Alt-az field rotation
Keep the field from turning during the exposure.
Translate a local field-rotation rate into edge motion and a sub-exposure ceiling.
Purpose and method
What this calculator does
How long can I expose on an alt-az mount? Convert an alt-az field-rotation rate into edge trailing and exposure limits.
In words: Stars at the field edge move by field radius multiplied by rotation angle.
Change a value and the result updates immediately. Rotation rate changes continuously with target position and is fastest in difficult sky regions. Use the worst rate across the proposed exposure or a derotator.
Working geometry
Rotating field edge
Field rotation moves stars farther at the frame edge and imposes an exposure-time ceiling.
Inputs
Rotation, field and sampling
Calculated now
Rotation limit
- Edge motion rate
- 0.1745 ″/s
- Maximum exposure
- 8.59 s
- Predicted edge trail
- 3.49 px
- Proposed exposure
- Exceeds limit
Governing equation
In words: Linear angular motion at the field edge equals field radius multiplied by rotation angle in radians.
Show the working
The calculator applies the equation above to the values currently entered, without rounding until this display.
- Edge motion rate
- 0.1745 ″/s
- Maximum exposure
- 8.59 s
- Predicted edge trail
- 3.49 px
- Proposed exposure
- Exceeds limit
Rotation rate changes continuously with target position and is fastest in difficult sky regions. Use the worst rate across the proposed exposure or a derotator.