What your Seestar saves, and which files to keep

A sub is one exposure from the camera, for example 10 seconds of starlight. A Seestar takes many subs in a session and combines them into one stacked picture as it goes. FITS is the file format astronomers use for images: the pixels plus a block of text lines, the header, saying how and when they were taken.

A thin line beside a passage says how far its claim is proven: measured, implemented, illustrative, inferred. Colored numbers carry the same mark. A passage with no line is stated. See how claims are marked.

The short version

• Switch on the app's option to save individual frames before you start the session. ZWO's own FAQ for the S30 Pro says the subs are saved only if that option is on when the session starts.
• Keep the FITS subs. Each one holds the starlight of a single exposure with its own start time, so a star's brightness can be measured sub by sub.
• The stacked image is for looking at and sharing. A stack has already averaged the subs together, so the night's changes from sub to sub are gone from it.

The files a Seestar writes

Which files you get depends on the mode, the model and one setting. This is what ZWO and other owners describe; I haven't checked every model and firmware myself.

Turn on sub saving

The setting lives in the Seestar app. ZWO's FAQ calls it the option to save individual frames; its exact label and where it sits can change between app versions, so look for that wording in your app's settings. Three habits make it hard to get wrong:

1. Check the switch at the start of every session, before you press start. ZWO says it must be on before the session starts, so subs from a session that started without it are not there to recover later (inferred).
2. A few minutes in, look for the target's _sub folder in the app's album or on the telescope's storage, and check it is growing.
3. Copy the subs off the telescope after the night. ZWO's file-transfer tutorial describes joining the telescope to your home Wi-Fi in station mode and opening its address in your computer's file manager. ZWO's file-storage guide also describes copying the files to a computer over a USB data cable, and owners on ZWO's user forum report finding them in a MyWorks folder.

What a FITS header records

The top of every FITS file is plain text: one line per fact, a keyword, an equals sign and a value. Here is the kind of header a sub carries. Every value below is made up, and the location lines are masked (illustrative).

An illustrative header. Every value is made up; the site lines are masked.
TELESCOP= 'S50_xxxxxxxx'          / which telescope (your unit's ID)
EXPTIME =                 10.0  / exposure, seconds
GAIN    =                   80  / sensor gain setting
FILTER  = 'IRCUT'                 / which filter was in
CCD-TEMP=                 21.5  / sensor temperature, deg C
FOCUSPOS=                 1600  / focuser position
DATE-OBS= '2026-01-01T03:00:00'   / when this sub started (UTC)
RA      =                10.68  / the target you asked for, degrees
DEC     =                41.27  / the target you asked for, degrees
SITELAT = '+##.####'              / where you observe from: latitude
SITELONG= '-###.####'             / where you observe from: longitude

Which keywords appear, and their exact names, vary by model and firmware. Open one of your own subs in any FITS viewer to see yours.

TELESCOP

Which telescope took the sub. On a Seestar it carries the unit's own ID, and the telescope lookup on the variable-star page reads it from this line (implemented).

EXPTIME, GAIN

How long the sensor collected light, and how strongly it amplified it. Both are needed to compare one sub's brightness with another's.

DATE-OBS

When the sub started. A light curve, a star's brightness plotted over time, needs this for every point.

CCD-TEMP, FILTER, FOCUSPOS

The sensor's temperature (it sets the dark signal a calibration frame takes off), the filter, and the focus. They help explain why one night looks different from another.

RA, DEC

Roughly where on the sky the telescope was pointing. On my S30 Pro frames these are the target you asked for, not the solved center of the frame, and the two were 0.2 to 1.4 degrees apart, so treat them as a starting hint, not the answer (measured). That's from the same optics Experiment as the plate solution below.

Some subs also carry the Seestar's own plate solution. Plate solving means matching the stars in a frame against a star catalog to work out exactly where it points; the result is written as a WCS, the standard recipe for turning pixel positions into sky positions. Plate solving has the longer story. Don't count on it in a sub: in my early S30 Pro captures only 97 of 3,120 frames had one (3%), in two unbroken runs on two nights, and the headers don't say why those frames and not the others. The stacked images the app makes mostly do carry one (measured). That's from What a Seestar S30 Pro's optics do to star positions.

Why subs matter

• Time. A stack has one time for the whole session. Subs each have their own, so a star's brightness can be followed through the night: a variable star's rise and fall, or the small dip of a planet crossing its star. The light curves on Seestar Collective light curves are measured from frames shared by the Seestar Collective and drawn as 10-minute averages (implemented).
• Your telescope, found again. That page's telescope lookup takes the ID from your TELESCOP line. The page holds only a SHA-256 key of each ID (a one-way fingerprint), so it can match yours without ever listing anyone's (implemented).
• Second chances. A sub with a satellite trail, a cloud or a wind shake can be dropped and the rest restacked. Once it's inside a stack, it can't be taken back out.

How big is a night

One Seestar S30 Pro FITS sub is 16,594,560 bytes, about 16.6 MB, read from a 10-second sub of M 31 (measured). That's from the Frame Receipt Format Note. Almost all of it is pixels at 2 bytes each, so a model with a smaller sensor writes smaller subs (inferred).

At that size, a night adds up quickly (illustrative):

Plain arithmetic: subs times 16.6 MB, before the thumbnails and previews. Plan to move each night's subs off the telescope before its storage fills.

What to do with them

1. Keep them

   Copy the .fit subs to a computer or a drive after each night. The JPG thumbnails and previews can go.

2. Clean them before you post them

   Clean my files moves the observing site in your FITS headers by up to 5 km before you share them anywhere. It runs in your browser and uploads nothing (implemented). For everything else a file can carry, read What's in your files before you share them.

3. Share a night

   Send a night of subs to SkyCruncher. Only FITS files and your notes and logs are sent: the JPGs, previews and videos in the folder stay on your computer, and the site in each header becomes its 6 km cell's center first (implemented). The files go to private storage. What does become public is a dot on the sky map: the target, its position on the sky, the number of frames and the hours of exposure (implemented). See Privacy and what the sky map shows.

4. Report a variable star

   Look up your telescope on Seestar Collective light curves, then follow Submitting to the AAVSO to add your measurements to the AAVSO's database under your own observer code.

Sources

• ZWO, Seestar S30 Pro FAQ (third party): the stacked image, saving individual FITS subs, the modes. Checked 2026-10-20.
• ZWO, Transfer files using Wi-Fi (third party): station mode and reaching the files from a computer. Checked 2026-10-20.
• Owners on the AstroBin forum (third party): Seestar S50 file organization, on the _sub folder, thumbnails and previews. Checked 2026-10-20.
• ZWO, How to find Seestar file storage (third party): copying the files over a USB data cable. Checked 2026-10-20.
• Owners on ZWO's user forum (third party): How do you retrieve a photo from the Seestar?, on the MyWorks folder seen over USB. Checked 2026-10-20.
• Frame Receipt Format: the size of one S30 Pro sub.
• What a Seestar S30 Pro's optics do to star positions: how many subs carry a plate solution, and how far the header's RA and Dec sit from the solved center.
• Seestar Collective light curves: the TELESCOP lookup and its SHA-256 keys.
• Send FITS files to SkyCruncher and Privacy: what an upload sends, how the site is moved, and what the public sky map shows.