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Proposing Guidance

"Proposing Guidance" is a summary of key considerations when proposing for ALMA time. Links to more detailed descriptions of topics are provided.

 


What is ALMA?

 

 

ALMA is an interferometer of 50 x 12 m antennas (the 12-m Array), 12 x 7 m antennas (the 7-m Array) and 4 x 12 m Total Power antennas (the TP Array) located at an altitude of 5000 m in northern Chile. It operates in the submillimeter/mm range, from 3.6 mm to 315 µm (84 - 950 GHz).

ALMA is operated by the Joint ALMA Observatory in Chile. The interface between the ALMA observatory and the astronomical community is provided by the three ALMA Regional Centers (ARCs).

An overview of the ALMA observatory, instrument and science goals can be found here.

Learn more

Beginners: check the principle of interferometry and ALMA basics in the ALMA Primer.

Advanced: check the technical aspects of ALMA operations in the Technical Handbook.

The ALMA Science Portal is the entry to the ALMA web.

Read the ALMA Proposer's Guide (Online version, PDF Version) and use the Observing Tool to prepare your proposal.

The overview guides to the ARCs.

ALMA Science Capability

 

ALMA excels at observing the cool Universe, including dust at temperatures from a few degrees about absolute zero to ~200 deg K, and an extraordinary wealth of molecular and atomic lines, some probing regions of even higher temperatures.

The primary science drivers for ALMA are our cosmic origins: the formation and evolution of galaxies, stars and planets as well as the origin of the elements, complex molecules and of the organic building blocks of life itself.

 

Learn more

The ALMA Cycle 4 Capabilities are summarized in Appendix A of the ALMA Proposer's Guide.

The ALMA Primer gives some introductory ideas to ALMA science opportunities.

Examples of ALMA science themes and highlights can be found here.

Some of ALMA's science capabilities are illustrated here.

Developing a Research Program with ALMA

 

Before investing a lot of time in preparation of a proposal you can get a sense of whether the Cycle 4 ALMA capabilities are appropriate for your goals.

In brief: for Cycle 4 observations at wavelengths of about 3.1, 2.1, 1.3, 0.87, 0.74, 0.44, 0.35 mm will be available. Angular resolutions of down to 0.066" and 0.024" will be reached at wavelengths of 3.1 and 0.35 mm, respectively. Polarization will be offered for on-axis continuum observations in Bands 3, 6 and 7. Note that polarization is not available with the ACA or with mosaic'ed observations.

Key elements you should consider before developing a proposal:

What rms sensitivity is required to achieve your science?

What angular and spectral resolution do you need?

What are the size scales of the sources of interest to you?

Depending on the configuration ALMA may resolve (or filter) out the larger angular scales of emission in extended sources

What is the total area you wish to map? Will you need a mosaic to achieve your goals?

Do you need to observe the polarization structure of your source?

What elevation will your sources have at the ALMA site?

Learn more

Beginners: an overview of the key parameters to develop a research program is provided in the ALMA Primer.

Your experiment will depend on the assumed water vapour column density, which is frequency and elevation-dependent. See, for example, Figure 5 in the ALMA Primer

A detailed explanation of each of these parameters is given in Chapters 5-9 of the ALMA Technical Handbook.

The size scales of interest in your source will govern your observational setup. A useful short (4 minute) video on this topic is available here.

Advanced: a detailed explanation of the key parameters to develop a research program is given in Chapters 5-9 of the ALMA Technical Handbook.

Estimating sensitivity and integration time

 

The ALMA Sensitivity Calculator (ASC) will help you with this step.

In radio astronomy, source fluxes are generally expressed in terms of flux density using Jansky (Jy) units, or brightness temperature, Tb, in Kelvin.

Line fluxes are generally expressed in terms of velocity-integrated flux, S, in units of Jy km/s, or in intensity, I, in units of K km/s

Learn more

Chapter 9 of the ALMA Technical Handbook explains the operation of the ASC.

Beginners: You may need to decide whether to setup your experiment in terms of Tb or flux density. See also the knowledgebase article How do I convert flux measurements given in Jy km/s or K km/s into the peak flux density required by the OT?

Formulae for deriving expected line fluxes can be found in Solomon & Vanden Bout, 2005, Annual Review of Astronomy and Astrophysics (ARAA), 43, 677, section 2.

Imaging considerations

 

Angular resolution, Largest Angular Structure (LAS) and configurations

The ALMA 12-m Array can be configured into several sizes, from 150 m in the most compact configuration up to 16 km in the most extended configuration in full operations. These numbers refer to the longest distance (or baseline) between any two antennas in the array. 

The length of the longest baselines governs the smallest angular resolution you will get in your image.

The length of the shortest baselines governs the Maximum Recoverable Scale (MRS) that the 12-m Array will detect. The Largest Angular Structure (LAS) of interest in your source must be smaller than the MRS of the 12-m Array configuration in order for your image to contain all the emission of interest. 

It is very important not to confuse LAS with target size. The 12-m Array will not be sensitive to emission larger than the MRS: it will spatially filter out that emission, leaving holes (sometimes called bowls) on scales larger than the MRS. This is true even if you make a mosaic that covers a larger target size than the MRS.

If the LAS of your target is larger than the MRS with any configuration of the 12-m array, the OT will recommend you the use of the Atacama Compact Array (ACA or Morita Array). The ACA is made up of the 7-m Array and the Total Power Array. Depending on the range of angular scales important for your science, you may need the complete ACA, or just the 7-m Array.

Target elevation

Targets as far as declination +40 deg or -70 deg, corresponding to a maximum elevation of 25 deg at the ALMA site can be observed. However, the sensitivity and quality of your images will be negatively affected. 

Visualizing your images

Since interferometric observations must be Fourier transformed before an image can be made, it is sometime not intuitive to visualize what your images will look like.

You can simulate your images with the CASA simulator tool "simalma” and the ALMA Observation Support Tool (OST).

Learn more

 

 

Chapter 7 of the ALMA Technical Handbook provides full description of interferometric imaging with ALMA and conditions for the need of the ACA.

Table A1 of the Proposers’ Guide contains a list of the coarsest and finest achievable angular resolution with ALMA at seven representative frequencies. The achievable MRS for those frequencies with and without the ACA is also listed in Table A1 of the Proposer’s Guide.

Beginners: if you expect there may be slightly extended emission choose a small MRS, but larger than your LAS. LAS and MRS are explained and illustrated in the ALMA Primer, and a useful short video.

 

 

 

 

 

 

 

 

 

 

 

Figure 7.5 of the ALMA Technical Handbook shows the loss of sensitivity due to shadowing for sources observed at low elevation.

Figure 7.7 of the ALMA Technical Handbook shows the difference in the image quality for sources observed at high and low elevation.

Spectral resolution and bandwidth

 

The bandwidth for continuum observations is 7.5 GHz. Spectral lines can be observed with resolutions ranging from 0.1-111 km/s at 84 GHz to 0.01 - 10 km/s at 950 GHz. 

Spectral lines and their frequencies can be researched using the spectral line database Splatalogue, which is also available from within the OT.

Spectral scans e.g. for redshift or chemical surveys are available since Cycle 3. 

Learn more

The spectral resolutions (and corresponding bandwidths) available with ALMA are listed in Table A5 of the Proposer’s Guide.

An overview of Splatalogue is available here.

Check out this link to learn more about the wealth of molecular lines that have been observed to date.

Scheduling considerations

Weather

The opacity (primarily determined by the amount of Precipitable Water Vapour – PWV) and the phase stability of the atmosphere limit when ALMA can be used at certain frequencies. High frequency observations are most challenging with respect to weather conditions and therefore only possible during 10-20% of time along the year. The lower phase stability also precludes observations above … GHz during daytime.

  

Configuration schedule

The angular resolution and LAS needed to observe your target will determine the antenna configuration or configurations and consequently the period of the year when your target will be observed.

If you are planning a high frequency observation check carefully that the period needed for its observation does not coincide with the Altiplanic winter (due to the requested angular resolution) or daytime (due to the requested angular resolution and the target visibility at the ALMA site). 

Learn more

Figure 1 of the Proposer’s Guide shows the yearly distribution of time available for high frequency observations at the ALMA site.

 

 

 

 

 

  

Table 3 of the Proposer’s Guide shows the tentative configuration schedule for Cycle 4.  Table A1 shows the mapping of antenna configurations to angular resolution and LAS for several representative frequencies.

Preparing your ALMA proposal

 

Investigators must use the software package, the ALMA Observing Tool (OT), to prepare and submit their proposals.

To prepare and submit an ALMA proposal, the Principal Investigator and all co-Investigators must be registered ALMA users. The registration tab can be found at the top banner of this page.

ALMA proposals are grouped into one of the following categories::

1. Cosmology and the high redshift universe

2. Galaxies and galactic nuclei

3. ISM, star formation and astrochemistry

4. Circumstellar disks, exoplanets and the solar system.

5. Stellar Evolution and the Sun


Proposal preparation involves development of the Scientific Justification, optimizing the observational parameters in the OT, and providing a Technical Justification within the OT

Proposals are subject to peer review for scientific value, and to expert assessment for technical feasibility.

Learn more

Beginners: The OT Phase 1 Quickstart Guide will give you a high level overview to using the OT.

OT Video tutorials: a General Overview, and 10 Easy Steps to Creating a Proposal.

The OT User Manual will give you a more in-depth guide to the OT, including installation instructions in Chapter 2, and OT basic concepts in Chapter 3.

The ALMA Technical Handbook gives complete explanations of how ALMA operates.

The ALMA proposal review process is described in Section 6.5 of the Proposer's Guide

Scientific Justification

 

The Scientific Justification is a free format document which is uploaded as a pdf into the OT. It is recommended you use the provided Proposal Template to ensure you include all required aspects of your proposal

Learn more

The guidelines for writing your Scientific Justification are given in Section 6.2 of the Proposer's Guide.

Technical Justification

 

Each Science Goal requires an accompanying Technical Justification. This allows to confirm the technical feasibility of your experiment, i.e. if your ALMA proposal is capable of meeting the stated Science Goals in terms of e.g. resolution or frequency access.

The Technical Justification is a form within the OT where you must justify the observational setup (sensitivity, imaging and correlator configuration), as well as non-standard requests.

Any item that requires written justification will be highlighted for you when you 'validate' the proposal in the OT. 

Learn more

The guidelines for writing your Technical Justification are given in the Proposer's Guide, Appendix B

Check the ALMA Primer for additional technical considerations when preparing your proposal.

Beginner: check the ALMA OT video tutorial 4 "The technical justification".

Science Goals

 

Proposal preparation consists of setting up one or more Science Goals (SG) using the Observing tool (OT). The Science Goal contains the complete observational setups: spatial coordinates and imaging characteristics, frequency band, spectral windows and spectral resolutions, sensitivity requirements and integration time for one or more science targets.

ALMA will use the Science Goals to build one or more Scheduling Blocks (SBs), which provide the controlling inputs for the telescope control, which is the reason the Science Goals must have strict limitations.

Doppler correction types

Three doppler corrections types can be selected, depending on the source velocity. Two approximations to the true relativistic velocity are generally used: radio velocities for Galactic targets and optical velocities for extragalactic observations.

Field setup & source sizes

The Observation mode for each science goal depends on the expected structure and distribution of your targets.

A number of unresolved, but widely separated targets (but within ~10 deg) are best observed with Individual Pointings.

Targets which are larger than 1/3 of the Field of View (FOV) of the 12-m Array will require a Nyquist sampled Rectangular Mosaic to obtain uniform sensitivity across the field due to the sensitivity roll off towards the edges of the primary beam.

Targets which contain size scales which are larger than the Maximum Recoverable Scale (MRS) of the most compact configuration of the 12-m Array will also require ACA observations; either the 7-m Array alone, or the entire ACA.

ALMA operations will automatically select the array configuration that will deliver your required resolution - thus you will not choose the configuration; instead you specify the angular resolution you need, and the Largest Angular Structure (LAS) expected for your targets.

Spectral Setup

The OT can set up standard baseband and spectral window (spw) frequencies for continuum observations, avoiding parts of the spectrum with high atmospheric opacity.

Spectral lines and their frequencies can be selected from Splatalogue, which is available standalone, and also from within the OT, for plotting on your graphical spectral setup display

Mixed mode spectral observations, using different spectral resolutions in each spectral window are possible since Cycle 3

Spectral scans are available since Cycle 3, benefitting redshift or chemical surveys, for example.

 

Learn more

Scheduling Blocks and Science Goals are described in OT Quickstart Guide and the OT User Manual

A more detailed explanation of the way that ALMA observations are set up can be found in the Technical Handbook, Chapter 8.

The detailed guidance and limitations for ALMA proposals are in the Proposer's Guide

 

 

 

 

Radio velocities become inaccurate at large redshifts. A summary of velocity systems and frames is available here.

 

 

 

Beginners: check OT tutorial video 3 for step-by-step assistance with field setup.

LAS, MRS and spatial filtering are explained and illustrated in the ALMA Primer, and also in a short video.

Advanced: Scheduling Block design and calibration requirements are described in the ALMA Technical Handbook Chapters 8 & 10.

More field-setup information is given in  Section 5.3.2 of the OT User Manual.

If you have targets spread further afield than ~10 degrees, use additional Science Goals to include them.

The ALMA Technical Handbook Chapters 7 & 8 provide full descriptions of interferometric observing and imaging.

 

 

 

 

 

Beginners: check OT tutorial video 2 for step-by-step assistance with spectral setups. 

Advanced: details of spectral window and baseband set up are given in the ALMA Technical Handbook, Chapter 6. More spectral-setup information is given in  Section 5.3.3 of the OT User Manual.

If you cannot fit all of the lines you want to observe for every source in your Science Goal, consider splitting them into two or more SGs.

Integration times are calculated for the first line in a nominated spectral window. Ensure your estimated integration time is appropriate for other target spectral windows that might have a higher opacity.

What if I need assistance from an Expert?

 

You can ask questions via the ALMA Helpdesk, which also has a public KnowledgeBase containing many articles that address common questions.

You can attend a Workshop, Webinar or Community Day Event if your local ARC hosts one or more of these learning opportunities. You can also plan a visit to your local ARC or ARC node for assistance.

Learn more

The KnowledgeBase is accessible without registration.

For Helpdesk questions, register here.

The ARC Guides and ARC web sites will help you with visit and training event planning