APT Micrometeoroid Avoidance

Certain special requirements can force observations into the portion of a target's visibility that is within the micrometeoroid avoidance zone. Such observations need to be minimized to reduce risk to the observatory. APT flags such observations for review by users prior to submission.

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See also: JWST Micrometeoroid Avoidance Zone APT and the Micrometeoroid Avoidance Zone Video Tutorial


Users need to be aware that placing certain special requirements on observations can force them to be scheduled into the portion of a target's visibility that is within the micrometeoroid avoidance zone (MAZ). While observing within the MAZ is not strictly disallowed, such observations need to be minimized to reduce risk to the observatory and should be strongly justified if required for your science use case.

APT will flag proposed observations that use certain special requirements that can force schedulers to use the MAZ visibility regions of the proposed targets and show users a warning message. A diagnostic in APT will help the user to understand the situation and determine whether mitigation is possible within their science constraints. After careful consideration of the warning, proposers can retire the warning by entering an explanation into the text box provided on the proposal Information page. Details are provided below.

Special requirements and the MAZ

Nearly all celestial targets will have plenty of visibility outside the MAZ, and schedulers will be primarily responsible for scheduling observations of those targets to minimize MAZ observing. However, if an observer specifies certain special requirements, schedulers could be forced by those constraints to schedule observations in a target's MAZ visibility. These situations are flagged by APT with a warning so that proposers can assess whether mitigation is possible within their science requirements.

The primary category of special requirements that can impact the need for MAZ observing is timing special requirements. However, not every use of a timing special requirement causes the problem. A problem only arises when timing constraints on an observation, or between 2 or more observations or visits, forces scheduling of observations into the portion of the target's visibility that is within the MAZ. Since most targets have many days or weeks of non-MAZ visibility, most timing constraints on observations will not cause a problem.  

As described in the MAZ overview article, low ecliptic latitude targets are the most severely impacted by avoiding the MAZ, "losing" one of 2 available visibility periods each year. Figure 1 shows an example target entry in APT, indicating where the target's ecliptic coordinates are provided for reference. However, even a target at 0° ecliptic latitude has a minimum of 53 days of visibility outside the MAZ. If such a target had 2 observations with a timing link that forced a second observation into the MAZ visibility period, it would receive a warning in APT. The proposer would need to consider whether shortening the separation to <53 days is a viable option or whether the need for MAZ scheduling is required by their science case. 

Figure 1. An APT target entry showing where you can locate ecliptic coordinates of your target

The APT target listing in the form editor provides an easy reference for finding the ecliptic coordinates (in particular, the ecliptic latitude) of your targets.
A second main category of special requirements that can push observations into the avoidance zone are absolute aperture position angle (or range) constraints.  In many respects, these special requirements are really "timing constraints in disguise," as the position angle of the JWST field of view varies with time. In this case, an APT warning message will alert users to assess whether a modified strategy (perhaps modulo 90° or 180° or simply a different position angle ) removes the need to observe in the avoidance zone.  

These are just 2 generic examples.  Additional examples and thoughts about mitigation are provided in a separate section below.

APT warning and diagnostics

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

It is up to the proposer to determine if they can adjust their special requirements to avoid the MAZ or whether observing in the MAZ is required for their science case.

The primary tool within APT that can provide this insight is the Visit Planner. By running the VP and opening the visit constraint graphics, the user can access the Meteoroid Safe Zone windows for comparison to the other scheduling constraints such as the total Field of Regard (FOR) visibility, Guide Star Availability, etc. Figure 2 shows an example Visit Planner run for the target shown in Figure 1 showing these constraints, but prior to adding any special requirements.

Figure 2. An APT Visit Planner output for the target in Fig. 1 showing the constraints in detail

As of APT 2022.7, released for Cycle 2, the APT Visit Planner contains a new informational constraint line called Meteoroid Safe Zone. In this example, the Visit line shows 3 visibility periods for the time frame shown. However, the Meteoroid Safe Zone constraint only shows two. This is because the middle visibility region is within the MAZ, and so is not shown. While scheduling in the middle visibility period is not disallowed, it is strongly discouraged.
There is an important difference between the Meteoroid Safe Zone constraint and the other constraints shown in the Visit PlannerThe Meteoroid Safe Zone constraint is informational only, and is not used to constrain the actual schedulability of the visit.  However, the Meteoroid Safe Zone constraint graphic will show you how much of the total visibility lies within the MAZ region and whether your proposed visit(s) intersect that region of visibility.

Let's continue this example as follows; the proposer in this case decides they need a timing special requirement on their observation, as shown in Figure 3.

Figure 3. One of several timing special requirements in APT that can, in principle, force observation scheduling in the MAZ

In this example, the user has determined that the observation must occur within a 20-day window in March 2023, which is specified using the BETWEEN DATES special requirement.
After placing this special requirement, the proposer then runs the Visit Planner again, and sees the output shown in Figure 4.  
Figure 4. The Visit Planner output after placing a timing special requirement on the observation

After placing a timing special requirement, the Visit Planner shows that the target is schedulable (green check mark in the red oval at left), but the observation is in the region with no green bar in the Meteoroid Safe Zone constraint, meaning it is within the MAZ. (The purple bars indicate visibility periods that are NOT impacted by MAZ, for quick visual reference.) Note also that a warning has been added to the visit and an error has appeared in the Proposal Information section. See Figure 5.
The target is schedulable, but in a region with no green bar in the Meteoroid Safe Zone bar, which indicates it is visible but within the MAZ. Let's look at the warning and the error by hovering the cursor over the red X and the yellow caution sign (see Figure 5).
Figure 5. The MAZ warning and error

The warning on the visit flags the user to the MAZ usage. The user should consider adjusting their constraints if possible to avoid MAZ usage. If the science requires MAZ usage or the warning cannot be otherwise mitigated, the user must resolve the error on the Proposal Information page by adding explanatory test in the box provided (see Figure 6). If the condition causing the warning is mitigated by the user, the warning and error will be removed upon re-running the Visit Planner.
Since MAZ usage is indicated in this case, the warning is a flag to the user to consider whether there are actions that can be taken within the proposed science requirements to alleviate the need for potential MAZ region observing. The details will depend on your specific science use case and requirements.

Examples and mitigation strategies

A more complete listing of APT special requirements that could potentially force MAZ observing in certain use cases includes the following:

After Date
Before Date
Between Dates
After Observation Link

Aperture PA Range (but not NIRSpec MSA Planned APA or Assigned APA)
PA Range
PA Offset Link

Background Limited
Moving Target Observing Windows

Each of these could impose MAZ scheduling, but the actual impact (or not) depends on the target, the observation specifics, and of course the details provided in the special requirement specification itself. In addition to the generic examples given above, here are a few additional examples to consider:

  • After Date, Before Date, or Between Dates: Depending on the details and the fixed date provided, the referenced observation could be forced into the MAZ.
    • Possible mitigation: Adjust the reference date such that the observation can be accommodated outside the MAZ.

  • Phase: Observation of a particular Phase of a periodic phenomenon could place the observation in the MAZ.  
    This should only occur for a period that is longer than or comparable to the target's visibility outside the MAZ, so most Phase cases should not create a problem. 
    • Possible mitigation for the remaining cases: If little flexibility in choice of Phase is available and the science requires it, explain the situation in the text box provided on the Proposal Information page and the warning message in APT will be resolved (see below).

  • After Observation Link: Specifying the separation of 2 or more linked observations can also force some of the observations into the target's MAZ visibility,
    • Possible mitigation: Adjust the separation in days such that the observations can be accommodated outside the MAZ.

  • PA Offset Link: It could be possible that the PA of an initial observation was outside the MAZ, but a link to a second observation at an offset PA would be forced into the MAZ visibility.  
    • Possible mitigation:  Consider specifying a PA or PA range on the initial observation such that the offset observation is not in the MAZ.

  • Background Limited: Restricting the schedulability based on the IR background already restricts the allowed plan window for a given target. In principle, the remaining visibility could overlap with or be entirely within the MAZ visibility.
    • Possible mitigation: Consider relaxing or eliminating the background limitation, if possible. Consider asking for more exposure time to get the required S/N (if possible), and provide an explanation in your proposal justification section. Or for the faintest targets where this is not practical, explain the need for MAZ observing on the Proposal Information page and dismiss the APT warning.

  • Moving Target Observing Windows: In principle, most moving targets should have significant visibility outside the MAZ, and scheduling should not be a problem.  However, applying certain moving target special requirements can force the timing into the MAZ portion of the target's visibility. For example, certain situations (e.g., a comet that only has visibility in the MAZ) might have limited time frames for observation that would require MAZ pointing. 
    • Possible mitigation: If this is known at the time of proposing, the situation will need to be justified and the warning can then be resolved in APT, as discussed below.

If the scientific need to observe within the avoidance zone arises after the fact (say, for a target of opportunity), the situation will need to be discussed at the time of activation.

Finally, it is possible that the APT warning about potential MAZ usage may simply be flagging a potential problem that has no impact (e.g., a false flag). There was no time to add detailed assessments beyond the generic warning in APT, and even if there was more time, there are simply too many factors for APT to assess them all accurately.  If the proposer's assessment is that the warning is not applicable to their use case, a note can be made in the Proposal Information text box and the warning message will be resolved, as described below.

Dismissing the APT warning

If your science requires observing in the MAZ, the error on the Proposal Information page can be addressed by entering explanatory text in the box provided (see Figure 6).

Figure 6. Excerpts from the APT Proposal Information page showing the error condition and long version of the error description

This example shows the error (red X) and text box on the Proposal Information page, for use if the MAZ warning cannot be resolved. Selecting either the Errors and Warnings tool in the top tool bar or the "errors and warnings" item at lower right (red highlights in the Figure) reveals the full text of the error, as shown here at the bottom of the figure.
Entering an explanation into the text box provided and then clicking outside the box will cause APT to remove the error condition on the proposal, as shown in Figure 7.  The error on the Proposal Information entry in the left menu, as well as the Meteoroid Zone Justification text box, have gone away and an informational icon has been added on any visit(s) that previously had a warning. (The text of the informational item is simply the previous warning message with a new informational tag.) This provides a record of what visits were affected for future analysis without causing a warning. However, the warning itself is removed and will not impact the appearance or the submitting of the proposal.
Figure 7. Excerpts from the APT Proposal Information page showing the resolved error and informational text box

In this example, explanatory text has been added to the text box, which removes the error condition (red X's) and changes the symbol on any affected visits to an "I" in a circle, indicating an Informational item. Viewing the errors and warnings as in Figure 6, you can see that the text entry has changed from being a warning to informational.  This is to provide future insight to the situation without causing an error to be present on the submitted APT file. The text entered here will appear in the STScI PDF file, to assist in tracking MAZ usage, but will not appear on the proposal PDF that goes to the peer review panels.

Latest updates
    Added link to APT-MAZ video help.
Originally published