Whether a Seestar owner needs calibration frames, and a step-by-step guide to shooting a full set for a checkup of your sensor. Once you have them, the calibration pipeline turns them into masters and a sensor sheet.
Every raw frame holds the light you wanted plus things the camera added. The electronics add a fixed offset to every pixel (the bias). Heat knocks loose electrons nobody asked for (dark current). The optics light the sensor unevenly, dimmer toward the corners and shadowed by dust (the flat field). Calibration takes these back off: subtract a bias frame (the shortest exposure, no light), subtract a dark frame (same exposure and temperature, no light), and divide by a flat frame (an evenly lit surface). Many frames of each kind are averaged into a master, so the correction doesn't add noise of its own.
If you shoot in the Seestar app, which is most of us: there's no bias or dark frame to shoot. The app doesn't offer bias frames, and darks you shoot yourself can't match subs the app has already processed. Flats are the open one. The app's Flat Shoot (Settings → Advanced Feature → Image Calibration) is the only flat that could reach your subs, and I'm testing whether it does being tested. This line changes when the result is in. Upload your subs as they are, and SkyCruncher models what's missing.
If you shoot over Alpaca, with NINA today or a SkyCruncher helper app later: a full set of bias, darks and flats tells you your own sensor's numbers. The step-by-step guide is below.
Your Seestar can also be run over Alpaca, the standard network interface that desktop astronomy programs use. Frames shot this way skip the app's processing, so they show what the sensor itself does: how noisy each read is, how fast heat builds up, where the hot pixels are. They don't calibrate stacks made in the app; think of them as a checkup for your sensor. Today it takes NINA, a free Windows program. A small SkyCruncher helper app that does it all for you is in the works planned. A web page alone can't drive the scope; it doesn't accept calls from websites.
Settings, then Station mode. Alpaca only runs in Station mode, with the scope and your computer on the same network.
In NINA, open Equipment, then Camera, and press refresh. The scope's telephoto camera should be in the list:
If it's missing, turn on Seestar Alpaca in the app's advanced settings and refresh again. Trust this list, not the switch: on my unit the switch wouldn't turn on while the scope was answering Alpaca anyway.
The scope runs one capture at a time. While the app is previewing, it refuses NINA with "capture is active".
Pick the telephoto camera and "Seestar S30 Pro_your ID Filter Wheel". Set gain to 200, the gain the app shoots at. Leave offset alone; the scope doesn't offer it.
Arm closed, filter wheel on Dark. The Dark filter is light-tight (about 3 ADU per 10 s) measured, so no cover is needed; just keep the room dim. In NINA's Sequencer:
Raise the arm so the lens points straight up, and set the filter wheel to IRCUT / no filter. The light has to shine down into the lens: lay the white paper over it, then set the tablet on the paper with its white screen facing down, at full brightness. With the lens straight up, it just rests there; no bands or tape needed.
For the white screen, open the white flat page on the tablet and tap once to start; it keeps the screen on. Tap again for a Go back button.
Screens flicker, which shows up as bands in short exposures (1.2 % at 0.04 s) measured. Dim with more paper, never with the screen, and keep exposures at 1 s or longer.
Not yet tried over Alpaca: the arm may need raising before NINA connects.
Turn the screen off and leave everything where it is. Filter wheel back to Dark.
Close the arm, cover the lenses with your solar filter cover, and put a dark cloth or a box over the whole scope: the cover alone lets light through, and the Dark filter sits only in front of the telephoto. Connect the wide camera in NINA.
The wide camera stops at 60 s and reports no temperature. NINA writes the telephoto's filter into its headers; ignore that.
One zip per session: about 3 GB of raw FITS on an S30 Pro. Don't calibrate or stack them first.
Sending it to us comes with the helper app planned.
S30 and S50: untested. On an S30, follow the same steps; its main camera is an IMX662 and the app shoots at gain 200. On an S50, shoot the whole telephoto set twice, at gain 80 and at gain 200 (its subs come at both); it has no wide camera. If the filter wheel has no Dark position, cap the lens instead. Times are estimates, not yet clocked on a full run.
| Frames | Count | Exposure | About |
|---|---|---|---|
| Telephoto camera | |||
| Bias | 20 | shortest (0.00003 s) | 1 min |
| Darks | 20 | 10 s, like the app's subs | 5 min |
| Dark ladder | 9 | 3 each at 30, 60, 120 s | 11 min |
| Flats | 30 | mean near 35 % of full scale | 4 min |
| Flat pairs | 20 | 10 levels × 2, 5 to 80 % | 8 min |
| Dark flats | 30 | the flats' exposure | 4 min |
| Wide camera | |||
| Bias | 20 | shortest | 1 min |
| Darks | 20 | 10 s | 5 min |
| Dark ladder | 9 | 3 each at 20, 40, 60 s | 6 min |
| Total | 178 | gain 200; about 3 GB | 45 min |
| Frames | What they measure |
|---|---|
| Bias | The pedestal, read noise, the converter's step size, and frame-to-frame steps in the black level |
| Darks at 10 s | A map of hot pixels, and the dark signal at tonight's temperature |
| Dark ladder | Dark current, from how the noise grows with exposure, and how the black-level clamp hides it |
| Flat pairs | Gain in electrons per ADU, so read noise in electrons and the full well |
| Flats | Vignetting, dust, and each pixel's own response |
Your Seestar does a lot to each frame before you ever see it. On my S30 Pro (firmware 8.46) I compared frames the app saved with frames I pulled straight off the sensor over Alpaca, the standard network interface that desktop astronomy programs use (see the guide). The app scales red by about 1.7 and blue by about 1.84, and it swaps out hot pixels: a 10-second dark over Alpaca has about 24,000 of them, and an app sub has almost none measured.
The app also has its own calibration settings: Flat Shoot (since app 2.5) and Dark Calibration (since app 3.3.0 on the S30 Pro) stated. What I don't know yet is whether they change the subs the app saves, or only the live stack you watch on your phone. ZWO's own notes disagree, and the files don't say. I'm testing it.
Either way, darks you shoot yourself can't help subs the app saved. Those subs have had their colors scaled and their hot pixels swapped before you ever see them, so your darks no longer match them.
Every number measured over Alpaca comes from one Seestar S30 Pro (firmware 8.46, app 3.3.2, Alpaca server 1.2.0-3). This page says what I measured on my unit, not what ZWO's firmware does in general.
