- Equatorial (RA/Dec) J2000 in hh:mm:ss ddd:mm:ss format
- Equatorial (RA/Dec) J2000 in decimal degrees
- Galactic in decimal degrees
- Equatorial B1950
- Ecliptic J2000
- Ecliptic B1950
- FITS Image Pixel
- Zero-based Image Pixel
and
HiPS
formats. You can also overlay catalogs. Click on the blue "Catalogs"
tab near the top of the page to initiate this process. See the Catalogs section for more on catalogs.
This section covers interacting with the images -- the visualization
tools.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.
Contents of page/chapter:
+FITS/HiPS Viewer
+Image Information
+Image Toolbar
+Extraction Tools
+Information on Color Stretches
+Information on HiPS features
+Footprints
+Breaking out of the pane (and going back)
+Image Navigation
+World Coordinate System (WCS) Alignment and Releated
Features
+Coverage Image
+Automatic FITS-HiPS-Aitoff Transitions
+"Catalog" loading
You can change the units of what is being read out, in terms of
coordinates or pixel values.
| If you click
on the label of the coordinates, "EQ-J2000" in the screenshot example
above, you get this pop-up, from which you can choose the coordinates
from among:
|
 | If you have a
FITS image loaded, you have an additional readout. Click
on the label of the readout, "Value" in the screenshot example above,
you get this pop-up, from which you can choose the pixel readout from
among:
is a
conversion tool between decimal and hex.)
|
In the lower left of the images, if you click on this:  , you get this pop-up. If you have a FITS image
loaded, at the top of this pop-up, it shows the whole image; the
orientation of the image is given with a compass rose. There is also a
zoom-in of the image at the location under your cursor. Underneath
that in the pop-up, whether you have a FITS or HiPS image loaded, you
can get a readout of the pixel size, a readout of location on the
image in two different coordinate systems, and a readout of the pixel
value. You can change the units of those values by clicking on the
name of the field: "Pixel Size", "EQ-J2000", "Image Pixel", and
"Value". Each results in a pop-up, as above.
|
|
You can make the cursor 'stick' on a particular place on the image --
tick the "Lock by click" box (either in the pop-up or in the lower
right of the image window), and then click on the image at your
desired location. When this is clicked, small "clipboards" appear near
the position readout. Click on that icon to copy that position to your
clipboard.
Here are two
examples of image labels. The former is from Spitzer Enhanced Imaging
Products (SEIP), IRAC channel 2, and the field of view is 11 arcmin.
The latter is far-ultraviolet (FUV) data delivered by the Local Volume
Legacy (LVL) project, and the field of view is 6.8 arcmin.
Images can have multiple planes;
the blue arrows shown here allow you to page through the planes. (This
is from the HerMES project and is Herschel SPIRE 350 micron data.)
For HiPS images, the FOV is the angular size of the width of the HiPS viewer. Even if the image as displayed is smaller than the window, the FOV readout is the width of the window, not the image. If you shrink your browser screen, the FOV can get smaller because the viewer gets smaller. If you load more than one image, the FOV can get smaller because two viewers must fit in the same pane. As a result, the HiPS FOV requested in the search panel is approximate.
For FITS images, the FOV label on FITS images works analogously to the FOV label on HiPS images. If you zoom out, the FOV will increase even when the FITS image is entirely within the viewer. That's because the FOV is what the viewer can show you based on the pixel size. If you drag the image so that it is only partially seen through the viewer, the FOV will not change. For FITS images, the cutout size is not the same as the FOV.
 | The target on which you searched is overlaid on the main image with a cross-hair marker. You can remove this (or change its color) from the layers pop-up, described below. |
This is the image toolbox when you have a FITS
image loaded:
And, this is the image toolbox when you have clicked on a HiPS image
you have loaded:
The two toolbars are different, but if the same icon appears, it has the same effect on the image. Many of the icons have a downward pointing blue triangle, which means that there are additional options in a drop-down menu that appear when you click on the icon.
We now discuss each icon in the order in which they appear.
Selecting a new
image
Tools drop down
 |
|
Saving the image . 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.
If the current image is a FITS file, you can save it as a FITS or PNG
or regions file to your local disk. If it is a HiPS file, your only
choices are PNG or regions file. 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 a FITS
file; see the "select region" icon below to crop, then click on the
save icon. Be sure to save the cropped FITS image (see annotated
figure). This feature is not available for HiPS images.
Note that if you overlay a large catalog on an image, then turn around and save a regions file from the catalog overlay, the full catalog may not be saved to the regions file. If you have >10,000 sources, not every source will be overlaid on the image, even if you zoom in, and thus will not be in the regions file. If you want to save your entire catalog as a regions file, save the catalog from the table pane.
The saved PNG is the same size as it is on your screen. If you want a big version, make the desired image big on your screen (view one-at-a-time) before saving the PNG.
Restoring everything to the
defaults
Viewing the image header
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 other filters):
An example of the HiPS properties window is here:
Rotating the image to any
angle
Rotating the image so that North is
up
Flipping the image on the y-axis
Add a coordinate grid
Measuring a distance
that appears
next to the image toolbar, or click on this icon a second time to remove
the distance tool. (You can also remove this layer via the layers
icon.) 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. ⚠ Tips and Troubleshooting: If you overlay a list of sources you created in ds9 regions format from your disk, it will only be overlaid on the current image, not all of the images you have loaded. If you want to have it overlaid on all the images you have loaded, create a catalog from your source list and overlay it as a catalog. Then it will appear on all of the images you have loaded, provided that the positions overlap on the sky.
Put a marker on the image
. 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 drop-down, enough that they have
their own section below.
Note that if you load both FITS and HiPS images at the same time, it
will include a region on the HiPS image that is the footprint of the
FITS images you have 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
clear what is going on:
Draw a line in the image
Make points in the image
Zoom drop down
Zooming in or out
If you click zoom in or out rapidly, a pop-up window appears to allow
you to more rapidly select the zoom level (field of view) 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: 
You can alternatively zoom using the mouse wheel (or drag forward and backward on a touchpad or magic mouse).
Note that there is a maximum (or minimum) allowed zoom level, and they are different for FITS and HiPS images. A notification will appear when you have reached the maximum (or minimum) allowed zoom level for a given image. To enlarge images more (or less) than that, please repeat your search to obtain new images with smaller (or larger) spatial extent. HiPS images are specifically designed for large areas, so if you need a big area, use HiPS. If you want to zoom in close enough to see individual original pixels, your best choice is FITS.
See also the section below on changing coverage images, specifically that on automatic transitions while zooming.
Zooming to a 1-to-1 size
Fit image to screen or fill
screen
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.
This is available for both FITS and HiPS images, though note that FITS images retrieved from IRSA using this tool are typically square, and HiPS images cover the sky, so fitting the image to the screen might not be what you want to do.
Color table drop down
The arrow in the upper right creates a pop-up window out of this drop-down menu so that you can leave the choices up while settling on the best option. Click to select the color table you want. Alternatively, you can use the "color bar" slider to move among the color tables. Below the color bar slider, there are sliders controlling the bias and contrast. Click or drag the slider to change the image display.
Color stretch drop down
Re-center the image drop down
Example of "Pan by table row" functionality: Load the tool. Search on M101, and ask for all SEIP channels (4 IRAC, 1 MIPS). When it returns, search on catalogs. Select a WISE/AllWISE catalog, and ask it for a polygon search covering the image that is loaded. When the catalog loads, go up to the image toolbox and be sure to select Align and Lock by WCS (see lock image discussion below for more information). Sort the catalog by RA by clicking on the top of the RA column. Note that the images are all now slewed to center the first object (the furthest east) in the catalog. Click on a source in the plot near the west edge of the image. Note that the images are all now slewed to center that newly selected source.
Other choices are to center on the target of the observation, center the image in the window, or center on a target of your choice. For the last of those, you can simply center on that target, or center and leave a marker on the image at that location.
The last option may or may not appear, depending on what you have been doing before getting to this screen. If it can, it gives you a choice to center on recent positions. Move your mouse over to the black arrow to select from a list.
When you have selected a region of the image, additional icons
appear above the image, and exactly which icons you see is a function
of whether you are working on a FITS or HiPS image, and whether or not
you have a catalog overlaid: 
These
icons will allow you to do several things:
Crop the image
Note that, if you have a rotated FITS image such that
a crop would have to bisect pixels, it will show you the region that
encompasses your selection. If you crop at that point, then, it will
crop in image space (such that pixels are not bisected). See the
figure below -- in the original image, north is up. This has been
rotated 45 degrees. The selected region is in white. The yellow
dash-dot line is the crop in pixel space that encompasses the selected
region.
Select sources
Filter sources
). 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 Tables section.
Zoom the image
Recenter the image
Obtain statistics
Search
appears next to the image toolbar
to remind you that you are in that mode. If you are refinifng positions
for a search, it will be 
. Either way, when you are done with the selection tool,
click on that red text to turn it off.
Viewing/changing the layers on the
image
This
reminds you of the target on which you searched -- here, it was M16,
where the coordinates were resolved by NED (as opposed to Simbad). The
two icons next in that row indicate, respectively, "copy this location
to the clipboard" and "center image on this position." The tickbox
here means "show this target" (as opposed to the tickbox at the left,
which means "show this image").
Where it's possible 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.
From here, you can click on your desired color in the top colorful
box. Immediately below that box, you can change the color and
saturation of the top box so that you can select from a different
range of colors. Below that, you can enter numerical Hex codes or RGBA
values (where the value for RGB is between 0 and 255, and A is in
units of percent, e.g., 50 = 50%). Finally, you can also select from
a pre-defined set of 15 colors by clicking on the small box. Note ha
the numerical codes update as you select different colors. Click "OK"
to implement your color choice, or click 'x' in the upper right to
close the window without changing the 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
pixel 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. 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.
Lock/unlock image color tables
and overlays
Lock/unlock images
Getting help
Specific information on Extraction Tools
Several tools allow you to extract information from images or image
planes, but only for FITS (not HiPS) files.
-- Extract down through image planes
-- Extract a line from the image
-- Extract points from the image
Here, we cover the basic approach, with specifics of each tool integrated as we go along.
When you click on one of these icons, you enter into the extraction
mode. Red text appears next to the image toolbar to remind you that
you are in this mode: 
When you are
done, to end this mode, click on this red "end extraction."
When starting out, the pop-up window that you get depends on the tool you pick.
| For the drill: | for the line: | and for the points: |  |
 |
 |
From this point, you can click on your image, or click and drag for the line tool. The pop-up then contains a plot of your extraction.
| For the drill: | for the line: | and for the points: |  |
 |
 |
Note that for the line, if you have more than one image loaded and visible, you can shift-click on a new image to see the same line on a new image. And, for the points, you can shift-click to change images without extracting points.
Once you have an extraction that you like, you can retain the extraction for further analysis, in any of three ways. "Pin chart/table" extracts the information as a table, just like any of the other tables in this tool, with an accompanying plot. You can then manipulate the table/plot just like any other table or plot in this tool. If the tool recognizes the extraction as a spectrum, you may have additional capabilities. Download as Table saves the table to your local disk with all the same options as a regular table. Download Chart saves the plot as shown, as a png file.
Once you pin or save your extraction, the tool leaves a "footprint" of
your extraction on the image so that you can remember what the
extraction was. NOTE THAT it is not interpolating
across fractional pixels here. It is averaging if you have asked it to
average, but particularly if your pixels are
large, if you draw a line that is diagonally across pixels, it will be
immediately obvious that it's not interpolating. This line gets
rendered as these pixels:
The point appears on the image at the lower left corner of the
relevant pixel.
You can pin as many different extractions as you want. Each one will result in new tabs with the corresponding table at the bottom of the screen. There are navigation aids within the tables section that may help.
Specific information on Color Stretches
This icon enables you to change the color
stretch of the displayed image. (This option is only available for
FITS, not HiPS, images.) When you click the button, a drop-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. If you
change the color stretch in the pop-up window, the drop-down menu
changes correspondingly.
Example: Display the pop-up for color stretch. From the main drop-down, pick 'Linear stretch to 99%'. Go back to the color stretch pop-up. 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. From the pop-up, pick a different stretch type -- try "histogram equalization." Select "refresh" to update the images. Go back to the drop-down menu. The last 7 items have changed to be based on histogram equalization, as opposed to the "linear" default.
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. By default, it stretches each band independently, and you can set the parameters in the stretch pop-up accordingly.
As described in Lupton et al. (2004) , a different algorithm may be useful for creating
3-band color images. Select "Hue preserving stretch" to invoke this
option. This stretch should be a brightness-independent
color-preserving asinh stretch, though in practical terms, it seems to
work best for optical images.
It may be useful to scale individual channels; sliders allow you to do so. The Q parameter has another slider. For a linear stretch, Q=0; increase Q to change what features are emphasized. Pedestal values can also be set to allow the level assigned to "black" to change.
Specific Information on HiPS Features
General Information
HiPS (hierarchical progressive survey) images are different than FITS images, and as such, the choices of what you can do with the HiPS images are different than what you can do with the FITS images. Choices you can make for HiPS images are sometimes different than what you get for 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; the complete list of
HiPS images available from the IRSA Viewer search page includes (once
the "IRSA Featured" checkbox is unchecked) many HiPS images from CDS .
HiPS images 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 change the stretch of HiPS images from within IRSA Viewer. This is why there may be multiple versions of some data sets in the list of HiPS images.
You can't save HiPS images from within IRSA's tool. To download your own copy, you will have to track down the original source of the image.
Overlays on HiPS Images
Catalog searches (see Catalogs section) on top of HiPS images are limited to a maximum of 5 degrees.
Once you have loaded a HiPS image into the IRSA Viewer, if you click
on the layers icon (), you will have
new, HiPS-specific choices in the layers:
Auto: This option overlays a position grid, with the
tile numbers marked in the center of each box. As you continue to zoom
in, when smaller tiles are needed, they are drawn, with the new tile
numbers marked. You may not zoom beyond HiPS Norder level 14 tiles.
The numbers after the "/" is in the "NESTED" (as opposed to RING or
NUNIQ) numbering system; see the IVOA
standards document for
more information.
Grid Match Image Depth: If you select this option, the grid will adjust to a new level when you zoom in and a new level of HiPS image both exists and is used for the display.
Grid Level Lock: Selecting this option yields an additional numerical drop-down menu. The higher number you pick, the smaller the grid boxes are that are drawn. When this option is selected, the boxes stay the same size regardless of how zoomed-in on the image you are.
A multi-order coverage map (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 ask for the
MOC to be displayed via the layers dialog and
selecting:
⚠ Tips and Troubleshooting:
This menu allows you to do two things: Change the HiPS Image, and add MOC overlays.
Things to note:
Things to note:
Footprints
The marker icon () has a drop-down menu with
several possible options:
Any
of the options with an arrow on the right can expand to additional
subsidiary choices, e.g.,:
We now describe these various footprints 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 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.
⚠ Tips and Troubleshooting
The first overlay choice (simply
called 'marker') is a red circle.
The remaining markers are all footprints from various telescopes:
Spitzer, SOFIA, HST, JWST, and Roman. HST, JWST and Roman are derived
from information provided via MAST (see http://gsss.stsci.edu/webservices/footprints/help.html
.) For JWST and Roman in
particular, they are pre-launch values.
Spitzer/IRAC 3.6 and 4.5 micron
footprints. These two footprints are placed separately
from each other. 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. Change the color, delete, or add
more copies of the IRAC footprints from the layers pop-up.
SOFIA footprints.
Several different SOFIA footprints are available; the graphic here
shows a selection of them. The available footprints (all of which are
placed separately) are:
HST footprints.
You can overlay the whole focal plane footprint, shown here, or
individual instrument footprints (NICMOS, WFPC2, ACS/WFC, ACS/HRC,
ACS/SBC, WFC3/UVIS, and WFC3/IR). 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 footprints from
the layers pop-up.
JWST footprints.
You can overlay the whole focal plane footprint, shown here, or
individual instrument footprints (FGS, MIRI, NIRCAM, NIS, and NIRSPEC).
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 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.
Nancy Grace Roman Space Telescope
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 Roman
documentation for
specifics on the apertures. Change the color, delete, or add more
copies of the Roman footprint from the layers pop-up.
Breaking out of the pane (and going back)
Panes: If you have both images and catalogs
loaded into the IRSA Viewer, the screen is broken up into panes - one
for images, one for catalogs, and one for plots from the catalog. If
you have more than one image loaded in, the image pane is further
subdivided.
Make it big! 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.
Go back the way it was: 
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.
Special case of images only: If you have only images loaded in, then the images are taking up all of your browser window, and it is already, by default, in this expanded mode. There's no 'close' arrow in the upper left since there is nothing else loaded in.
Removing things: 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 catalog tab in the window pane view.
Also see the next section on image navigation.
Image Navigation
Single or Tiled Images
When you have many images loaded in, you can have icons like this:
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 at once," e.g., tiled images. Whether the
images (tiled or not) take up all the space or not depends on whether
you are viewing in panes or in the full-screen mode (see immediately
above on Breaking out of the pane).
Image List
Depending on what you have loaded, you may have an additional icon:
Clicking on this icon brings up a
list of the images you have loaded, with some additional information
on each one. This list is a table like any
other in this tool. The power of this table is best demonstrated
by an example.
Example: Load the tool. Search on M101, with the default image size.
Select all bands from the following data sets: SEIP, DUSTiNGS, LVL,
MIPS_LG. Search. When the tool comes back, click on the image list
icon above and obtain this pop-up:
This table shows that it found
images of M101 from SEIP, and some LVL bands, but nothing in DUSTiNGS
or MIPS_LG. You can use the table to omit the failed images all at
once: click the down arrow at the top of the "Status" column, tick the
box next to "Success" and then click "filter." The failed images are
removed all at once from the table and your display. Now, click the
top of the "Wavelength" column. The table is sorted by wavelength, and
the images in your display are as well.
You can remove selected rows, or delete the failed images with one click; click on the corresponding button on the lower right of this window.
The "help" links in the far right of this table take you to a master list of all data sets available in IRSA Viewer, from which you can obtain standard information about the data sets (mission, wavlengths, links to more information about the program or delivery, and more).
Paging through single image views
If you have many images loaded in and click on the single big square
to view one image at a time, you are provided with navigation aids in
the upper right, like this:
The arrows allow you to scroll through your list of images, sorted as
specified in the image table. The green dot in the list of blue dots
shows you where your currently displayed image is in the set of loaded
images. The "auto play" tick box triggers automatic scrolling through
each of the loaded images.
Removing things: 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 catalog tab in the window pane view. Closing the upper-right image leaves your mouse on or near the x for the next image that fills that corner, allowing multiple images to be closed with minimal mouse movement.
World Coordinate System (WCS) Alignment and Related
Features
As described above, there are two different lock buttons on the image
toolbar, one () for the color
table/stretch/overlays, and another, different one () for WCS matching. This section describes
the image locking in more detail.
The main purpose of having lock buttons at all is to make it easier to change color tables for everything at once, or to zoom/scroll all the images at once.
To use either of the locking buttons: Select an image by clicking on it; note that your selected image is outlined in orange. Lock by color and then change the color table in that selected image; all the images change color tables. Lock by position and all the images align to the same scale as your selected image.
Aligning images by position on the sky is likely to be the most common use of locking. You can align FITS and HiPS images to each other. When you have locked the images, if you zoom, click-and-drag, etc., then all the images move together. This option only makes sense if all of your loaded images are of the same region of sky. Unlike in other IRSA tools, it's quite possible in IRSA Viewer to 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, which is why locking by position of any sort is an option distinct from locking color tables.
When aligning images, you can specify how the images align and for
how long. Clicking the lock images icon produces this drop-down
menu:
The first set of options aligns the images only once; the second set of options makes the alignment persist ("lock") when you move the images.
You can align by the images' WCS (world coordinate system, e.g., RA and Dec), by the target (align by target on the screen, regardless of position in the sky), by the pixels according to the origin of the coordinate system in the image header, or by the pixel at the image center. The most common choice is likely the WCS align and lock.
Here are examples of different alignments, left to right: align by
WCS, by pixel origin, and by pixel at image centers.
Note that aligning by WCS puts North up, and aligned so that each image has the same angular scale.
In contrast, here is an align by target - several different spiral
galaxies, but the target used for each image is in the center of each
image tile.
Coverage Image
If you have launched IRSA Viewer by initially doing a catalog search
(as opposed to an image search), when the catalog is loaded, it will
pick an image on your behalf, called a coverage image.
If you have launched IRSA Viewer with images and then do a catalog search, it will generate the coverage image for you and create a new tab under your loaded images.
The next subsection covers more details about the coverage image, specifically how it automatically changes to accomodate your zoom level.
Automatic FITS-HiPS-Aitoff Transitions
Coverage images can have many choices;
it could look something like either of these screen shots. In this
section, we cover these choices from left to right.
Type of image.
images are
created specifically to enable fast, dynamic resolution changes; zoom
in and find smaller pixels than if you zoom out. HiPS images are
designed to cover large areas of sky efficiently. HiPS images are
limited to the color and stretch set by the person who originally made
them. Choices of what you can do and what you can manipulate with HiPS
images can be different than your choices for FITS images.
FITS images are best for small regions of the sky. HiPS images are best for large regions of sky. Aitoff projections are best for viewing the entire sky at once. If you tick the "Auto" box and then zoom in or out, the viewer will automatically toggle between image types as needed. Zoom in enough, and it will swap from HiPS to FITS. Zoom out again enough, and it will swap from FITS to HiPS.
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
clear what is going on:
Coordinates
The drop-down menu allows you to select between coordinate systems. Used in conjunction with the image readout and/or the coordinate layer button (both described above), you can change what coordinate system it uses.
"Catalog" loading
As discussed in the catalogs
section, you can upload several different types of files into the
tool, including files that have images, or mixtures of images and
tables.
If you load in a regions file but have no images loaded, the tool will "hold onto" the regions file until you give it an image on which to overlay the regions.
If you load in a mixture of images and tables, it will load both the images and tables into different window panes.
If you select multiple images, you have the option of loading them all
into diferent frames or all into the same frame, where you can page
through the images.
Uploading a multi-HDU FITS file containing a mixture of tables and
images. (Note that in this case, all planes are selected and the lower
left gives a choice for loading all the images into one window or one
extension per window.)
and the result after loading that (note blue arrows to scroll through image planes):
Uploading a FITS file with multiple image planes where only one plane
is selected:
This simply loads the image as if you have loaded an image (not a catalog) from disk.
Add a compass rose 
Read in a DS9 Regions file
Selecting a region drop down