Shooting a Mosaic: When One Frame Is Not Wide Enough

Sooner or later you point a telescope at something that will not fit. The North America Nebula spans four degrees; a 500 mm refractor with an APS-C sensor gives you about two and a half. The Veil is worse. You can buy a shorter scope, or you can shoot the object in pieces and join them afterwards.

Joining them is the easy part. Shooting them so they can be joined is where a mosaic is won or lost, and almost every failed mosaic failed at the telescope rather than at the computer.

Overlap is not optional

Two panels have to share sky. Not touch — share.

The stitcher works out where each panel belongs by matching the stars it can see in both. With no shared stars there is no way to know how the panels relate, and no amount of clever software invents that information. Two panels that merely abut are two separate images.

Plan for 15 to 20% overlap on every edge. That sounds wasteful and it is not. Three things eat into it:

• Field rotation and drift. Over an hour on a panel your framing moves. The overlap you planned is the overlap you had at the start.
• Optical distortion at the corners. The outer few percent of most fields is the part you least want to trust as the join.
• Vignetting. Panel edges are the darkest part of a frame, and a seam made from two dark edges is a visible dark band whatever the blending does.

Below about 10% overlap the star matching starts to fail intermittently — it works on one pair and not the next, which is the most confusing way for it to fail.

Shoot each panel as a complete session

A panel is a finished image before it is a mosaic tile. Its own lights, its own calibration, its own stack.

The temptation is to throw every frame from every panel into one stack and let the software sort it out. It cannot. Stacking aligns frames on their shared stars, so a set of frames that only partly overlap will align on the stars they have in common and smear everything else into arcs. What comes out is not a mosaic; it is a mess with a sharp middle.

So: stack panel one. Stack panel two. Then stitch the results. In Akastroid that is Deep Sky for each panel, then Mosaic for the join.

Match the panels at the telescope, not afterwards

Every panel should be shot with:

• The same exposure length and gain. Different exposures mean different noise and different star sizes.
• The same filter. Obvious, and still a mistake people make on a two-night mosaic.
• Roughly the same total integration. A panel with half the time is visibly noisier, and there is no seam blending that hides a noise difference across a join.
• Roughly the same sky conditions. A panel shot through high cloud or with the Moon up sits at a different sky level, has softer stars, and shows as a rectangle.

Stitching can correct the level difference between panels — Akastroid measures each panel’s background and matches it to the reference, which is what stops the join reading as a step in brightness. It cannot correct a difference in noise, star size, or transparency. Those are decided when you press the shutter.

What the stitch actually does

Three steps, in this order:

1. Placement. Stars in each panel are matched against the reference panel, which gives the offset and rotation to put every panel on one canvas. A panel that overlaps nothing in the main group is reported rather than forced into place — the honest answer to “these two do not touch” is to say so.
2. Level matching. Each panel’s background is brought to the reference’s, so no panel is a brighter or darker patch.
3. Feathering. Rather than a hard edge, the panels cross-fade over a ramp. A wider ramp hides a seam more thoroughly and costs a little detail either side of it; the default is about 12% of a panel, which is invisible on most data.

Planning the grid

Two by two is the sensible first mosaic. Four panels, four times the integration, a field twice as wide in each direction.

Work out your field of view first:

field (arcmin) = 3438 × sensor size (mm) / focal length (mm)

An APS-C sensor is 23.5 × 15.6 mm. At 500 mm that is 162 × 107 arcminutes — 2.7 × 1.8 degrees. For four degrees of nebula with 20% overlap you need two panels across, and the arithmetic tells you that before you waste a clear night finding out.

Then decide the panel centres and enter them into whatever you use to slew. Most planetarium software will draw a mosaic grid for you; the numbers matter more than the tool.

Two by two is four nights, realistically

This is the part nobody mentions. A mosaic multiplies your integration requirement by the number of panels. Four panels at three hours each is twelve hours for one image — and every panel needs enough time to be as deep as the others, so you cannot stop early on one and make it up later.

If you have two clear nights a month, a four-panel mosaic is a two-month project. That is a fine thing to commit to deliberately and a miserable thing to discover halfway through.

When not to bother

• The object fits. If it occupies more than about a third of your frame, you have framing to spare and a mosaic buys nothing.
• You have not filled one panel with signal yet. A noisy mosaic is four noisy images in a wider arrangement. Depth first.
• Your mount will not repeat. If slewing back to a panel centre lands you somewhere different each time, your overlap is a guess.

In Akastroid

Stack each panel in Deep Sky and export or keep the result. Then Mosaic: add two or more panels, and it places them by their stars, matches the levels and feathers the seams. The first panel you add is the reference — the others are matched to it, so make it the one shot in the best conditions.

The stitched field arrives as an ordinary session, with the same styles, the same histogram handles and the same export as anything else. It is a wide image at that point, not a special kind of object.

Mosaic stitching is part of Akastroid Pro.