Field Rotation Planner

How to read A/B codes

Quick start (3 steps)

Enter location, date, timezone, and target Right Ascension and Declination (RA/Dec).
Set step size and mount type, then optionally add imaging constraints.
Review q(t), dq/dt, best-window highlight, and CSV export.

Inputs

Latitude (deg) Longitude (deg, east +) Date Step (min)

Time zone mode IANA time zone Coordinated Universal Time (UTC) offset override (min) Mount type

RA Dec RA unit Dec unit

Imaging constraints (optional)

Pixel scale (arcsec/px) Resolution X (px) Resolution Y (px) Allowed blur (px)

Best window highlight (optional)

Start (HH:MM) End (HH:MM)

Results

Max |dq/dt|—

Min |dq/dt|—

Estimated max exposure—

Reference time—

Resolved TZ—

Parallactic angle q(t)

Rotation rate dq/dt

Series table

Local time	Alt	Az	q (deg)	dq/dt (deg/min)	Max exp (s)

Definitions & notes

This tool defines q as the angle from celestial north toward zenith, positive toward east (counterclockwise on tangent plane).
We define field rotation as q(t) - q(reference).
We compute dq/dt from unwrapped q values to avoid wrap spikes.
The max exposure estimate includes rotation blur only and ignores tracking or seeing errors.
If you need imaging inputs, calculate pixel scale in Imaging planner (A9: Imaging planner).

How to use Field Rotation Planner effectively

Use this page when Alt-Az imaging is possible but rotation blur may set the real exposure limit. It helps answer “how long can I expose here?” rather than “is the target visible at all?”

Best workflow

Set site, target, and time first so the sky path is realistic.
Add your imaging scale only after the geometric rotation curve looks sensible.
Compare the rotation-limited exposure with your planned sub length before you commit the run.

FAQ

What should I enter first?

Start with the target, site, and observing time. Exposure limits are only useful after the sky geometry is correct.

Does this replace Observability time plotter?

No. Observability time plotter tells you when the target is up under your constraints. Field Rotation Planner tells you whether Alt-Az rotation makes your planned sub length unrealistic.

What does the max exposure estimate include?

It only covers rotation blur from the changing parallactic angle. It does not include tracking, seeing, wind, focus, or mount-specific errors.

When should I open another tool next?

Open Imaging planner when you need field size or pixel scale, Finder Chart Pro when you need a chart, and Observability time plotter when the main question is timing rather than blur.

Next steps

Observability time plotter
Confirm the usable time window before worrying about rotation-limited subs.
Imaging planner
Get pixel scale and field size so the exposure estimate is tied to a real setup.
Finder Chart Pro
Build the chart you will actually use after exposure limits are clear.
RA/Dec to Alt/Az
Inspect the underlying sky path directly if you only need coordinate conversion.