The two cameras aboard HMI each take 4096 x 4096 full disc images of the sun every 3.875 seconds, but offset by 1.875 seconds, resulting in a "Level 0 filtergram" every 1.875 seconds. The HMI "Level-1" products are simply the individual 4096 x 4096 images taken from level-0, but with flat-field images applied and fits to the limb computed to find the disk center position, along with some ancillary metadata such as SDO velocity and location from the SDO Flight Dynamics Service included.
This data is available for all science analysis, but requests for more than tiny amounts will need to be coordinated with
Hao Thai since the data is offline after 30 days since observation. The data is not available
from jsoc, only from jsoc2 which requires a password for access.
Proper interpretation of the data will usually require understanding of instrument details and instrument observing sequence details. Therefore, we recommend contacting Sebastien Couvidat before starting large analysis projects using hmi.lev1 data. The data can be accessed via the standard JSOC export tools.