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Tutorial: correcting field rotation on an altazimuth mount

This tutorial corrects a long capture and a time-lapse of the Sun taken on an altazimuth mount. It uses a real SER capture, 15_20_57.ser (1920 by 1080, 100 frames with real timestamps), and shows the effect by giving its frames the timestamps of an hour-long session, because the capture itself lasts only 9.5 seconds. The method is in Field rotation. Skip all of this on an equatorial mount: it has no field rotation.

1. Decide whether you need it

The rotation rate is 15 degrees per hour * cos(latitude) * |cos(azimuth)| / cos(altitude). Open the capture in the app, turn on Advanced settings, set Field rotation to Alt-az and enter the site: the line under the controls says how far this capture rotates. For 15_20_57.ser (9.5 s, 51.5 N) it is 0.023 degrees, 0.09 pixels at the limb of a 445 pixel disc: nothing to correct. A capture of ten minutes near noon turns by about two degrees, 16 pixels at the limb: correct it.

2. Correct a long capture

You need timestamps in every frame (SER) and the site: latitude north, longitude east, height in metres (use your own; the example uses 51.5, -0.1, 30).

helia stack session.ser -o sun.fits --preset white --field-rotation \
    --site 51.5,-0.1,30

In the app: Advanced settings, Field rotation: Alt-az, the three Site fields, then Stack as usual. If your picture is a mirror image (a star diagonal), tick Mirror image or add --field-rotation-mirror; if the stack is not as sharp as it should be, try the other setting.

The Stack step with field rotation on

What to expect: the run prints rotation: lines with the angle covered and the shift at the limb, and the FITS history gets a FIELD-ROTATION step. Settings used for the measurement below: the 100 frames of 15_20_57.ser stamped one every 36 s (an hour), site 51.5, -0.1, the Sun at 47 degrees: 6.3 degrees in total, turns of up to 3.2 degrees, 25 pixels at the limb; best half of the frames kept.

Granulation band (1 to 4 px), % of the mean Frames as they were Rotating capture Corrected
Global alignment 1.286 0.706 1.296
Alignment points 1.311 0.915 1.321

Grouped bars of the granulation band (1 to 4 px) at the disc centre in per cent of the mean. Global alignment: frames as they were 1.286, rotating capture 0.706, corrected 1.296. Alignment points: 1.311, 0.915 and 1.321.

Granulation band at the disc centre for 100 frames of 15_20_57.ser given the timestamps of an hour, from the table above (higher is better).

3. Align a time-lapse

Stack each capture to FITS (the file carries DATE-AVG), then turn the images to one orientation:

helia derotate sun-1100.fits sun-1130.fits sun-1200.fits -o aligned \
    --site 51.5,-0.1,30

The table printed shows each image's time, the parallactic angle and the turn applied. Images come out as sun-1100-derot.fits and so on, turned to the middle image. For TIFF or PNG give the times: --times 2026-03-20T11:00:00,2026-03-20T11:30:00,.... Stack or crop afterwards: the corners the turn uncovers are filled with the edge level.

What can go wrong

What you see Why What to do
needs the time of each frame The capture has no timestamps (AVI, video) Use a SER capture; or for stacked images give --times
needs the observing site No --site and no HELIA_SITE Give the site
the stars did not show which way the field turns A star field whose subs give no clear lock, or two that are too alike Give --field-rotation, with --field-rotation-mirror if the picture is a mirror image, to name the sense yourself
The limb is still smeared Wrong turn direction, or a clock far off Toggle the mirror setting; check the computer's clock when you captured
Nothing changes The capture is short, or the mount is equatorial Nothing to correct

Reference: every option

Setting CLI API (config.field_rotation, with site) App Range Default
Correct the rotation helia stack --field-rotation field_rotation (an object; {} for the defaults) Advanced settings, Field rotation: Off / Alt-az on or off off
Reference orientation --field-rotation-reference reference middle (fixed) middle, first, last middle
Mirror image --field-rotation-mirror mirror Mirror image on or off off
Stop the automatic correction of FITS subs --no-field-rotation not offered not offered on or off off (it is automatic when the header names target, site and mount)
Site --site LAT,LON,ALT or HELIA_SITE site or the server's HELIA_SITE Site (N, E, M) latitude -90 to 90, longitude -180 to 180, height -1000 to 10000 m none
Time-lapse helia derotate IMAGES -o DIR with --site, --times, --reference (middle, first, last or an image number), --mirror, --overwrite not offered not offered middle

Results appear in summary.field_rotation in the API.