OBSERVER_RING_OPENING_ANGLE and OBSERVER_RING_ELEVATION differ for Uranus

[rfrenchseti]

In the ring_summary tables, OBSERVER_RING_OPENING_ANGLE does not do anything special for Uranus, while OBSERVER_RING_ELEVATION does. What is the rationale for this? Should they be made consistent?

  OBJECT                        = COLUMN
    COLUMN_NUMBER               = 59
    NAME                        = OBSERVER_RING_OPENING_ANGLE
    DATA_TYPE                   = ASCII_REAL
    START_BYTE                  = 649
    BYTES                       = 8
    FORMAT                      = "F8.3"
    UNIT                        = "DEGREES"
    VALID_MINIMUM               = -90.
    VALID_MAXIMUM               = 90.
    NULL_CONSTANT               = -999.
    DESCRIPTION                 = "Observed ring opening angle at the center
      of the ring system. This is the angle from the ring plane to the
      direction of outgoing photon to the observer. It is positive on the
      north side of the ring plane and negative on the south side.

      This value is defined by the instantaneous geometry at the time of the
      observation; it is independent of the field of view."
  END_OBJECT                    = COLUMN

 OBJECT                        = COLUMN
    COLUMN_NUMBER               = 36
    NAME                        = MINIMUM_OBSERVER_RING_ELEVATION
    DATA_TYPE                   = ASCII_REAL
    START_BYTE                  = 410
    BYTES                       = 8
    FORMAT                      = "F8.3"
    UNIT                        = "DEGREES"
    VALID_MINIMUM               = -90.
    VALID_MAXIMUM               = 90.
    NULL_CONSTANT               = -999.
    DESCRIPTION                 = "Observer ring elevation is the angle,
      measured at the ring intercept point, between the ring plane and the
      direction of the outgoing photon to the observer. The value is
      positive on the side of the ring plane defined by prograde rotation;
      note that this is the north side of the ring plane for Jupiter, Saturn
      and Neptune, but the IAU-defined south side for Uranus.

      Ring intercept points that are obscured by the planet or a moon within
      the observed field of view are ignored. For observations capturing
      reflected sunlight (i.e., with wavelengths in the visual or
      near-infrared), ring intercept points that are shadowed by the planet
      are also ignored.

      This column tabulates the minimum value of observer ring elevation
      within the field of view of the observation."
  END_OBJECT                    = COLUMN

[matthewtiscareno]

Conversation on a subthread of #rms-data-chat on our Slack workspace on April 4:

Matt Tiscareno (he/him):

Interesting that Ring Center Opening Angle and Observer Ring Elevation are actually defined this way in PDS3. I wonder whether that was an oversight at the time, or whether there is a reason. We could ask Mark Showalter.

Rob French:

The definitions in PDS3 were made up by Mark, since these are files he generated from scratch.

Matt Tiscareno (he/him)

So we might want to change the definition of Ring Center Opening Angle so that we basically multiply it by -1 for Uranus observations. This would affect about 500+ Voyager observations and a few Hubble NICMOS observations.

Rob French

We can open an issue to discuss that. For now we should keep it consistent.

Mark Showalter

The distinction between "prograde" and "north" has been less of an issue till now. BTW, all of those "north-based" OPUS quantities were necessary to deal with the fact that different Cassini teams were defining ring incidence and emission angles differently. In the case of Uranus, do we want "north-based" to mean IAU north or positive rotation north? Off the top of my head, I don't recall what north-based means for the Voyager images from Uranus. It shouldn't be hard to check though. If we want "north" to mean positive rotation north, we might want to rename the quantity and/or update the definition, This really needs to be sorted out!

Rob French

I've created this issue where people can add discussion. Obviously it's possible to change this just in OPUS, but it would be much better to change the archived metadata files. And it would be kind of annoying to have the archived files be some using one definition and some using another.