SOFIA Science Data Archive: Images

SOFIA data includes many FITS images, but the SOFIA Archive provides HiPS images for context setting. This chapter covers some basics about images, including an introduction to HiPS images and coverage images; visualization tools are covered in another chapter.

Contents of page/chapter:
+Introduction
+HiPS Images: General Information
+Coverage Image
+Upper Left HiPS menus

 

Introduction

There are basically two kinds of images you can find in the SOFIA Archive, FITS and HiPS . More information about HiPS is in the next section below.

FITS images you may be most familiar with have a header and an image. But FITS images can be far more complicated, and most SOFIA FITS files are more complicated. A FITS file is a header plus a "data unit", or an HDU. A single HDU can be an image, a spectrum, a table, or a data cube. Thus, a single HDU can have multiple data planes. FITS files can have multiple HDUs, each of which can have multiple data plane. FITS files can also be a mixture of images and tables and cubes. Many of the SOFIA high-level data files are multi-plane, multi-HDU, mixed images and tables in the same file. A description of how to use the SOFIA Archive to interact with these complicated data files is in the spectra section, because most of these complicated files are spectra.

Much more detail about interacting with images in general can be found in the Visualization section.

HiPS Images: General Information & Definitions

HiPS stands for hierarchical progressive surveys, and these kinds of images are multi-resolution HEALPix images (where HEALPix stands for Hierarchical Equal Area isoLatitude Pixelation). (Also see IVOA docs on HiPS .) In practice, what this means is that you can interact with images of a very large chunk of sky, and as you zoom, the pixel size changes dynamically. HiPS images are fundamentally different than FITS images, and as such, what you can do with the HiPS images are different than what you can do with the FITS images.

The whole point of HiPS images is to provide on-demand resolution changes. Zoom out, and it loads large pixels. Zoom in, and it loads smaller pixels. HiPS images are designed to cover large areas of sky efficiently. If you need to visualize many degrees, this is the image type to use.

There are HiPS images from all over the world available via the web; the complete list of HiPS images available from the images search page includes (once the "IRSA Featured" checkbox is unchecked) many HiPS images from CDS .

HiPS images typically have the color and stretch set by the person who originally made them. The color table can be remapped within this tool. But, you cannot, in general, change the stretch of HiPS images with as much flexibility as you can with FITS images. This is why there may be multiple versions of some data sets in the list of HiPS images.

HiPS maps typically come with a Multi-Order Coverage map (MOC). A MOC is a format developed by the International Virtual Observatory Alliance to specify sky regions. In this context, a MOC tells you via a simple boolean yes/no, is there sky coverage from this data set in this region. You can overlay a MOC from one data set onto a completely different data set's HiPS image.

Things that define a HiPS image. When selecting a HiPS image, the tool gives you a table. The table that appears has several columns that summarize important things about the available HiPS images:

Coverage Image

The SOFIA Archive provides for you a "coverage image", which is basically a way for it (and you) to keep track of where you are working on the sky. This may initially be surprising. There is a legend that appears in the upper right of the coverage image, and there is a quick link to the layers icon; see layers section of the Visualization chapter. You can collapse the legend to make it smaller by clicking on the arrow in the upper right of the legend box.

You may have a coverage image like this -- this is an example of a coverage image showing the results of a search on M42. The background HiPS image is a 2MASS image.

M42 was a very popular target! The polygons correspond to different images found by the search; see the legend in the upper right. The orange ones correspond to the currently-selected ones in each corresponding instrument tab. There are many more pointings down in the 'murk' in the center. The circle with crosshairs is the position used for the search.

This coverage image is a DSS (optical) HiPS image, showing a less popular target, M17. The polygons are largely FORCAST images (one is HAWC+), and the points are FIFI-LS pointings -- see the legend in the upper right.
This coverage image is 2MASS again, of M101. There are polygons for FIFI-LS coverage, a single point for a GREAT pointing, and many small squares that are point sources from a WISE catalog overlaid -- see the legend.
Finally, this coverage image is IRAC-1, of the Galactic Center. The larger polygons are HAWC+, FIFI-LS, and FORCAST coverage; the FORCAST data in particular are from this SOFIA Legacy program . The many-thousand-source Gaia catalog from this region is overlaid in red. Because there are so many sources in the catalog, the tool is rendering the catalogs in a hierarchical fashion, similar to how HiPS images work. What that means is that the tool has shown the HEALPix cells and the total number of sources in that cell. I've clicked on three cells so it has rendered the individual sources in those cells. You can control what this threshold is and how it renders the cells from the layers pop-up.

In all cases, you can interact with the coverage image in pretty much exactly the same way as you would any other image loaded into this tool; see the the visualization chapter for much more about those tools. The thing that makes a coverage image a little bit different than any other image you might load, however, is that it can automatically adapt, even beyond what a HiPS image can intrinsically do. The next subsection includes more details about how it can automatically change to accomodate your needs and zoom level.

⚠ Tips and Troubleshooting

 

Upper Left HiPS menus

In the upper left of the coverage image, there are two drop-down menus. The first is covered immediately below; the second is covered here.

The first drop-down menu looks like this.

There are two sections here, "Data Options" (see immediately below) and "HiPS to FITS Conversion."

Under "Data Options", you can change what data are shown.
Change HiPS: Changing HiPS images
The choices made by any given creator of a HiPS image may result in any particular region being saturated or too faint to see. Color stretches are set by the creator of the HiPS map and cannot be changed; color tables can be somewhat changed by this tool, but that may be insufficient for your needs. If the HiPS map as shown does not suit your needs and you wish to change the HiPS image, click on the "HiPS/MOC" menu, and then click "Change HiPS". It brings up this pop-up:

Things to note:

  • To select a new image, click on the row corresponding to the new HiPS image you want, and click "Change HiPS" on the bottom left.
  • To cancel without selecting a new image, click "Cancel."
  • This is an interactive table, so all the filtering and column manipulation tools apply here too. You can filter down the columns to find the image you want to use.
  • To learn more about each HiPS map, click on the i with the circle in the second column. It will spawn another window with standardized information about the HiPS map.
  • By default, it shows HiPS maps corresponding to IRSA data collections. To see a more comprehensive list, unclick the box marked "IRSA Featured."
  • A blank HiPS image (basically a blank canvas) is also available from the list of HiPS images.

Add MOC Layer: Adding a MOC Overlay
It is often useful to see what other data are available. Looking at a Herschel/PACS far-IR HiPS map? Overlay a Chandra MOC to see what X-ray data might be available to go with your IR data. Click on the "HiPS/MOC" menu, and then click "Add MOC Layer." It brings up this pop-up:

Things to note:

  • Coverage (the first column) is the fraction of the sky covered by that MOC.
  • To select a new MOC, click on the row corresponding to the new MOC you want, and click "Add MOC" on the bottom left.
  • To cancel without selecting a new image, click "Cancel."
  • This is an interactive table, so all the filtering and column manipulation tools apply here too. You can filter down the columns to find the image you want to use.
  • By default, it shows MOCs likely to be most interesting for IRSA users. To see a more comprehensive list, unclick the box at the top left.
  • If you have your own MOC FITS file, you may upload it via the "Use my MOC" tab on the top center.
  • If you try to upload a MOC FITS file via the upload tab, it will behave as if you have uploaded it here.

Under "HiPS to FITS Conversion", you can control whether the viewer will automatically toggle between image types as needed.
By default, the coverage image is most likely a HiPS image. FITS images are best for small regions of the sky, and HiPS images are best for large regions of sky.
Auto Zoom-in to 2MASS K_s FITS:
If you select this, then when you zoom in very close to a target, it will automatically convert to a FITS image when you get close enough.
Switch to 2MASS K_s FITS image:
If you select this, then it will jump directly to a FITS image centered on the currently selected target.
Note that if you swap between HiPS and FITS and back again, it will include a region on the HiPS image that is the footprint of the FITS images you had just loaded. A label appears at the center of that footprint, which may be disconcerting if you are not zoomed out enough to see the region itself. Here is an example, zoomed out so it is more clear what is going on:

The second drop-down menu looks like this.

 There are three sections in this menu.
Orientation
Under "Orientation", you can control whether the coordinates are in Galactic, Equatorial J2000 (RA/Dec), or Ecliptic J2000. This basically is a quick way to orient which way is up on your screen. Note that this can be used in conjunction with the image readout and/or the coordinate overlay layers button.
Center Galactic
If you select "Center Galactic", the HiPS image slews (or maybe twists is a better word) to put Galactic North up, the Galactic Center in the center of the field of view, and the Galactic Plane horizontally across your field of view. This is useful if you have been zooming or scrolling around to look at individual sources and need to bring it back to a familiar orientation quickly. "Center Equatorial" does the same thing for the celestial poles, equator, and ra/dec=0 (those defined by RA/Dec). "Center Ecliptic" does the same thing, but for the ecliptic plane, poles, and coordinate 0,0. This feature can be most useful when comparing sky coverage (MOCs) to those from other missions or surveys that 'work' in alternate coordinate systems. If you are looking at a coverage map, and you've clicked this menu choice when you were zoomed in closely on a target that isn't 0,0 in your coordinate system, to "find" your position, click on a row in whatever table you have loaded on the left to bring your target back into view.
Projection
Under "Projection", you can control whether the display is in Spherical or Aitoff coordinates. Aitoff projection works better for all-sky displays.