Frequently asked questions
How the forecast is put together, what your rig changes, and how to plan around the sky you can see.
The forecast
What is The Quiet Sky?
A forecast for astrophotography and stargazing that runs in your browser, anywhere in the world. Choose where you observe and compare the next seven nights, hour by hour. Add a target to check the cloud between you and that object, record the skyline at your site, and find usable imaging time for your telescope, camera and filters.
Is this useful for visual observing as well as imaging?
Yes. The cloud, seeing, transparency, Moon and darkness forecasts are useful for visual astronomy and stargazing with binoculars, a telescope or the naked eye. Use the cloud forecast map to look for clear skies around your site. The night scores and usable windows are tailored to imaging, so visual observers should also check the individual weather fields.
How is this different from an ordinary cloud forecast?
An ordinary cloud forecast is for your location. Your telescope usually points across the sky, so the cloud in front of it may be much farther away. The Quiet Sky follows the line of sight to your target and checks the forecast where it crosses the low, mid and high cloud layers. By default, these are represented at 1.5, 4 and 10.5 km above sea level. Cloud well away from your site can block a low target even when the sky overhead is clear.
Who gets sightline cloud, and is there a limit?
Sightline cloud needs an account. You can check up to 5 different targets a day. Around the dark part of the night, the target's path is sampled in 20-minute steps while it is at least 5° above the horizon. That is a calculation interval, not a weather-update schedule. If the target is too low or sightline data is unavailable, the app shows overhead cloud and labels it as such.
How is each night scored?
Each hour gets a score out of 100. Cloud and twilight set the starting point, then the Moon, transparency, seeing, wind and dew take points off. Rain, or a high chance of it, can cap the result. The usable window is the longest unbroken stretch of dark hours that meets your rig's threshold. The defaults are 55 for broadband and 50 for narrowband; you can change them.
What do Quiet, Settled, Broken, Unsettled and Closed in mean?
The label is based on the night's best unbroken run of hours. Quiet needs 4 hours scoring 75 or more. Settled needs 3 hours at 55 or more. Broken needs 2 hours at 35 or more. Unsettled means short windows at best, and Closed in means overcast, or otherwise unlikely to be usable for imaging.
Where do seeing and transparency come from?
7Timer!, run by the Shanghai Astronomical Observatory, provides them for roughly three days ahead where data is available. Where it has no data, we estimate seeing from the jet stream and atmospheric instability, and transparency from humidity at the ground and higher up. Copernicus Atmosphere Monitoring Service forecasts account for smoke, dust and haze. These can lower the transparency estimate but never improve it. Use seeing to compare nights at your site, rather than to predict star size.
Which weather models does it use?
The hourly forecast comes from Open-Meteo, which serves the open models of national weather services including the UK Met Office, DWD, ECMWF, NOAA, Environment Canada and Météo-France. Environment and Climate Change Canada's GEM model is also shown beside each hour as a second opinion. The cloud map is painted from the finest regional model Open-Meteo has for your area. Every source is credited on the Sources and licences page.
How do you choose the weather model for my region?
Each night we compare candidate cloud forecasts with reports from weather stations in regions with saved sites. We check a shortlist on most nights and the full configured set weekly. The results help choose a model or blend for each region, alongside checks on coverage, data freshness and update frequency. Where no regional choice is available, the app uses Open-Meteo's default forecast.
How far ahead does it forecast?
Seven nights, starting tonight. Each night runs from sunset to sunrise in the site's own timezone.
Targets and planning
Which targets can I plan for?
You can search over 13,000 objects, including the NGC and IC catalogues, Sharpless, van den Bergh, Barnard and LBN nebulae, and planetary nebulae. You can also look up objects by name through SIMBAD. The Targets page shows what is well placed tonight, this week or this season. Also up tonight offers a shorter list of popular targets.
Can it allow for trees, roofs or a balcony that block my view?
Yes. Sign in, open Visible sky on a saved site (the ◐ button) and mark what blocks your view. On iPhone or Android you can trace the skyline with the camera and compass. On a computer you can draw it, enter compass sectors such as clear from 120° to 220° above 20°, or import an .hrz horizon file. Without an account, the horizon tool traces your skyline and saves it as a horizon file for N.I.N.A. If a balcony or eaves block the high sky, add the height you can't see above. Hidden targets score zero, so those hours are left out of usable windows, target suggestions, projects and night plans. The default 2° safety margin allows for small differences in where you set up.
What if my phone's compass is a few degrees out?
If you know when a target disappeared behind your roof, add it as a sighting: for example, I lost the Eagle Nebula at 01:00. The app turns your trace to match. This helps when the phone's compass is a few degrees out, which is common near metal or buildings. You can save up to 5 observing spots at each site, such as the garden and an upstairs window, and choose which skyline the forecast uses.
Can I see how a target fits my camera?
Yes. The framing preview shows a real survey image at your rig's field of view. In the Sky Atlas you can rotate the frame or lay out a mosaic of up to 6 by 6 panels. You need an account to save it.
What does my rig change?
Your focal length and pixel size determine how much poor seeing and transparency affect the score. Narrowband filters, particularly Hα and SII, reduce the effect of the Moon and twilight. Your gust limit, dew heaters and minimum altitude change the score and the usable window, and your mount type determines whether you see meridian flip or field rotation advice. Pick your telescope and camera from the gear list or type in your own numbers. Without a rig, the forecast uses a 500 mm APS-C starter rig.
Can I export to N.I.N.A. or ASIAIR?
Yes, in beta. A saved frame or mosaic exports as a target list that N.I.N.A. and ASIAIR can import. ASIAIR takes targets only, so you enter the exposure groups from the sheet the export provides.
Can it plan a whole night?
Yes. The night planner works with one rig, one site and up to three projects per night. Each project keeps your sessions and the time you want to collect through each filter. You can import exposure details from FITS or XISF files; the files are read on your device, without uploading the images.
Accounts, alerts and price
What does The Quiet Sky cost?
All features are currently free. Any future pricing changes will be announced in advance.
Do I need an account?
No. You can check the forecast, scores, Moon and darkness times, cloud map, targets and framing without signing in. An account adds sightline cloud, saved rigs, sites synced across your devices, alerts, projects and the night planner.
Will it tell me when it is going to be clear?
Yes. With an account, save a site and set an alert for the conditions you need. You'll get a push notification when a matching window is coming, at most once a day per alert. More than two days ahead, Environment Canada's model must show the clear spell too. On iPhone, add The Quiet Sky to your Home Screen first.
Does it work outside the UK?
Yes, anywhere in the world. Each site uses its own timezone, and night times account for daylight saving changes. At high latitudes in summer, when the sky never gets astronomically dark, the app uses nautical or civil darkness instead.
Does it know about light pollution?
Yes. Each site shows its sky brightness, an approximate Bortle class and the darkest direction nearby, from David Lorenz's light pollution atlas, which covers latitudes from 65° south to 75° north. You can override the Bortle class if you know your sky better.
Can I install it as an app?
Yes. Install it from your browser or add it to your Home Screen. It opens like an app, but you still need an internet connection for forecasts.