The Difference Between The Mosaicking Features of Skyview and Montage

One of the questions we are often asked is what is the difference between Montage and the mosaic capabilities offered by the  Skyview Virtual Telescope at NASA’s High Energy Astrophysics Archive Research Center.

My colleague John Good, the Montage architect, describes the difference as follows:

“Montage is much more slavishly flux conserving than Skyview. By default Skyview uses an interpolation between neighbor input pixels to determine the output value of a pixel.  It only has one mode that is close to flux conserving and this only works exactly for certain projections where pixel edges can be treated as straight lines even after reprojection (though not a bad approximation most of the time).

Montage has been extensively tested and shown to conserve flux (to the floating-point round-off limit) for all projections.  Pixel edges are treated as spherical coordinate curves.  Also, reprojected areas for fractional pixels are preserved exactly so there are no edge effects at all when mosaicking.

Montage can be used with all standard projections in the WCS libraries. Going by the Skyview documentation, it only supports seven.  Similarly, Skyview only supports a few specific coordinate systems while Montage allows full precession (e.g. Ecliptic B1983.5 -> Equatorial J2011.0).

Montage includes an extensive set of tools for background matching over the complete set of overlapping input images based on an iterative relaxation technique

Montage is written in C for speed. The newer version of Skyview is written in Java.  When dealing with complex spherical trigonometry, C is much faster.  Skyview often looks fast, but this is mostly due to the default resampling and background handling.

Finally, Montage is broken down into a set of modules that can be
run intependantly and processing can therefore be heavily parallelized with very little effort.  In fact, Montage has been used extensively by the IT community to test large-scale parallelization and workflow environments.”

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9 Responses to The Difference Between The Mosaicking Features of Skyview and Montage

  1. Gus says:

    The criticisms of SkyView and the advantages of Montage are accurate of course but if this is suppose to be a “difference” article than there is almost no acknowledgement of SkyView’s features and characteristics that are not present in Montage. There is no mention of SkyView’s relative simplicity, ease of use for quick look purposes, no mention of useful non-WCS projections present in SkyView but not Montage (TOAST), that SkyView code is extensible for many of the mosaicking features discussed here, or a recognition that as a command line, web based tool the number of surveys that can be quickly cutout or mosaicked via SkyView is significantly greater than Montage. As is the message of this article the advantages of Montage are Many (with a capital M) but that should be the title of the post: “The advantages of Montage versus SkyView for image mosaicking.”

  2. Alberto says:

    I must say I agree with Gus. In my use of both I found Montage more cumbersome to use than SkyView. SkyView, besides providing the ability to generate TOAST projections is also relatively easy to extend and provides one with the ability to have (modern) web services something that I don’t think Montage can do with ease. I think the tools are comparable in features. How about users? How many people use montage? skyview?

  3. Eli Bressert says:

    I’m using python-montage, which makes using Montage much faster and less cumbersome. I have tried both SkyView and Montage and prefer Montage in general.

  4. astrocompute says:

    Thank you for your comments. I apologize if I appear to have sleighted Skyview, which was not my intent. The question of which one to use depends on your needs, I think. If I were asked to put it in a nutshell, Skyview is best for quick look mosaics, Montage is best for larger scale, flux conserving mosaics. Montage also rectifies background radiation to a common level across the mosaic, which I believe is not offered in Skyview. I would, however, disagree that Montage cannot easily support bulk cutouts or modern web services as easily as Skyview, as Montage users commonly use Montage this way (we do it ourselves at IPAC, in fact, in some of our end user services).

    I don’t know about the number of Skyview users. Montage has had over 6,000 downloads. It is used by astronomers and by the IT community in developing next generation cyberinfrastructure.

  5. Stephen Chu says:

    A nightmare for a laymen to get Montage working and python wrapper on windows all working. Skyiew works easily but I dont think has as many options as montage may have but I will never know for sure if I cant get Montage to work on windows assuming that is actually possible.

    • astrocompute says:

      Please understand that Montage does not run natively on Windows -it was designed to run on all common *nix platforms. I recommend running it through a Unix emulator such as cygwin.

      • Steve says:

        I could never get it to run on windows even with cygwin and I have tried recently. I had problems getting it to run on debian 8 or windows 7 even though this blog says it has run on debian 3 and win 2000 under cgygin. Could just be me but without some detailed tutorials on at least getting setup on my operating systems I cant get in to use montage. So I have been relaying on Skyview but it has lots of edge effects which is making it only partially usefull. If anyone has got Mosiac to work on windows under cygwin or debian 8 maybe they can post on this blog how they set Mosiac up?

      • astrocompute says:

        Hello, I am replying to your comments. Please note that in the past Windows has not been supported by the Montage project – it was run under Cygwin as a proof-of-concept, not as a supported platform (most astronomers work on Linux and Mac OS X). If you wish to run Montage under Windows, there is help at hand. We have recently developed a set of Windows executables for Montage, and we are currently testing and evaluating it before making a public release. If you are willing to use it on a shared risk basis, you may download the Windows version at We would be happy to receive feedback. Please note you no longer need to use an emulator – the modules all run on Windows natively.

        The Montage front page at contains links to Tutorials – this one will get you started: We also have a full API for all modules – the API is the same for Windows and Linux.

        If you have further questions, I will be happy to help: please submit a ticket to our help desk – the link is on our home page: see the Support tab.


        Bruce Berriman (Montage PI).

  6. Steve says:

    Thats great! Thanks. I will be giving it a go!

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