JWST APT Coronagraphic Sequence Examples
Examples of specifying MIRI and NIRCam standard coronagraphic sequences
See also: High Contrast Imaging Overview
This article walks through two examples of specifying a coronagraphic sequence, one for MIRI and one for NIRCam. Use the File --> JWST Demonstration Proposal option with the selections MIRI Coronagraphic Example and/or NIRCam Coronagraphy Example, as directed below. A standard coronagraphic sequence involves a set of linked observations, and these simple examples demonstrate this process by showing the appropriate Special Requirements needed for the linking. More complicated combinations using multiple filters and/or coronagraphs are also possible, as described in JWST Coronagraphy Strategies. Also, there additional more complex ways of specifying PSF reference star observations than are shown in these simple examples below. These will be covered in a future article.
A MIRI example
The APT demonstration file MIRI Coronagraphy Example can be loaded and viewed as one walks through the example below. See MIRI Coronagraphic Imaging and related pages for detailed information about coronagraph with MIRI.
The example proposal contains two targets, BET-PIC (which represents the science target) and DEL-DOR (which represents the PSF reference star). A standard coronagraphic sequence involves two observations of the science target and at least one observation of the PSF reference star, all linked together in a non-interruptible sequence. (This is done to minimize any thermal or other changes that could cause the PSF to vary significantly.) These observations have already been added to the example file, which looks like Figure 1. Note the template sections labeled Target Acquisition Parameters, Coron Parameters, and PSF Reference Observations, which will be described further below.
The two observations of the science star are observed with an offset in roll angle between the two (called a roll dither), which is limited by JWST observing constraints to be <14 degrees. In our example, we are observing with a single MIRI 4QPM/filter combination.and we have fixed the allowed range on the initial science observation. (This is only necessary if known structure around a given target, say a disk or a known planet, needs to be positioned so as to avoid structures in the instrument field of view, in this case, the boundaries of the quadrants in the 4QPM). In the example, Figure 2 shows the Special Requirements that have been set to control this sequence:
Finally, even though the setting of the SEQ NON-INT special requirement has grouped the observations together, information must be provided to the Data Management System as to how to connect the PSF reference star observation to each of the science observations. The section at the bottom of each Observation template gets used for this purpose. Note that in APT, one may have to use a scroll bar to see the bottom section of the template. For Observation 3 (the PSF reference star observation), simply select the appropriate check box:
Then for each of the science observations, this reference observation needs to be selected from the pick list provided, as shown in Figure 5.
It is not required that the user view the observations in Aladin, but it can be useful as a sanity check, both that the angle has been selected properly and that the roll dither has been specified as intended. In the example in Figure 7, we have selected the two science observations (that is, in the form editor, select observation 1, then shift-select observation 2, which should highlight both observations, then choose "View in Aladin" from the top tool bar in APT). Since we allowed a range of 10-14 degrees for the offset, Aladin shows the mean, which is a 12 degree offset, in the display.
A NIRCam example
The APT demonstration file NIRCam Coronagraphy Example can be loaded and viewed as one walks through the example below. Refer to the NIRCam Coronagraphy and related support pages for detailed information.
The example proposal contains two targets, BET-PIC (which represents the science target) and ALF-PIC (which represents the PSF reference star). In this example, we step up the complexity only slightly from the previous MIRI example by having the sequence contain observations with two coronagraphs/filters instead of one. In this case, following the recommendations of the coronagraphic strategies article, all of the observations are done at the initial position angle (roll 1) before moving to the offset roll dither position angle (roll 2). Of course, a single observation in each setup is used on the PSF reference star, so they are put together at the end of the sequence. Hence, this sequence contains a total of six observations instead of three, all of which must be schedulable together in order to be valid. Here is what the special requirement look like for this situation:
First notice the observation order shown. Observations with two different coronagraph/filter combinations are done at roll 1 prior to the roll dither, then both of the observations are repeated after the roll. Finally the PSF reference star is observed in both configurations. In this case, setting the Special Requirements is a bit different for the observations with each of the masks, but all six observations are linked in the sequence. Figure 9 shows the Special Requirements from this example.
As a word of caution, the "SEQUENCE ... NON-INTERRUPTIBLE" special requirement indicates that the specified set of observations will in done in "increasing observation number" order. In the example, the observations are shown in order 1-6 in the form editor window at left. However, for reasons of general editing in APT, users are allowed to "drag and drop" observations in the form editor. If a user reorders the observations in a sequence using this method, it does not change the order of actual sequencing of the observations, which is done via the observation number. Hence, users should check that their desired order for the sequence is consistent with the ordering on the listed observation numbers on the individual observations.
The specification of PSF reference observations and the proper attachment of them to each of the science observations proceeds exactly as outlined above in the MIRI example.
As shown in Figure 10, even with a sequence of six observations, the Visit Planner has been able to verify that there is a time when all six observations can be scheduled together. Again, as with the MIRI example above, the fairly narrow range of allowed absolute PA placed on observation 1 results in a rather small window of schedulability, so this type of restriction should only be placed when necessary for the science.
Finally, if one desires to perform a sanity check on the angular offset between the two rolls on the science target, one can select the relevant observations in the form editor and "View in Aladin", as shown in Figure 11.