# Acquire a cross-shaped dither sequence

The complete source is included below.

```powershell
python -m nextastro_obs_client.examples.dithered_sequence --ra 12.5 --dec -30 --target "Student field" --filter V --seconds 5 --offset 15 --count 2
```

Acquires a centre and four sky-offset positions. Each offset starts from the named target, avoiding cumulative tangent-plane drift. East/north are sky arcseconds, including at high declination. The example requests a return to the supplied target when it exits; existing cancellation/operator stop can prevent that return.

Student extension: Record offset metadata, align and combine the images in the student's analysis environment.


## Python source

```python
"""Expose at a sequence of sky offsets and return to the initial pointing."""
from nextastro_obs_client.examples._common import parser, target_arguments, point, execute, return_after


def run(obs, camera, args):
    point(obs, args)
    with return_after(lambda: point(obs, args)):
        for east, north in [(0, 0), (args.offset, 0), (0, args.offset), (-args.offset, 0), (0, -args.offset)]:
            # Offsets are incremental tangent moves; each new position starts at
            # the named target so the requested pattern has a common sky origin.
            if east or north:
                point(obs, args)
                obs.mount.offset(east_arcsec=east, north_arcsec=north)
            for image in camera.expose(seconds=args.seconds, filter=args.filter, count=args.count, target_name=args.target):
                print(east, north, image.path)


def main(argv=None):
    p = parser("Dithered sequence")
    target_arguments(p)
    p.add_argument("--seconds", type=float, required=True)
    p.add_argument("--offset", type=float, required=True, help="Sky arcseconds")
    p.add_argument("--count", type=int, default=1, help="Frames per position")
    return execute(p, run, argv)


if __name__ == "__main__":
    main()
```

Shared argument and session helpers: [Example support code](support.md).