Show HN: Slopcap

SlopCap records video of the planets, the Moon and the Sun from ZWO, QHY, Player One, ToupTek and MallinCam astronomy cameras, on Windows, Linux and macOS. It gets every frame onto the disk, shows a live picture that keeps up, and helps with everything that decides how sharp the result will be: collimation, focus for each colour, atmospheric dispersion, and knowing what you're pointed at. Its recordings go straight into AutoStakkert!, PIPP and WinJUPOS. It can also send the camera's picture to other computers on the network, point to point, for a projector at an outreach night or a warm room indoors.

Saturn above: 5,000 frames of 6.7 ms from a ZWO ASI585MC on 4 August 2022 at 07:03 UT, the best 25 % stacked in AutoStakkert! and lightly sharpened. It was recorded before SlopCap existed; the Jupiter in the screenshots below is a recording from the same summer, replayed through SlopCap as if it were the camera.

Records every frame. Frames go from the camera into memory set aside in advance and on to the disk without waiting on each other. Should anything fall behind, SlopCap counts what the camera lost and what the disk lost, separately, and shows it. A ZWO camera has been recorded at 168 frames a second with nothing dropped, on Linux and on Windows.

A live picture that keeps up, colour-corrected and stretched on the graphics card, with a histogram, the camera's raw readout per colour channel, and the sharpness and dispersion of the planet as you watch. Right-click the planet to centre the region of interest on it.

Gain at the camera's sweet spots: buttons for unity gain, lowest read noise and the high-conversion-gain switch, from what the camera itself reports.

Names recordings the way WinJUPOS expects (2022-08-04-0702_8-Saturn.ser, from the time at the middle of the recording), with a text file of every setting next to each. Leave the target on Auto and SlopCap works out whether it's looking at the Moon, Mars, Jupiter, Saturn, Uranus or Neptune from the image and the clock.

Collimation on a defocused star: it measures how far the secondary's shadow sits from the ring's centre and points the way to turn.

Focus, colour by colour, with an electronic focuser, below.

ADC: with an atmospheric dispersion corrector, it measures how far the planet's red and blue images sit apart and says which way to move the levers.

The mount: nudge the planet back into the frame with guide pulses, through INDI or with SlopCap's own drivers for the common mounts, below.

Point-to-point video to other computers, below.

A log of the night: every setting, recording, drop, focuser move and decision, for working out afterwards what happened.

Focus, colour by colour

Through a Barlow lens, an ADC or any other glass in the light path, red, green and blue light don't come to focus in quite the same place. On a colour camera that means one focuser position can't be perfect for all three, and judging focus by eye on the combined picture mostly judges green. SlopCap scores each colour on its own.

Live: three bars (red, green, blue) show each colour's sharpness as you turn the focuser, against the best that colour has reached so far, smoothed over the last few frames so the seeing doesn't make them jump.

The sweep finds each colour's best position. Pick a range (Coarse ±1500 or Fine ±350 steps around where the focuser is, following it as it moves, or your own), a step size and how many frames to take at each step. SlopCap moves the focuser through the range, always finishing each move in the same direction so the focuser's slack is taken up the same way every time, waits for it to settle, and scores every frame. Started off to one side of focus, it doesn't stop at the end of the range with the curve still climbing: it carries on past that end until the curve comes down again, then goes to the best focus. The camera stays in view beside the chart, and on the other tabs the curves fill in over a corner of the live preview.

Then choose what to focus on, and Go to moves there with the same final approach as the sweep:

Every sweep's scores, frame by frame, are saved as a CSV next to the recordings.

The screenshot shows a sweep against INDI's focuser simulator, with a test camera whose red, green and blue are sharpest at different positions.

Low in the sky the air acts as a weak prism: it lifts a planet's blue image a little above its red one and smears the detail. With an atmospheric dispersion corrector (ADC) between the telescope and the camera, the ADC tab measures how far red sits from blue, live and to a fraction of a pixel, and says what to do next: spread the levers, close them, or turn the ADC itself. Knowing the site and the target, it also shows how much dispersion the air is causing at the planet's altitude right now, so you know what to expect.

The mount panel nudges the planet back into the frame with guide pulses, shows where the telescope points, and parks, unparks and switches tracking. SlopCap reaches the mount through an INDI server, or drives it itself, with nothing else to install:

Mounts without guide pulses of their own get timed slow moves instead. The drivers are written from each maker's published protocol and tested against simulations of them, but haven't been tried on these mounts yet. A night at the telescope says what to put in Settings for each.

The computer at the telescope has the camera; the people are often somewhere else. One SlopCap shares its camera, others watch it, directly over the local network: no server in between, no cloud, no account. Any SlopCap, Windows, Linux or Mac, can be either end.

Windows slopcap-windows-x64.zip: the program is one file, slopcap.exe, with no installer and the camera drivers inside

Linux slopcap-linux-x64.zip: unzip and run, Qt and the camera drivers included (Ubuntu 26.04 or newer)

macOS slopcap-macos-arm64.zip: SlopCap.app (Apple silicon, macOS 27 or newer), with the camera drivers inside

Builds are on the Releases page, always as zips with a README for that system, the license and the third-party licenses. Installing SlopCap covers each system: first launch, camera drivers, the focuser and the mount.

Installing SlopCap Windows, Linux or Mac: getting it running, cameras, focuser and mount

A night at the telescope the screen, the camera, collimating, focusing, the ADC, recording

Remote viewing sharing a camera, watching one, picture quality, firewalls

Files, settings and logs where recordings, settings and the session logs are

Troubleshooting when something doesn't work

Build notes (building from source on each system, the tests, local CI, the packages, how the code is laid out): docs/developing.md. The design and what's still to come: PLAN.md.

SlopCap is free software under the GNU General Public License, version 3. Copyright (C) 2026 Daniel Mead. The camera SDKs, Qt and the IBM Plex fonts that come with it are other people's work under their own licenses; each download has them in licenses/ (what's there).