A mosaic covers a large object with several overlapping frames that are stitched together afterwards. Panels need about 15% overlap, and each panel needs its own exposure time, so a 2 × 2 mosaic takes about four times as long as a single frame.
When you need a mosaic
If an object is longer than the short side of your frame, it will not fit in one image. The Andromeda Galaxy is more than three degrees long, the Veil Nebula and the North America Nebula are about two to three degrees across: wider than the frame of most smart telescopes.
How a mosaic is made
Some smart telescope apps can plan and shoot a mosaic automatically. With others you image the panels one by one and stitch them yourself, for example in Siril or other free tools.
The object is divided into a grid of panels that overlap by about 15%, so the software can match them.
The telescope images each panel in turn, stacking each one separately.
Software aligns the panels on the stars and blends the overlaps into one large image.
Mosaic planner
Andromeda Galaxy is larger than the frame of some smart telescopes. A mosaic covers it with several overlapping frames that are stitched together afterwards. Pick a telescope to see the layout.
Seestar S30 · 2.14° × 1.21°: 2 × 1 = 2 panels Each panel needs its own exposure, so plan about 2× the imaging time of a single frame.Seestar S30 Pro · 3.99° × 2.24°: 2 × 1 = 2 panels Each panel needs its own exposure, so plan about 2× the imaging time of a single frame.Seestar S50 Pro · 2.45° × 1.38°: 2 × 1 = 2 panels Each panel needs its own exposure, so plan about 2× the imaging time of a single frame.DWARF mini · 2.13° × 1.2°: 2 × 1 = 2 panels Each panel needs its own exposure, so plan about 2× the imaging time of a single frame.DWARF 3 · 2.93° × 1.65°: 2 × 1 = 2 panels Each panel needs its own exposure, so plan about 2× the imaging time of a single frame.Vespera II · 2.5° × 1.4°: 2 × 1 = 2 panels Each panel needs its own exposure, so plan about 2× the imaging time of a single frame.Vespera 3 · 2.5° × 1.4°: 2 × 1 = 2 panels Each panel needs its own exposure, so plan about 2× the imaging time of a single frame.Vespera Pro 2 · 1.6° × 1.6°: 3 × 1 = 3 panels Each panel needs its own exposure, so plan about 3× the imaging time of a single frame.Seestar S50 · 1.29° × 0.73°: 6 × 1 = 6 panels Each panel needs its own exposure, so plan about 6× the imaging time of a single frame.Origin Mark II · 1.32° × 0.75°: 3 × 2 = 6 panels Each panel needs its own exposure, so plan about 6× the imaging time of a single frame.Odyssey · 0.75° × 0.56°: 8 × 2 = 16 panels Each panel needs its own exposure, so plan about 16× the imaging time of a single frame.Odyssey Pro · 0.75° × 0.56°: 8 × 2 = 16 panels Each panel needs its own exposure, so plan about 16× the imaging time of a single frame.eQuinox 2 · 0.76° × 0.57°: 8 × 2 = 16 panels Each panel needs its own exposure, so plan about 16× the imaging time of a single frame.eVscope 2 · 0.76° × 0.57°: 8 × 2 = 16 panels Each panel needs its own exposure, so plan about 16× the imaging time of a single frame.
Calculated with 15% overlap between panels and a 10% margin around the object, with the frame lined up along the object's long axis. When the object is longer than the short side of the frame we always count at least two panels, because on an alt-az mount the frame turns during the night. Whether a telescope has a built-in mosaic mode depends on its app and firmware. You can also shoot the panels separately and stitch them yourself.
What it costs
Every panel needs its own exposure, so the time grows with the number of panels: two panels need about twice the time for the same noise level, six panels six times. Plan large mosaics over several nights, and make sure the object stays high enough for all of them.
Mosaic or wider telescope?
If most of what you want to photograph is large, a telescope with a wider native field saves a lot of time. If you mostly image small galaxies but want the occasional big nebula, a narrow field plus the odd mosaic is a good compromise. Every target page on this site shows how many panels each telescope needs.