Does Montage Conserve Flux When Changing the Image Resolution?

This question has been asked several times. It was first posed on-line by Loren Anderson on the Astrobetter page on image mosaics:

“An important note on Montage is that it does not conserve flux when changing the image resolution. For example, if you want to rebin the image to a pixel size three times larger, the flux of all point sources will decrease by a factor of 9 (3^2). Multiplying by the ratio of the pixel areas, (new_pixel_size/old_pixel_size)^2, will get you back to the original flux. This can be done via the “-x” flag in Montage’s mProject. A “conserve flux” flag in Montage, as in MIDAS, would be nice though.”

My colleague John Good responded:  “Montage is very careful to conserve flux in how it redistributes pixel content (in fact the method used for the slower reprojection algorithm does so using spherical angles even at the pixel scale). The problem arises in not being able to know whether the units of the data represent a flux or a flux density. If the former, she is correct but most of the datasets we have encountered in practice have been flux densities, where such scaling would be inappropriate.

However, if there are enough people that need rescaling like this, we would be happy to include it. It’s a trivial addition; we just try to avoid too many extraneous flags. Anyone desiring additional functionality can write to me directly.

By the way, whenever possible you should try to use mProjectPP, as it is 25-30 times faster than mProject. The former can handle any projection pair; the latter is more restrictive but for small areas can often be used even with some of the odder projections through a little mathematical trickery (creating a “distorted” tangent-plane projection that fits the image locally).”

This material first appeared on the Astrobetter site.

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1 Response to Does Montage Conserve Flux When Changing the Image Resolution?

  1. aadqwd says:

    Skyview has an option to preserve flux during resampling

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