AstroViewer brings FITS thumbnails to Windows and Linux

Astrophotographers generate a lot of files, and most of those files are not especially helpful when viewed through a normal file manager.

Open a folder containing a night's worth of FITS frames and you are often greeted by rows of nearly identical dark icons. A clean exposure, a frame ruined by clouds, and an image captured after a bad slew can all look the same until each file is opened in astronomy software.

AstroViewer is built to remove some of that friction.

The free desktop application supports FITS, SER, TIFF, and DNG files, but its most immediately useful feature may be the ability to generate astronomy-friendly thumbnail previews. Instead of treating a raw astronomical exposure like an unreadable dark rectangle, AstroViewer applies an automatic display stretch and background flattening so the contents of the frame can be recognized before the file is opened.

For imagers sorting through a long session, that could save a surprising amount of time.

FITS thumbnails make a night's folder easier to understand

On Windows, AstroViewer installs a thumbnail handler that allows FITS, SER, TIFF, and DNG captures to appear as image previews directly inside File Explorer.

The previews are not simple raw renders. Astronomical data often keeps most of its useful signal near the bottom of the available brightness range, which is why an unstretched FITS frame can appear nearly black on a regular screen. AstroViewer automatically stretches the preview and applies its Flatten process to reduce smooth light pollution gradients and vignetting.

The result is a folder that can be reviewed visually.

Clouded frames become easier to spot. Bad pointing becomes obvious. A clean exposure can be separated from a poor one without loading every file into a larger processing application.

Plate-solved frames can go a step further. When standard World Coordinate System data is available, AstroViewer can draw constellation figures directly on the thumbnail. That turns the file browser into a quick visual record of where the camera was pointed throughout the night.

For Windows users, this is the feature that gives AstroViewer a clear identity. It is not trying to replace a full processing platform. It is trying to make the space between capture and processing less tedious.

The viewer offers more than file previews

Opening a file brings up a more complete inspection environment.

AstroViewer can automatically stretch still images while leaving the underlying pixel data unchanged. Users can adjust black, midpoint, and white levels, choose linear, logarithmic, Asinh, or midtone transfer curves, and view mono images through several false-color palettes.

The application also includes a Flatten option for reducing smooth background gradients and a PSF correction option intended to tighten stars and recover detail lost to optical or atmospheric blur.

These tools should not be confused with a full image-processing workflow. AstroViewer does not replace calibration, registration, stacking, advanced gradient removal, color calibration, or the deeper processing controls found in applications such as PixInsight or Siril.

Its role is narrower and more practical.

It gives the imager a way to inspect data, check what was captured, identify poor frames, make quick display adjustments, and export an accessible version without opening a full processing project.

The built-in histogram includes a hover readout, while the statistics panel reports minimum, maximum, mean, median, noise, and median absolute deviation values. Capture settings such as exposure, gain, offset, and binning can also appear when those values are present in the file header.

A gallery strip along the bottom of the window makes it possible to move through other images in the same folder. Multiple files can be selected for deletion or exported as full-resolution PNG images, making the viewer useful for culling a session before stacking begins.

Plate-solved files become an interactive sky record

AstroViewer makes particularly good use of plate-solving information.

When a FITS file contains a standard WCS solution, the viewer can calculate the celestial coordinates beneath the cursor and display a live right ascension and declination readout. It can also annotate the image with named stars, Messier and Caldwell objects, constellation names, and constellation figures.

This is more useful than simply decorating the image.

For wide-field imaging, all-sky cameras, meteor work, constellation photography, or a session where the exact framing is no longer obvious, the overlays provide immediate context. The image becomes both a photograph and a record of where the camera was looking.

The labels respond to the actual contents of the frame. AstroViewer checks whether cataloged objects appear in the data rather than placing markers over empty areas simply because an object falls within the calculated field.

The application can also read pointing, observation time, and location information when those values are included in the file metadata. FITS headers are viewable, and FITS still-image headers can be edited from within the program.

TIFF and DNG files can carry some of the same information, although plate-solving data may need to remain in an accompanying WCS sidecar file. Keeping that sidecar beside the original capture allows the overlays and sky-coordinate features to continue working after the files are copied to another computer.

SER support gives planetary and lunar imagers another option

AstroViewer is not limited to deep-sky still images.

SER files open as video, with playback controls, frame stepping, a scrub bar, adjustable playback speed, frame numbers, and per-frame timestamps. Mono, color, and Bayer SER data are supported at 8-bit and 16-bit depths.

The viewer intentionally opens video frames as recorded instead of automatically stretching them. Planetary and lunar videos are normally exposed so the subject already occupies a useful portion of the available brightness range. Automatically stretching those frames could exaggerate background noise or wash out brighter details.

Users can still enable stretching when needed.

SER videos can also be exported as numbered 32-bit floating-point FITS frames. Those files can then be passed into dedicated stacking software for further work.

This keeps AstroViewer in its proper place. It can inspect and prepare the data, but it does not attempt to become another planetary stacking application.

AstroCapture is optional, not required

AstroViewer is presented as the desktop companion to AstroCapture, an Android application that controls supported ZWO ASI cameras from a phone or tablet.

AstroCapture can record FITS, TIFF, DNG, and SER data, plate-solve captures on the phone, and write WCS information that AstroViewer can later use for annotations and coordinate readouts. The two applications share catalog and constellation information, creating a path from capture in the field to inspection on the desktop.

AstroViewer does not require AstroCapture, however.

It can open standard files created by other capture applications and can use compatible WCS information written by other plate-solving tools. That makes it relevant to imagers who already have an established capture setup and only want a faster way to browse and inspect their data.

Windows support is further along than Linux

The current Windows installer is a relatively small, self-contained download and installs for the current user without requiring administrator rights. It registers the supported file associations and the File Explorer thumbnail handler during installation.

The application does not automatically contact the internet during normal use. Update checks only occur when the user selects the update command, and downloaded updates are verified before installation.

A Debian package is also available in beta and has been tested on Ubuntu 26.04. Linux users should treat that build as an early release, especially because desktop integration can vary between distributions and file managers.

The availability of a Linux build is still encouraging. Astronomy software has become more cross-platform over the last several years, and a lightweight viewer has a place on capture computers, observatory systems, and processing workstations that do not run Windows.

A focused tool for a common astrophotography problem as AstroViewer brings FITS previews

AstroViewer does not need to become another complete processing suite to be useful.

Its strongest idea is much simpler: an astrophotography folder should not be a wall of anonymous black files.

By combining stretched thumbnails, background flattening, plate-solved overlays, header inspection, statistics, SER playback, and quick exports, the application creates a more practical bridge between capturing data and processing it.

Experienced imagers may already have several ways to inspect individual files, but file-manager previews could make the first review of a long session much faster. Newer imagers may appreciate having a focused viewer that explains the data without immediately dropping them into a complicated processing environment.

There is also value in the application's restraint. AstroViewer is not presented as a stacker, calibration tool, or replacement for a mature astrophotography workflow. It is the program used to see what was captured, identify the frames worth keeping, and move the useful data toward the next step.

For a free application, that is a clear and worthwhile job.