Thumbnails, previews and metadata columns for XISF and FITS files in Windows Explorer.
AstroThumbs teaches Windows Explorer to understand and visualize astronomical image files in the XISF and FITS formats. Once installed, it works everywhere Explorer does, with no extra program to run and no other software required:
.xisf, .fits,
.fit, .fts and .dpproj file shows a
real image instead of a blank icon, in every icon view and size.Linear (unprocessed) data is displayed with an automatic screen stretch similar to the one PixInsight uses, so a raw subframe looks in Explorer like it does on your processing screen, not like a black rectangle. Already-stretched images are shown as they are. One-shot-color mosaics can be rendered in color, and multi-image XISF files show their main image.
AstroThumbs also handles .dpproj files, the project format of
DeepParallax and DeepParallax Studio. A project embeds its layers whole, so
nothing outside the file is needed: the thumbnail, the preview and the columns
all come from the project's own source image, and they keep working if the
original file it was built from is moved, renamed or deleted.
Where a project holds several layers, the one shown is the source image the project itself designates, which in a starless plus stars session is the starless layer rather than a separate composite.
Run the installer and follow the prompts. AstroThumbs requires 64-bit Windows 10 or Windows 11.
| Choice | What it means |
|---|---|
| Install for all users (default, needs administrator approval) | Everything works: thumbnails, the preview pane and the metadata columns. |
| Install for the current user only | Thumbnails only. Windows reads preview-pane and column registrations from machine-wide locations, so those two features require the all-users install. |
| Restart Explorer now (default: off) | Restarting Explorer loads AstroThumbs straight away, so thumbnails and columns work without waiting for the next sign-in, at the cost of closing open Explorer windows. Files Windows has already stored a thumbnail for keep it either way, until the thumbnail cache is cleared. |
The installer registers all five file types: .xisf,
.fits, .fit, .fts and
.dpproj. Any of them can be turned off later under File
types in AstroThumbs Settings. The .fit extension is shared
with Garmin and other sport devices, which use it for activity files; those are
recognized by their content and left untouched, keeping their normal icon.
To uninstall, use Settings > Apps or the uninstall entry in the Start menu. File icons return to Windows defaults.
In Details view, right-click any column header and choose More..., then tick the columns you want. AstroThumbs columns appear alphabetically in that list. They sort and group like any other Explorer column, so a folder of subframes can be ordered by exposure, filtered by frame type, or grouped by session.
| Column | Content |
|---|---|
| Object | Target name from the file (OBJECT). |
| Filter | Filter name (FILTER). |
| Exposure | Exposure time in seconds. |
| Frame type | Light, Dark, Flat, Bias, as recorded by the capture software. |
| Sensor temp | Sensor temperature in degrees C. |
| Binning | Binning, shown as 1x1, 2x2, and so on. |
| Gain / Offset | Camera gain and offset settings. |
| Telescope / Camera | Instrument names from the file. |
| Pixel size | Pixel size in microns, including binning. |
| Image scale | Arcseconds per pixel, computed from pixel size and focal length when both are present. |
| RA / Dec | Image center coordinates. Also available as decimal degrees for copying into other tools. |
| Bayer pattern | Color filter array pattern, for one-shot-color data (RGGB and similar). |
| Airmass | Airmass at capture time. |
| HFR / FWHM / Guide RMS | Focus and guiding quality figures, when the capture software recorded them. |
| Session | Observing session date. Shifted back twelve hours, so everything captured during one night groups under a single date instead of splitting at midnight. |
| Compression | XISF compression codec, when compressed. |
| Sample format | Pixel data type: UInt16, Float32, and so on. |
| Images | Number of images stored in a multi-image XISF file. |
| Observer | Observer name from the file. |
| Format | Container format of the file: XISF, FITS, or a DeepParallax project. |
Standard Windows columns are filled too: Dimensions, Title and the photographic Exposure time work for these files like they do for ordinary photos. Values come from the file's own keywords; a column stays empty when the capture software did not record that item.
Open AstroThumbs Settings from the Start menu. Changes apply to thumbnails and previews drawn from then on. Previews follow immediately, while thumbnails Windows has already stored keep their old look until they are refreshed, as described below. The Help button opens this document in your default browser.
Windows stores every thumbnail it is given and does not ask AstroThumbs for that file again. A tile therefore keeps whatever it looked like when it was first drawn: changing a setting, installing a new version, or reading a file that was slow the first time will not change a tile you have already seen.
There are three ways to have thumbnails drawn again. Prefer the narrowest one that covers what you need.
| Scope | How | Notes |
|---|---|---|
| One file | Right-click the file and choose Refresh thumbnail. | Immediate on a local disk. The tile updates by itself, with no need to refresh the folder. |
| One folder | Right-click the folder, or the empty background inside it, and choose Refresh AstroThumbs thumbnails. | Handles the astronomical files in that folder, and does not descend into subfolders. A progress window shows what it is doing and can be canceled; you can carry on working while it runs. On a network drive allow a few seconds per large file. |
| Everything | Refresh thumbnails in AstroThumbs Settings. | Clears the thumbnails Windows has stored for every kind of file, not only astronomical ones, and reloads AstroThumbs. Explorer closes and reopens, and everything is redrawn as you browse. Use this after installing or updating AstroThumbs. |
On Windows 11 the two right-click entries appear under Show more options rather than in the first menu, which is where Windows places entries added by programs other than itself.
Choose which extensions AstroThumbs handles: .xisf,
.fits, .fit, .fts and
.dpproj. Changing these re-registers the shell extension and asks
for administrator approval on an all-users install.
| Setting | Effect |
|---|---|
| Auto-stretch images (default: on) | Applies the automatic screen stretch to linear data. Off shows the data exactly as stored, which for unprocessed frames is almost black; the setting exists for collections of already-stretched images. |
| Brightness (default: Standard) | How bright the automatic stretch renders the sky background. Standard matches PixInsight's default. Darker and Brighter are one step either side. |
| Color from Bayer data (default: Automatic) | Automatic renders color when a file declares a Bayer pattern, except for calibration frames, which stay monochrome. Always color also treats undeclared one-shot-color files as RGGB mosaics when their pixels show a mosaic structure; genuinely monochrome images are never affected. Always mono renders everything in grayscale. |
| Label thumbnails with the file format (default: off) | Puts a small XISF, FITS or DP badge on the
thumbnail. Useful in a folder that mixes astronomical files with
ordinary pictures, particularly if Windows is set to hide file
extensions. The label reports the format AstroThumbs actually
recognized, so a FITS file saved as .fit still reads FITS.
It is left off small thumbnails, where there is no room for it to be
legible. |
| Label position (default: Bottom right) | Which corner the badge sits in: Top left, Top right, Bottom left or Bottom right. Astronomical images are not uniformly busy, so a corner that covers empty sky in one image covers the subject in another. The setting is available only while the label itself is switched on. |
| Setting | Effect |
|---|---|
| Always use a dark background (default: on) | Astronomical images sit better on a dark backdrop. Off follows the Windows light or dark theme instead. |
| Show image details beside the picture (default: on) | The panel with coordinates, dimensions and format next to the preview. It hides itself automatically when the pane is narrow. |
AstroThumbs starts with a 15-day free trial. During the trial everything works exactly like the licensed product: nothing is watermarked and nothing is held back. When the trial ends, thumbnails return to default icons, columns show no values, and the preview pane explains how to license.
To license: open AstroThumbs Settings, click Enter license key..., and enter the e-mail address the license was issued to together with the key from your registration e-mail, exactly as received. The key is bound to your e-mail address, not to a computer: it is valid on every machine you use, entered the same way on each. Licenses are permanent within a major version, and updates within that version are free.
This message may appear when trying to preview certain files. It does not affect thumbnails. It comes from Windows, not from AstroThumbs, and it appears for every kind of file rather than only for astronomical images.
Windows marks files that arrive from outside the computer: a browser, a mail client or an archive extractor records that the file came from the internet. Since the Windows security updates of October 2025, File Explorer refuses to show a preview for any file carrying that mark, because a previewed file that points at external content can be used to leak credentials. Explorer applies that rule before it asks AstroThumbs for anything, so AstroThumbs is never consulted and cannot override it. That is the right arrangement: a preview handler able to talk Windows out of a security check would itself be the vulnerability.
Thumbnails and metadata columns are not affected. Only the preview pane is, so a folder of downloaded files still shows its pictures and still sorts by exposure or filter.
To preview one of these files, clear the mark: right-click it, choose Properties, tick Unblock at the bottom of the General tab, and click OK. Explorer sometimes keeps showing the message for a moment afterwards; restarting Explorer clears that, which the Refresh thumbnails button in AstroThumbs Settings does as part of its work.
For a whole folder of downloaded data at once, open that folder in PowerShell and run:
Get-ChildItem -Recurse | Unblock-File
The underlying rule is a Windows zone policy rather than an AstroThumbs setting. An administrator who needs previews restored for a trusted network share should change it in Windows, following Microsoft's guidance, and not expect any AstroThumbs option to affect it.
.fit is ticked under File types, then Apply settings.Questions, problems, suggestions: www.deepskycolors.com/contact-us.
AstroThumbs is a Deep Sky Colors product by Rogelio Bernal Andreo. XISF is a format of Pleiades Astrophoto; FITS is a standard of the astronomical community. AstroThumbs is an independent product and requires neither PixInsight nor any other software.