Making Mosaics Using MONTAGE
Contents
Why Do Astronomers Need to Build Image Mosaics?
Astronomers need to build mosaics because astronomical structures are often much larger than the field of view of a CCD camera. Moreover, observing and data processing have become so efficient that telescopes and satellites can cover large areas of the sky quickly, and meaning astronomers have a super-abundance of data to build into mosaics.
The Montage image mosaic tookit (http://montage.ipac.caltech.edu) was built to read a set of images in FITS format and create a mosaic of them. Figure 1 shows an impressive three-color mosaic of the Pleiades, a cluster of very young stars in Taurus, computed by Montage from visual data obtained from the Space Telescope Science Institute (STScI).
Figure 1: A mosaic image of the Pleiades computed with Montage. The mosaic is 3-degrees on a side, and the blue, green and red channels represent blue, red and near-infrared images from the Digitized Sky Surveys (DSS) at the Space Telescope Science Institute. Image courtesy of Dr. Inseok Song.
How Are Mosaics Built?
Building a mosaic can be complicated, a consequence of how data are acquired. Astronomers generally wish to study regions at many different wavelengths because the way the radiated energy of a source or structure varies with wavelength is a powerful probe of its physics. To obtain images at different wavelengths, astronomers are obliged to use a variety of telescopes and instruments, ground-based and space-based. The cameras on these telescopes observe the sky with different pixel patterns – the pixels differ in size and shape, and the centers of pixels that nominally point to the same point on the sky are invariably slightly misaligned. That is, the pixels are not precisely co-registered on the sky. Montage was built to make the images at all wavelengths look as if they were measured on the same instrument and telescope with the same size pixels, and with all the pixels co-registered. Montage then combines the images at a given wavelength to create a mosaic. Thus, the astronomer is presented with a panchromatic mosaic of the region, with the pixels laid out in the same pattern for all wavelengths. This mosaic is ready for the astronomer to analyze.
Figure 2 illustrates this process for a three-color mosaic of the Large Magellanic Cloud (LMC) made with IRAC and MIPS data measured by Spitzer. The mosaic on the bottom right is made up of 350,000 frames of Spitzer data measured at two epochs in 2005. The images on the left show how the MIPS and IRAC cameras (the square overlays) have different fields-of-view and pixel patterns on the sky. Note also that for both MIPS and IRSAC, the pixel patterns are rotated a little relative to each other on the two observation dates. Montage takes account of these differences in fields-of-view, pixel patterns and rotations to produce a mosaic where all the MIPS and IRAC data have the same pixel pattern on the sky.
Figure 2: A mosaic of 350,000 Spitzer observations of the Large Magellanic Cloud LMC) in three-colors (bottom right). The observations were made in Summer and Fall 2005. The images on the left show the different fields of view and pixel patterns of the input IRAC and MIPS images at the two observing epochs. Images courtesy Dr. Margaret Meixner (STScI)
Finally, there is also an atmospheric phenomenon that must be taken into account in building mosaics of ground-based data. Ground-based images are invariably plagued by airglow, and it often can be as bright as astronomical signals. This airglow is variable in space and time, but there is no physical model that we can use to correct for its effects. Montage does not try to remove the airglow, but instead minimizes the differences in airglow between images, and so the airglow appears uniform across a mosaic of the images.
How Can I Start Building Mosaics?
There is an easy way to build mosaics that doesn’t require you to know anything about how Montage processes images, and that is to use the web-based image mosaic service at:
http://hachi.ipac.caltech.edu:8080/montage/
This service allows you to build mosaics from three major wide-area surveys:
- The Two-Micron All Sky Survey (2MASS), a survey of the entire sky in three Near-Infrared bands.
- The Sloan Digital Sky Survey (SDSS), which maps one-quarter of the sky in the blue, visible and red.
- The Digitized Sky Surveys (DSS) from the Space Telescope Science Institute (STScI), which cover the whole sky in the blue, visible, and red.
Visit http://hachi.ipac.caltech.edu:8080/montage/docs/datasets.html to learn more about these surveys and their datasets.
The service builds mosaics for one wavelength at a time, up to a maximum size of 2 degrees on a side. In Spring 2010, we plan to extend the service to include more datasets, larger mosaics, and to offer multi-color mosaics.
How Do I Use The On-Line Mosaic Service
Figure 3 shows a screenshot of the user interface for the on-line mosaic service. We recommend that you refer to this screenshot throughout this section. First, you have to create an account for yourself (one-time only). Go to the web page and click on the “Create Account” hyperlink – it is near the top of the interface, just under the “Image Mosaic Service” banner - to set up an account and password (there is no anonymous usage). When you login, you will be able to change your preferences (e.g. request e-mail notification of job completion), and you can monitor the status of your jobs. You may have up to 10 jobs running at one time, with the rest placed in a queue. The output from your completed jobs is kept for three days and then purged.
When you are logged in, simply full in the coordinate/object, select a survey, enter a region size, select the pixel resolution (the size of the mosaic’s pixels on the sky), select a coordinate system and click “Submit”. Figure 3 has example parameters filled in. You can try this as a first example – it will generate a mosaic of the Galaxy NGC 5584.
Figure 3: A screenshot of the Montage on-line web service user interface, with the required fields filled out. New users first need to create an account for themselves by clicking the “Create Account” link.
The service will take you to a web page that shows progress and finally link you to a web page giving you the mosaic. The tutorial pages at:
http://hachi.ipac.caltech.edu:8080/montage/docs/request.html
give details on how to monitor jobs and download and visualize your mosaics.
Can I Use Montage on My Desktop?
The code is freely available for download via a click-wrap license at:
http://montage.ipac.caltech.edu/docs/download.html
To date, there have been over 3,000 downloads by astronomers. It runs on all common flavors on Linux/Unix, and on Mac OS X. It does not run under Windows, but can be made to run by using a Unix emulator program, which you would need to install. Please contact the Montage Help Desk (http://irsa.ipac.caltech.edu/cgi-bin/Helpdesk/nph-genTicketForm?projname=Montage&projmail=montage@ipac.caltech.edu) if this interests you.
By design, Montage is built not as an application, but as a toolkit that contains modules for performing the steps needed to build a mosaic. This design allows astronomers to build individual components into their processing environments, and allows them to customize some components for their own needs. You would need to learn what the components do and how to specify inputs and outputs to them, but you would still not need to know the details of how Montage computes the mosaics. You would also benefit from a knowledge of Unix scripting so that you can build automated scripts to produce mosaics. These scripts need not be complicated. For example, here is the script that was used to build the mosaic of the Pleaides in Figure 1:
#!/bin/bash
# Pleiades Image creation BASH script.
# Inseok Song, 2007
for bands in DSS2B DSS2R DSS2IR; do echo Processing ${bands};
mkdir $bands;
cd $bands;
mkdir raw projected;
cd raw;
mArchiveList dss ${bands} "56.5 23.75" 3 3 remote.tbl;
mArchiveExec remote.tbl;
cd .. ;
mImgtbl raw rimages.tbl ;
mProjExec -p raw rimages.tbl ../pleiades.hdr projected stats.tbl ;
mImgtbl projected pimages.tbl ;
mAdd -p projected pimages.tbl ../pleiades.hdr ${bands}.fits ;
cd .. ;
done
mJPEG -blue DSS2B/DSS2B.fits -1s 99.999% gaussian-log \
-green DSS2R/DSS2R.fits -1s 99.999% gaussian-log \
-red DSS2IR/DSS2IR.fits -1s 99.999% gaussian-log \
-out DSS2_BRIR.jpg
If you are interested in modifying this script for your own mosaic, we recommend getting in touch with us and we can help you get started.
Feedback
We want to make this service useful to educators, so if you have comments, questions, suggestions and improvements, please get in touch with us at the Montage Help Desk:
Montage was funded by the National Aeronautics and Space Administration's Earth Science Technology Office, Computational Technologies Project, under Cooperative Agreement Number NCC5-626 between NASA and the California Institute of Technology. The web-based mosaic service uses software developed by IPAC for the US National Virtual Observatory, which is sponsored by the National Science Foundation.
The technical content of this page was prepared by Dr. G. Bruce Berriman (Montage Project Manager). The wiki page was prepared by Ms. Amy Beekley (NExScI Technical Writer).
