Aperture Position Angle Special Requirements

Aperture Position Angle Special Requirements (e.g. APERTURE PA RANGE, SAME APERTURE PA) are used to restrict the scheduling of JWST observations by position angle constraints. The Astronomer’s Proposal Tool (APT) is used to enter the requirements into the proposal.

Introduction

 Format definitions...

Purple text indicates the parameter is Limited Access.

Boldface italics type indicates the name of an APT parameter or a value for a parameter.

(warning)Red text indicates restrictions on a parameter.

(red star) Black text indicates an important note.

Brown text indicates notes for the developers.

Green text indicates the name of the parameter used by Commanding.

Items in brackets - <value> - are required values.

Items in square brackets - [<value>] - are optional.

All Aperture Position Angle special requirements indirectly restrict the times when observations (and/or their component visits) can be scheduled. These should be used to provide the schedulers at STScI with enough constraints to ensure that the observations are properly scheduled. Special requirements should not be used unless necessary to accomplish the scientific objectives of the proposal.

(red star) As noted in the Observation Specifications article, observations have an implicit GROUP WITHIN 53 DAYS special requirement, and visits within an observation are required to be executed at the same aperture position angle.

The Aperture Position Angle special requirements are summarized in Table 1, and a detailed description of each requirement is provided in the following subsections. All Aperture Position Angle values are specified in degrees or decimal fractions of degrees. Note that an observation-level special requirement applies to ALL the visits within that observation.

(red star) All Aperture or V3 PA Special Requirements are a limited resource and should be used only when necessary.

See JWST Observatory Coordinate System and Field of Regard for more information on the JWST coordinate system.


 Table 1. Supported Formats for Aperture Position Angle Special Requirements 

Aperture Position Angle Special Requirements

Observers who need to limit the position angle of the aperture on the sky (for example to place a jet in a slit) will need to determine and specify a range of acceptable position angles for the y-axis of the aperture with respect to north (measured through east) around the target. For instance an aperture position angle of 0 means that the observation should be scheduled with the y-axis of the aperture aligned with the north celestial pole and an aperture position angle of 90 means that the observation should be scheduled with the y-axis of the aperture aligned due east. (This y-axis is for the undistorted Ideal Coordinate System. It is documented with diagrams for each instrument in JWST Instrument Ideal Coordinate Systems.

Note that JWST must remain oriented with respect to the Sun (such that the Sun is within +5.2o angle of the spacecraft V1/V3 plane) to prevent solar illumination of any of the cold parts of the Observatory. As JWST orbits the Sun, this requirement for the Observatory translates into a position angle on the sky which is a function of time. In order to achieve an Observer specified aperture position angle and satisfy spacecraft roll constraints, an observation must be scheduled within a fairly narrow time interval (10-53 days, depending on the target’s ecliptic latitude).  The observer must take this into consideration when specifying additional timing constraints (e.g. BETWEEN, BEFORE, AFTER) or the observations might be unschedulable. For a discussion of normal roll and how it changes with time based on ecliptic latitude, see the section on Normal and Off-Normal Roll .

Notes to developers:

Assuming that small differences in position angle will be allowed for visits within observations, the user will need to be able to specify an override APERTURE PA RANGE on individual visits.

Commanding will be providing information on the defining aperture for each template so that we know which apertures y-axis to consider when there are multiple apertures).

 APERTURE PA <min-angle> TO <max-angle>

 V3 PA <min-angle> TO <max-angle>

(red star) V3PA and APERTURE PA are not separate and independent requirements, but are different views of the same requirement (i.e. regardless of which position angle you enter, both angles will be displayed).

This requirement specifies that a specific range of aperture or V3 position angles is required for the visits within the current observation. <min-angle> and <max-angle> denote a region within which the position angle must fall at the time of the observation; (warning) both limits must be between 0 and 360 degrees. Both angles are measured in a counterclockwise direction (measured North through East) around the target, so if the position angle range crosses zero, the <min-angle> value would be greater than the <max-angle> value. Otherwise, <min-angle> should be less than <max-angle>. If necessary, it is possible for <min-angle> and <max-angle> to be equal, thus specifying a narrow time range that the observation must be made, but this is very restrictive to scheduling.  In general, the size of the region between the two limits should be made as large as possible to make scheduling easier.

This special requirement can list multiple acceptable angle ranges:

APERTURE PA <min-angle1> TO <max-angle1>

APERTURE PA <min-angle2> TO <max-angle2>

APERTURE PA <min-angle3> TO <max-angle3>

APERTURE PA <min-angle4> TO <max-angle4> . . .


Note for many observations, two angle ranges can have a 180 degree offset and still be schedulable. If this is possible (for greater scheduling flexibility), please specify the additional possible angles. It is also possible for the position angle region to be larger than 180 degrees.


(red star) If a visit uses multiple targets, the direction of North from the first target will be used to interpret position angle range special requirements. The spacecraft will not roll between targets within a visit, so the position angle range at the subsequent targets may be slightly different from that of the first target.


  APERTURE PA OFFSET FROM <observation> BY <min-angle> TO <max-angle>  

V3 PA OFFSET FROM <observation> BY <min-angle> TO <max-angle>   

(red star) V3PA OFFSET FROM and APERTURE PA OFFSET FROM are not separate and independent requirements, but are different views of the same requirement.

This requirement specifies that an aperture or V3 position angle, relative to an aperture or V3 position angle of another <observation>, is required for the visits within the current observation. The <min-angle> and <max-angle> values denote a region of permitted position angle offsets. (warning) <min-angle> and <max-angle> must be between –180 degrees and +180 degrees. If necessary, it is possible for <min-angle> and <max-angle> to be equal,which greatly restricts the time the observation can be scheduled. For this reason, the size of the region between the two limits should be made as large as possible to make scheduling easier.

Both angles are measured in a counterclockwise direction, so if the position angle region crosses a point 180 degrees from <observation>, <min-angle> should be positive and <max-angle> negative. Otherwise, <min-angle> should be less than <max-angle>. It is possible for the position angle region to be larger than 180 degrees.

Note that if a different aperture is used in the two observations linked by an APERTURE PA OFFSET, and those apertures are oriented differently in the telescope, the observations will be taken at different rolls of the telescope. For example, if Observation 1 was taken with with MIRI imaging, the reference angle is for that selection, while if Observation 2 is to be obtained with NIRCam imaging, the reference angle is for that aperture. So by specifying APERTURE PA OFFSET 2 BY 30 TO 35 one is asking for the NIRCam imaging Field-of-View to be placed counterclockwise by 30-35 degrees from the MIRI imaging Field-of-View in Observation 1.   

To specify that the current observation be scheduled at the same position angle as another observation, use the SAME APERTURE PA AS or SAME V3 PA AS special requirement.

(red star) Note that only one APERTURE PA OFFSET FROM or V3 PA OFFSET FROM may be specified for an observation. If you need to have multiple OFFSET FROMs for an observation, you can create two different observations and note in the Observing Description and Observation Comments that only one should be executed.

Normal and Off-Normal Roll  

Aperture PA constraints roughly translate into timing constraints. Normal Roll is defined as the sun lying in the V1-V3 plane when the telescope points at the target of interest. Since Normal Roll is defined with respect to the sun, the observatory V3 position angle for target’s Normal Roll is a function of time. When scheduling an observation at Normal Roll, the time used is typically the mid time of the observation. Targets near the ecliptic have two values of normal roll (+90 degrees and – 90 degrees with respect to the NEP) separated by roughly 6 months in time.  Near an ecliptic pole, all position angle values are available and the normal roll varies by about one degree per day. See JWST Position Angles, Ranges, and Offsets for more details.   

In general, excluding other constraints, the off-normal roll is limited to less than ±5.2 degrees. (Actually you can achieve ± 7 degrees at large solar elongations and ±3.5 at small solar elongations.) If the spacecraft is unable to roll far enough "off-normal" to satisfy the required difference in position angle between the two observations, they must be scheduled apart in time in order to schedule both observations at valid roll angles. It should be noted that because of the small allowed off normal roll and the V1-Sun angle range of 85 to 135 degrees, many position angles are unavailable. See JWST Position Angles, Ranges, and Offsets for the Offset Angle from V3 for each instrument.

(red star) WARNING: For any detailed orientation requirements, please describe your requirements clearly in the proposal text and give angles and offsets. Please feel free to contact jwsthelp for assistance in preparing your orientations.


  SAME APERTURE PA <observation list>

SAME V3 PA <observation list>

(red star) SAME V3PA and SAME APERTURE PA are not separate and independent requirements, but are different views of the same requirement.

Sometimes any aperture or V3 position angle (or any position angle within the range of the APERTURE PA RANGE or V3 PA RANGE special requirement) is acceptable the first time an object is observed, but must then be the same for subsequent observations of that target. This special requirement requests that the visits in the current observation be made at the same aperture or V3 position angle as the visits in <observation list>. If timing  special requirements are also used, then an incompatibility may result or the observations may be difficult to schedule.

Note that if the observations being linked with the SAME APERTURE PA requirement are from different instruments, the apertures being constrained can have offsets from each other, and constraining them to be SAME APERTURE PA can have impacts on schedulability.  At the least, such observations may be forced apart in time (in order to align the reference axes on the sky the same way), and in certain cases (for instance for an observation near the ecliptic plane) the linked observations may be unschedulable.  See JWST Position Angles, Ranges, and Offsets for more information.

  VISITS SAME PA

APT will add this requirement to any observation which has multiple visits to assure that all visits will be executed at the same position angle. Examples of these types of observations are mosaics, observations too long to fit into a visit, and observations whose pointings are far enough from each other to require separate guide stars.

(red star) Note that this requirement is not editable.

MSA SELECTED APERTURE PA <min-angle> TO <max-angle>

APT will add this requirement to any observation using the NIRSpec MSA once an aperture position angle has been selected by STScI. It may be necessary to update the MSA Plan to match the selected aperture position angle.

MSA PLANNED APERTURE PA <min-angle> TO <max-angle>

APT will add this requirement to any NIRSpec MSA observation created using the MSA Planning Tool, which does not yet have a selected aperture position angle as indicated by an MSA SELECTED APERTURE PA requirement.

Change log

 Click here to expand...

Version 4 (April 16, 2019)

  1. Editorial change (moved change log to main article).


Version 3 (September 26, 2017)

  1. PR 88095  - revised V3 PA requirements to make non-LAP


Version 2 (May 9, 2017)

  1. PR 87316  - added MSA APA requirements


(January 24, 2017)

Reworded Nominal/Off-Nominal Roll section with help from several people.


Version 1 (July 6, 2016)

This is the converted Word File from Chapter 7.