRCam SW observations (i.e. observations 10 & 11). Version 2.1 of the JWST ETC produces the following estimations:

  1. Exposure time of 1795.99s; extracted SNR of 1.2e-3 and 42 partially saturated pixels.
  2. Exposure time of 1795.99s; extracted SNR of 1.9e-3 and 23 partially saturated pixels.
  3. Exposure time of 785.74s; extracted SNR of 8.9e-4 and 12 partially saturated pixels.
  4. Exposure time of 1795.99s; extracted SNR of -0.00 and 2 partially saturated pixels.
  5. Exposure time of 1883.64s; extracted SNR of 0.00; 172 partially saturated pixels and 34 fully saturated pixels.
  6. Exposure time of 1883.63s; extracted SNR of 0.00; 115 partially saturated pixels and 8 fully saturated pixels.

For the above settings, we should observe the disk at a SNR of ~3 per pix or more interior to ~5". By binning pixels, we may be able to detect the disk at ~10", near the outer edge of the subarrays. We find that for equations 1, 2, and 4 the SHALLOW4 readout pattern could be an option, however would result in ~200 partially saturated pixels. The exposure settings chosen instead were verified in the ETC to not produce any warnings other than a few dozen pixels partially saturated. 


Figure 5. ETC outputs for our NIRCam MASK335R+F335M and MASK210R+F182M coronagraphic mask + filter combinations

Column 1 shows the SNR, Detactor and Saturation plots for the MASK335R/ F335M calculation, respectively. Column 2 shows the SNR, Detector and Saturation plots for the MASK210R/ F182M calculation, respectively.


For the MIRI calculations we specify the following exposure settings (in the form Coronagraph, Filter, Subarray, Readout pattern, Groups per integration, Integrations per exposure):


  1. FQPM F1550C, MASK1550, FAST, NGROUPS = 100, NINTS = 55
  2. LYOT F2300C, MASK2300, FAST, NGROUPS = 100, NINTS = 55.

Resulting in the following estimations:

  1. Exposure time of 1318s and extracted SNR of -1.10 with no saturation.
  2. Exposure time of 1282s and extracted SNR of -1.34 with no saturation.

The reference PSF star (Alpha Pic) is 0.9 mag brighter than the science target, so we reduced the observations to be roughly half as long in each filter. These parameters are: 

  • BRIGHT2, NGROUPS = 10, NINTS = 40 (for our NIRCam LW PSF observations in the F250M, F300M and F444W filters).
  • SHALLOW4, NGROUPS = 10, NINTS = 20 (for our NIRCam LW PSF observations in the F335M filter).
  • RAPID, NGROUPS = 4, NINTS = 50 (for our NIRCam SW PSF observations).
  • FAST, NGROUPS = 100, NINTS = 30 (for both MIRI PSF observations).

 


Implementing into APT

We will now discuss how to implement this program into the APT. We encourage that you read the JWST APT article and its related links to familiarize yourself with its functionality. We assume that the user has reviewed these articles and therefore understands how to construct a basic JWST observing proposal (including how to add targets, observations and visits, use observation templates and folders etc.), as well as how to run the APT Visit PlannerAPT Smart Accounting and JWST APT Aladin Viewer. Further help with APT training or problem resolution can be found in the JWST APT Help Features and JWST APT Training Examples and Video Tutorials articles. 

Defining targets

Our program consists of two fixed targets: the science target star Beta Pictoris (i.e. * bet Pic) and PSF reference star Alpha Pictoris (* alf Pic). We use the APT fixed target resolver tool to look up both targets from their Archival names and commit them to our proposal (which will auto-populate their J2000 Coordinates). We then enter the following information into the targets' templates:

Number

Name in Proposal1

CategoryDescription (key words)J200 Coordinates2 Proper motion4Epoch5
ICRS3 RAICRS3 DecRA (mas/yr)Dec (mas/yr)
1Beta Pic (target star)CalibrationCoronagraphic05 47 17.0877-51 03 59.444.6583.102000.0
2Alpha Pic (PSF reference)CalibrationCoronagraphic06 48 11.4551-61 56 29.00-66.07242.972000.0


 

1 Adding designations in the "Name in the proposal" field to clearly indicate which targets are intended for science and which are PSF-reference stars, can be particularly useful down-the-line!
2 J2000 Coordinates: The year of the equator and equinox to which the coordinates are referred must be J2000 (Julian). It is not necessary to apply precession corrections to coordinates from positional catalogs.
3 For observations it is vital that you provide positions derived in the ICRS reference frame. 
4 APT does not currently retrieve target proper motions from SIMBAD; users will need to manually enter target proper motion into APT.
5 The “Epoch of position” is the date of the data from which the position of a star with proper motion was measured, or to which it has been corrected.  Any epoch may be given but it must be correctly specified. The “Epoch of position” may or may not be the same as the date of “Equinox for Coordinates” (J2000). Remember that the “Epoch of position” is the date the target position is referred to, whereas the “Equinox of Coordinates” is the date of the coordinate frame, which changes because of the precession of the Earth's axis. 

 

Specifying observations

We can now begin fleshing out our proposal; because we have decided to split our MIRI and NIRCam coronagraphic observations into two different coronagraphic observation sequences (at different epochs), we can create two separate observation folders — e.g., "MIRI Coronography of Beta Pictoris" and "NIRCam coronagraphy observations of Beta Pictors"— in which to collect all the observations that pertain to each of the instruments' sequences. Our observations can then be specified, within the appropriate observation folder, by selecting a target and instrument observing mode. Each observing mode has a corresponding APT template that allows the user to specify paramaters appropriate to that mode of operation. Each observing mode has a corresponding APT template that allows the user to specify parameters appropriate to that mode of operation (see the MIRI Coronagraphic Imaging Template and NIRCam Coronagrahic Imaging Template guides). 

It is recommended that we create each observation template for each sequence first, just specifying the instrument, template, target and chosen label. We can then later come back and fill in the details of the specific observation. These steps will make it easier to make various connections, such as developing the PSF-reference observations, or adding necessary special requirement links to the observations.

We will create a total of 6 observations in each folder (i.e., 6 MIRI observations using the MIRI coronagraphic imaging template and 6 NIRCam observations using the NIRCam coronagraphic imaging template). We will chose to lable each the observations (so that they can be easily identified in the observation folder) as follows:



Visit 1:1


Visit 1:1


Visit 2:1


Visit 3:1


Visit 4:1


Visit 5:1


Visit 6:1




Visit 7:1


Visit 8:1


Visit 9:1


Visit 10:1


Visit 11:1


Visit 12:1



Note that while we may create these observations in any order, it is essential that we assign the "Observation number" for each observation as stated above (i.e. organized according Observing Strategy we previously devised). This is because within the APT, observations occur according to increasing observation number.

When crafting each observation within the appropriate instrument coronagraphic imaging template, we will have control over 3 primary parameters:

  1. coronagraphic mask + filter combination
  2. small-grid dithering type
  3. detector read out pattern and exposure time (via the number of groups and integrations).

The allowed values for each instrument template are documented and maintained in the MIRI Coronagraphic Imaging Template Parameters and NIRCam coronagraphic Imaging Template Parameters pages.

We will define our Target Acquisition parameters and Coronagraphic ("Coron") parameters according to our advance work in the ETC/ CVT. Note that, of course, pairs of roll-dithered observations will share the same exposure parameters (e.g., Obs2/ "Beta pic - 1550 4QPM Roll 1" will share the same exposure parameters Obs 3/ "Beta Pic - 1550 4QPM Roll 2").

In particular, we must remember that as we determined using the CVT, our chosen orientations for the MIRI coronagraphic observations place disk midplane in MIRI quadrants 2 and 4, so we shall define our "Acq Quadrant" in the "Target Acquisition Parameters" to be quadrant 1. 

As we discovered from our previous work in the CVT, the orientation selected for our observations places the disk midplane in MIRI quadrants 2 and 4. Because Beta Pic is edge-on, we will chose tp perform TA in the quadrants that the disk does not reside in, thus defining our "Acq Quadrant" in the "Target Acquisition Parameters" to be quadrant 1. Furthermore, as we do not expect to perform science in the TA quadrant, we do not require a 2nd exposure with the MIRI observations to treat persistence.

We will perform the observations in an order to minimize overheads. As mentioned previously, because Beta Pictoris is one of the brightest disks in the sky we do not require the use of small grid dithers. 

The final section of the observation templates, labeled "PSF Reference Observations", is where we will explicitly set the links between PSF reference observations and science observations to be used in data processing. First, we go through each of the observation templates with which our PSF reference star, Alpha Pic, is the target (Obs. #5-8) and indicate that they are a PSF reference observations by marking the "This is a PSF reference Observation" checkbox. Then, for each of the dedicated science target (Beta Pic) observations (i.e. Obs. 1-4, and 10-13), we make an appropriate selection from the "PSF reference observations" list, indicating the observation that should be used for PSF subtraction in the data-processing system. Associations should be pretty straightforward to make because (1) we clearly labeled which observations are PSF references (so won't accidently select a science observation) and (2) the APT only displays observations with matching coronagraphs (and filters in the case of MIRI) within this list (so we won't accidently select the wrong PSF reference observation). 

In order to fulfill the guidelines of our Observing Strategy, our program requires the placement of special requirements (SR) on our observations. A variety of Observation-level Special Requirements may be chosen; these requirements apply to all Visits in the Observation.

For the observations in the "MIRI coronagraphy of Beta Pictoris" observation folder, the following special requirements will be placed (and should appear in APT) as follows:

  • All observations shall be grouped by a "Sequence, Non-Interruptible" SR to ensure the observations execute together and in the order provided, (1) to minimize possible thermal variations differentially affecting the acquired PSFs and (2) to minimize the number of rolls and slews.
    • "Sequence Observations 1, 2, 3, 4, 5, 6, Non-Interruptible"
  • Because we are performing a roll dither on our science targets, we place an "Aperture PA offset" SR between the two science observations in each of the wavelength channels by 10° to 14°.
    • "Aperture PA Offset 4 from 1 by 10 to 14 Degrees (Same offsets in V3)"
    • "Aperture PA Offset 3 from 2 by 10 to 14 Degrees (Same offsets in V3)"
  • In order to position the disk in quadrants 2 and 4, well away from the 4QPM boundaries, we set an absolute "Aperture PA Range" SR to the range 340°–355° for the observations at "Roll 1". Specifying this orientation for roll angle 1 also yields good target positioning at roll angle 2, upon applying the PA Offset SR.

    • "Aperture PA Range 340 to 355 degrees (V3 335.550295 to 350.550295)"

For the "NIRCam Coronagraphy of Beta Pictoris" observation folder, the SRs placed are as follows:

  • All observations shall be grouped by a "Sequence, Non-Interruptible" SR to ensure the observations execute together and in the order provided, (1) to minimize possible thermal variations differentially affecting the acquired PSFs and (2) to minimize the number of rolls and slews.
    • "Sequence Observations 7, 8, 9, 10, 11, 12, Non-Interruptible"
  • Because we are performing a roll dither on our science targets, we place an "Aperture PA offset" SR between the two science observations in each of the wavelength channels by 10° to 14°.
    • "Aperture PA Offset 11 from 10 by 10 to 14 Degrees (Same offsets in V3)"
    • "Aperture PA Offset 12 from 13 by 10 to 14 Degrees (Same offsets in V3)"
  • In order to position the disk midplane near the diagonal of the NIRCam coronagraph subarray, we set an "Aperture PA Range" SR between 345°–360° for the observations at "Roll 1". This orientation for Roll 1 will also yeild good target positioning at Roll 2, given the defined Aperture PA offset SR.
    • "Aperture PA Range 345 to 360 degrees (V3 345.186639 to 0.186639)"

Running the Visit Planner

Once the observations for our coronagraphic sequences have been fully specified, we are able to run the APT Visit Planner and ensure that our sequences are able to execute in their entirety. Upon verifying the scheduability of our program, we run the Smart Accounting tool on our observation folder. The tool will identify any excess major slews assumed by APT and reduce them to the minimum needed. Because our set of observations is in a non-interruptible sequence, it will obviously only need one major slew at the beginning of the sequence. Smart Accounting will catch and correct this, thus reducing our reported overheads.

According to these tools, we find that the observability of our targets (w.r.t. the observatory as a function of time) and guide star avaibility, combined with the overlap of all of our special requirements, results in a scheduiling window of ~2 weeks in length for each instrument. Below is a bar graph (at the observation level) that shows a roll up of the available times that satisfy the constraints (at the visit level) of each observation in our program. 

If you are using the instrument handbook, you may view this bar graph here

 

Upon running Smart Accounting, it is now possible to view the revised total resource assessment of our program in the proposal cover page. It is instructive to inspect the "times" report and/or the "Smart Accounting" report generated by APT for each observation in the proposal. These files provide a more detailed breakdown of where the various overheads are being charged, and will help us in determining the tradeoffs in efficiency for different observation strategies.

The largest contribution to JWST observing overheads are the number of major slews associated with a JWST program, and activities associated with each visit (including initial slews, guide star acquisitions, mechanism motions, frame resets, small angle maneuvers, and visit clean-up activities). Programs that minimize the number of major slew and the number of visits will typically achieve a higher efficiency (science time/total time) than programs with large numbers of slews and visits.  

We have specifically chosen the specified ordering of observations (i.e. changing the filters/occulters before changing the target, or rolling the observatiory) in order to minimize slew overheads. We conclude, that the total time reported by the APT 25.4.2 for this program is 18.84 hours; 9.21 hours of which is dedicated science time.  

It is now that we draw the reader's attention back to the two observing strategies we mentioned in previous sections. 

Now consider the second proposed strategy: whereby all the observations, both using MIRI and NIRCam, are scheduled together in one long, non-interruptible sequence.

  1. Using the CVT, find an orientation that aims for maximum spatial coverage of the disk (but also ensures the disk is not coincident with any instrumental obstructions), in the FOV of both instruments. Hint: you'll want to comprimise!  
  2. Impose the new APA range SR on the observations.
  3. Run the visit planner and smart accounting tools, and compare the difference in overheads.
  4. Try re-ordering the observations, and determine the impact this has on these overheads.
  5. Compare the window of scheduability of this observation strategy.
  6. Determine which observational strategy you think is optimal for our program, and why?

 


 


Related links

JWST User Documentation Home
NIRCam Coronagraphy of HR8799 b
MIRI Coronagraphic Imaging
NIRCam Coronagraphic Imaging
NIRCam Coronagraphic Recommended Strategies
Selecting suitable PSF reference stars for JWST High Contrast Imaging
SearchCal: JMMC Evolutive Search Calibrator Tool
JWST Coronagraphic Visibility Tool Help
JWST Exposure Time Calculator - ETC
Astronomers Proposal Tool


References

JWST technical documents


Block has been set to pdf-hide. Contact Shireen if you enter references







Last updated

Published February 10, 2018


 

The format for 'Last updated' is shown below. Enter updates above this internal"Comment" box. Only enter major updates (not typos or formatting changes).


Updated April 5, 2017

  • Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aliquam fermentum vestibulum est. Cras rhoncus. 
  • Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Sed quis tortor. 

Published March 2, 2017