This is the process by which we demonstrate that the data produced by ALMA is valid. The main way of doing this is by making observations of objects that have already been observed by other telescopes and comparing our results with those existing data sets.

This process is getting underway now (January 2011) as we set about testing the validity of data produced with the basic capabilities that have been commissioned so far. To start with we will be using about 8 antennas on baselines of ~20 to 100 meters with receiver bands 3, 6, 7 and 9. Scientific Verification will continue in stages for the next several years as additional capabilities are brought into operation. When we have data sets that we consider to be valid we will release them to the community - i.e. make them available at locations where it can be down-loaded - both in processed form (calibrated images and data cubes) and as raw data (measured visibilities together with the calibration observations). It is hoped that this will be helpful to prospective users, both to give an idea of what the telescope is able to do and to provide example data sets to use when learning how to process ALMA data.

These are the requirements that we plan to test in the current phase:

• Continuum Sensitivity - demonstrate that the noise level is as expected, given the system temperatures and collecting area, that we do detect known faint sources and that the system does not generate spurious sources, even with long integration times. At this stage the 5-sigma limits in one hour should be around 0.25mJy at 100GHz, 1.5mJy at 340 GHz and 5mJy at 675GHz.
  
  
• Line Sensitivity and Resolution - as for continuum, but now demonstrating that faint known lines are detected and that spurious features are not a limitiation. We also need to demonstrate that the spectral resolution is as expected (channel spacing between about 30 kHz and 15 MHz can be selected and the spectral response should have a predictable form) and that the bandpass calibration is accurate.
  
  
• Imaging Fidelity and Dynamic Range - with the limited UV coverage available at present, we expect the "fidelity" of the images to be modest but we need to confirm that the limitation is indeed set by the UV coverage and not by instabilities in the system or inaccuracies in the measurements. High dynamic range (even as high as 1000:1 at 100GHz and 100:1 at 675GHz) should be possible on maps of objects that are bright enough and have suitable structure to do self-calibration, and we need to confirm that.
  
  
• Amplitude Calibration Accuracy - demonstrate that we find the correct fluxes for both continuum and spectral line objects. At this stage our goals are 5% accuracy at 100GHz, relaxing to 15% at 675GHz.
  
  
• Positional Accuracy - demonstrate that we are measuring the positions of known object correctly (including moving sources - e.g. solar system bodies). Errors should be (much) less than 1/10th of the synthesized beam width.
  
  

Other topics that we will be moving on to a little later include polarization, small mosaics and zero-spacing (single-dish) data.

We are in the process of drawing up the list of objects that will be used to perform the verification. The attached file - ALMA Scientific Verification - gives more details including a listing of different types of object and shows the sources that we have chosen so far. Note that we need to check the performance over the whole frequency range (84GHz to 720GHz at this stage) and in general we will need to use different objects in different bands.

We are looking for suggestions from the community for sources to add to this list. The main critieria are that there are existing good data (ideally in numerical form, but this is not essential) in one of the frequency bands we are using and that the object has properties that will enable us to make quantitative tests of one or more of the above requirements. Obviously the objects need to be visible from the ALMA site (latitude -22 degrees) and for the present phase it would be best if they transit at night during the coming months (LST ~ 5 to 15 hrs). Since the data will be released publically, making suggestions will not give you any special rights to the data but we will make sure that credit for the suggestion is given when the data are released and we will expect to involve you in the discussion of issues like the quality of the existing data. Please send your suggestions to This e-mail address is being protected from spambots. You need JavaScript enabled to view it