The ALMA Sensitivity Calculator (ASC) will calculate the necessary integration times for a given sensitivity (or vice versa) for your ALMA observing project. Input and output parameters are explained below, but for an in-depth explanation of how the ASC works, please consult Chapter 9 of the ALMA Technical Handbook.
The ASC has been configured so that inputs are allowed that conform to the final design specification of ALMA e.g. the maximum number of allowed antennas for the 12-m array is 50 and the highest resolution must not exceed that corresponding to a maximum baseline of 16 km. This allows users to prepare for future Cycles.
Please be aware of the following limitations:
- Integration times refer only to the on-source time (no overheads are taken into account)
- For DSB receivers (B9 and 10) the contribution to the system temperature from the image sideband is assumed to be twice that in the signal sideband
- The ASC is not able to add the source contribution to the system temperature and therefore the times for particularly bright sources e.g. planets or the Sun, will be underestimated
- The effects of shadowing are not taken into account
- Users will unfortunately find that for certain frequencies, the ASC will complain about the default angular resolution for the 7-m array. A workaround is to simply enter zero if sensitivities in K are not being used
In order for the ASC to work, your browser must be java-enabled. We strongly recommend that the official Oracle version of Java 8 be used. You can test your browser's Java installation here. Note that Chrome versions 45 and higher no longer support the NPAPI Java plugin and thus will no longer display the ASC.
- Polarization: There are two options: "Single" and "Dual". Single Polarization allows for very high spectral resolution at the cost of sensitivity. Users will usually want "Dual" for continuum and for spectral line modes not requiring the highest spectral resolution modes.
- Bandwidth Per Polarization: 7.5 GHz should be selected for continuum observations. For spectral line observations, the width of the resolution element in which the user is interested should be entered.
- Observing Frequency: This must be set within the boundaries of ALMA bands 1 to 10; the seven bands available for science observations are highlighted below. Bands 1 and 2 may become part of ALMA as part of a future development plan.
- Band 1: 31 GHz - 45 GHz
- Band 2: 67 GHz - 90 GHz
- Band 3: 84 GHz - 116 GHz
- Band 4: 125 GHz - 163 GHz
- Band 5: 163 GHz - 211 GHz
- Band 6: 211 GHz - 275 GHz
- Band 7: 275 GHz - 373 GHz
- Band 8: 385 GHz - 500 GHz
- Band 9: 602 GHz - 720 GHz
- Band 10: 787 GHz - 950 GHz
- Water Vapour Column Density: The "Automatic Choice" option will select an appropriate value according to the input frequency. Alternatively, the user may override this with their own choice. Note though that the OT will always use the automatically-chosen option when calculating a project's estimate time. See the Weather page for further information on weather statistics and atmospheric transmission.
- Number of Antennas: For Cycle 4, this defaults to 40 12-m array, 10 7-m array and 3 TP array antennas.
- Resolution: A value should be entered here if the user is interested in surface brightness sensitivity.
- Sensitivity/Integration Time: One of these quantities needs to be specified in order to calculate the other. Choosing the unit "Kelvin" will cause the ASC to use surface brightness units and will thus require that an angular resolution be entered in order to give sensible results.
Some users may experience problems with the calculator, depending on the OS, Java version/vendor and browser. The following may be helpful:
- Is your Java installation up to date? The Oracle Java page will tell you which version of Java is being used by applets. We strongly recommend that Java 8 be used
- OpenJDK e.g. IcedTea, variants of Java do not appear to work well with the ASC, although they have not been tested for a long time. The user is generally advised though to install the Oracle version wherever possible
- The Chrome browser no longer supports the Java plugin and so cannot be used to view the calculator
- The Alt modifier key no longer works on Macs. For other systems, holding this key down and changing the unit i.e. from Jy to mJy, will update the numerical value accordingly
- Sometimes, problems can be solved simply by reloading the page or restarting the browser
Any questions about the ASC should be addressed to the Helpdesk (registration required). We are particularly interested to hear about any undesirable behaviour and bugs as well as any improvements that you would like to see.
This java applet was adapted from the ALMA Sensitivity Calculator in the ALMA Observing Tool by Hiroshi Yatagai. The Observing Tool is the software tool that will be used for the preparation of ALMA Observing Projects. The Tool is built by the Proposal and Observing Preparation System team within the ALMA Computing IPT.