Imaging planner
Dark window, moon and target altitudes for any night. Targets are ranked by time above your minimum altitude in real darkness, less what the moon takes away.
The night
Selected target
Best targets tonight
| Target | Type | Hours | Max alt | Transit | Moon |
|---|
How to use the imaging planner
- Set your location with Use my location, or type latitude and longitude. Check the time zone matches where you will shoot.
- Pick the evening you plan to image. The night runs from that evening to the next morning.
- Set a minimum altitude. 30° is a good default. Use 20° for a clear, dark horizon or 40° under city light.
- Read the dark window and the moon, then pick from the ranked list. Click any target to see its altitude through the night.
Questions
What is astronomical darkness?
The sun is more than 18° below the horizon, so twilight no longer brightens the sky. Deep-sky imaging works best inside this window. Near midsummer at high latitudes it never arrives, and the planner falls back to nautical twilight and says so.
How high should a target be for imaging?
Above about 30°. Lower down you look through more air: at 30° there is twice as much as overhead, and at 20° about three times. That brings more haze, light pollution and blurring from turbulence.
How much does the moon matter?
It depends on its phase, how far it is from your target and your filter. A bright moon near the target ruins broadband images of galaxies. Emission nebulae shot through narrowband or dual-band filters cope far better, and the ranking takes that into account.
When is a target best placed?
At transit, when it crosses the meridian and reaches its highest point. The planner shows the transit time and the hours the target spends above your minimum altitude in real darkness.
Which targets are in the list?
All 110 Messier objects, the Caldwell catalog and 53 popular NGC and IC targets. Each has a target guide with its best months.
Method
Darkness
The dark window runs from the end of evening astronomical twilight to the start of morning twilight, when the sun is 18° below the horizon. At high latitudes in summer the sun never gets that low. The planner then falls back to nautical twilight (sun at -12°) and, failing that, to sunset and sunrise, and says so.
Altitude
Catalog positions are J2000. They are precessed and nutated to the date, then altitude comes from local apparent sidereal time:
sin(alt) = sin(lat) sin(dec) + cos(lat) cos(dec) cos(LST - RA)
Refraction is ignored. It lifts objects by about 0.03° at 30° altitude. Against a full reduction with astronomy-engine the error is under 0.02°. Sun and moon use astronomy-engine directly, and the moon is topocentric.
Ranking
Each target is sampled every 10 minutes inside the dark window. Samples below the minimum altitude count for nothing. The rest add up as:
score = sum of step (h) x sin(alt) x (1 - P) P = moon illumination x (0.35 + 0.65 x clamp((90 - separation) / 75, 0, 1)) when the moon is up
- sin(alt) favors targets high in the sky, where there is less air and less light pollution.
- The moon penalty is halved for emission nebulae, which you can shoot through narrowband filters.
- Named objects get a factor of 1.15 and objects under 5′ a factor of 0.8, so a 2′ planetary does not top the list for a 60 mm refractor.
Data
Target list: all 110 Messier objects, the Caldwell catalog and 53 popular NGC and IC targets. Positions and sizes come from OpenNGC (CC-BY-SA 4.0). Ephemerides come from astronomy-engine. Times are shown in the time zone you pick, which defaults to your device's.