How Should I Mosaic My SCUBA-2 Data?

I've been asked this a number of times so I thought I'd write something down. Currently the SMURF iterative map-maker calculates models for each chunk of data independently (a chunk is a continuous sequence so many times that will be a single observation but it might be smaller than an observation if a flatfield is inserted mid-way through). It then combines them using a weighted coadd to give you the final map. This means that there is nothing to be gained in throwing all your observations at MAKEMAP apart from it taking much longer to make a map before you can see the result.

In terms of flexibility it is better to make a map from each observation separately. You can then look at each map to see how it looks before combining it. You may also want to remove any low frequency structure at this point as well. To combine these maps into a single map you have two choices using Starlink software:

1. Use KAPPA WCSMOSAIC with an interpolation or rebinning scheme of your choice. Make sure that you set the VARIANCE parameter to true to enable variance weighting of your mosaic.
2. If you have already ensured that the maps are made on the same grid (you can use the REF argument in MAKEMAP to ensure this) then you can try CCDPACK MAKEMOS. This can be used to do median or clipped mean stacking and does no interpolation or smoothing. If your images are not aligned on the same grid it is probably better to remake them so that they are, but if that is difficult you can use the KAPPA WCSALIGN command to match up the grids first. Use the GENVAR and USEVAR parameters in MAKEMOS.

In the future it may be that the map-maker will be able to do a better job handling all the data at once itself instead of external mosaicking but at the moment this is not the case.

If you want to keep a track of exposure times you need to track that separately. We will cover that in a follow up post.