(replace the "X" in /dev/sdX with the actual drive letter that's shown by the dmesg command after you have inserted your SD card into a card reader.)
Optionally, compress the image with a file compression tool of your choice. (For best compression results, I first delete all FITS files from the SD card and run
zerofree
on the
loopback-mounted SD-card image
to fill unused file system blocks with zeros. This brings the backup image size down from an uncompressed 64 GBytes to around 6 GBytes.)