Efficient map-making

Finally, a brief note about map-making. People are making some really quite big maps. SPECX tries to hold the entire cube in virtual memory at one time, and in fact with various permutations of INTERPOLATE and ROTATE, may want to have up to 3 copies of the cube resident. If excessive paging is to be avoided, then it is desirable that all these fit into the available physical memory of the machine (the working set). The usual consequence if they do not is that the whole machine grinds to a halt, without any apparent error...

You can minimize the amount of virtual memory required by only mapping those spectral channels of interest. Use TRUNCATE (or DROP-CHANNELS) to dispense with spectral channels lying far away from the line; use BIN-SPECTRUM where you have higher resolution than you need. Even if you have plenty of memory to spare, you can greatly speed up the map-making process by using as few channels as possible. To reinforce this point, when you do an OPEN-MAP you are now asked explicitly for the number of spectral channels in the cube. You can also eliminate one cube from memory altogether by using `interpolation-on-demand' (an option in INTERPOLATE-MAP), albeit at the cost of slightly increased time to make any particular map.

As noted in the manual, SPECX maps are really channel maps, and no information is stored in the header about individual spectra. Thus if you want to map data taken in the other sideband from that of the map header, or taken with a different spectrometer etc, you first need to INVERT, SHIFT, TRUNCATE etc to get your spectrum into the right form. There is a new macro that does this automatically: it can be invoked by the command symbol CONVERT-TO-MAP-FORMAT this macro is stored along with all the other standard macros in the directory with logical name "specx_command".) Just use this command before you ADD-TO-MAP to convert your current spectrum to the same frequency scaling etc as the map header.