GILDAS/CLASS Mar26-Jul26 error in main beam efficiencies

We have identified a software error in the IRAM 30m main-beam efficiencies implemented in GILDAS/CLASS releases Mar26 through Jul26.

Following the repainting of the IRAM 30m telescope in summer 2025, the telescope efficiencies were remeasured during winter 2025/2026. The measurements themselves are correct; the error was introduced in the GILDAS/CLASS implementation of the new efficiencies. The problem can affect IRAM 30m observations dated on or after 1 September 2024 when the CLASS command

MODIFY BEAM_EFFICIENCY /RUZE

was used with its built-in default parameters in one of these releases. Ta* data are unaffected. The incorrect main-beam efficiencies are lower than the correct values by approximately 15-35%, depending on frequency, as shown in the figure below.

The affected GILDAS releases are Mar26, Apr26, May26, Jun26, and Jul26. The problem is corrected starting with Jul26b. If you have one of the affected releases installed, we recommend that you replace it with the latest released version of GILDAS and no longer use the affected release, even if your current data are not affected.

If your data are affected, we recommend repeating the reduction starting from the Ta* data with the latest GILDAS release. If this is not possible, please contact us at gildas@iram.fr so that we can help you determine the appropriate correction.

The IRAM science software team sincerely apologizes for the inconvenience.

Additional information for potentially affected users

1. Which GILDAS version am I using?

Start CLASS from a terminal:

shell-prompt> class

The GILDAS version is displayed on the first line, for example:

GILDAS Version: sep26 (01sep26 05:00 cest) (x86_64-debian12-gfortran) executable tree

In this example, the version is sep26. The affected releases are mar26, apr26, may26, jun26, and jul26. The problem is corrected starting with jul26b. If CLASS reports one of the affected releases, please update your installation to the latest released version of GILDAS and stop using the affected release. This prevents it from being inadvertently used for future reductions.

2. How do I update GILDAS?

Please install the latest released version of GILDAS following the standard installation instructions on the GILDAS web page.

3. How do I know whether my data are affected?

Your data are affected by this bug only if all of the following are true:

• they are IRAM 30m data;
• the observation date stored in the spectrum is 1 September 2024 or later;
• MODIFY BEAM_EFFICIENCY /RUZE was applied using one of the affected GILDAS releases (mar26, apr26, may26, jun26, or jul26);
• the command was used without supplying explicit B0 and Sigma values, so that CLASS used its built-in defaults.

Simply using an affected GILDAS release for other CLASS operations does not by itself affect the data. Commands such as:

LAS> MODIFY BEAM_EFFICIENCY value

and

LAS> MODIFY BEAM_EFFICIENCY /RUZE B0 Sigma

are not affected by this issue. Observations dated before 1 September 2024 do not use the erroneous set of default Ruze parameters.

4. What should I do if my data are affected?

First, update your GILDAS installation to the latest released version. We then recommend repeating the data reduction starting from the Ta* data. This is the preferred procedure because it provides the safest and most reproducible correction. If you cannot repeat the reduction starting from the Ta* data, please contact us at gildas@iram.fr. We will help you assess your particular reduction and determine the appropriate correction.