Observing Scenarios

Below are basic CHARIS observing scenarios and demonstrations during engineering observations. 

The recommended baseline observing sequences can be optimized if the target of interest is a disk or a point source observation.

Point Source Observation:

  1. Observe target unocculted with bright Astrogrid on
  2. Observe target occulted without changing astrogrid brightness with the same exposure time
  3. Observe target occulted without changing astrogrid brightness, increasing exposure time to nearly saturate the astrogrid.
  4. Observe target occulted with dimmest astrogrid brightness, same exposure time as step 3
  5. Increase exposure time to near saturation limit for brightest speckles/astrogrid brightness.
  6. Begin observing sequence, obtaining as much field rotation as possible. Due to telescope vibration limitations it is advised to image the target on both sides of transit to maximize ADI at the lowest inner working angles.

Typical Disk Sequence:

  1. Observe a bright target star in the same part of the sky to establish a PSF reference. both unocculted and occulted with astrogrid on and off.
  2. Move to Target
  3. Observe target unocculted with bright Astrogrid on
  4. Observe target occulted without changing astrogrid brightness with the same exposure time
  5. Observe with astrogrid with increased exposure time for good SNR on astrogrid spots
  6. Turn off astrogrid
  7. Increase exposure time to for brightest speckles/astrogrid. Do not saturate speckles. Do not exceed 30s exposure time, particularly for high-zenith targets.
  8. Begin observing sequence, obtaining as much field rotation as possible. Due to telescope vibration limitations it is advised to image the target on both sides of transit to maximize ADI at the lowest inner working angles.

General Observing Advice for Target Scheduling:

  • Typical Field Rotation:
    A typical observing sequence should have sufficient field rotation if you wish to have significant leverage in ADI post-processing. The duration is dependent on how high overhead the target is, how stable the ExAO PSF is, seeing conditions, and your specific post-processing requirements. However a baseline field rotation of ~45 degrees is recommended.
  • Typical observing duration:
    For most objects, a single ADI sequence will span 45 minutes to and hour on both sides of transit to get sufficient field rotation. If your target requires significant field rotation It is recommended that you observe in the same mode on both sides of transit. For most targets that go high overhead, plan for at least 15-20 minutes of time on either side of transit to provide insufficient quality data for ADI sequences. This is because of telescope vibrations that currently limit the stability of the PSF from SCExAO PyWFS loop, which we expect to improve over time. When in doubt, please e-mail SCExAO and CHARIS teams for more information.