- 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
Contents of page/chapter:
+FITS/HiPS Viewer
+Image Information
+Breaking Out of the Pane (and Going Back)
+Image Toolbar
+Color Stretches
+Image Layers
+World Coordinate System (WCS) Alignment and Releated
Features
+Region Selection
+Footprints
| 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 three examples of image labels. The first is
from Spitzer Enhanced Imaging Products (SEIP), IRAC channel 4, which
is 8 micron data, and the field of view is 12 arcmin. The second is
from data delivered by the Local Volume Legacy (LVL) project, and it
is from MIPS channel 3, which is 160 micron data; the field of view is
1.5 degrees. The third is also from the LVL project, but it is
far-ultraviolet (FUV) data, and the field of view is 32 arcseconds.
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, sometimes called a "reticle." You can remove this (or change its color) from the layers pop-up, described below. |
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.
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.
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 before saving the PNG.
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.
Restoring everything to the
defaults
Viewing the image header
For comparison, an example of the HiPS properties window is here:
Rotating the image so that North is
up
Flipping the image on the y-axis
Add a coordinate grid
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.
Zoom
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 in the Images chapter on changing coverage images, specifically that on automatic transitions while zooming.
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.
Zooming to a 1-to-1 size
Color table drop down
 |
|
Color stretch drop down
Re-center the image drop down
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.
Selecting a region drop down
Image Layers: Viewing/Changing the
Layers on the Image
Lock/unlock images
Getting help
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.
 | The top of the menu either says "Color and overlays locked" or "Color and overlays unlocked" -- by default, all of the (FITS) images that you have loaded are locked together for color and overlays. What that means is if you change the color stretch (via this menu), then the color stretch for all the (FITS) images are changed. (Or, if you add a layer to one image, or change the color table of one image, then the change is made to all the images; see other sections of this chapter.) If you don't want this to happen, select "Color and overlays locked" to unlock it. Select the text again to lock it again. |
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.
The number that appears circled 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 brown).
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 image.
Here (on the right) is an example of a well-populated layers pop-up;
in real life, this is scrollable to see several more layers). From
this pop-up, you can:
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.
Note the target description: |  |
| 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 any of the small boxes. Note that the numerical codes update as you select different colors. Your choices are implemented as soon as you select them. Click 'Close' to close the window, or click 'x' in the upper right. |
 |
| 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. Your choices are implemented as soon as you
select them. Click 'Close' to close the window, or click 'x' in the
upper right.
|
 |
Note that if you load both FITS and HiPS images at the same time, it
can include a marked layer 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:
Once you have loaded a HiPS image, if you select
the HiPS image and 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.
(See here for more information on MOCs in general.) A MOC tells you via a simple boolean yes/no, is there sky coverage from this data set in this region. The choices here are:
for position matching. This section describes
the image locking in more detail.
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 (zoom, etc.) 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.
When you click this icon, you can
select a region of the image, from which then you can do a whole host
of things to the image and to the catalog you may have overlaid upon
it.
| First, from the drop-down, you are given a choice of a rectangular selection or an elliptical selection: |
 |
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: 
These
icons 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 (and cancel
selection)
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
appears next to the image toolbar
to remind you that you are in that mode. When you are done with the
selection tool, if your other actions don't turn off the selection
tool, click on that text to turn it off.
) has a drop-down menu with
several possible options:
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 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.
Add a compass rose 
Measuring a distance
Read in a DS9 Regions file
This reminds you of the target on which
you searched -- here, it was M101, 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."