Creation of exposure map

[jslavin]

Kathy Reeves, and probably others are interested in creating temperature maps using xrt_teem and composite images - that is images where two or three images have been combined to create a composite with larger dynamic range. So these images have two or three different exposure times. xrt_teem uses the DN (effectively counts) in an image to assess the uncertainty in the derived temperature and emission measure. The IDL code thus requires the input images to be unnormalized. In the xrtpy version we allow normalized images but then multiply by the exposure times to get the unnormalized images. For a composite image then, we'd need to multiply each pixel by the exposure time appropriate to that pixel. Thus the need for an "exposure map". I wrote some code to create such a map for a composite image, but currently it's not very portable. It assumes that the user has access locally to the XRT archive.

So my question is, is such code worth including in XRTpy? I think it would be useful for some. If so, how do we get around the non-portability?