OT tutorial SISS
The objective of this tutorial is to describe the key observational parameters which you would need to define to propose for ALMA observations (spatial resolution, sensitivity, spectral setup), and to let you play with the ALMA OT so that you feel comfortable enough to craft your own proposal for the next Call for Proposals (next spring!).
This worksheet is intended as a step-by-step guide on how to prepare a mock proposal for the last ALMA Call for Proposals (Cycle 4), by using the ALMA proposal preparation and submission tool ('Observing Tool' or OT). Feel free to skip any of the proposed steps which are not applicable to the science of your practice proposal, and note that many special modes are not described in this basic tutorial. We recommend the OT User Manual for further reference.
Before starting to craft a proposal, you should have an estimate of some characteristics of your source (or at least estimates). These include (but are not limited to):
- The coordinates of the source(s), its velocity towards the observer (in km/s or redshift)
- The extent or size of the emission/absorption regions (which will in form your necessary spatial resolution, as well as your largest angular scale, in arcseconds ")
- If it is an emission/absorption line experiment, the width of the frequency structure that you want to detect (in km/s or kHz/MHz/GHz)
- Its continuum emission flux, based on your own models or previous observations (in Jy/beam at a given frequency)
- A list of transitions (lines) of interest observable with ALMA which are present (or expected to be) in the source, and their estimated brightness (in Jy/beam). You can use the Splatalogue to determine which lines of interest are encompassed within the ALMA bands. Be careful to include the sources' redshift/velocity when you do your line search.
If you already have a preferred source (or a set of sources) in mind, please use it for this tutorial. You can also go ahead and just make up something that is sensible to you.
Now that you know a little more about your source(s), you should decide what are the scientific goals which you want to achieve, and what measurements would allow you to meet your scientific goals. For example, measuring the continuum distribution with a spatial resolution of 0.5". Or obtaining a 10 sigma detection of HCN. Or both. The measurements must be achievable within the capabilities offered by ALMA. We will assume that you are proposing for a Cycle 4 project, for which capabilities are listed here.
If you would like to check out some ideas of what is possible with ALMA in Cycle 4 in order to get you started, check out the Did You Know page here.
You should in particular decide:
- The spatial resolution do you want to reach
- The signal to noise do you want to achieve (on a line detection or on the continuum detection)
- The frequency range(s) which you want to observe.
- For continuum observations, the choice of frequency is usually driven by the optimal compromise between signal to noise and spatial resolution. You can play with the continuum reference frequency in the OT to determine which is the best option depending on your goal.
- For line observations, this choice is driven by the sky frequencies of the lines would be the most suited for your scientific goal (based on your own models, previous publications, line parameters from Splatalogue)
- The maximal spatial scale which you want to be able to retrieve
It is very likely that you will need to go back and forth and modify your original goals while preparing the proposal, to satisfy constraints on ALMA observational setups, or optimize the signal to noise.
Starting your proposal in the OT
Now you can open the OT to prepare your proposal
- type ALMA-OT.sh in a terminal
- select 'Create a new proposal' in the pop-up window. You will see a tree structure on the left panel of the OT display, which describes the structure of the proposal.
- click on 'Proposal' in the tree structure
- on the main panel, write some general information on the proposal (Title, Proposal Type, ...).
Determine the science goals
An ALMA proposal is composed of one or several science goals (SG). In practice, each SG (or pseudo-SG) corresponds to observations obtained with a single receiver and correlator/receiver setup (except for spectral scan mode) of sources which are sufficiently nearby in the sky (~<10 degrees apart) to be observed with the same phase calibrator. However, a single science goal can be used with several arrays or configurations (for example, ACA and 12-m array, or extended 12-m and compact 12-m configurations). The telescope observation instructions (scheduling blocks or SBs) are produced based on the SGs inputs.
- Create one (or several) scientific goals by clicking on the 'target' button on the task bar of the OT ('New phase 1 science goal')
- In the project tree, click on the newly created SG. You will see that the SG is divided in 6 panels (General, Field, Spectral, Calibration, Control, Justification)
- In the 'General' panel of the SG, give a distinctive name to the science goal
- The next sections will help you to fill each panel of the SG
Field panel (source definition)
- In the tree structure click on the 'Field Setup' section of a SG
- In the 'Field Setup' tab, define the name, coordinates and velocity (in km/s or z) of a source, as well as some expected properties
- Note that the expected fluxes densities are defined 'per beam'. This corresponds to the flux encompassed in a resolution unit of the size of your requested angular resolution. For example, if you know that your source is a 4"-wide disk emitting 1 Jy of continuum emission total, the peak flux density per beam of 0.4" will be 10 mJy.
- if your source size may extend beyond half of the primary beam of the telescope (which depends on the observing frequency), or if it is composed of several regions of interest which are more than half of the primary beam away from each other, you will need to observe multiple pointings within your source. In that case:
- you may prefer to define yourself the coordinate of each pointing. Select 'individual pointings' on the 'target type' line. Add as many pointings as desired by clicking on 'Add' at the bottom of the 'Field Center coordinates' panel. You can define the coordinates of each pointing in RA/Dec or offsets from the coordinates defined above. By clicking 'custom mosaic', these pointings will be imaged as a single mosaic
- you may prefer to set up a regular pattern of pointings so as to ideally cover a given area around the source center. Select 'Rectangular field' on the 'target type' line. You will need to define the size of the area in two directions (p and q), and the spacing between pointings. The OT will suggest a number of pointings with the 12-m array, and - if necessary - a number of pointings with the 7m array.
- In the 'Spatial' tab (top of the panel), you can obtain a graphic view of the chosen pointing pattern. You will need to upload a fits file of the source or make an image query.
- If you want to add additional sources to the SG, click on 'Add Source' at the bottom of the panel
- You can flip through the different defined sources by using the tabs at the top of the panel
Spectral Setup panel (receiver and correlator setup)
- In the tree structure on the left panel in the display, click on the 'Spectral Setup' section of a SG
- On the 'Spectral type' line, you need to define if this is a line project (spectral line or spectral scan) or a continuum project
- If it is a pure continuum project, you will probably want to maximize the observed bandwidth and use the low resolution-large bandwidth mode of the correlator (Time Division Mode or TDM). Define the receiver band corresponding to your chosen frequency, and you'll be given a default suggested frequency and a correlator setup (4 2-GHz wide spectral windows). You can change the average sky frequency (the corresponding rest frequency is shown below).
- In the case of a line project, you will need to manually define each of the desired spectral windows within the four basebands. Each baseband encompasses at most 2 continuous GHz of the sky spectrum, and can be split in up to 4 spectral windows. There are several rules restricting how basebands and spectral windows within basebands can be setup with respect to each other.
- Add spectral windows in each baseband. To do that in a given baseband, you can either:
- click on the 'add' button below the baseband panel. An additional line appears above, in which you write the desired central frequency directly in either the 'rest' frequency' or 'sky' box
- click on the 'Select Lines to Observe' button below the baseband panel. A line selection window pops up, showing a selection of lines from the Splatalogue catalogue. You can refine your line search by selecting on the left the desired ALMA band, the range of sky frequencies, or the maximum energy state of the transitions. Once you have identified the line you want, highlight it and click on 'Add to selected transitions'. You can then continue your line search and select up to 4 transitions. When you click on 'ok' (bottom right), a spectral window will be created for each of the selected transition. Any unacceptable selection will be highlighted in red in the 'Spectral Setup Errors' box.
- Define the spectral resolution/bandwidth and spectral smoothing of each spectral window. This is mainly driven by the expected width of the observed spectral lines, and how well you need to resolve them. Note that all spectral windows within a baseband must share the same resolution before smoothing.
- Add spectral windows in each baseband. To do that in a given baseband, you can either:
- Select which one of the spectral windows will define the representative frequency for which parameters such as noise, signal to noise and resolution are calculated.
- At any time you can have a graphic view of your spectral setup plotted over the atmospheric absorption spectrum in the 'Spectral' tab on the top of the panel.
Control and performance panel
The signal to noise which you want to achieve drives the required sensitivity, which necessitates a certain amount of observing time to be reached. Determining the necessary sensitivity is (pretty) simple. You have an idea of the signal to noise you want on your detection (S/N ratio). You have an estimate of the flux density (signal) of the source per beam: for a continuum detection this is the peak continuum flux density. For a line detection, this is the peak flux density averaged over the lowest spectral resolution unit on which you want to achieve a detection (usually, the expected FWHM of the line). The sensitivity (noise) hence corresponds to the flux density per beam divided by the S/N ratio. If the SG includes different lines and continuum detections, each leading to different desired sensitivities, the desired sensitivity for the SG will be defined as the most stringent of these sensitivities.
- In the tree structure on the left panel in the display, click on the 'Control and Performance' section of a SG
- Write down your desired sensitivity (in Jy)
- Write the corresponding bandwidth. For a continuum project, this is the aggregate bandwidth over all spectral windows. For a line project, this is the lowest spectral resolution unit on which you want to achieve the detection.
Now you can define the imaging performance parameters which are necessary to perform your project
- Enter the desired angular resolution
- Enter the largest angular structure (LAS)
Control and performance panel: observing time and arrays
- Click on 'Time Estimate'. You will obtain a breakdown of the estimated time to be spent on source and on calibrators, as well as which arrays and configurations are needed to perform the proposed measurement
Depending on your desired angular resolution and the maximal characteristic scale which you want to image, the time estimator may indicate that a combination observation from different arrays or array configurations is needed. An extended configuration of the main array (the 12-m array, composed of 40 12-m antennas) may provide an excellent spatial resolution, but filter out large scales. This would be compensated by adding observations, not necessarily simultaneous, in a more compact configuration of the 12-m array, or observations by the ACA (Atacama Compact Array, composed of 9 7-m antennas). You will find more information to understand the concept of largest recoverable scale here https://science.nrao.edu/science/videos/largest-angular-scale-and-maximum-recoverable-scale.
If you think that the final time estimate is too high with respect to what you could hope being awarded, there are many ways to drive it down. Remember that the observing time varies as the square of the sensitivity, so a small change in requested sensitivity can significantly change the time request.
- Increase the requested angular resolution. With a larger synthesized beam, the (resolved) sources' expected flux per beam increases. For a constant desired S/N ratio, the corresponding desired sensitivity will be increased.
- Decrease the size of the area to be mapped - hence decreasing the number of necessary pointings.
- Increase the spectral resolution unit for line detection.
- If possible, change the frequency for a frequency offering a better S/N ratio (often at the expense of spatial resolution)
- Eliminate scientific objectives which are too time-expensive
You may need several iterations to hone the most optimal observation parameters.
You have now defined most of your observational setup. To verify if all parameters are set in a correct way, run a proposal validation (check mark icon on the main task bar). This will indicate if there are some errors in the proposal and parameters need to be changed.
If this was a real submission [INFORMATIONAL ONLY]
There are additional steps to undertake to transform a mock-up proposal into a real one. Those steps *should not be performed* during this exercise, but for your information only, here they are:
- add yourself as a PI and collaborators as co-Is. You can only add people who are registered in the ALMA userbase. You can register at almascience.org ('Register' link at the top-right corner)
- justify your choices of observational parameters in the 'technical Justification' tab
- attach a pdf of your scientific justification
- generate a pdf file of your proposal. In the Tool section at the top of the window, click on 'Generate a pdf'
- Submit it: In the File section at the top of the window, click on 'Submit'