How do I change the MEM option when I get this error?
# DMCOPY (CIAO): WARNING: Creating large image: 134 MB. Current max set at 50 MB. Increase maximum using [opt mem=n] or increase blocking to reduce size.
The output files are completely valid; the warning is just there to let you know that the files that are being created are large.
To avoid the warning, specify MEM=135 by adding [opt mem=135] to the input filename; e.g. for dmcopy:
unix% dmcopy "acis_evt2.fits[ccd_id=1][bin sky=1][opt mem=135]" ccd1_bin1.fits
Be aware that the DM tools may ask for a lot of RAM when making large images, so keep your machine's capabilities in mind when using this option; see ahelp dmopt for more information.