FITS Viewer

The FITS Viewer shows FITS images and spectra, fast, with the world coordinates (WCS) of every pixel. It reads single images, multi-extension files, compressed files (.fz) and tables that hold a spectrum. It works without signing in. Open it from its Home tile, or Go ▸ FITS Viewer (⌘3).

1. The tabs, one per file, and New tab (⌘T).
2. Link Crosshair: the crosshair follows the same sky position in every tab.
3. Sync Zoom: every tab shows the same patch of sky.
4. Blink: alternates two tabs.

Below them, the side panel has the file's extensions, the display controls, and the Open, Header, Bookmarks and Marks buttons. The image fills the rest, with a bar under it.

Opening a file

• Open (or Open FITS File… in an empty tab) chooses a file on your Mac.
• Drag a FITS file onto the viewer.
• With no file open, the viewer lists the files you opened recently (RECENTLY OPENED); click one to open it again.
• From Research (Open in FITS Viewer), from Storage (Open in FITS Viewer), or from the file browser.

A file with a third axis, such as a spectral cube, asks first: Open as… FITS Viewer (2D) or Cube Viewer (3D) (see Cube Viewer).

Each file opens in a tab. ⌘T opens an empty tab; ⌘W, or the × on a tab, closes it.

The display

The side panel, from the top:

• HDUs: the file's extensions, with each image's size or each table's name. Click one to show it.
• Stretch: how values become brightness: Linear, Log, Sqrt, Squared or Asinh.
• Colormap: Grayscale, Inverted, Heat, Cool, Viridis, Inferno, Magma or Plasma.
• Cuts: the values shown from black to white, with the Min and Max sliders or fields. Auto sets them from a little below the background to where the brightest percent begins.
• Crosshair, Go To and Zoom: see below.
• Export Figure…: see Figures.

A file that lacks standard WCS shows WCS approximate: its coordinates are estimated and may be off. An image whose pixels all have the same value shows a 0…1 range instead.

Moving around the image

• Zoom: 25% to 800%, Fit (the whole image), 1:1 (one image pixel per screen pixel), and N (North Up: turns the image so north is up and east is left, keeping it all in view). The View menu has Zoom In (⌘+), Zoom Out (⌘−), Actual Size (⌥⌘1) and Zoom to Fit (⌥⌘0). Pinch or scroll on the trackpad to zoom and pan, or type a percentage in the zoom field at the bottom right and press Return.
• The bar under the image shows the RA, Dec and Value of the pixel under the pointer, and the image's size and pixel scale.

The crosshair

Click the image to place the crosshair. The panel then shows its RA, Dec and value:

• Copy copies the coordinates.
• Clear (Esc) removes the crosshair.
• Search Here (⇧⌘L) searches the CADC archive at that position, in Search. It needs WCS.

Go To puts the crosshair on a position you type: RA and Dec, in degrees or sexagesimal, then Go. A position off the image says so.

Header and image info

Header shows, in the side panel, a summary of the image (Dimensions, Pixel range, WCS, Scale, Center, Orientation, Field of view) and every header card, with Filter keywords… to find one.

Bookmarks

Bookmarks keeps positions you want to come back to. Place the crosshair, type a Label (optional), and save it. Each bookmark has Go To and Delete.

Comparing images

With two or more tabs open:

• Link Crosshair: placing the crosshair in one tab puts it at the same sky position in the others, through their WCS. If a tab has no exact WCS, or shows another part of the sky, a note says so.
• Sync Zoom: every tab shows the same angular size of sky.
• Blink (⇧⌘B): alternates the current tab with another. Pause and Resume (Space), A and B to show one (← and →), a slider for the interval (0.5 to 5 seconds), and Stop (Esc). Images without WCS blink unaligned.

Marks

Marks are your annotations on an image: circles, boxes, callouts with a line, and text. Verbinal keeps them for each file (and each extension) on this Mac; open the file again and they are there.

Open Marks in the side panel:

1. Turn on Draw, choose a Shape (Circle, Box, Callout or Text), then click or drag on the image.
2. Drag the shape to move it, a grip to resize it; double-click to rename it.
3. Style the selected mark: Colour, bold (B), label size, and outline thickness.

The panel lists every mark, with Filter marks…. Export saves them as DS9 Regions… or JSON…; Clear All removes them all from this image, after asking.

Right-click a mark for Edit Label, Copy Position, Centre on Mark, Search Here, Export Figure Around Mark…, Export Marks as DS9 Regions…, Export Marks as JSON… and Delete Mark (⌫).

Spectra

A table that holds a spectrum, such as an HST x1d file, opens as a plot: flux against wavelength, with their units, and a ±1σ band from the error column when there is one. Files with several orders plot them all. Export Figure saves the plot as PNG 2×, PNG 4× or PDF….

Figures

Export Figure… makes a publication figure of the image, with a preview:

Setting	Choices
Region	Whole image, View on screen, or Around a mark
Theme	Journal light or Cockpit dark
Font	Sans, Mono or Serif
Text color	Auto, White, Black, Cyan or Amber
Text scale	A slider
Annotations (header + legend)	A header and a legend on the figure
Marks	Your marks on the figure
Transparent background	For a figure laid over something else

Then PNG 2×, PNG 4× or PDF…. Figures are saved in your Downloads folder, under a name with the date and time.

What an assistant can do here

It can open files, switch tabs and extensions, set the stretch, colour map, cuts and zoom, turn North Up, go to a position, read pixel values and the header, save bookmarks, blink and link tabs, search at the crosshair, draw, edit and export marks, read spectra, and export figures. It sees the image as you do.