How is the field of view calculated?

Field of view in degrees equals the arctangent of the sensor dimension over twice the focal length, doubled, converted to degrees. A common approximation is 57.3 times sensor size divided by focal length, which the tool uses in its full trigonometric form for accuracy across wide fields.

Why do mosaics need overlap?

Overlap gives the stacking software shared stars to register and blend adjacent panels seamlessly, and it covers vignetting and edge distortion that you crop away. Without overlap, gaps or hard seams appear between panels. Fifteen to twenty-five percent is the usual range.

How does overlap change the panel count?

Each panel only contributes its effective field of view, which is the full field times one minus the overlap fraction. The tool divides the target size by that effective field and rounds up, so higher overlap means more panels to cover the same area.

What target size should I enter?

Use the angular size of your subject in degrees. For example, the North America Nebula spans roughly 2 by 2 degrees, the Andromeda Galaxy about 3 by 1 degrees. Add a little margin so the target is comfortably framed rather than touching the edges.

Does this account for sensor orientation?

It assumes the sensor's width maps to the target's width and its height to the target's height. If you rotate the camera, swap your sensor width and height inputs. Planning in the orientation you will actually shoot gives the correct column and row counts.

What is the Astrophotography Mosaic Panel Calculator?

Enter sensor size, focal length, target dimensions in degrees, and overlap percentage to compute your camera field of view and the number of panels and grid layout needed for an astrophoto mosaic. For imaging large nebulae. It runs free in your browser on Gera Tools, with nothing uploaded.

Astrophotography Mosaic Panel Calculator

Name: Astrophotography Mosaic Panel Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Imaging a target larger than your sensor’s field of view means shooting a mosaic — a grid of overlapping panels stitched into one frame. This planner computes your single-frame field of view from the sensor and focal length, then works out how many columns, rows, and total panels you need to cover the target with your chosen overlap.

How it works

Field of view for each sensor axis is found by trigonometry, then each panel’s effective coverage is reduced by the overlap so adjacent frames share a margin:

FOV_deg       = 2 × atan(sensor_mm / (2 × focal_mm)) × 180/π
effective_FOV = FOV_deg × (1 − overlap_fraction)
columns       = ceil(target_width_deg  / effective_FOV_width)
rows          = ceil(target_height_deg / effective_FOV_height)
panels        = columns × rows

The arctangent form stays accurate even for wide-field optics where the simple small-angle approximation drifts.

Why mosaic imaging?

Many of the most rewarding deep-sky targets are simply larger than any practical single-frame field of view. For example:

The North America Nebula (NGC 7000) spans roughly 2.0 × 1.7 degrees
The Andromeda Galaxy (M31) covers about 3.2 × 1.0 degrees
The California Nebula (NGC 1499) stretches about 2.5 × 0.67 degrees

A full-frame camera at 500 mm focal length gives roughly 4.1 × 2.7 degrees — wide enough for many targets, but a narrower 1,000 mm telescope cuts that to about 2.1 × 1.4 degrees, meaning even M31 would overflow the frame. Mosaic imaging solves this by assembling a larger composite from individually manageable panels.

Worked example

A full-frame 36 × 24 mm sensor at 500 mm focal length gives:

FOV_width  = 2 × atan(36 / (2 × 500)) × (180/π) ≈ 4.1°
FOV_height = 2 × atan(24 / (2 × 500)) × (180/π) ≈ 2.7°

To cover a 6 × 4 degree region with 20% overlap:

effective_FOV_width  = 4.1° × (1 − 0.20) = 3.28°
effective_FOV_height = 2.7° × (1 − 0.20) = 2.16°
columns = ceil(6 / 3.28) = 2
rows    = ceil(4 / 2.16) = 2
panels  = 2 × 2 = 4

Four panels are needed. If the target were 10 × 6 degrees — roughly the size of the Vela Supernova Remnant — the same setup would need ceil(10/3.28) = 4 columns and ceil(6/2.16) = 3 rows, giving a 12-panel mosaic.

Choosing your overlap

The overlap percentage is a trade-off:

Too low (under 10%): Registration software may struggle to find enough stars in the shared border, leading to seams or failed stitches.
Typical range (15–25%): Provides reliable registration stars and corrects for vignetting at the edges of each frame. Most astrophotographers use 20%.
High (30%+): Very secure registration but significantly increases the total number of panels — a 30% overlap on a 3 × 3 grid that would otherwise need 9 panels might now require 16. Use higher overlap when targeting regions with sparse stars or when using cameras with significant corner vignetting.

Practical planning tips

Match rows to declination: When planning panels on the sky, rows run east-west and columns north-south. Panels at different declinations have slightly different plate scales in altitude, so keep individual panel exposures short enough that tracking drift stays within acceptable limits.

Consider sensor orientation: This tool assumes sensor width maps to target width. If you rotate the camera 90°, swap your sensor width and height inputs. Many imagers rotate to get the target to fit with fewer panels.

Add margin to your target dimensions: Enter the target angular size with a few extra tenths of a degree on each axis so the outermost stars of the target are comfortably inside the innermost panels rather than crowding the edges.