Go / no go for deep skyGo / no go for deep sky imaging
M91
spiral galaxy in Coma Berenices
M91 is a faint barred spiral in Coma Berenices that went missing for a time because of an error in Messier's catalog. It has low surface brightness, so it needs a dark sky and long exposure.
Save your telescope or lens on the planner to lay your camera frame over this diagram.
// Key facts
- Catalog
- M91
- Type
- spiral galaxy
- Constellation
- Coma Berenices
- Size
- 5′ across
- Brightness
- Magnitude 10.2
- Distance
- about 53 million light-years
- Peak month (any location)
- Mar
- Typical prime months
- Jan to Jun
- Difficulty
- Advanced
- Filter
- No filter (broadband); a light-pollution filter is optional
Pick a saved location or share your device location. Your coordinates are used only to calculate tonight and are not stored. To save locations, use the planner.
How to image M91
- At 5 arcminutes, use 1,500 mm or more.
- Dark skies and long total exposure matter more than most targets here.
- Best in spring evenings from northern sites.
Framing: what focal length?
M91 is small, so a "60% of the frame" focal length would be impractical. Here is how much of an APS-C frame it covers at common long focal lengths; expect to crop.
| Focal length | Share of the frame width |
|---|---|
| 1,000 mm | about 6% |
| 2,000 mm | about 12% |
| 3,000 mm | about 19% |
How high does it climb from your latitude?
The highest point M91 reaches in the sky. Above about 60° is excellent, 30° to 60° is workable, and under 30° you are looking through a lot of air.
| Latitude | Highest point | Rating |
|---|---|---|
| 55°N (Scotland, Alaska) | 50° | Workable |
| 45°N (Minneapolis, Milan) | 60° | High in the sky |
| 35°N (Los Angeles, Tokyo) | 70° | High in the sky |
| 25°N (Miami, Cairo) | 80° | High in the sky |
| 35°S (Sydney, Santiago) | 41° | Workable |
Also worth imaging
Want a GO / NO-GO verdict for tonight, with clouds, moon, seeing and a full dark-window timeline?
Open the planner