Any exposure taken with LIRIS can be displayed in the RTD, which consists of a control window and a DS9 display.

One of the most frequent applications is to subtract a previous, offset exposure from the current image, yielding a sky-subtracted image (see the OBS-REF option below). Thereby faint objects are better visible. This operation affects only the displayed data (i.e. no changes are made to the data stored on disk). Further display options are available if the store mode is other than  DIFF, in which case e.g. the pre-read can be shown as well.

The RTD control window contains, from top to bottom:

• Observation data Shows the file name of the image actually displayed in DS9.
• Reference data Shows the file name of the image loaded as reference data (see below).
• Status shows the status of the RTD. Usually, this will say that the RTD is waiting for a new image to arrive. It can also be ''scanning'' if the Data detection is selected to be on (clicking on the corresponding diamond), or ''data detection off'' if Data detection is off.
• Data directory shows the directory where the data is being stored.
• Data detection If you click on the on box, the new images are automatically loaded into the RTD.
• Display state This section offers several options to display the data, which are activated by clicking on the diamonds on the left:
• OBS-REF Displays the post minus pre of the observation data minus the post minus pre of the reference data
• OBS/REF pre Displays the pre-read of the observation/reference data
• OBS/REF post Displays the post-read of the observation/reference data
• OBS/REF post-pre Displays the post minus pre of the observation/reference data
  

• Refresh Click on this button to refresh the DS9 display.
• OBS -> REF Load the current observation data as reference data.
• Quit Click on this button to quit the RTD.
• Load OBS/REF data These buttons can be used to load any fits files of the Data directory as Observation data/Reference data: type the file name of the image in the corresponding box and click the corresponding button.
  

Star profile: When you activate this task, you can measure the image seeing. Switch  File detection off, then point the cursor on the star you want to measure. Press ''b'' on the keyboard to move the little green box onto the star, then ''q'' to perform the measurement and to quit. The results will be displayed in the \bf Star parameters section.

The DS9 data display window

This is a typical DS9 image display window but in addition it turns red the pixels which value in the post minus pre of the data set is higher than 40000 ADU to alert observers to saturation (45000 ADU). The pixels are also turned red when either the value in the pre or in the post is higher than 43000 ADU. Non-linearity is ~1% over the range 2000-20000 ADU (in the post-pre) . A viewgraph is available here, as well as further detector characteristics.

Troubleshooting:

To restart the RTD, i.e. if it crashes:

SYS> startudasrtd

This command kills the current ds9 session and the RTD control panel before relaunching them. Should the old windows not get terminated, close them manually.