# HOW-TO Derive the read noise¶

## What is the read noise?¶

The read-out noise is the noise introduced in the data by the read-out process of CCDs. It is measured from pairs of bias exposures. The rms scatter of the differences between two bias exposures is computed and divided by $$\sqrt{2}$$; this is the read noise in ADU.

## Querying¶

The read noise value is stored in the database using the ReadNoise class. A query using the select method such as the following will select the most recently created, valid ReadNoise object available for the given instrument, date and chip:

awe> rn = ReadNoise.select(instrument='OMEGACAM', date='2017-10-01', chip='ESO_CCD_#65')
1.99976016871049


To derive the read noise using the DPU (for all 32 CCDs of OmegaCAM simultaneously in this example) you can do the following (again fill in instrument and date as appropriate):

awe> # Example using distributed processing

awe> # Example using a Task (single CCD)