Image Overlay

The Image Overlay section lets you load a reference image — such as a lab flat-field or an on-sky frame — and align it with the computed focal-plane layout. This makes it easy to verify that the optical model matches the real detector geometry.

Table of contents

1. Loading an Image
2. Transform Controls
   1. Position
   2. Orientation
   3. Reflection
   4. Proxy Size
3. Actions
4. Workflow Tip

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Loading an Image

1. Expand the Image Overlay section in the sidebar (or scroll down to it).
2. Click the file input (or drag and drop) to load a PNG or JPEG file.
3. The status line updates to say Image loaded… X µm/pixel, using the mosaic scale parameter to convert pixel size to physical units.

The mosaic Scale parameter (µm px⁻¹) in the Mosaic section determines the physical-to-pixel conversion. The alignment process updates this value initially to fill the loaded image inside the mosaic. Modifying the Scale parameter in the Image Overlay section adjusts the mosaic scale.

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Transform Controls

All parameters are applied simultaneously when you click Apply transform.

Position

Parameter	Unit	Description
Offset X	mm	Shift the image horizontally in the focal plane
Offset Y	mm	Shift the image vertically in the focal plane

Orientation

Parameter	Unit	Description
Rotation	°	Rotate the image around its center
Scale	×	Uniform scale factor (1.0 = no scaling)

Reflection

Option	Effect
None	No flip
Flip Y	Mirror horizontally (left ↔ right)
Flip X	Mirror vertically (top ↔ bottom)
Flip both	Rotate 180° (equivalent to flip X + flip Y)

Proxy Size

The Proxy size (pixels, 128 – 4096) controls the resolution of the image thumbnail sent to the browser for display. A smaller proxy renders faster; a larger proxy reveals more detail. The original image data is not modified. Setting the proxy size to the original image dimensions (or larger) will display the full-resolution image, but may cause slow rendering and increased memory usage in the browser.

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Actions

Button	Description
Apply transform	Process the image with the current transform values and display it in the plot
Reset transform	Revert all transform fields to their defaults (no offset, no rotation, scale = 1, no flip)
Remove image	Unload the image and clear it from the plot

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Workflow Tip

A quick alignment workflow:

1. Load the image.
2. Set Rotation, Offset X / Y and Scale to match the mosaic layout.
3. Click Apply transform.
4. Iterate until the mosaic layout aligns with the image.
5. Adjust instrument parameters (e.g. focal length, detector position) to refine the alignment of spectral lines with those present in the image.