Session planner

How do you choose an astrophotography target?

The planner above remains the main tool: it combines date, location, altitude, the Moon, and your equipment's field of view. The notes below explain the criteria that help prepare a deep-sky session, without replacing the calculation shown higher on the page.

Which astrophotography target should you image tonight?

A good astrophotography target is not only visually appealing: it must be high enough, long enough, and compatible with your focal length. Altitude matters because a low object travels through more atmosphere: seeing worsens, the light-pollution gradient grows, and usable time shrinks.

Visibility time above your threshold (often 20° to 30°) tells you whether a session is worth it. An object that peaks for an hour then drops toward the horizon is a poor fit for a long integration. A circumpolar object, or one sitting well in the south at nightfall, leaves more room to stack frames.

In summer the Milky Way offers many nebulae and star-forming regions. Winter is also rich in nebulae, especially around Orion and Monoceros. Spring is generally the best season for galaxies, including the Virgo, Ursa Major, and Leo fields. The planner already filters by date: the goal is not to shoot everything, but to drop what will not rise high enough.

Take the Moon into account

The Moon is the factor that most often ruins a poorly prepared session. Its phase brightens the sky and lowers the contrast of faint objects. A full Moon does not forbid all astrophotography, but it pushes you toward contrasty targets, often nebulae with a narrowband filter, rather than galaxies or faint hydrogen wide fields.

Angular separation matters as much as phase: a weakly lit Moon sitting on your target still throws a hard gradient across the field. A comfortable gap (tens of degrees) gives more latitude, especially if you aim at a galaxy without a narrow filter.

The planner shows both pieces of information. Before you commit, check whether the Moon stays away during the hours when the object is high. If it does not, switch targets instead of hoping processing will save the sky background.

Match the target to your equipment's field

Apparent size and sensor field have to agree. A small galaxy at a short focal length drowns in the frame; a large nebula in a long Newton overflows. Field of view depends on effective focal length and sensor size: that is exactly what the planner computes when you enter your setup.

The filter also changes the choice. A dual-band filter (such as L-eXtreme) favours emission nebulae and tolerates a peri-urban sky better. It does not help a galaxy: there, sky background, the Moon, and altitude weigh more. Matching the target to the filter avoids hours of subs on an object processing cannot rescue.

Light pollution, the site's Bortle class, and the season add up. Under an average sky, a narrowband nebula stays reachable; a faint galaxy will need more altitude, more time, and no Moon. The planner does not invent the sky: it tells you what is observable. You still judge whether field, filter, and visibility make a useful session.

Nebula, galaxy, or wide field?

An emission nebula is easy to plan: it is often large, responds to filters, and remains photographable even when the Moon is not zero. A galaxy needs a cleaner sky, good altitude, and sampling that fits its size. A wide field (Milky Way, large complexes) depends mainly on a short focal length and framing that serves the composition, not only the catalogue name.

Start with what the tool lists as visible, high enough, far enough from the Moon, and fitting your field — or justifying a mosaic. Astrophotography planning is not a trophy list: it is a filter so you do not point at an object that is too low, too wide, or too close to the Moon. The rest is your session.