sbig-snap should support color imaging sequences

[garlick]

now we would need to:

sbig cfw 1 # select L filter
sbig snap
sbig cfw 2 # select R filter
sbig snap
...

[garlick]

Perhaps something like this:

sbig snap --cfw-seq 1:60,2:20,3:20,4:20

producing 4 FITS files, each selecting a different cfw slot for a different exposure time