Explore the sky around your target.

Sky Atlas is an interactive map of the whole sky. It does not need an image. You pan and zoom like a planetarium, search for objects, and click them to see where they are from your site at the time you choose. You can also import your own plate-solved images and see them in place on the map.

• Pan and zoom the whole sky. Drag to move and scroll to zoom from 360° down to 2°, or closer on the Moon. Step 04
• Search every downloaded catalog. One search window covers named stars, Messier, NGC, IC, nebula catalogs, HASH planetary nebulae, and the Moon, including catalogs you have hidden. Step 05
• Pin object details. Click an object to see its RA, Dec, altitude, azimuth, constellation, and links to SIMBAD, ADS, VizieR, and HASH. Step 06
• Set your site. Pick your observing location on a world map and save it for next time. Step 07
• Control time. Set the UTC time, step by hours or days, or pause the sky. Step 08
• Add reference layers. Equatorial grid, alt-az grid, constellations, and the ground and horizon. Step 09
• Go deeper. Show stars down to magnitude 15 from Hipparcos, Tycho-2, or Gaia DR3, and switch on up to ten deep-sky catalogs. Step 10
• Find the Moon. See the live Moon for your site and time, and zoom in close. Step 11
• Style the sky. Tune the Milky Way background and constellation lines and labels. Step 12
• Overlay your own images. Group your sky-registered images into named surveys and show them on the atlas with opacity, brightness, and blending controls. Steps 13 and 14

Images

None are needed to use the atlas. To add custom overlays you need sky-registered PNG, XISF, TIFF, or FITS images.

Plate solving

Only needed for overlays. Images with a valid embedded WCS, or PNG/TIFF files with a matching FITS file next to them, import directly. Others are plate-solved during import; an astrometry.net API key in CAst's settings gives the solver another option.

Internet

Needed the first time you load star catalogs and deep-sky catalogs, and for the map and place names in the Observer Location dialog. The bright starting view and the Messier catalog are packaged with CAst.

Disk space

Small for the default depth. The deepest Gaia DR3 setting (magnitude 15.0) is estimated at about 4.8 GB.

Graphics

Stars and labels are drawn on the GPU. If GPU star rendering is unavailable, CAst switches to a slower compatibility renderer and says so in the status bar.

1. On the launcher, click Sky Atlas (“Explore the sky and objects with an interactive all-sky map.”). If CAst is already open in another mode, choose Mode > Sky Atlas.
2. The atlas fills the window, centered on Orion with a 60° field. Stars down to about magnitude 5 are shown.
3. Move the pointer to the bottom edge of the window. A dock of icon buttons slides up. Move the pointer away and it slides back down.

Hover over a dock button to see its name. From left to right they are Search the sky, Date and time, Pause time, Observer location, Toggle equatorial grid, Toggle alt-az grid, Toggle constellations, Toggle ground and horizon, Custom sky map overlays, Sky Atlas settings, Sky View options, and Toggle full screen. The full list is in R1.

The footer along the bottom of the atlas shows your observer site, the field of view, and the RA/Dec and Alt/Az of the centre of the view.

• Drag with the left mouse button: pan
• Scroll: zoom in and out
• Hover over an object: highlight it and show its name
• Click an object: select it and pin its details
• Double-click an object: center the view on it

Zoom glides smoothly between a 360° and a 2° field. As you zoom in, fainter stars fade in until you reach the Limiting magnitude set in Step 10. Wide views keep stars small so the constellations stay readable.

To return to the opening Orion view, click Sky View options in the dock and then Reset View.

1. In the dock, click Search the sky. A centered window titled Search the sky opens with the note “Every catalog is included, even layers you have turned off.”
2. Type a name, catalog ID, or alias, for example M31, Vega, NGC 7000, or StDr 1.
3. While CAst looks, the status line reads Searching…. Then it shows the number of matches, most relevant first.
4. Each row starts with the object type, such as Galaxy  Andromeda Galaxy. The second line shows the catalog, the magnitude, and the constellation.
5. Click a result, or press Enter to go to the highlighted one.

• Enter: go to the highlighted result
• ↓ or ↑: move from the search box into the result list
• Esc: close the search window

The atlas glides to the object, selects it, and pins its details in the panel at the top left. If the object belongs to a catalog you have switched off, it is still marked and pinned so you can look at it.

1. Click a star or a deep-sky marker, or choose a search result.
2. Read the panel at the top left. It shows the name and magnitude, up to four alternate names, the object type, and the RA, Dec, Alt, Az, and Constellation rows.
3. Click a literature link to open it in your browser: SIMBAD for the object record, ADS for papers sorted by citation count, VizieR for catalog entries, and HASH for HASH planetary nebulae (catalogue V/163 in VizieR).

Alt and Az depend on your site and time, so set those in Step 07 and Step 08. They update while the clock runs. Click an empty part of the sky to clear the selection; the panel hides when nothing is selected.

1. In the dock, click Observer location. The Observer Location dialog opens with a world map on the left.
2. Drag the map to your area. Scroll, or click + and -, to zoom.
3. Click your site. A marker appears with its coordinates, and Latitude and Longitude fill in.
4. Type your Elevation in metres.
5. Check Name. CAst fills in a place name from the clicked coordinates. Type your own name if you prefer.
6. Leave Keep this site in quick-switch presets checked to keep the site in Saved Sites.
7. Click Save Location.

The footer now shows your site name, and Alt/Az values, the horizon, and the Moon use the new location. CAst remembers the site the next time you open Sky Atlas.

All controls of this dialog are listed in R5.

1. In the dock, click Date and time. A popup opens with a UTC date and time box.
2. Type a date and time, or click the box's arrow to pick a date from the calendar. All times are UTC.
3. Or step the time with -1 d, -1 h, +1 h, and +1 d. Click Now to return to the current time.
4. Click outside the popup to close it.

The sky clock runs in real time when Sky Atlas opens, so stars rise and set as they would outside. To freeze the sky, click Pause time in the dock. The button then reads Play time; click it to let the sky move again.

Each of these dock buttons switches one layer on or off. A highlighted button means the layer is on. All four start off.

1. Click Toggle equatorial grid for RA/Dec lines.
2. Click Toggle alt-az grid for altitude and azimuth lines around your horizon.
3. Click Toggle constellations for constellation lines and names.
4. Click Toggle ground and horizon to draw the ground below the horizon for your site.

To change the color, width, and opacity of constellation lines and labels, see Step 12.

1. In the dock, click Sky Atlas settings. The Sky Atlas Settings dialog opens on the Stars page.
2. Drag Limiting magnitude to the faintest stars you want, up to 15.0. The Star catalog download box below shows which catalog that needs, about how many stars, and the download size.
3. Click Deep-Sky Objects in the list on the left.
4. Under Catalogs to show, check the catalogs you want, for example Barnard and Sharpless for dark and bright nebulae.
5. Under Visibility, set Object limiting magnitude and Label limiting magnitude if you want more or fewer markers and names.
6. Click OK.

The atlas previews star and object changes while the dialog is open. After OK, if catalog data is missing, a banner with a progress bar and a Cancel button appears at the top of the atlas. You can keep exploring; the sky refreshes when the new catalog is ready.

Every control on both pages, with ranges and defaults, is in R2.

1. Make sure your site and time are set (Step 07 and Step 08).
2. Click Search the sky and type Moon. Luna and Earth's Moon also work.
3. Press Enter. The atlas centers on the Moon and pins it.
4. Scroll to zoom in. With the Moon selected or centered, you can zoom to a field about 0.24° wide.

The Moon's position is computed for your site and time (topocentric), so it shifts against the stars as you change either. The magnitude in the panel is estimated from the lit fraction of the disc.

1. In the dock, click Sky View options.
2. To change the Milky Way background, choose a Preset: Atlas, Dark sky, or Source-like. Fine-tune with Opacity, Exposure, and the tone sliders.
3. To hide the Milky Way, uncheck Milky Way.
4. To style constellations, check Constellations, then choose Lines, Labels, a Color, and the width, opacity, and size sliders.
5. Click outside the popup to close it.

Star brightness, size, and twinkling are on the Stars page of Sky Atlas Settings, not here. Every control in this popup is listed in R3.

A custom survey is a named group of your own sky-registered images, for example all your H-alpha frames. CAst places each image on the atlas using its WCS.

1. In the dock, click Custom sky map overlays. The Custom Sky Map popup opens.
2. Click Edit Surveys. The Custom Sky Map Surveys dialog opens.
3. Click New Survey. A survey named Survey 1 appears in the Surveys list.
4. Type a Name. Optionally choose or type a Filter, such as Ha, OIII, or Luminance.
5. Click Add Image... and choose a PNG, XISF, TIFF, or FITS file.
6. Wait while CAst reads the WCS or plate-solves the image. The image then appears in the Images list.
7. Repeat Add Image... for each image. Each click imports one file.
8. Click OK.

How CAst places the image:

1. If the file has a valid embedded WCS, CAst uses it.
2. For PNG and TIFF, CAst first looks for a FITS file with the same base name in the same folder (.fits, .fit, .fts, or .fz) and uses its WCS.
3. Otherwise CAst plate-solves the image, using Gaia matching seeded from the image metadata or astrometry.net.

To change a survey later, open Edit Surveys again. Remove Selected removes the selected image, and Delete Survey removes the whole survey after a confirmation.

1. In the dock, click Custom sky map overlays.
2. Choose your survey from the Survey dropdown. CAst turns on Show custom sky overlays and draws the survey's images.
3. Search for an object inside your image, or pan to it, to see the overlay in place.
4. Adjust Opacity to blend the image with the atlas, and Brightness and Saturation to match the background.
5. If the image has a black frame, check Treat black as alpha to make black and near-black pixels transparent, and raise Feather to soften the edges.
6. Uncheck Show custom sky overlays to hide the overlay without deleting it.

The dock appears when the pointer is near the bottom edge of the atlas, and stays while one of its popups is open.

Search the sky

Opens the search window.

See Step 05.

Date and time

Opens the UTC time popup with -1 d, -1 h, Now, +1 h, and +1 d.

Pause time / Play time

Freezes or resumes the sky clock.

Default: running.

Observer location

Opens the Observer Location dialog.

See R5.

Toggle equatorial grid

RA/Dec grid.

Default: off.

Toggle alt-az grid

Altitude/azimuth grid.

Default: off.

Toggle constellations

Constellation lines and names. Same as the Constellations checkbox in Sky View options.

Default: off.

Toggle ground and horizon

Ground and horizon for your site.

Default: off.

Custom sky map overlays

Opens the Custom Sky Map popup.

See R4.

Sky Atlas settings

Opens Sky Atlas Settings.

See R2.

Sky View options

Opens the Milky Way, constellation, and Reset View popup.

See R3.

Toggle full screen

Switches the CAst window in and out of full screen.

Footer

From left to right: the observer site name (Not set if no site is saved), the field of view as width × height in degrees, the RA/Dec of the view centre, and the Alt/Az of the view centre.

Mouse

• Drag: pan
• Scroll: zoom, 360° to 2° (about 0.24° on the Moon)
• Hover: highlight and label
• Click: select and pin details
• Double-click: center on the object

Opened from Sky Atlas settings in the dock. Changes preview on the atlas as you move them. OK saves and starts any needed download, Cancel reverts, and Restore Defaults resets every control in the dialog.

Stars page

The Star catalog download box shows the catalog, star count, and size estimate for the chosen magnitude, and the folder after Saved under:.

Deep-Sky Objects page

Catalogs to show

One checkbox per catalog: Messier, NGC, IC, VdB, LDN, LBN, Barnard, Sharpless, RCW, HASH PN.

Default: Messier and NGC on.

Object limiting magnitude

Marks objects brighter than this limit.

Range 1.0–16.0, default 10.0.

Label limiting magnitude

Shows names only for objects brighter than this.

Range 1.0–16.0, default 8.0.

Dense catalogs (HASH PN, LDN, LBN, Barnard, Sharpless, RCW) show labels only when you zoom in: named objects below about a 28° field, unnamed ones below about 10°.

Opened from Sky View options in the dock. These settings return to their defaults when CAst restarts.

Max mag

Shows the current star limiting magnitude. Change the limit in Sky Atlas Settings.

Reset View

Glides back to the opening 60° Orion field.

Milky Way

Choosing a Preset sets Exposure, Contrast, Saturation, Black point, and Gamma together. Moving one of those sliders switches the preset to Custom. Setting the resolution to Off hides the Milky Way.

Constellations

Custom Sky Map popup

These settings are saved between sessions.

Show custom sky overlays

Shows or hides the selected survey.

Default: off. Choosing a survey turns it on.

Edit Surveys

Opens the Custom Sky Map Surveys dialog.

Survey

The survey to display. Shows No surveys yet until you create one.

Opacity

How strongly the images cover the atlas.

0–100%, default 85%.

Brightness

Image brightness.

0.35x–2.00x, default 1.00x.

Saturation

Color strength.

0.00x–2.40x, default 1.00x.

Feather

Fades the image edges into the atlas.

0–100%, default 0%.

Treat black as alpha

Makes black and near-black pixels transparent so the overlay blends into the atlas sky.

Default: off.

Custom Sky Map Surveys dialog

Surveys

Your surveys, shown with their filter in brackets.

New Survey

Adds a survey named Survey 1, Survey 2, and so on.

Delete Survey

Deletes the selected survey. Asks first if it contains images.

Name

The survey name. Every survey needs one.

Filter

Optional. Choose Luminance, Clear, R, G, B, Ha, OIII, or SII, or type your own.

Images

The images in the selected survey. Hover to see the source file path.

Add Image...

Imports one PNG, XISF, TIFF, or FITS image.

Remove Selected

Removes the selected image from the survey.

OK / Cancel

OK saves your changes to the surveys; Cancel discards them.

Each imported image is cached in its survey folder as overlay.png, overlay.wcs.fits, and overlay.json, downscaled to a long edge of 2048 pixels. Each survey folder also has a survey.json manifest.

Map

Drag to pan, scroll to zoom, click to place the marker. Needs an internet connection for the map tiles.

- / +

Zoom the map out or in.

Center on marker

Returns the map to the selected point.

Name

Filled in from the clicked coordinates using OpenStreetMap. Type your own name to keep it; clear the field to turn auto-fill back on.

Latitude / Longitude

Degrees, north and east positive. Typing moves the marker.

Elevation

Metres above sea level, −500 to 12000.

Keep this site in quick-switch presets

Adds or updates the site in Saved Sites when you save.

Default: on.

Saved Sites

Your saved sites. Load fills the form with the selected site; Remove deletes it.

Save Location

Makes the site active and saves it.

Star catalogs

The opening view uses the brightest stars, so it appears quickly. Deeper catalogs load only when you ask for them in Sky Atlas Settings, or the first time when no Hipparcos stars are on disk yet. Hipparcos to magnitude 9.5 is the base set once it has been downloaded.

Downloaded catalogs are saved in a sky-atlas folder next to the program. The exact path is shown in Sky Atlas Settings after Saved under:.

Deep-sky catalogs

HASH PN objects are labeled with their usual names when HASH provides one, including amateur discoverer names such as StDr-1, Pa, Kn, and Ou, and keep the HASH number as an alias. Search accepts StDr-1, StDr 1, and StDr1. CAst downloads these catalogs from CDS/VizieR when you enable them; it does not redistribute the HASH database.

What is saved

Saved between sessions: Sky Atlas Settings, the observer location and saved sites, custom surveys, and the Custom Sky Map display settings. Not saved: the dock layer toggles, the clock state, and Sky View options.