Use Case: Offline: Reduce & Image Auto-Correlation (Single Dish) data

Interferometric Use Case for ALMA (based on ALMA SW Memo 11, Science Requirements and Use Cases, modified to include offline processing requirements).

Goal:   Reduce and create an image using on-the-fly (OTF) or single/multi-field auto-correlation data.

Contact Author:   D. Shepherd

Role(s)/Actor(s):
Primary:   The PI (follows the basic course for this UseCase)
Secondaries:  
  Archive (needed to download data into offline package or disk/tape.)
  Offline (to do the actual data reduction and imaging)

Priority:   Critical

Performance:   Response to user inputs in near real time. Typical reduction session for an experienced user should not take more than a few hours.

Frequency of Use:   Perform this Offline Use Case for each auto-correlation mapping or single-field experiment run by ALMA.

Preconditions:

1. Observations have been made by ALMA.
2. WVR corrections have been made on-line and stored in the Archive file.
3. Archive file includes all necessary calibration & source observations (e.g. not just a partial scheduling block).
4. Auto-correlation data has been downloaded from the Archive or is available on tape or disk in ALMA FITS format.
5. If an OTF map is being reduced, the map should be large enough along the scan direction to include regions on each side where there is no source emission. (Note: this may not always be the case, e.g. in the galactic center.)
6. If combining single dish and interferometric data: calibrated, interferometric uv data is available on disk (downloaded from the archive or reduced in a previous UseCase.

Basic Course:

NOTE 1: All steps in the Basic Course should be able to be saved to a master script. Alternatively, the user can start from a master pipeline script downloaded from the Archive, and modify it to run this entire sequence or just parts of the sequence.

NOTE 2: Generally, OTF mapping requires a slew across a designated field with an observation of an OFF-position at the end of the slew. The single OFF-position will be subtracted from all scans in that slew. As such, calibration (from the user perspective) is similar to that needed for single-field total power ON-OFF observations (classical position switching mode). Frequency-switching may also possible in OTF mode if the line emission is sufficiently narrow. Beam-switching modes (nutating subreflector or electronic switching between dual feeds), if available for ALMA, are more applicable for single fields or small multi-field mosaics.

1. Fill the data from the Archive, tape, or disk into the Data Reduction Package.
   
2. Edit data automatically using system-supplied heuristics developed by the Pipeline subsystem. Use, e.g., pointing & weather information in the decision process to determine location of obvious bad data and flag it.
   
3. Calibrate the data:
   • Compute system temperatures (Tsys) using total power on sky and ambient load measurements (previously measured).
     
   • Calculate antenna temeratures (Ta*) from Tsys measurements (apply atmospheric opacity correction) & subtract OFF-source (position or frequency) data from ON-source data. This step produces a first bandpass correction - flattening out the bandpass; continuum emission is still in the data. A spectrum is generated from each observed OTF point.
   • Do a Jy/K conversion based on observations of a known flux density calibrator (if available) or bootstrap the flux density scale based on pre-determined values.
4. Identify spectral lines (manually or using header information [or possibly a Lovas catalog?]) and identify all continuum channels (channels can be interspersed between a dozen or more lines).
   
5. Edit data automatically using system-supplied heuristics based on calculated statistics. Edit based on Tsys values determined above or use channel-by-channel comparisons or pre-determined clip levels to eliminate bad channels. Note: Ensure that line emission regions are not included when calculating statistics or when flagging data above, say, a 5 sigma RMS level.
   
6. Average continuum channels together to create a single channel (will be used later to create a continuum-only map).
   
7. Fit a baseline to the spectra to remove continuum & any residual effects. If the bandpass is stable with respect to time, a single bandpass solution can be derived and applied to all spectra. Otherwise, derive a fit as a function of time.
   
8. If user desires, edit data interactively. Interactive editing of individual spectra may only be possible for single fields or small multi-field mosaics. Given the large number of spectra in a typical OTF map, interactive editing may not be a viable option. Thus, automatic editing may be more critical to the reduction process. Examples of processes required to manually edit auto-calibration data:
   • Average all spectra in an OTF map or user-specified segments of data (e.g. every 100 or 1000 spectra or over specific time ranges) to increase S/N to allow better identification of bad channels or RFI.
     
   • Interactively select data by e.g. time, antenna, channel and flag it.
     
9. Given that ALMA observations will likely include many spectral lines, if desired, split out user-specified or pre-determined channels into separate datasets to create smaller datasets with separate lines. This step should be possible any time after the baseline fit (continuum subtraction).
10. Grid the line-only and continuum-only data to create image cubes using actual pointing positions in the gridding process.
    
11. Remove first-order calibration errors (stripes) from each image cube by selecting line and continuum-free regions on both sides of the map and fitting a 2D spatial baseline along the scan direction.
    
12. Remove residual calibration errors (low-level stripes or other systematics) if needed. The method to be used could be a Fourier transform of the data to identify and flag bad spatial baselines and then inverse FFT back to the image plane. If this doesn't work, other TBD methods may have to be developed.
    

1. Analyze the image cube for science results. (details are TBD)
   
2. Generate a publication-quality figure with annotation, overlays, and comparison images if desired.
   

1. Write out calibrated images to FITS files on disk or tape.
   

Postconditions:

1. Publication-quality figures are generated.
2. Image statistics and analysis results are available to user.
3. Calibrated images are stored for later use (e.g. if this data will be combined with interferometric data).
4. Calibration script is available (if desired by the user).

Issues to be Determined or Resolved:   None at this time.

Notes:   This Use Case was created by D. Shepherd. It was created to help test the Offline software. Relevant SSR Use Cases from SSR Memo 11 are: 4.6.2 (Retrieve Archived Data); 4.7.4 (Reduce Auto-Correlation OTF Map [in pipeline mode]); 4.7.5 (Position switched mapping); 4.7.6 (Frequency-switched mapping).

Last modified: 04aug03