IRSA Viewer: Visualization

Contents:
+Basic FITS Viewer
+Visualization Tools -- Image Options
+Footprints -- overlaying markers
+Breaking out of the pane (and going back)
+World Coordinate System (WCS) Alignment

Basic FITS Viewer

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. In IRSA Viewer, it looks like this:
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.

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.

Finding out more about the displayed image.

The top of the display (upper left of the image in tiled view, above the image in single-image view) gives a title of the image. It indicates the survey origin (instrument and channel, in black), and the current zoom factor (in smaller, green font).

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).

Visualization Tools -- Image Options

This image toolbox is always present as a row of tools just under the IRSA banner. 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. 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 that if you overlay a catalog consisting of tens of thousands of sources, then turn around and save a regions file from the catalog overlay, then you will have fewer sources in the regions file than you have in the full catalog.

Selecting a new image.

Clicking on this icon is the same as selecting the blue "Images" tab. Click on this icon to add a new image through a new search, or to change the current image to a new image.

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 blue '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. (Ex: 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.)

Rotating the image to any angle

This feature 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 blue 'x' in the upper right of the pop-up.

Rotating the image so that North is up.

Images retrieved from the IRSA Viewer are commonly already oriented such that North is up, or close to it. However, when interactively investigating images, or loading images from other sources, 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. 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 (save the cropped FITS image via the save icon described above);
• obtain statistics from the image on the region;
• select the catalog sources overlaid on the image within the region (highlights the sources in the list and plot);
• filter the overlaid catalog down to the sources within the enclosed area;
• zoom the image to fit the selected area;
• recenter the images on the selected area.

The statistics option results in a pop-up that looks something like this:

Note that it calculates the location of the minimum and maximum fluxes, and the aperture and flux-weighted centroids. 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. You can choose to impose a filter via this selection mechanism; the filters icon changes above the catalog to indicate that there is a filter applied (). To clear the filters, click on the cancel filters icon (which also appears after you impose filters): . There is much more on filters in the catalogs section.

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.

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 dash-dot line around it means that it is 'active', so you can move (click and drag the marker) or resize it (click and drag the dash-dot boundary). You can change the color of the marker (and change the label) 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.

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 blue '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. If you click this icon, you will get a pop-up window with a list of all the layers you have on top of the 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 get used for it. 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. For catalogs or the search target, you can also select the symbol shape and size:

To adjust the size, type in the symbol size in pixels or use the up/down arrow keys to change the size by one pixle at a time. Click OK to implement your choices. To delete a layer, click on 'delete'. Some layers do not have that option; to remove that layer, click on the corresponding icon from which you added that feature. To make this pop-up window go away, click on the blue 'x' in the upper right of the pop-up.

Lock/unlock images

Depending on what you've done to this point, and what view you have of the images you've loaded, you may have this "lock images" icon appear in your toolbar. Clicking on this icon locks all the images you have loaded for zooming, scrolling, etc. Clicking it again unlocks the images. This option is discussed in more detail in the WCS 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.

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 blue '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.

Each of these marker choices, when overlaid and/or selected as 'active', has a dot-dash square around it. If it is asymmetrical (most of them are), it has an additional "appendage" and a red plus at the center of the footprint:

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; if you wait a few seconds, they vanish.

You can add multiple copies of the same marker. You can also change the label of the marker from the layers pop-up (described generally above).

Note that if you have images of very different resolutions loaded (e.g., IRAS and really anything else), sometimes it struggles to render the marker on each image. You may need to place markers one image at a time.

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. The footprints for HST, JWST, and WFIRST 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 the marker to make it 'active', then grab and drag the dash-dot line to resize or rotate 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, one example of which is given here:

JWST focal plane footprints.

JWST Footprint	Description
	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. The footprint can be moved or rotated. Click the marker to make it 'active', then grab and drag the dash-dot line to resize or rotate 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, one example of which is given here:

Breaking out of the pane (and going back)

For some purposes, it is useful to individually view just the table, or the images, or the plots, as large as possible. In any pane, this icon appears in the upper right of the pane. Clicking on it will expand the pane into a larger window, as big as possible given your browser size.

If you have more than one image loaded in, and you make the image pane big, from this view, you now have some additional options that appear near the top of the new window.

First, the currently displayed band (sometimes using abbreviations), magnification factor, and rotation angle (if relevant) is shown. Zoom in or out using the visualization toolbox described above.

Then, 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 individual window panes).

If you have more than one image loaded, perhaps you wish to see, e.g., just two of the images tiled. Click on the last, "list" icon in this icon cluster to bring up a pop-up window:

This popup window lists all of the images you have loaded, and if you uncheck the corresponding box, that image, while it is still loaded, will not be shown if you, say, scroll through the images, or look at them in tiled mode.

If you have more than one image as well as catalogs and plots loaded, an abbreviated version of this one-at-a-time/tiled icon appears in the image window pane, even in the multi-pane view: . Using this, you can view just one of the images you have loaded in, or tile them.

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

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

Finally, on the right hand side, the arrows, labels, and green/blue dots (which only appear when you are viewing one image at a time) allow you to navigate through your set of images. The green dot inamongst the blue dots indicates where you are in the sequence of images, and the arrows and labels indicate which image is before or after the currently viewed image. Click on the arrows or the dots to move around in the sequence. (When multiple images are loaded just within the images window pane, the arrows are truncated to fit so you just have the arrows; no dots or image names.)

World Coordinate System (WCS) Alignment

Near the right hand side of the image toolbar, you may have this icon: This icon enables locking images of all bands for zooming, scrolling, etc. This option only makes sense to check off if all of your images are of the same region of sky, but if you are in that situation, this option can be quite useful. When you have selected this icon, if you zoom, click-and-drag, change the color table, etc., the changes are made to all of the images simultaneously. This box is not checked by default in the IRSA Viewer, because you very likely will have images loaded from many different targets or other situations where you likely you don't want all of the displayed bands to change together. In other tools at IRSA, this option is turned on by default.

Expanded View WCS Checkboxes

In the expanded view ("broken out of the pane"), you have up to two checkboxes that may appear as viable options: "WCS Match", "Target Match", and "Auto Play." () However, since you may have many images from several sources, covering different regions of sky, these WCS options may not make sense or may not seem to have an effect (e.g., may not seem to affect the displayed images in any sensible way). They are:

WCS Match

This box is available whether you are looking in single frame or tiled frame view. Clicking on this box will align all of the displayed images to be aligned such that north is up and zoomed such that all images are scaled to the same angular scale. Your search target is indicated by the blue circle. In essence, this is a one-click way of doing the following: clicking the "Lock images" 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 obtained at various epochs. 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 is neither square nor 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.

Target Match

This box is available whether you are looking in single frame or tiled frame view. Clicking on this box will align all the images with the target in the middle of the displayed image. 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 obtained at various epochs. 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.

Auto Play

This box is only available in single frame view. This option blinks through all of the images you have loaded (and aligned). You may find that you have 'extra' images loaded, beyond what you wanted to blink. To remove those from the blink sequence, stop the blinking by unchecking the "Auto Play" box, click on the list icon immediately to the left of the checkboxes, which brings up a "Choose which" pop-up. Uncheck the images you don't want to show, click on "ok", and then go back and check the "Auto Play" box to blink the remaining images. (See above for more details on this process.) Once it loads and blinks through all the images at least once, it holds the images on the screen for 1 second before showing the next image.

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