Talon XObservatory (Xobs) Graphical User Interface (GUI) Summary

Talon GUI (Xobservatory / Xobs) Summary:

Sky Map (box widget)

The Sky Map box depicts the relative positions of the telescope (an icon represented by a small, open, white circle), the Sun (an icon represented by a small, solid, yellow circle), and the Moon (an icon represented by a small, grey circle, or semi-circle, dependent on the Moon's phase).  For directional orientation, the upper Northwest quadrant of the Sky Map is marked with the abbreviation NW.  The relative positions of the telescope, Sun, and Moon change within the Sky Map box, depending on where the telescope is pointed, and the locations of the Sun and Moon.  The outer edge of the Sky Map circle represent the local horizon, the inner circles at 30-degree intervals, and the inner, center dot the Zenith.

Positions:

The Positions panel of the Xobs GUI indicate the Current, Target, and Difference coordinates of the telescope and dome.  

There is one row for the current position, which indicates where the telescope is in real-time, and may be static when the telescope is not in motion, or updating when the telescope is slewing or tracking.

The second row indicates the Target coordinates for the telescope and dome.  These fields will be blank when the telescope is not in motion, or populated when the telescope is commanded to move to a particular target.  These fields represent the intended end-point of the telescope and/or dome motion based on manually entered slew/track coordinates, commanded motions from button-presses (i.e. Service/Zenith, Stow), or the result of a Lookup operation from the Xobs GUI or Xephemeris planetarium application.

The third row indicates the Difference in the telescope and dome position as a function of the distance between the Target and Current coordinates.  These fields may be blank when the telescope is not in motion, and the Target and Current position coordinates are equal, populated and static when the telescope is not in motion and the Target and Current position coordinates are not equal, and populated and updating when the telescope is in motion moving to the Target coordinates.

The specific fields are described as follows:

RA (J2000) - the Right Ascension (RA) coordinate for the J2000 Epoch in hours, minutes, and seconds (HH:MM:SS.S)

Dec (J2000) - the Declination (Dec) coordinate for the J2000 Epoch in degrees, minutes, and seconds (+/-DD:MM:SS.S)

HA - the Hour Angle (HA) of the telescope's polar axis in hours, minutes, and seconds East (+) or West (-) (+/- HH:MM:SS.S)

Altitude - the pointing Altitude of the telescope above the horizon in degrees, minutes, and seconds (DD:MM:SS.S)

Azimuth - the Azimuth angle of the telescope, East of North in degrees, minutes, and seconds (DD:MM:SS.S)

Dome Az - the Dome Azimuth, East of North in degrees, minutes, and seconds (DD:MM:SS.S)


Logo Section (box widget):

The Xobs GUI contains a bitmap logo panel in the upper right hand corner - this is a non-functional, static object.

Control Section:

The Xobs GUI Control Section is a panel that supports the control actions necessary for a variety of general "mode" functions for the telescope and dome.  Each of these control functions is described in the following list.

Stop - This is a red button labeled 'Stop'.  A left-click with the mouse on this button stops any in-progress command, or commands in the Xobs execution queue.  It is not an emergency-stop (e-stop) in the traditional sense.  It is a "soft" stop function that causes Talon to halt any in-progress operation, including tracking, slewing, dome rotation, dome shutter operation, mirror cover open/close, and dome automation functions.  Basically any item controlled by Talon will be halted if Talon is commanding a motion or action for that object.  The 'Stop' button is an immediate mode control - it does not alter the state of the Talon system, telescope motion control, or any of the maintained internal system state - it only affects operations in progress when the command button is clicked by the mouse.  There are five separate hardware emergency-stop (e-stop) controls that physically interrupt power to the motion control axes; one on the control room console shelf above the Talon console, one on the control room foyer entry door, two on the telescope pier, both East and West, and one on the dome entry door.  Effecting a physical, hard e-stop requires pressing one or more of these e-stop buttons, AND clicking the Talon 'Stop' button.

Quit - The 'Quit' command button gracefully exits the Talon Xobs GUI.  It clears any pending commands in the Talon command queue, and signals the underlying Talon daemons that the particular instance of Xobs is gracefully exiting.  It does not shut down the telescope system, it only exits the Xobs GUI.  Shutting down the TCS completely requires a second step - after Talon Xobs 'quits', the Linux shell command 'killTel' must be executed to cleanly and completely shut down the TCS and telescope.

Find Homes - The 'Find Homes' command button brings up a dialog pop-up that exposes five additional active command buttons, and three inactive command buttons (greyed-out, Rotator, Focus, Filter):
 
 - Scope HA/Az - This command button causes the telescope to perform its 'home' operation on the Hour Angle (HA)/Azimuth axis.  The telescope will slew East in hour angle/right ascension to approximately 2 - 3 hours, then execute a series of motions East and West (+/-) in this axis, decrementing its motions by a factor of ten at each step until the servo/encoder loop in this axis closes, and the position accurately defined and recorded.  The telescope will remain in its 'found' position at the end of the homing operation.  The approximate position of the HA axis after homing is complete will be a Right Ascension (RA) of 2:05:00, and an Hour Angle (HA) of -0:48:05.5.  This position varies slightly over time, and is refined with each homing operation.

 - Scope Dec/Alt - This command button causes the telescope to perform its 'home' operation on the Declination (Dec)/Altitude axis.  The telescope will slew North in hour angle/right ascension to approximately 40-degrees (north of Zenith), then execute a series of motions North and South (+/-) in this axis, decrementing its motions by a factor of ten at each step until the servo/encoder loop in this axis closes, and the position accurately defined and recorded.  The telescope will remain in its 'found' position at the end of the homing operation.  The approximate position of the Dec axis after homing is complete will be a Dec coordinate of 47:05:11, and Altitude of 71:06:30.  This position varies slightly over time, and is refined with each homing operation.

 - Dome - This command button will cause the dome to move clockwise (CW), East of North to find the dome's home position, via actuation of a limit switch in the dome control system.   This causes the dome shutter power contactors to align, which in turn allows power to be supplied to operate the dome shutter for open/close operation.  The 'home' position for the dome is Dome Azimuth (Dome Az in the rightmost column of the Positions Panel) is 110:34:51.  This position is invariant, and does not change between homing operations.

 - All - This command button simultaneously performs the homing operation of the HA, and Dec axes of the telescope, along with the Dome Azimuth.  When activated by a left-button click, the telescope will move in both axes, and the dome will rotate clockwise (CW) East of North until all three home positions are located by Talon.  

 - Close - This command button closes the 'Find Homes' dialog pop-up panel.

The telescope and dome must be 'homed' before any other operations, other than use of the software hand-paddle, can be performed.  Slewing, tracking, dome automation, and shutter operation are dependent upon Talon's successful completion of the 'Find Homes' action.
 
Find Limits - The 'Find Limits' is similar to the 'Find Homes' command button function.  In this case, rather than performing homing operations on the axes and dome, this command button collection attempts to identify hardware limit switches on the HA, and Dec axes, and the Dome azimuth.  This is a maintenance and testing action set, and does not need to be used other than for initial configuration and maintenance diagnostics.  It will be hidden in a future release of the Xobs GUI, and accessible only as a maintenance-level authenticated action.

Test - The 'Test' command button accesses diagnostic functionality that is not implemented in this release of the Talon TCS.  It currently returns a 'Not Yet Implemented' pop-up message if activated by a left mouse click.

Reload - The 'Reload' command button causes Talon to re-read/reload all of its configuration (*.cfg) files.  In future releases of the Xobs GUI, this will be hidden as a maintenance-only, authenticated function.  Activating this command button causes Talon to perform a soft initialization, and requires a complete recover sequence including homing of both axes on the telescope, the dome, and a loss of all current state.  Its use may leave the telescope non-functional, and in an indeterminate state.  Use of this function requires specialized knowledge of recovery of the telescope's operational state, motion-control system, and precise sequence of operations - otherwise the telescope and TCS will at best be left in an indeterminate state, and at worst inoperable state requiring Observing Support intervention to restore the telescope into a usable state.

Calib Axes - This command button brings up the telescope/TCS axis calibration dialog panel.  Use of the Axis Calibration panel requires the Talon TCS companion application XEphemeris (Xephem), and is used to refine the TCS/telescope mesh pointing model.

Axis Calibration requires selection of a minimum of two, and a maximum of eight bright stars using XEphemeris, identifying each star in turn  via IRAF-ICE and a test integration/image on the PFC, moving the identified star to the center of field via the Talon software hand-paddle, and iterative cycles of an image integration via the PFC and IRAF-ICE until the specified star is centered in the PFC field of view.  Once a star has been centered, it is 'Marked' using the 'Mark' command button in the Axis Calibration dialog panel.  Once a minimum of two, and a maximum of eight stars have been identified, centered, and marked in this process, the 'Solve' command button computes the telescope pointing mesh solution.  The mesh solution is populated in the dialog panel in a series of seven fields for the Pole, Axis Reference, and Pole Offset variables.  If the mesh solution is determined to be satisfactory, it can be 'Installed' in the current TCS/telescope model by use of the 'Install' command button in the Axis Calibration dialog panel.  The panel also includes 'Reset', 'Undo', and 'Close' buttons.  The 'Reset' button restores the mesh model to its last known state.  The 'Undo' button performs a step-by-step undo of the last commanded action in the Calibrate Axis panel, and is used to 'unmark' or step back through the actions performed in that instance of the Calibrate Axis session.  The 'Close' button exits from the Calibrate Axis dialog panel.

This is an advanced command dialog panel, and is intended primarily for use by Observing Support personnel to refine the telescope's pointing model.  It is currently exposed in this release of the Talon Xobs GUI, but will become an authenticated, maintenance only action set in a future release of the interface.

CAUTION - WARNING - ATTENTION!

Incorrect use of the Calibrate Axis dialog panel will at best create inaccuracy in the telescope's pointing model, causing the telescope to point incorrectly.  At worst, it will cause the telescope to become unusable.  DO NOT USE the Axis Calibration dialog without first consulting, and obtaining approval from a member of the Observing Support staff, and make sure that the existing files:

/usr/local/telescope/archive/config/home.cfg 

and the directories

/usr/local/telescope/archive/calib, /usr/local/telescope/archive/pointmesh

have been backed up to a safe location.

No Confirm - the 'No Confirm' control action is a button 'toggle' that turns off, or on (enables/disables) 'Confirmation' dialog pop-ups on actions that normally require, and request YES or NO positive confirmation.  The default state is 'enabled', which allows any commanded Talon action that normally requires pre-execution confirmation to request positive consent before that action is executed.  It is recommended to leave this 'enabled', to prevent unintended actions from taking place in the event of an accidental or incorrect command action.  Disabling (i.e. toggling 'No Confirm' to on) also disables help tips in the Xobs GUI.  Whenever this is toggled 'on', the Red indicator for 'Confirm' in the Status panel display will be present.
 
Auto Focus - the 'Auto Focus' command button has no function in this release of Talon.  Currently, all instrument and detector functionality is managed and controlled via IRAF-ICE.  Talon controls only the telescope axes and dome functions, while IRAF-ICE controls the PFC filter wheel, PFC focuser, and the LF1 detector (in upcoming releases the dome flat lamp, filter home, and focus reset).  Pressing this button causes Talon to emit a message in the 'Message' box "No focus motor configured."  This button will be removed in future releases of Talon.

Batch Mode - the 'Batch Mode' control action is a button 'toggle' that enables, or disables Batch execution mode.  Talon has the capability to operate autonomously in a hybrid mode of operation that can execute a scripted observing session, a scheduled list of targets, or a combined scripted/scheduled observing session.  For human observer direct interaction with the telescope, the 'Batch Mode' operation should be disabled (the default state).  When 'Batch Mode' is enabled (toggled on), it is not possible to command the Talon Xobs GUI to interact with the telescope - instead the telescope TCS expects to take input from the Scheduler and related scripting interface.

Paddle - the 'Paddle' command button displays the Talon software hand-paddle.  There is no physical hand-paddle present on the telescope.  In lieu of a physical hand-paddle, Talon uses a virtual, software-defined hand-paddle.  The software hand-paddle contains the following functional controls:

 - Coarse scope - this button toggles the hand-paddle for coarse motion-control over the HA/RA and Dec axes of the telescope.  When it is toggled on (enabled), the left-hand section of the hand-paddle panel changes to four Arrow buttons, marked +Y (North in declination), -Y (South in declination), +X (East in HA/RA), and -X (West in HA/RA), when the telescope is not tracking, and N, S, E, and W respectively when the telescope is in tracking mode.  Pressing one of these arrow-icon buttons causes the telescope to move at slew speed in the indicated direction.  The button presses attempt to simulate an analog control, and can be 'nudged', or pressed continuously to simulate a continuous press on a real hand-paddle.  The 'nudge' and 'hold' modes of use have the effect of moving the telescope in the commanded direction.  Be aware that the telescope is not directly visible from the control room (in local mode operation), and in remote operation usually monitored by the dome camera system, and that the camera system has latency/lag associated with it.  Use care to not overrun the desired position.  The telescope has hard limits in both the RA/HA and Dec axes, button presses will not move the telescope past these hard limits.  Be aware that moving the telescope to the East or West limit in HA/RA can cause liquid nitrogen (LN2) to spill from the PFC/LF1 cryogen dewar, especially if it has been recently filled.  At either the East or West limit, the dewar will lose approximately half of its LN2, requiring a refill before the end of the observing night.

 - Fine scope - this button toggles the hand-paddle for fine motion-control over the HA/RA and Dec axes of the telescope.  Similar to Coarse mode, these arrow icons have an identical function, save for moving the telescope at 'Search' speed as opposed to slew rate.  This mode allows for fine pointing adjustments to position an object in the PFC/LF1 field to a desired location.

 - Focus/Filter - this button toggle is not implemented for the PFC/LF1 instrument/detector - if toggled on, it displays a greyed-out/disabled icon set for filter wheel rotation and focuser travel.  It will be removed in a future release of Talon Xobs.

 - Roof/Dome - this button toggle creates a left-hand panel button icon set for Dome Open, Dome Close, and Dome CCW (counter-clockwise, West of North), and CW (clockwise, East of North) dome azimuth rotation.  Do not overlap the Dome shutter Open or Close operations - if you press Open, allow the dome to open completely BEFORE pressing the Close arrow.  Overlapped operation may leave the shutter in an indeterminate, or 'stuck' state that requires manual intervention to recover.

 - Cover - this button toggle creates an icon set to open, or close the telescope's M1 primary mirror cover.  Pressing the 'Open' arrow icon causes the Talon TCS to open the mirror cover.  Pressing the 'Close' arrow icon causes the Talon TCS to close the mirror cover.

 - Close - this command button closes the software Hand-paddle.

The software hand-paddle, once activated by the 'Paddle' control button will remain active, and 'float' in the display window.  It can be moved around by a left-click mouse 'grab' action on the widget's top frame, and dragged around the user display as desired.

CAUTION!

The software hand-paddle defaults to the last-used toggle mode within a Talon Xobs session - for example, if Roof/Dome was selected, and the hand-paddle is closed, and re-opened by the 'Paddle' control, it will return in 'Roof/Dome' mode - make sure the correct/desired mode is toggled on (enabled) before commanding a paddle action.

Sounds - This button toggle enables, or disables sounds and tones potentially emitted by the Talon host computer 'talon' located in the 30"/0.8M control room.  It has no real function, as there is no audio system implemented on the 'talon' host.

Status:

The Status panel section of the Talon Xobs GUI indicates current status of various functions.  Each of the icons in the Status panel is a virtual 'indicator' light, which can be one of Off (grey), Green, Red, or Yellow based on the current state of the item it monitors.

Batch - if 'Batch Mode' is enabled, and the telescope is executing an autonomous observing session, this indicator will be Green (autonomous session active), Yellow (indicating either start-up, shutting down, or non-fatal fault), or Red (stopped and/or fatal fault).

Tracking - This indicator will be Green when the telescope is tracking, Yellow when moving/slewing to track, or Red for a tracking fault.  Look in the 'Messages' panel window for related information about tracking faults.

Slewing - This indicator will be Yellow when the telescope is slewing to a commanded position.

Homing - This indicator will be Yellow when the telescope or dome are performing a homing operation on one or more axes.  The indicator will be red if the homing operation faults.  Check the 'Messages' panel window for information related to a homing fault.

Limiting - This indicator will be Yellow when the telescope or dome are performing a limit-finding operation, and Red if the operation faults. Check the 'Messages' panel window for information related to a limit-finding fault.

Weather - this indicator is not active in this release of Talon.  In a future release, it will be Green if there are no closure cautions or conditions present (i.e. weather OK/good), Yellow if there is a closure caution condition(s) present (i.e. increasing dust, wind gusts, increasing humidity), and Red if closure conditions exist.  It will be tied into the Mt. Locke weather system, acting in concert and companion with the 'wx', Weather display, and Weather Closure conditions panel in the dome control room.

Confirm - This indicator turns Red whenever the 'No Confirm' Control toggle has been enabled, and is Off/grey otherwise.

Roof/Dome:

The 'Roof/Dome' panel section provides indication and control over the Dome shutter and Dome automation functions.  The Dome Open and Close command buttons, and virtual indicator 'lights' , as well as the Dome auto on/off toggle, and status indictor are contained in this panel.

Open Indicator - this virtual indicator 'light' is Green when the dome upper and lower shutter are open, Yellow when the upper and lower shutter are in transit to the open state, and Red in the event of an Open fault.  

Open - this command button causes the upper and lower dome shutter to open.  If the dome is in the home position, the upper and lower shutter will open.  If the dome is out of the home position, the dome will seek in azimuth to its home position, to allow the shutter power contacts to align, which provide power to the dome shutter mechanism, and the dome will then open.

Close - his command button causes the upper and lower dome shutter to close.  If the dome is in the home position, the upper and lower shutter will close.  If the dome is out of the home position, the dome will seek in azimuth to its home position, to allow the shutter power contacts to align, which provide power to the dome shutter mechanism, and the dome will then close.

Close Indicator - this virtual indicator 'light' is Green when the dome upper and lower shutter are closed, Yellow when the upper and lower shutter are in transit to the closed state, and Red in the event of an Close fault.  

Dome Auto Indicator - this virtual indicator 'light' is Green when the Dome auto system is off (disabled), Yellow when the dome automation system is on (enabled), and Red in the event of a dome automation fault.

Auto - this button toggle enables (turns on) dome automation, or disables (turns off) dome automation.  Its default state is off/disabled.  Toggling dome automation on/enabled allows the dome slit to synchronize with the telescope's pointing and tracking operation, causing the dome slit to align with the telescope's objective end.  The dome automation does not provide continuous tracking motion, it periodically moves the dome in azimuth to maintain reasonable alignment with the telescope's tracking motion.  The current algorithm for dome synchronization provides unpredictable operation when the telescope is at or near the Zenith, due to the singularity associated with tracking through the Zenith - it can cause the dome position to 'oscillate' in this condition, in which case it may be temporarily disabled until the object and telescope are a few degrees off the Zenith.  The algorithm sometimes fails to predict the shortest path required to re-synchronize with the telescope after a slew/point operation to a new object - it is best to turn off dome automation if a long slew (large zenith distance move) is required, then manually rotate the dome into position with the software hand-paddle, enable dome automation after the pointing/slewing operation is complete.

Az - this field allows manual entry of the target dome azimuth position when dome automation is turned on (enabled).

Telescope:

The Telescope panel provides a number of button and field entry based command operations over the telescope, including Zenith/Service, Stow, Slew, Track, Here and Lookup.  These are described individually in the following list.

Service/Zenith - this command button causes the telescope to position itself at the Zenith (HA of 0:00:00, Dec of 32:17:38).  This position is useful to allow access to the PFC/LF1 electronics mounted at the Cassegrain focus of the telescope, or for refining the telescope's point mesh model using a bright star near the Zenith.  By default, this command action requires confirmation, if 'No Confirm' is not enabled, it will present a red pop-up YES/NO dialog window requiring the user to confirm the command before execution.

Stow - this command button causes the telescope to position itself at the stowed position (HA of 0:00:00, Dec of -35:00:00).  This position is the desired/default 'park' position for the telescope, and allows the PFC/LF1 dewar to be filled.  By default, this command action requires confirmation, if 'No Confirm' is not enabled, it will present a red pop-up YES/NO dialog window requiring the user to confirm the command before execution.

Slew - this command button will cause the telescope to slew to the coordinates in the RA, HA, and Dec, Alt, Az fields in the panel.  The telescope will slew (within its hardware limits) to the supplied coordinates, but not track.  It will move to the identified coordinates and stop there.

Here - this command button will populate the RA, HA, Dec, Alt, Az, and Ep fields with the current coordinates of the telescope.  This is sometimes useful for identifying or confirming where/what the telescope is pointed at.

Lookup - this command button locates an object in the Talon TCS catalog (supported in the adjunct application XEphemeris).  If an object name is entered into the 'Source name:' field, and 'Lookup' is activated by a left mouse click, it will populate the RA, HA, Dec, Alt, Az and Ep fields with the objects coordinates (if found).  It does not cause the telescope to slew or track, it only locates the coordinates from the object database in XEphemeris.

Track - this command button causes the telescope to slew to the coordinates in the location fields, and begin tracking once the slew/pointing operation is complete.  If the telescope is already at the object location identified by the coordinates, it begins tracking.  The telescope will not track an object below the local horizon, or outside its defined hardware limits.

Source Name:   The Source Name field allows entry of an object by common name, or catalog name.  For example, entering 'Alpheratz' in the Source Name field and pressing enter, or clicking Lookup will cause Talon to look up Alpheratz in the XEphemeris catalog, and populate the coordinate fields with RA, HA, Dec, Alt, Az and Ep data.  Entering 'NGC3242', 'M33', 'Sun', 'Moon', 'Jupiter', or similar catalog or common object identifiers will similarly populate the coordinate fields as appropriate.  If an object name cannot be located, or is otherwise ambiguous, for example 'Orion', it will return a message in the 'Message' panel window to the effect 'Orion:  not found anywhere in catalogs'.

RA field - the Right Ascension (RA) for an object for the Epoch (Ep) identified or specified in the Ep (Epoch) field - defaults to J2000 Epoch

HA field - the Hour Angle (HA) for an object for the Epoch (Ep) identified or specified in the Ep (Epoch) field - defaults to J2000 Epoch

Dec - the Declination (Dec) for an object for the Epoch (Ep) identified or specified in the Ep (Epoch) field - defaults to J2000 Epoch

Alt - the Altitude (Alt) above the horizon for an object calculated or specified in degrees, minutes, and seconds (DD:MM:SS.S)

Ep - the default (J2000) Epoch, or specified epoch (i.e. EOD, 1950, etc.)

Az - the azimuth angle of an object East of North, or specified target angle East of North for pointing the telescope

Not all combinations of field entry are meaningful, and the Talon TCS will validate information entered into these fields on various criteria - it will report invalid input or incompatible coordinate specifications in the 'Messages' panel window.

Site Information:

The Site Information panel displays various fields as they relate to the location of the 30" / 0.8M telescope at McDonald Observatory, including Local Time, UT, UT Date, etc.  The individual fields are described in the following list.

Local - this is the Local Time, expressed in 24-hour format as HH:MM:SS.

UT - this is Universal Time (UT), expressed in 24-hour format as HH:MM:SS.

UT Date - this is the Universal Time Date (UT Date) expressed in international format as DD-MMM-YYYY.

LST - this is Local Sidereal Time (LST) expressed in 24-hour format as HH:MM:SS.

JD - this is the Julian Date (JD), expressed in decimal format as NNNNNNN.nnn

Moon - this is the current phase of the Moon, expressed as percent visible, cardinal direction, and elevation angle in terms of Zenith Distance.

Sun - this is the current cardinal direction of the Sun, and elevation angle in terms of Zenith Distance.

Dusk - this is the time in UT, expressed as HH:MM for civil twilight (dusk)

Dawn - this is the time in UT, expressed as HH:MM for dawn

Weather (NYI) - these fields are local weather conditions, not yet implemented in this release of Talon Xobs.  They will ultimately be populated by data from the Mt. Locke weather system in a future release of Talon Xobs.

Messages:

The 'Messages' panel window is a scrollable list of textual status messages from the various Talon TCS components.  The message window contains a scroll bar on the right-hand side of the window that allows the messages for that instance and session of Xobs to be be examined.  The 'Messages' panel window is a composite 'view' of several of the Talon daemon process log files, and the data over multiple Talon sessions is persisted in the associated on-disk log files.

Erase - this command button erases all messages in the current Talon Xobs 'Messages' panel window.   Its effect is local to the currently running Xobs instance, and does not erase or otherwise destroy any of the data in the associated log files - it merely depopulates the message window (clears the messages) for the current Talon Xobs session.

Last - the 'Last' command button causes the Xobs GUI to echo the last message in the 'Messages' panel window buffer - since the window is a view onto multiple message/status sources, it is possible that the items in the window are not in temporal order, and can be out of sequence.  The 'Last' button command echoes, or repeats the last item to arrive in the message window buffer, which is sometimes useful to determine what the last status item in sequence was.