See your astrophotography
right in Windows

Installation

FITS Explorer is distributed through the Microsoft Store as a single package that installs both the desktop app and the Windows Explorer shell extensions.

🔄 What gets installed

The Interface

FITS Explorer has a simple left-hand navigation. Open a file with Open a file..., by dragging it onto the window, or from Windows Explorer.

📂 Opening a file

• Click Open a file... on the Home page, or press Ctrl+O, and pick a file. The dialog lists every supported format together, and each one on its own.
• Drag and drop a file from Windows Explorer onto the window.
• In Windows Explorer, right-click a file and choose Open with, FITS Explorer. The app opens straight on the Image page.
• From the Explorer preview pane, right-click and choose the Open entry for FITS Explorer.

Windows Explorer Integration

Once FITS Explorer is installed, Windows Explorer gains three capabilities for FITS, XISF and SER files, and for Siril sequences.

👁️ Using the Preview Pane

The preview pane draws only the part of the picture that is visible, at the pane's resolution, with the stretch measured once on the whole image, so panning and zooming never change its brightness.

The choices made in the pane last as long as it shows that file. Playing a video is the app's job, the pane steps frame by frame.

A stretch, a debayer or a linked stretch picked from the right-click menu applies to the preview you are looking at. To keep it, choose Use these choices for all previews: FITS Explorer opens on its Settings page with your choices saved as the Stretch in Windows, Debayer and Linked stretch settings, and every file you select in Explorer then uses them. Debayer and Linked stretch are the app's own settings too, so the app follows them as well. A change made in Settings shows up in a preview that is already open, there is no need to close Explorer or to keep FITS Explorer running.

Viewing Images

The Image page is the app's viewer. The picture opens fitted to the window, stretched and in color, the right way up.

🔍 Zoom & fit

Use the Zoom box to jump to a preset, or type a percentage. Hold Ctrl and scroll to zoom around the mouse pointer, or press Ctrl++ and Ctrl+-. Zoom percentages are relative to the fitted view, so 100% shows the whole picture. Press Ctrl+1 for the actual pixels, one pixel of the file per screen pixel, drawn as sharp squares past that, so you can judge stars and noise.

The fit you choose last is the one every file opens with.

✋ Pan

When zoomed in, drag the picture, scroll the wheel to move up and down, or hold Shift and scroll to move sideways.

🔄 Turn & mirror

Turn the picture a quarter at a time with the two rotate buttons (Ctrl+R, Ctrl+Shift+R), and mirror it with the flip menu. Exports follow what you see. A new file opens upright again.

📊 Histogram

The Histogram button (Ctrl+T) shows the distribution of the picture's values with the black point, midtones and white point of the stretch in use. Drag them to set the stretch by hand, see Stretch & Color.

📄 Files that hold no picture

Some FITS files hold only tables, spectra or radio interferometer data. They open anyway: a note says what they hold, for example the binary table EVENTS (1,706 rows), and their headers are listed below it. The picture controls are hidden for such a file, and Explorer shows no thumbnail for it.

Stretch & Color

A raw astronomical frame stores light linearly, so shown as it is it looks almost black. The stretch brings the faint signal up to a visible picture without changing your file.

🌗 Stretch modes

🖐️ Setting the stretch by hand

Open the histogram and drag its three handles. The values show beside it. Reset goes back to the mode's own stretch. Save as Custom keeps the curve as the Custom stretch: this file uses it, and so does any file you set to Custom. A stretch set by hand lasts as long as the file is open.

💾 Each file keeps its stretch

A file you have never opened starts with the Default stretch of Settings, Auto unless you change it. Choose another mode in the toolbar and that file reopens with it next time. The memory follows Recent files: a file that leaves the list forgets its stretch, and nothing is kept while recent files are off.

🔗 Linked stretch

For color pictures, the link button applies one curve to red, green and blue, which keeps the camera's color balance. Unlinked, the default, each channel is measured on its own, which gives a neutral gray sky even under light pollution.

🌈 Debayer

A one shot color camera records a mosaic of red, green and blue pixels. FITS Explorer turns it into color. From file uses the pattern the file declares (BAYERPAT and its offsets). Pick one of the four patterns to correct a file that declares the wrong one, or Off to see the raw mosaic. Darks, flats and biases stay gray unless you turn that off in Settings.

Videos & Cubes

A SER video holds thousands of frames, a FITS cube a frame per wavelength or per exposure. The Image page shows one at a time.

🔢 Choosing a frame

Type a frame number in the Frame box, or use its arrows. Each frame is measured on its own when you go to it.

▶️ Playing

The Play button plays the frames at the rate they were recorded when the file says it, as SER videos with time stamps do, and at 25 frames per second otherwise, at most 60. It keeps the stretch of the frame it started on, so the video does not flicker, and it skips frames rather than slow down when your computer cannot draw them all. From the last frame, it starts over. Typing a frame number stops it.

Header & File Information

FITS and XISF files carry a header, every keyword the capture program wrote: exposure, filter, gain, sensor temperature, telescope, coordinates and more.

📑 The header pane

The Header button (Ctrl+H) shows the keywords beside the picture, each with its comment as a tooltip. SER videos have no header of their own, the pane shows what their 178 byte header says.

ℹ️ Properties

The ⓘ button (Alt+Enter) sums the file up: size, format, compression, sample format, frames, and the capture details read from the header.

🪟 In Windows Explorer

The Details view and the file's properties show its dimensions, bit depth, exposure, focal length, date, camera, object, observer and software. With Better Shell Integration, columns of their own add object, filter, frame type, exposure, gain, offset, sensor temperature, binning, telescope, camera, Bayer pattern, stacked frames, image scale, coordinates, airmass, format, compression, sample format, frames, session and software, so a night of lights, darks and flats sorts itself out.

Exporting Pictures

Export as images and Export as PDF write what the Image page shows, stretched, in color, turned and mirrored, at as many sizes as you want.

⚙️ Export settings

🗂️ What to export

Image, as shown writes the picture of the Image page. For a video or a cube, the Frames box writes frames too, each in a file of its own, chosen by number or by time, see Choosing Frames. Each frame is measured on its own. When several pictures are written, the template must contain {layer}, which names each one.

📏 Export sizes

Each size is the longest side of the output, in pixels. Add as many as you want.

🏷️ Metadata in the files

Every file says it was made by FITS Explorer and its version, which you cannot remove. The Author, copyright, title and description section lets you sign your files: your author and copyright are kept from one export to the next, the title and description start from the header, the object and the filters, telescope and camera, and anything you type replaces them. JPEG, TIFF, PNG, JPEG XR, HEIC and PDF carry them all. GIF keeps the application's name in a comment. BMP and DDS have no room for metadata.

🏃 Running an export

Click Export. A progress bar shows how far it got, with two ways to stop:

An export of more than 500 files, many frames times many sizes, asks you to confirm before it starts.

Export as Video

Export as video writes the frames of a SER video or a FITS cube as an MP4 video or an animated GIF, to share a planetary capture or check the seeing.

Every frame is drawn with the stretch of the frame shown on the Image page, so the video does not flicker. MP4 sides are rounded down to even numbers, which H.264 requires. The video says it was made by FITS Explorer in its encoder tag, its Encoded by property and, when you give no description, in its comments.

Convert

The Convert page writes the open file again as FITS or XISF, its samples exactly as they are and its header keywords carried over, so the new file goes on to calibration and stacking like the original. XISF to FITS, FITS to XISF, SER frames to either, and a compressed .fz to a plain FITS.

🏷️ What the new file says

Every keyword of the original header is carried over, except those that describe the old file's own layout, its compression or its checksums. A FITS file says it was converted by FITS Explorer and its version in a HISTORY line, an XISF file in its creator application property, with its creation time. Your author goes into the FITS AUTHOR keyword, your copyright, title and description into comments, and all four into the matching XISF properties. Left empty, the original header is kept as it is.

Compose a Color Image

Combine images of one field taken through different filters into a color picture. Each filter is stretched on its own and given a color, then the colors are screened together, the way the layers of a Photoshop composite are blended.

➕ Adding files

Click Add files..., drop files onto the page, or drop several files on the window and choose Compose. Two to six files. Each gets a card with its filter, read from the header, its color, its stretch and its histogram.

🎨 Palettes

A palette assigns the colors from the filters the headers name. Change any card's color or stretch, untick a card to leave it out, or drag the handles of its histogram.

🧰 Options

📐 Alignment

The images must lie on the same pixel grid. When every file says where it points in the sky (WCS keywords), they are shifted onto each other and the picture is the area they all cover. Without sky coordinates, images of the same size are laid over each other as they are, which works for frames taken minutes apart. Images taken at another scale or rotation need registration in a stacking program first, the page says so.

💾 Saving

Choose the Format: a Photoshop document, 16 bits or 8 bits, one layer per filter blended with Screen plus the neutral background, to finish it there, or any picture format for a flattened image. The output folder, template ({name} the object, {palette}, {date} of the observation), overwrite and metadata work as on the export pages. Click Compose.

Siril Sequences

A Siril .seq file lists the images of a sequence, their selection and their registration. It holds no picture itself.

FITS Explorer shows a sequence through its reference image, or the first selected image when it has no reference, read from the files beside it: the numbered FITS files of the sequence, the SER video of the same name opened on the reference frame, or the single FITS file that holds them all. Thumbnails, the preview pane, the properties and the app all show that image, and a SER based sequence can be played and stepped through like the video itself.

Better Shell Integration (administrator required)

FITS Explorer can register its file types with Windows itself. This turns on thumbnails, the preview pane, file properties and FITS Explorer's own Details columns, plus a system-wide decoder so other Windows apps can open FITS, XISF and SER files. Because it changes the machine, Windows asks for administrator approval when you turn it on or off.

🛠️ Install, update, uninstall

Everything is done from Settings, Windows Explorer, Better shell integration. The card shows the current status and the matching buttons.

Recent Files

The Recent files page keeps a list of files you have opened, so you can go back to them quickly. It also holds the stretch each of them was last shown with.

Settings

Appearance

Viewing

Windows Explorer

The About section shows the app version and a link to System Information, useful when reporting an issue.

Diagnostics

The last row of About is Diagnostics, a report to send support when something goes wrong, in the app or in File Explorer.

1. Choose Problems only, or Everything when support asks for more detail.
2. Make the problem happen again.
3. Click Save report. One file lands on your desktop, ready to attach to an email.

Supported Formats

FITS

Every image unit of a file, 8, 16, 32 and 64 bit integers and 32 and 64 bit floats, signed or unsigned, frames and cubes. A whole file gzipped is read too. Tile compressed FITS (.fz) with RICE_1, GZIP_1, GZIP_2, HCOMPRESS_1 and PLIO_1, quantized or not, lossy HCOMPRESS smoothed when the file asks for it.

XISF

Mono and color images, every sample format, compressed with zlib, LZ4, LZ4HC or Zstandard, with or without byte shuffling, and the thumbnail an XISF file may carry.

SER

8 to 16 bit videos, mono, Bayer mosaic, RGB and BGR, with their time stamps when the writer kept them.

Siril sequences

Shown through their reference image, see Siril Sequences.

Export

Pictures in the formats registered on your Windows system, PDF, MP4, animated GIF, layered PSD from the Compose page, and the data itself as FITS or XISF from the Convert page. A green dot marks formats that keep metadata.

How It Decides

Some choices FITS Explorer makes for you are not obvious. Here is what they are and why, so a picture that looks different from another program's can be explained.

🌗 The automatic stretch

Auto measures each channel's median, the level of the sky, and its spread around it. It puts the black point 2.8 times that spread below the median and brings the median up to a quarter of full brightness, the screen transfer function astronomy software uses. Strong puts the black point 1.8 times the spread below and the median at 35%. The measure is taken once on the whole image, so zooming and panning never change the brightness, and a sample of the pixels is enough for it, the result is the same.

🌕 Why a planet shows linear in Auto

When the brightest 2% of a channel already reach a quarter of its range while its background is dark, the image is a planet, the Moon, the Sun, a photographic plate scan or a picture already stretched. The automatic stretch would turn it into a white disc, so Auto and Strong show it linear. Deep sky data stays far below that level, nebulae and galaxies included, and keeps its stretch. A Custom stretch you saved is always applied as it is, and you can still drag the histogram of such an image.

💾 Why a file reopens with another stretch than the default

The stretch you chose for a file in the toolbar is remembered with the recent files. The Default stretch of Settings is only for files you never opened, and for thumbnails. Changing the toolbar never changes the default.

🔃 Which way is up

The FITS standard stores the bottom row first, so FITS images are turned over by default, as astronomy software shows them. A file is read top row first instead when its ROWORDER keyword says so, when it was written by ZWO, ASIAIR or Seestar software, which store the top row first without saying it, or when its sky coordinates show north up and east left only that way. XISF and SER files are always top row first. Turn Honor the row order off to see the rows exactly as stored.

🌈 Color without a Bayer keyword

When a frame of one channel declares no Bayer pattern, FITS Explorer looks at the camera that took it: ZWO cameras whose name ends in MC, QHY color models and digital cameras are read as RGGB, Seestar as GRBG. Other frames stay gray. Darks, flats and biases stay gray by default, a mosaic is easier to judge as it is.

⬛ Borders without data

Mosaics and registered stacks fill the areas they have no data for with a constant, often 0, or 50000 in some archives. FITS Explorer finds that constant along the image's border and leaves those pixels out of the stretch measure, so an empty border never darkens or brightens the picture.

🎞️ Playback and video stretch

Playing a video and exporting one both keep the stretch of the frame shown, because measuring every frame would make the brightness pump from one frame to the next. Exporting frames as separate pictures measures each one, since they are seen one at a time.

🔁 What a conversion changes

Only what the target format needs. FITS stores the bottom row first and XISF the top row first, so the rows are written in the target's order, and the keywords that depend on it are written again to say the same thing: ROWORDER, the Bayer pattern (an RGGB mosaic of even height reads GBRG once its rows are reversed) and the plate solution (CRPIX2 and the terms of the second axis). XISF has no signed or scaled integers, such images become floating point, which holds every value exactly. FITS keeps scaled integers with their own BSCALE and BZERO. An XISF float image says the range of its values in its bounds, some programs expect floats between 0 and 1 and scale other values on reading, the values in the file are the original ones. A FITS cube of frames says so with CTYPE3 = 'FRAME', so three frames are never read as a color picture.

🎨 Compose stretches each filter on its own

Each filter of a composition gets its own stretch, with a darker background than a single image, because the skies of three filters add up when they are screened together. The neutral background option then evens out the color of the sky.

File Name Tokens

The File name template of the export pages names each file with these tokens, case-insensitive. Write {{ for a literal brace.

Export as images and as PDF

Export as video

Convert

Compose

Choosing Frames

The Frames box of the export pages takes a list of frames and ranges, by number or by time, separated by commas. It is checked as you type, and the line below says what it picks or which item is wrong.

Ranges may overlap, a frame is written once. Times need a file that says when its frames were taken, SER videos with time stamps, and are converted with the recorded rate. A range cannot mix a frame number and a time. A frame past the last one is an error, a time past the end of the video stops at the last frame.

Keyboard Shortcuts

In the FITS Explorer app

In the Windows Explorer preview pane

Limitations

FITS Explorer is a fast viewer, converter and decoder, not a processing program. Knowing where it stops helps set expectations.

✏️ View & export only

Your files are never modified, conversions write new files. It does not calibrate, register or stack: compose images that are already aligned, from a stacking program or taken minutes apart.

🔁 Conversions

FITS files are written uncompressed, an XISF file uncompressed or with zlib, LZ4 or LZ4 HC. Zstandard XISF files are read but not written, Windows and .NET have no Zstandard encoder to build on. XISF files with several images open on their first one, as a FITS file with several image units does. Stop and keep keeps the files finished, a file being written when you stop is removed rather than left incomplete.

🎨 Compose

Alignment shifts images onto each other, it does not rotate or scale them. Images of another scale or rotation, or of two nights without sky coordinates, need registration first.

🎬 Video

MP4 frames are at most 4096 pixels on their longest side. GIF plays at most 50 frames per second and suits short clips.

🧾 Siril sequences

A sequence shows its reference image, not the stack. Its images must stay beside it.

Tips & Tricks

🗃️ Sort a night of subs

Install Better Shell Integration, open the folder in the Details view, right-click a column header, More..., and add Object, Filter, Frame type, Exposure and Sensor temp. Group by Filter or Frame type to see a session at a glance.

🪐 Find the steady seconds of a planetary capture

1. Open the SER video and press Play to watch the seeing.
2. Note the times where the planet holds still.
3. On Export as video, type those times in Frames, 0:42-0:55, and export a short MP4 to share, or the same frames as pictures from Export as images.

🌌 Want data to try?

The Moana Project shares the raw frames of a remote telescope under a Creative Commons license, narrowband and RGB sets ready for the Compose page. Credit them as "Moana Project" when you publish a result.

🌙 Dark mode

FITS Explorer follows your Windows theme by default, a dark theme is easy on night-adapted eyes. For a fixed look, choose Light or Dark in Settings.

Troubleshooting

🔒 "File system error (-2143322102)" when opening a file

If opening a file shows its path and File system error (-2143322102), your free trial has ended. Windows refuses to start an app whose Store licence is no longer valid, and that number is the Store's licensing error, 0x803F800A. Nothing is wrong with the file. Get FITS Explorer from the Microsoft Store and the files open again.

🖼️ Thumbnails only show in the icon views

Windows draws thumbnails in Explorer's icon views only. A folder in Details or List view shows the plain file type icon instead, whatever the file is. Your Pictures folder opens in Large icons, which is why images look right there, while Documents, Downloads and a USB stick open in Details view. The folder and the drive make no difference, the view does.

In a folder where thumbnails are missing, open the View menu of the Explorer toolbar and choose Medium icons, Large icons or Extra large icons. Holding Ctrl and turning the mouse wheel does the same, and Windows remembers the choice for that folder.

👁️ The preview pane stays empty

The preview pane belongs to Explorer, not to FITS Explorer, and Windows keeps it closed until you ask for it, in each Explorer window separately. Press Alt+P, or use View → Preview pane, in the window where you want it. A second window needs the same.

On Windows 11 the Details pane and the Preview pane cannot be open at the same time, so turning the details pane on closes the preview. The Open and Save dialogs of other programs have no preview pane at all, whatever is installed.

🥇 Thumbnails or previews don't show (another app is the default)

1. Open Windows Settings, Apps, Default apps.
2. Find and click FITS Explorer.
3. For each extension you want, choose FITS Explorer as the default.

🚫 Preview stays blank for files downloaded from the internet

Starting with Windows 11 25H2, Microsoft blocks the preview pane for files that carry the "Mark of the Web", files downloaded from the internet. This affects every file type and FITS Explorer cannot override it.