Concept for an Observation Builder for Array and Correlator Configuration

Transcription

1 NRC-E Memo# Concept for an Builder for Array and Correlator Configuration NRC-E Memo# 012 Brent Carlson, February 20, 2001 ABSTRACT The WIDAR correlator design for the E is very flexible and, as such, it has a vast parameter space within which it can operate. This is advantageous in that it provides astronomers with a flexible instrument that can be used to maximize the science output from a particular observation given the system s hardware resource limitations. owever, it is problematic in that the high degree of flexibility provided by the correlator can make it difficult for end-users (astronomers) and software designers to use and implement the almost unlimited number of modes that the correlator provides. This memo is an attempt to present an initial concept that may allow the end-users to take advantage of the full flexibility of the correlator and allow software designers to envision ways in which all of the modes the correlator provides can be programmed. Since the correlator is intimately linked with the entire array, a representative concept for array configuration is presented that is admittedly simplistic in nature and should be judged accordingly. Introduction The WIDAR correlator design [1] for the E provides a large parameter space within which an almost unlimited number of modes or combinations of modes can be envisioned. Each station/antenna input to the correlator can handle eight, 2 Gz analog bands. Each analog band can be sliced up into 16 digital sub-bands and each of these sub-bands can be any width and placement 1 in the analog band. Anti-aliasing techniques ensure that adjacent sub-band results can be seamlessly stitched together with no systematic biases but with some SNR degradation in the overlap region. If desired, overlap can be eliminated by changing the sub-band s digital filter shape, but at the cost of loss of bandwidth at the sub-band edge. The correlator contains 16 sub-band correlators 2 that can be used to correlate each sub-band, or that can be used in concert to correlate less than 16 sub-bands, but with more spectral points per sub-band. Each subband, depending on bandwidth, can be correlated to produce anywhere from 64 spectral points to ¼ million spectral points. Multiple independent and/or overlapping sub-arrays can also be defined. The advantages and problems associated with this kind of flexibility have been stated informally by various people on a number of occasions. The Builder concept in this memo demonstrates that it should be possible to 1 Within the integer sub-band constraints imposed by the signal-processing design. 2 In the current (as yet un-memo d ) design, up to 18 sub-band correlators are defined for redundancy.

2 NRC-E Memo# easily configure and use the correlator (and thus the entire array) anywhere within the vast parameter space available. Context Figure 1 is a diagram that defines the context within which the Builder concept defined in this memo resides. Astronomer Builder Modifications, t-schedule Science Case/ Proposal On-the-fly modifications File Builder Proposal Review File "s Ready Pool" File File File Dynamic, Goal-Oriented Scheduler Images Real-time Imaging Ack Config Systems and Correlator Figure 1 Diagram illustrating the context within which the Builder described in this memo resides. In the diagram, the astronomer who wishes to use the uses an Builder 3 to create a observation file. In this concept, the observation file should contain all of the information necessary to completely configure the array and the correlator (antenna pointings, receivers, local oscillators, sub-band digital filters, correlator chips etc.). This builder should be platform independent 4, be usable by any astronomer, and should not require the user to have specialized knowledge of systems. Indeed, in the builder concept presented, the word correlator is never even used. The observation file is included with the astronomer s science case for review 5. Once the observation is approved, the file goes into the s ready pool a pool of observations that are ready to be executed by the dynamic goaloriented scheduler 6. A possibility (shown with dashed lines in the diagram) is that the astronomer could change an observation s parameters on-the-fly while her observation is in progress. This would entail reading the observation file into the builder, making 3 Definitely not a new concept. owever, the astronomer s view of the necessarily requires that this builder contain details of the correlator s capabilities and that is why it is being described. 4 On a laptop, over the web etc. etc. 5 This is not new, is speculative, and is only one possible model. 6 The scheduling concept intended by NRAO for the E.

3 NRC-E Memo# allowed modifications, and writing it back out for reconfiguration by the scheduler. It may be impractical to do this with a dynamic scheduler since it may not be possible to give the astronomer enough advance notice to allow her to be involved. Straw-Man Builder A straw-man Builder GUI (Graphical User Interface) screen is shown in Figure 2. It contains four major components: the Reference Image window; the Array Settings window; the Spectrum Allocator window; and the Spectrum Planner window. Reference Image Window Spectrum Allocator Window Spectrum Planner Window Beam: Reference Image DEC 23:31 v= -10e4 km s-1 P=2 O Spectrum Allocator 2CO N3 2O 23:32 23:33 23:34 Src: Cas A () CAL RA 01:20: Vel: 10 km-1 DEC 23:02: Ampl: 10^-6 Jy 23:35 RA 01: cm 21 cm 5 cm 1.4 cm 0.7 cm Spectrum Planner v el res: 0.01 km s-1 -- BW: 1 Gz -- SBW: 12.5 Mz R1 v el res: 0.01 km s-1 -- BW: 1 Gz -- SBW: 12.5 Mz L1 O O O 2CO 20 N3 21 cm 5 cm 1.4 cm 0.7 cm Array Settings FOV RES SENS OBS. TIME Ar ray (arcmin ) (arcsec) (Jy) (ours) D C B A v el res: 0.1 km s-1 -- BW: 2 Gz -- SBW: 128 Mz R2 v el res: 0.1 km s-1 -- BW: 2 Gz -- SBW: 128 Mz N3 2O N3 2O 75% v el res: 0.1 km s-1 -- BW: 2 Gz -- SBW: 128 Mz R3 v el res: 0.1 km s-1 -- BW: 2 Gz -- SBW: 128 Mz L3 L2 A Array Config u Sens. Map v 1.2 cm 1.2 cm PLOT Test Image Image Attributes: Qual: 95% DR: 10e+5 #vis: 10e10 v el res: 0.01 km s-1 -- BW: 1 Gz -- SBW: 2 Mz R4 v el res: 0.01 km s-1 -- BW: 1 Gz -- SBW: 2 Mz L4 Beam: LST 01:12: Array Settings Window Timeline Figure 2 Straw-man Builder GUI showing the four main windows and the Timeline. The Reference Image and Array Settings windows are representative of what might be required for a complete package. The Spectrum Allocator and Spectrum Planner windows are where the astronomer decides how to use bandwidth effectively setting the desired correlator configuration. The Timeline allows for different array and bandwidth usage during the course of the observation and is not described in any more detail in this memo. The Reference Image and Array Settings windows are not specifically required to configure the correlator but are concepts that it is believed are required for a complete package. The Spectrum Allocator and Spectrum Planner windows are where the

4 NRC-E Memo# astronomer decides what to do with available spectrum resources effectively configuring the correlator. The Spectrum Allocator and Spectrum Planner could be implemented independent of the Reference Image and Array Settings windows with an appropriate interface. The bottom window is an Timeline that allows different array and spectrum configurations to be defined within the desired observing time. Not shown (and something that could go in a generic top-of-the-screen tool bar) is an observation wizard which, when implemented, should allow any astronomer to configure the array and spectrum usage (correlator) to meet desired science requirements. Full-scale implementation of this wizard may require utilization of artificial intelligence algorithms. The following figures describe each of these windows in more detail. The Reference Image and Array Settings windows are included since they are representative of what might be required for a complete package. The Spectrum Allocator and Spectrum Planner windows are where the available bandwidth and correlator resources are planned in detail. There are many places where there is considerable coupling between windows. For example, grabbing a spectral line in the Spectrum Planner window and moving it will cause the entire associated analog band to be moved accordingly in the Spectrum Allocator. Select tool: Allows phase-center cursor to be moved. This phase-center will be the phase-center for the observation. Allows changeable item to be modified by double-clicking (may not be possible here). Zoom tool: Zoom in: left mouse button. Zoom out: right mouse button. Source selector: Allows selection of source for reference image and setup of display such as plotting current array settings sensitivity contours on the image. Can be from database, or can allow addtion/deletion of source to/from database. This could be a null source or an artificial source. Mosaic-tool: Allows definition of multiple fields. Each field will have its own phase-center cursor and FOV indicator. Velocity space tool: Allows travelling through the image s velocity space if a spectral-line observation. old down left mouse to increase velocity, right mouse to decrease velocity. Measuring tool: Measure image structure extent in angular or absolute units. Calibrator source tool: provides a list of nearby calibrators. Can select/deselect calibrators for the observation. Can provide more detailed information on each calibrator. FOV indicator of selected wavelength/analog band. Image information at currently selected phase-center. Reference Image: Previously observed image from some instrument that astronomer is interested in for this observation. Could be a "null" image. RA Beam: O Reference Image Src: Cas A () RA 01:20: DEC 23:02: CO 20 N3 Phase-center cursor DEC 23:31 23:32 23:33 23:34 CAL Vel: 10 km-1 Ampl: 10^-6 Jy 23:35 01: Image spectral content/observed bands: Provides more spectral information about image. Double-click on items to find out more. Select, pan, and zoom tools provided. 21 cm 5 cm 1.4 cm 0.7 cm Figure 3 Details of a representative Reference Image window. This window is intended to provide the astronomer with information sufficient for planning the observation. This is representative only of what might be required in a complete package and is probably somewhat simplistic.

5 NRC-E Memo# Array Settings window: Allows control/setup of array parameters and testing of those settings on test images. Sliders for array configuration, field-of-view, resolution, sensitivity, and observing time. These are all interlocked and interlocked with observing bandwidth (i.e. changing one necessarily changes the others). Settings are for currently selected analog band(s). May want a slider for hardware integration time as well. Array Sensitivity Map: Brightness temperature sensitivity over the current Test Image. Affected by Array Settings sliders, band-width etc. etc. Could display other maps such as resolution/beam shape/size? D C B Array Settings FOV (arcmin) RES (arcsec) SENS (Jy) OBS. TIME (ours) Array config indicator: Shows array configuration and selected antennas. Click the "select antennas" button to select antennas for a sub-array. This may be a time-variable function, so allows selection of subarrays on a time scale. Double-click in window to enlarge or change display. Array Config A A u Sens. Map u-v tracks indicator: Plots u-v tracks for the array configuration, selected analog bands, spectral resolution, and observing time. v PLOT: Left-clicking this causes the Test Image to be generated. PLOT Test Image Image Attributes: Select Test Source for the Test Image: Allows selection from a catalog of test sources for the Test Image. Could allow selection of the Reference Image (or part of the reference image) as well. Qual: 95% DR: 10e+5 #vis: 10e10 Select image processing attributes such as contrast/enhance, (clean/self-cal algorithms??) etc. etc. Beam: Zoom tool: Zoom in: left mouse button. Zoom out: right mouse button. Pan tool: hold left mouse button to pan through the image. Test Image: this is a test image when the PLOT button is hit. This is an indicator of how the array configuration etc. affects the image. (Right-click in image to toggle between display of image and the ideal image??) Test image attributes: Some measure of image quality compared to the ideal image; image dynamic range; # of visibilities used in generating the image (and expected # of visibilities out of the correlator). Figure 4 Details of a representative Array Settings window. This window is intended to provide the astronomer with the ability to configure the array and test that configuration s effect on a Test Image. This is only representative of what might be required in a complete package and is probably somewhat simplistic.

6 NRC-E Memo# Selector: Double-clicking allows a changeable item to be modified. Single-click allows an item (such as a line, a band) to be selected). Velocity or z indicator: Indicates the object s velocity or z-shift so lines are shifted in wavelength accordingly. Double-click to change or lock to reference image. Spectrum Allocator window: Allows definition of what part of the spectrum to observe. Double-click in blank part of window to enlarge. Right-click on any parameter to lock it. Bandwidth tool: old left button and drag to define required bandwidth region. To delete bandwidth, select it and hit the delete key. Grab tool: Use this to grab bandwidth and move it around. Grabbing a spectral line is the same as panning. O 2CO N3 2O Zoom tool: Zoom in: left mouse button. Zoom out: right mouse button. Pan tool: old left mouse button to pan in frequency/wavelength. v= -10e4 km s-1 P=2 Polarization: Number of polarization products desired. Double-click to change. Spectrum Allocator 74 cm 21 cm 5 cm 1.4 cm 0.7 cm Spectral-lines of interest: Click to select; use delete key to delete. Wavelength/frequency indicator: Double click to change showing wavelength or sky frequency or band designator (e.g. L ) Bandwidth indicator: (E.g.) These Green indicators apply to the Green band in the Spectrum Planner. Top is R; bottom is L. If P=2 or 4, then locked together. Can be moved or deleted. "Lights up" when selected. Double-click to change attributes (BW, #quant bits etc). Shift-click to select more than one. Affects indicators in Ref. Image window and Array Settings window. Spectral-line palette: Click to add pre-defined or custom spectral lines and widths to the diagram. Also add/remove reference (v=0) lines. Select antennas: Allows selection of antennas for this particular spectrum allocation. Antennas different than the antennas could have different analog baseband characteristics. New antennas spectrum allocations automatically sync. with current allocations. Figure 5 Details of the Spectrum Allocator window. This window is used to perform overall planning of where the eight available analog bands will be placed within the observing capability of the. Predefined spectral lines, and user-defined spectral lines can be placed in this window with a user-defined velocity shift. The user can then place the analog bands for optimum coverage of the spectral lines of interest. Analog band placement will be reflected in more detail in the Spectrum Planner window (next Figure) along with correlator sub-band and spectral resources.

7 NRC-E Memo# vel res: 0.01 km s-1 -- BW: 1 Gz -- SBW: 12.5 Mz R1 vel res: 0.01 km s-1 -- BW: 1 Gz -- SBW: 12.5 Mz L1 O O vel res: 0.1 km s-1 -- BW: 2 Gz -- SBW: 128 Mz R2 vel res: 0.1 km s-1 -- BW: 2 Gz -- SBW: 128 Mz L2 N3 2O N3 2O vel res: 0.1 km s-1 -- BW: 2 Gz -- SBW: 128 Mz R3 vel res: 0.1 km s-1 -- BW: 2 Gz -- SBW: 128 Mz L3 1.2 cm 1.2 cm vel res: 0.01 km s-1 -- BW: 1 Gz -- SBW: 2 Mz R4 vel res: 0.01 km s-1 -- BW: 1 Gz -- SBW: 2 Mz L4 Select tool Zoom tool: Zoom in: left mouse button. Zoom out: right mouse button. Pan tool: old left mouse button to pan in frequency/wavelength. Grab tool: old left mouse button to grab and move sub-bands, and grab and move spectral lines (which effectively moves the analog band). Add sub-band tool: When active, sub-bands can be added by clicking the left mouse button. Sub-bands snap to allowed locations. Spectrum % utilization graph: This bar graph displays how much of the system s spectral resources have been used up by the current sub-band and spectral channel settings. 75% Spectrum Planner Spectrum scale: Selectable display in wavelength, velocity, or frequency. Double-click on any one to change. Single Spectrum Planner Element: Each one of these applies to one analog baseband. If the analog baseband is not active in the Spectrum Allocator, this is "greyed-out". Doubleclick to enlarge. Click in window to make it the active element. Right-click on any parameter to lock it. Interference spectrum: Known interference spectrum for this analog band span. Double-click to enlarge. Tracks analog band zooming/panning. Active sub-bands: Any sub-band showing up here is active Can move these, define their width, velocity/spectral resolution, delete, add, and copy them. May be able to select F bandshape characteristics if desired as well as which sub-bands get "phased-up (and what the phasing characteristics ar pulsar gating etc. A selected sub-band i high-lighted (magenta). Double-click to change attributes. Bandwidth summary: Selected sub-band velocity resolution, analog bandwidth, selected sub-band bandwidth. Polarization and pair number indicator: For single polarization this is still labelled as such. Analog band indicator: Approximate shape of analog bands. Used only for indication. Border "lights up" when this is the analog band that we are currently working in. Figure 6 Details of the Spectrum Planner window. This is the window within which the correlator configuration is really mapped out. This window allows definition and placement of sub-bands as well as the number of spectral channels allocated to sub-bands and analog bands. As can be seen, there are virtually an unlimited number of ways that sub-bands can easily be defined in this window completely matching the correlator s flexibility.

8 NRC-E Memo# Some important functions in the Spectrum Planner window are: Grabbing a spectral line and moving it causes the entire analog band to move (i.e. changes the local oscillator frequency) and is reflected by appropriate movement in the Spectrum Allocator window. This feature can be used to prevent spectral lines of interest from ending up in the overlap region between sub-bands where there is additional SNR degradation. The interference spectrum window will help astronomers decide if the spectral line of interest will be clobbered by interference. If so, hardware integration and back-end image processing may have to be adjusted accordingly. To change or view more detail on some item in any sub-window, double-click on it. To view a larger version of a sub-window, double-click in a blank part of the subwindow. Right-click on items to lock them (or unlock them if they are locked) so they can t be disturbed. This allows important parameters to be set and locked before modifying or experimenting with less important parameters. The current displayed capability is always within the correlator s capability. If the user tries to define something outside of the correlator s capability then an error message is generated (if it just can t be done), or a warning message is generated if it will have a detrimental effect on some other parameter s setting. Conclusions This document presented a concept for an Builder. This builder contains a Reference Image window, an Array Settings window, a Spectrum Allocator window, a Spectrum Planner window, and an Timeline. The Reference Image and Array Settings windows are only representative of what might be required for a complete package. The Spectrum Allocator and Spectrum Planner windows are where the full flexibility of the correlator can be easily explored and utilized by an astronomer planning an observation. The Builder generates a observation file that completely describes how the correlator (and the array) are configured for the observation. References [1] Carlson, B., A Proposed WIDAR Correlator for the Expansion Very Large Array Project: Discussion of Capabilities, Implementation, and Signal Processing, NRC-E Memo# 001, May 18, 2000.

9 NRC-E Memo# Acknowledgements Many thanks to Chris Brunt, Peter Dewdney, Sean Dougherty, Lewis Knee, and Tony Willis for providing constructive feedback on this straw-man design.