Finder Chart: Visualization

The images meeting your search criteria are shown as the primary results of the search, sorted by survey. The magenta circle overlaid by default is the search position you submitted. If you searched on catalogs from the main front search page, then those catalogs are also overlaid on your images. You can add and overlay additional catalogs and add other layers to your image. Click on the blue "Catalogs" tab near the top of the page to add more catalogs. See the Catalogs section for more on catalogs.

Critical to many of the functions described in this visualization section is the concept of the "selected image." Finder Chart can return up to 34 images of the same patch of sky. Look at the array of images returned by a search. One (you may have to look for it) is "selected" -- it is outlined in orange. Click on a different image, and that image becomes outlined in orange as the selected image. By default, the Finder Chart images are locked together, so quite often what you do to one image is done to all the images. But certain other behaviors described here are still customized to each individual image. In order to affect a given image, you must select it.

Contents of page/chapter:
+Basic FITS Viewer
+Visualization Tools -- Image Options
+Footprints -- overlaying markers
+3-color images
+Catalogs and Layers
+Breaking out of the pane (and going back)
+World Coordinate System (WCS) Alignment

 

Basic FITS Viewer

All of the interactive image visualization tools work the same basic way, and here we describe these basic options, in roughly the order in which you might encounter them in the window.

Interactive exploration of the image with the mouse.
Move your mouse over any image that is loaded into the viewer. Details about the image and, specifically, the pixel beneath your mouse cursor, appear along the top of the window with a variety of useful items. Some information is updated in real time (such as coordinates); some information (such as flux densities) is updated when you stop moving your mouse for a second or two. The image can be interactively investigated in this fashion.

You can make it 'stick' on a particular place on the image -- tick the "Lock by click" box and then click on the image at your desired location.

The orientation of the image is given with a compass rose on the right, next to a zoomed-in view of the image under your cursor. If you can't see all of these two windows, enlarge your browser window slightly.

 

Finding out more about the displayed image.
The results are grouped by survey. It indicates the survey origin (instrument, channel) and, where possible, the date of observation in the upper left of each image. (Some images are coadds from individual frames taken at different times, up to months or even years, so a single date may be misleading. Investigate the FITS header of the image using one of the other buttons below to obtain the average date suggested by the producer of the FITS files.)

Some of this information (like the title of the image) is also given in the interactive exploration region described immediately above. This region also includes additional information about the image, such as the pixel scale and orientation. There is a shorter version of the image source in the upper left of each image as well.

Visualization Tools -- Image Options

There is a toolbox along the top of the IRSA Viewer window:

from which you can select a variety of options, now described. Letting your mouse hover over any of these icons will result in a "tool tip" that appears in order to remind you what the icon does. This information is also dynamically updated just to the right of the toolbox itself.

Saving the image.
The diskette icon will allow you to save the current image as a FITS or PNG or Regions file to your local disk or to the IRSA Workspace. Note that you control where the file is saved on your disk through your browser; your browser may be configured to store all downloads in a particular location on your disk.

Saved FITS images will not save the color stretches or overlays; it will just save the underlying FITS image. Saved PNG files WILL include any overlays or annotations you have placed on the image, but will not include the underlying FITS image. Saved regions files will not save the underlying image, but will just save the overlays as a DS9 Regions file. See the DS9 website for more information on the syntax of these DS9 region files.

Note that you can save the original or a cropped version of the FITS file; see the select region icon below to crop, and then save the FITS image.

Note also that if you overlay a catalog consisting of tens of thousands of sources, only the first 500 sources will be saved in the regions file, not the full catalog.

Zooming in or out.
Clicking on these magnifying glass icons zooms in or out of the image. The readout of how many times you are zoomed appears at the top of the visualization window.

If you click zoom in or out rapidly, a pop-up window appears to allow you to more rapidly select the zoom level you want. Select the desired level, or click on the 'x' in the upper right to make the window go away. Here is an example:

Note that there is a maximum (or minimum) allowed zoom level. A notification will appear when you have reached the maximum (or minimum) allowed zoom level for a given image. To enlarge things more than that, please repeat your search to obtain new images with smaller (or larger) spatial extent.

Zooming to a 1-to-1 size.
Clicking this icon will zoom the image such that one pixel in the image is one pixel on your screen.

Fit image to screen or fill screen
These two icons are designed to maximize the available space in your browser window. The first one automatically picks a zoom level such that the image entirely fits within the available space. The second one automatically picks a zoom level such that the image fills as much of the available space as possible (e.g., it is zoomed such that short axis of the window is filled with the image, whether that short axis is left-right or up-down).

By default, the images that are returned are frequently but not always centered on your search target. Clicking on these icons let you see the whole image that is returned, whether or not it is centered on your target.

Changing the color table.
This icon enables you to change the color table of the displayed image. When you click the button, a pull-down menu appears with a wide variety of color table choices. Select your new color table from the options shown:

Changing the color table stretch.
This icon enables you to change the color table stretch of the displayed image. When you click the button, a pull-down menu appears with a variety of choices. You can choose from a set of pre-selected options:

If you pick the first one, "color stretch", you can customize the stretch. A pop-up window appears with a histogram of the values in the image, and you can change the stretch type and range.

If you pick a color stretch from the pre-defined options, the pop-up window reflects this change. (Example: pick 'Linear stretch to 99%'. Go back to "color stretch". Note that it has filled out the stretch type and ranges to reflect the current choice. Then -- either with the pop-up window still up or not -- go back and pick a different pre-defined stretch from the standard options. Note that the values in the pop-up change to reflect this current choice.) If you have a 3-color image, you can change the stretch in each color plane separately; select the tab at the top accordingly for red, green, or blue.

Rotating the image to any angle
This tool allows you to rotate the image to any angle of your choice, in degrees. It will rotate the image counter-clockwise (to the left) from the current view, not necessarily the original image. For example, entering "45" in the rotation pop-up and hitting "rotate" will rotate the image 45 degrees counter-clockwise relative to its original orientation. Then selecting the icon again, and entering "180" in the pop-up (followed by hitting "rotate") will rotate the image an additional 180 degrees counter-clockwise. To exit the pop-up without making further changes, hit the 'x' in the upper right of the pop-up.

Rotating the image so that North is up.
Images retrieved from Finder Chart are commonly already oriented such that North is up, or close to it. However, when interactively investigating images, or in certain situations, you could find yourself in a situation where North is not necessarily up. Clicking this icon will orient the selected image so that North is up.

Flipping the image on the y-axis.
Clicking on this icon flips the image on the y-axis.

Re-center the image.
Clicking this icon will re-center the image on the center of the last query, or on the center of the image.

Selecting a region.
When you click this icon, at first, nothing seems to happen. (There is a notice with brief instructions that appears on the lower left of your browser window.) You can now click and drag in the image, selecting a box on the image. This box can be resized by grabbing and dragging the corners of the box. You can make a new box right away by holding down the shift key and clicking and dragging to select a new box. When you have selected a region of the image, additional icons appear above the image: These icons will allow you to (from left to right) crop the image to the selected region, obtain statistics on the region, select the catalog sources overlaid on the image within the region, or filter the overlaid catalog down to the enclosed sources. (The last two options will only appear if you have a catalog overlaid.) You can save the cropped FITS image via the save icon (described above). The statistics option results in a pop-up that looks something like this:

If you put your mouse over the row of the table in the pop-up, that location appears as an 'x' on the image. If you have a catalog overlaid, selecting sources with this tool highlights them in the catalog list. When you impose a filter, the filters icon changes above the catalog to indicate that there is a filter applied (in this case just one filter: ). To clear the filters, click on the cancel filters icon (which also appears after you impose filters): .

Measuring a distance.
When you click this icon, at first, nothing seems to happen. However, you can now click and drag to draw a line on the image, and the length of the line is displayed (in the middle of the line). The units for the measured distance (and the color of the overlay) can be changed from the "layers" icon (described below). You can calculate the difference in RA and Dec separately via the layers icon as well; find the layer associated with the distance measurement and tick the "offset calculation" box. When it displays the offset calculation, it will give you the angle in degrees in one corner, and the length of the line segment in the RA and Dec directions, in the units you have specified. Clicking on this icon a second time removes the distance tool. (You can also remove this layer via the layers icon.)

Put a marker on the image.
When you click this icon, a pull-down menu appears with several possible options:

The first overlay choice (simply called 'marker') is a red circle. Initially, it appears in the center of the images, and is meant to be moved to wherever you first click in the image. It looks like this: . The small circles in the four corners are "handles" -- you can grab the handles to resize the circle. You can also drag it to any location in the image. You can change the color of the marker (and add a label to it) via the "layers" icon (described below). You can also remove this layer via the layers icon. There are several additional options in the pulldown, enough that they have their own section below.

Show the directions of North and East
When you click this icon, arrows appear on the image showing which direction is North and which is East. Clicking on this icon a second time removes the compass rose. (You can also remove this layer via the layers icon, described below.)

Add a coordinate grid.
Click on this icon to overlay a coordinate grid on the image. Click it again to remove it. Customize the units of the grid (to, e.g., Galactic coordinates) via the "layers" icon (described below).

Read in a DS9 Regions file
When you click this icon, you get a pop-up window from which you can read in a DS9 regions file from your local disk. See the DS9 website for more information on the syntax of these DS9 region files. The supported regions are text, circle, box, polygon, line, and annulus. To make this window go away without doing anything, click on the 'x' in the upper right of the pop-up.

Viewing/changing the layers on the image.
If you've been following along by trying these various options, you now have an image with a lot of annotations on it. The number that appears in blue over the layers icon tells you at any given time how many layers you have on the currently selected image (the image outlined in orange). If you click this layers icon, you will get a pop-up window with a list of all the layers you have on top of the selected image. From the pop-up, you can turn layers off and on, at minimum, but you can often also change exactly what is displayed and what colors/symbols get used for it. Note that when you load 2MASS images, image artifacts ar automatically loaded but not displayed; they can be overlaid the the layers pop-up as well.. To add new things, though, you need to go to other options within the toolbar. Here is an example of a well-populated layers pop-up.

To change colors of a layer, click on the 'colors' link to be taken to a new pop-up from which you can select a new color. To delete a layer, click on 'delete'. Some layers appear that do not have that option; to remove that layer, click on the corresponding icon from which you added that layer. Alternatively, you can "show all" or "hide all" with the buttons on the lower left of the pop-up window. To make this pop-up window go away, click on the 'x' in the upper right of the pop-up.

If you searched on catalogs from the front page, you may have an intimidating number of layers right away. The interaction of the selected image, the layers themselves, and the layers pop-up can be very powerful but also very intimidating. This is discussed more in the catalogs and layers section below.

Restoring everything to the defaults
If you've played around a lot with the image, you may want to undo everything you've done. Click this button to restore everything to their original default values. Some layers may persist; remove them via the layers icon described above.

Lock/unlock images
For the Finder Chart interface, since one of the main purposes of this tool is to provide multi-wavelength views of the same source, this tool has "lock images" turned on by default. In other words, all the images that you have loaded are linked together for zooming, scrolling, etc. You can click on this icon to turn off this linkage among the images, enabling you to zoom/rescale/etc. images on an individual basis. Click it again to re-enable it. This option is discussed in more detail in the WCS section below.

Viewing the FITS header.
This icon will display a pop-up window with the FITS header of the background image. If you click on the columns of the FITS header in the pop-up, it will sort the keywords alphabetically by that column. This is useful for finding individual keywords in particularly densely populated FITS headers. Click the header again to sort in reverse-alphabetical order, and a third time to return to the default order. Below are examples of the original and sorted FITS header. To make this window go away, click on the 'x' in the upper right of the pop-up, or click "close" on the bottom.

Further, you can click on the gears in the upper right of the window to bring up a dialog box via which you can filter down the header keywords (using the same syntax as the other filters):

Getting help.
Clicking on this icon takes you to this help page.

Footprints

The marker icon () has a pull-down menu with several possible options:

We now describe these options here. For each of these choices, the markers appear initially in the center of the loaded images. The first mouse click you make in any of the images will move the marker to that location.

Each of these marker choices, when overlaid and/or selected as 'active', has a large circle surrounded by 4 small circles located at the edge of the large circle. These so-called "handles" allow you to resize and/or rotate the marker, depending on the nature of the marker. These handles only appear when the marker is selected as active, and they are in the same color as the marker itself.

From this marker icon pull-down, by default, it comes up with each option as "show". When you select that option, the marker appears in the layers pop-up (described above), from which you can hide/show the marker, or change its color. Note that after you add a marker, the pulldown menu here changes for that marker menu option to be "hide all" in front of that marker menu option. You can add multiple copies of the same marker using the layers pop-up (described generally above). You can also add a label to the marker from the layers pop-up.

The first overlay choice (simply called 'marker') is a red circle.

The remaining markers are all footprints from other space-based telescopes: HST, JWST, WFIRST, and Spitzer. Except for Spitzer, these footprints are derived from information provided via MAST (see http://gsss.stsci.edu/webservices/footprints/help.html.) For JWST and WFIRST in particular, they are pre-launch values.

HST focal plane footprint. Consult the HST documentation for specifics on which apertures are which. The footprint can be moved or rotated. Click and drag the center of the footprint. A circle appears with four small circles ("handles") around it. Grab and drag the small circles to rotate it, or drag the big circle to move it. Note that if you overlay the footprint on a very small image, nothing will appear to have happened. You need at least a 45 arcmin image to comfortably see the footprint. Change the color, delete, or add more copies of the HST footprint from the layers pop-up:

JWST focal plane footprints.

JWST FootprintDescription
JWST FGS Preliminary footprint
JWST MIRI Preliminary footprint
JWST NIRCAM Preliminary footprint
JWST NIRSPEC Preliminary footprint
JWST NIS Preliminary footprint
JWST entire Preliminary footprint (cross is boresight)

Note that if you overlay the footprint on a very small image, nothing will appear to have happened. You need at least a 30 arcmin image to comfortably see the footprints, especially the entire JWST focal plane. Please consult the JWST documentation for details about the footprints. In all cases, if the footprint is 'active', a circle near the middle of the footprint will appear with four small circles ("handles") around it. Grab and drag the small circles to rotate it, or drag the big circle to move it. Change the color, delete, or add more copies of the footprints from the layers pop-up, one example of which is given here:

WFIRST focal plane footprint. As above, the footprint can be moved or rotated. Click and drag the boresight (the cross hairs), which appears by default to the upper right of the array of squares. A circle appears, centered on the boresight, with four small circles ("handles") around it. Grab and drag the small circles to rotate it, or drag the big circle to move it. Note that if you overlay the footprint on a very small image, nothing will appear to have happened. You need at least a 60 arcmin image to comfortably see the footprint, and even then you will probably have to click and drag to see the entire footprint. Consult the WFIRST documentation for specifics on the apertures. Change the color, delete, or add more copies of the WFIRST footprint from the layers pop-up:

IRAC 3.6 and 4.5 micron footprints. These two footprints can be placed separately from each other. As above, the footprint can be moved or rotated. Click and drag the boresight (the cross hairs), which appears by default to the upper right of the array of squares. A circle appears, centered on the boresight, with four small circles ("handles") around it. Grab and drag the small circles to rotate it, or drag the big circle to move it. Change the color, delete, or add more copies of the IRAC footprints from the layers pop-up:

3-color images

In Finder Chart, a default search results in images in many bands of the same region of the sky. It can be useful to create 3-color images from these multi-color bands.

The images window pane consists of rows of images grouped by survey. Near the top left of the images window pane, you can find this icon: . If you click on this icon, at the end of each row, a 3-color image appears that has been created out of the bands going into that survey. Here is an example for HL Tau:

At the end of each set of survey images is a new, 3-color image. This image has been automatically generated with pre-selected bands as the color planes:

surveyredgreenblue
DSSDSS 2 IRDSS 2 redDSS 2 Blue
SDSSzru
2MASSKHJ
WISEW3W2W1
Spitzer/SEIPI3I2I1
AKARIWideL (140 um)WideS (90 um)n60 (65 um)
IRASIRAS-60IRAS-25IRAS-12

 

The "3-color" button is "sticky" in that if you turn it on, and then do another search, the new search will automatically make 3-color images.

To save the color PNGs, you can do either of the following: (1) Click on the image to select it (make it outlined in orange). Click on the diskette icon in the far left of the image toolbar. Select "png" for the file type to save, and click 'download.' (2) Click on the "download" button in the far upper left of the images window pane. Click on "include color images" in the download options pop-up and save as PNGs.

Catalogs and Layers

If you search on catalogs from the search results page, then Finder Chart will overlay these catalogs on your images as additional layers. In order to see the layers on any given image, you need to click on the layers icon () in the image toolbox (described above). See the Catalogs section for much more information on catalogs in general.

However, if you search on catalogs from the main search page, Finder Chart overlays the catalogs on your images right away, and it does so in a way that is quite powerful, but may be initially confusing. Critical to understanding what it is doing (and how you can change it) is the concept of the "selected image." Out of the potentially large number of images you have loaded, one is "selected" -- it is outlined in orange. Click on a different image, and that image becomes outlined in orange as the selected image. Different things are overlaid on different images (or image sets), so it is important to realize which image you have selected.

In order to explain what it is doing, let's look at a real example. Search on "HL Tau" and ask it to retrieve catalogs within the image boundary:

Here are the results obtained after a few seconds:

This is a single target search, so three panes are returned; one has the images (top), and the bottom has two panes, one with the catalogs and the other with the plots from the catalogs. The images have catalogs overlaid -- the DSS doesn't have a corresponding survey, so it has no catalog, but the SDSS images have the SDSS catalog overlaid, the 2MASS images have the 2MASS catalog overlaid, etc. for all the other surveys with catalogs. By selecting the SDSS catalog tab on the bottom, and then selecting a row in the SDSS catalog, that object is highlighted in the SDSS image and the SDSS catalog plot. It works the other way as well - select an object in the SDSS image or plot , and the object is highlighted in the SDSS catalog. This behavior is identical to that for catalogs in general.

What is potentially confusing here is that the catalogs arrive with the search results and are already read in. From the main search page, you could conceivably stumble into having (1) nothing overlaid; (2) only the central source overlaid, so it looks like nothing is overlaid; (3) so many sources overlaid that you can't see the image. From the layers pop-up, you can control which of these loaded catalogs are shown on the selected image (and in which colors, and what symbol shape). But what is shown on what image is a strong function of which image you have selected.

Perhaps you are interested in whether the WISE catalog has counterparts to an object seen in the 2MASS images. Here is how you add the WISE catalog to the 2MASS images.

  1. Select one of the 2MASS images by clicking on it -- it should be outlined in orange. For this example, click on the J band image.
  2. Select the 'layers' icon from the image toolbox:

  3. The layers pop-up shows all of the potential layers you can add to this particular image from things already read in. You can also add those layers to the entire row of 2MASS images. Right now, in the screen shot above, the 2MASS image has the query object shown, and the 2MASS catalog shown on the entire row of 2MASS images.
  4. To add the WISE catalog to just the J band image (selected in the screen shot above), locate the WISE catalog row in the layers pop-up, and select the radio button for "Overlay: image". Then click on the checkbox for "Show: WISE".
  5. The WISE catalog appears on just the J band image, in the color and with the symbol shape as indicated in the layers pop-up.
Using a similar approach, you can add the 2MASS catalog to all images, or just the IRAS catalog to just the WISE-4 image, or just the row of WISE images, etc.

From the main search page, you can choose to search the catalogs just over a small radius, and/or further constrain it to just find the closest source (one-to-one match) to your target. If you do this, the overlays of the catalogs will (most likely) all be right on top of the same, single source, and it will be much less clear that the catalogs are overlaid at all, much less that the catalogs are different on each set of images. But, it is behaving in the same way as what is described here!

Helpful tip: If you leave the layers pop-up window open while you click on different images to select them, you can see the contents of the pop-up layers window change to reflect the layers (and options) present in that selected image.

Breaking out of the pane (and going back)

By default in Finder Chart, the screen is broken up into 1-4 panes. The set of images corresponding to the target could be your only pane. If you searched on a list of targets, the list is on the left. If you searched on catalogs, the catalogs and plots are on the bottom. For some purposes, it is useful to view the images (or the tables) as large as possible. On the upper right of any pane, there is an expand icon . Clicking on it will expand these tables into a larger window, taking up as much space in your browser window as possible. On the upper right of the image pane, there is this icon: , which expands the window pane into a larger window, taking up as much space in your browser window as possible, but still leaving the images in tiled view.

There are a set of options just above the images, even when the images pane is smaller (and the catalogs/plots/other targets are visible):

The large "Close" arrow at the upper left is always available in the expanded views, and enables you to return back to the pane view.

Next, there is a cluster of icons that portray (in icon form) the different views you can have of the images you have loaded. The first icon (the big square) denotes "show one image at a time." The second icon (the cluster of four squares) denotes "show smaller images of all the images I have loaded, but still take up the whole browser window with the set" (as opposed to going back to the window panes).


If you are viewing one image at a time, the arrows allow you to click through your set of images in the order they are loaded.


These next two icons will generate a PDF and turn on the 3-color images, respectively.

The next portion of the window has some WCS (world coordinate system)-related options that will be discussed below.

World Coordinate System (WCS) Alignment

Locking/Unlocking Images

When viewing the default window pane search results, all of the images returned by your search (for a given target) appear together in the results window. If you make changes to one of the images (using the visualization toolbox described above), such as zooming in or changing the color table, the changes are made to all of the bands shown at the same time. If you don't want this to happen, if you want to make changes to just one image at a time all by itself, you need to unlock the images. By default; the images are locked; near the right hand side of the image toolbar, there is this icon: . Click on this icon to unlock the images. The icon changes: Click it again to lock the images again. Having the images locked by default makes sense in this tool, because it is designed to return many images of the same target. In other tools at IRSA (with the same look and feel), where the tool may not come up with images of the same target, this option is turned off by default.

WCS and Target Match Checkboxes

Near the top of the images, you have two checkboxes that appear as viable options: "WCS Match" and "Target Match". () Since the images as returned for a given target are already the same portion of the sky, all oriented north up (or close to it), selecting and unselecting these WCS options may not seem to have an effect (e.g., may not seem to affect the displayed images in any discernible way).

WCS Match
This tick box (when the images are locked together) aligns each image to the orientation, zoom, etc., of the image you have selected. (An image is selected by clicking on that image, and, once it is selected, the tile is outlined in orange. In single-frame view, the image you are looking at is the selected image.)

Target Match
This tick box (when the images are locked together) aligns each image to the orientation, zoom, etc., of the image you have selected and centers your search target in the images. (An image is selected by clicking on that image, and, once it is selected, the tile is outlined in orange. In single-frame view, the image you are looking at is the selected image.)

Both options align the images with north up, and aligned so that each image has the same angular scale. In essence, this is a one-click way of doing the following: ensuring the images are locked via the "lock" icon, clicking on the "rotate North up" icon, clicking on the "zoom to fit" icon, and clicking on the "center on search target." This is particularly useful if you are looking for changes between multiple images of the same target, or following a moving target as it moves across multiple images. However, since most of the images you might be investigating in Finder Chart are already rotated such that North is up, the changes visible in your browser may be minimal. Since, as of this writing, 2MASS images are only available in relatively small tiles, if you pick a target and a relatively large search radius and ask it for a 2MASS image, chances are excellent that you will obtain an image that may not be centered on your target. In this case, the WCS Match will align the images properly. Note that the alignment is made to the image you have selected, e.g., the only image shown in single-image view, or the tile that is outlined in orange in the tiled view.

Note that the WCS options are "sticky", which means that if you click one, and then unclick it a few seconds later, it doesn't necessarily automatically "undo" the changes it made; to "undo" it, you either have to redo your search entirely, or click the "return to defaults" icon in the toolbox.

To remove an image (or catalog) entirely, click on the small 'x' in the upper right of the image in the tiled view, or on the small 'x' in the corner of the image (or catalog) tab in the window pane view.