SkyCruncherNote ยท Sharing safely
What's in your files before you share them
Could your files give away where you live? Often, yes. Here is what in a night of frames points back to your home, and what SkyCruncher's tools change before you share them.
Updated
A thin line beside a passage says how far its claim is proven: implemented, illustrative. Colored numbers carry the same mark. A passage with no line is stated. How claims are marked.
A backyard site is a home address
Most capture programs, the Seestar's included, write the observing site into every frame they save: the latitude and longitude of the telescope, and often its height. They put it in the FITS header. FITS is the standard file format telescopes save their frames in, and its header is the block of text at the start of each file. Written to four decimal places, a latitude is good to about 11 m, which is one house on one street.
If you observe from a backyard, I'd treat that line as your home address. It goes wherever the file goes: a shared Drive folder, a forum post, a processing service.
Deleting the latitude and longitude isn't always enough, because the sky itself can act as a GPS. A star sits at a known position on the sky. If a header also records the exact time and how high and in which direction the telescope pointed, that is a sextant reading, and the site can be worked back out from it. Sailors found their position this way for centuries.
The header keys that reveal a site
Each line of a FITS header is a key (a short name of up to eight characters), a value and an optional comment. Programs don't all use the same names, but these are the common ones. The first three state the site outright. The rest are worked out from the site, so together with the time and the target's position they can give it back.
| Key | What it holds | Example |
|---|---|---|
SITELAT | The site's latitude | 40 |
SITELONG | The site's longitude | xxx.xxxx |
SITEELEV | The site's height above sea level, in meters | xxx |
OBJCTALT | The target's altitude above the horizon, which depends on where you are | xx.xx |
OBJCTAZ | The target's azimuth, the compass direction it lies in | xxx.xx |
AIRMASS | How much air the light came through, worked out from the altitude | x.xxx |
LST | Local sidereal time, the star time at your longitude | xx:xx:xx |
HA | Hour angle: the sidereal time minus the target's right ascension | xx:xx:xx |
(illustrative) Made-up example values, masked; the latitude is shown in whole degrees only. A real file writes the site to several decimal places.
The same facts turn up under other names, such as LAT-OBS and LONG-OBS, or as three Earth-centered coordinates, OBSGEO-X, OBSGEO-Y and OBSGEO-Z. Any one of them is enough to place you.
(implemented) Clean my files and Upload look for these other names too, not only the three in the table.
Photos and file names
A JPEG or other photo can carry a GPS position of its own, in its EXIF data (the metadata block a phone or camera writes into each photo), when the device that made it had location turned on. Camera raw files can carry one too.
File and folder names travel with the files. A folder named after your town, or one some program named with coordinates, says where you were as plainly as a header does.
(implemented) Upload skips JPGs, previews and videos: they are never sent. It converts camera raw files to FITS in your browser and moves their site like any FITS file's, and the original raw file is never sent. It also changes the parts of file and folder names that look like coordinates or match your site. It can't catch every way of writing a coordinate, and a town name in a folder is sent as it is.
(implemented) Clean my files keeps names as they are. It doesn't clean photos or camera raw files: it flags them and asks you to leave them out of the shared folder, or to remove their location first.
What Clean my files and Upload do on your computer
(implemented) SkyCruncher has two tools that move your site before a file leaves your hands. Both work in your browser: the exact site is never sent to us and never shown on the page. They move it by two different rules.
- (implemented) Clean my files is for files you share somewhere else, such as a Drive or Dropbox folder. It moves the site by a random direction and a random distance of up to 5 km. The offset is picked once for your site and kept in your browser, so in that browser every later file from within 5 km of that place moves the same way and lines up with the earlier ones. A new browser or device, a private window, or clearing this site's data starts a new offset. It uploads nothing.
- (implemented) Upload is for files you send to SkyCruncher. It writes your site as the center of the map cell it falls in, one of about 12.6 million equal cells of about 6 km each. Every file from the same place gets the same center, and nothing about your site is kept in your browser. Any other header value that still matches your site is removed. A file that Clean already moved is moved again, to its cell's center.
(implemented) Under both rules:
- (implemented) The elevation is rounded to the nearest 100 m.
- (implemented) Values worked out from the site are recalculated for the moved site or removed: the target's altitude and azimuth, zenith distance, airmass, sidereal time, hour angle and parallactic angle. The Sun's and Moon's altitude and azimuth are removed.
- (implemented) A file whose site can't be read and moved is not passed on. Upload lists it as "not sent, to protect your location". Clean my files says it couldn't clean the file and asks you to leave it out of the shared folder.
- (implemented) Only the header changes. Two
HISTORYlines are added, saying the site was moved and the pointing values recalculated, and the image data is kept byte for byte.
(implemented) Upload also scrubs notes, capture logs and gear lists in the folder. Site coordinates it recognizes become the cell's center, elevations are rounded, and home-folder paths and obvious user and machine names are replaced. You see every change before anything is sent, and a text file it can't scrub is skipped. Pointing values a mount writes in its log are kept as they are, so remove those lines first if that matters to you. Clean my files doesn't scrub notes or logs.
(implemented) When you upload, a dot goes on the public sky map: the target, its position on the sky, the number of frames and the hours of exposure, without your name, telescope or site. The privacy page has the details.
What is kept
(implemented) Everything else in the header is kept as it was written (apart from the values Upload removes because they match your site), because the frames are only useful for science with it: the image data, the times, exposure and gain, the camera and telescope details, and the target's RA and Dec (right ascension and declination, its position on the sky). So is any observer or site name your capture software wrote into the header: if it writes your name or your town, that goes too.
The telescope name (TELESCOP) is often unique to one unit, so it links your files to each other even with the site moved.
And a full night of frames still says roughly where on Earth you were, as any astrophoto does. Moving the site hides your street, not your region.
Before you share
What I'd do with a night's files before they leave my computer:
- For a shared folder, run the FITS files through Clean my files first.
- To send a night to SkyCruncher, use Upload: it moves the site itself.
- Leave phone photos and screenshots out, or remove their location in your photo app before you share them.
- Rename folders that carry your town or a coordinate.
- Open capture logs and delete mount pointing lines if you don't want them sent.
Read more
- What your Seestar saves, and which files to keep: the files a night leaves you with, before you decide what to share.
- How your observing site is moved: both rules in full.
- Upload on the privacy page: what is sent, where it goes, and how to delete a batch.
- Clean my files on the privacy page: where the offset is kept, and what it doesn't do.