Everything the app does, and the astronomy behind it.

Is tonight worth it

Four things decide whether you should get in the car, and they matter in roughly this order: cloud, the Moon, darkness, and where your target actually is. Sidereo's Tonight tab scores all four and gives you one answer, but the reasoning is worth knowing, because on a marginal night you will want to overrule it.

The Moon is the one people forget

A full Moon is roughly as disruptive as a small city on the horizon. It raises the sky background everywhere, and no amount of exposure fixes that — you are not photographing faint things through it, you are photographing moonlight. The number to watch is not just illumination but whether the Moon is above the horizon during your window. A 90% Moon that sets at 01:00 gives you a perfectly good second half of the night.

For the Milky Way or anything faint, aim for the Moon below the horizon, or under about 20% lit. For planets, the Moon barely matters — Jupiter does not care.

Darkness is not the same as sunset

The sky keeps getting darker for well over an hour after the Sun goes down, in three named stages:

In summer at high latitudes, true night may never arrive at all. Twilight Times under Tools gives you the exact boundaries for your location and date; Best Nights compares the coming week so you can pick rather than hope.

Light pollution, honestly

The Bortle scale runs 1 (a genuinely dark site) to 9 (inner city). It is worth setting yours accurately rather than optimistically, because Sidereo uses it to decide how many stars to draw. Set it to 4 when you are really in a 7 and the app will show you a sky you cannot see, which makes finding anything harder, not easier.

Finding things

There are three ways to find an object in Sidereo, and they suit different situations.

Sky View — point and read

Hold the phone up and objects are drawn where they actually are. The crosshair identifies whatever it settles on. There is also a camera-free planetarium mode, which is better indoors, better for learning the shapes, and much easier on the battery.

Why is Jupiter drawn so large? At its true angular size it is about two pixels, and impossible to aim at on a phone held at arm's length. Positions are accurate; the symbols drawn at those positions are deliberately enlarged. Every serious sky app does this, and any that claims otherwise is doing it too, quietly.

Sky Map — plan without pointing

A flat, orientation-independent view of the whole sky with pinch-zoom. Use this at the kitchen table to work out what is up; use Sky View outdoors to actually find it.

Find — guided pointing

Pick a target and Sidereo tells you which way to turn and how far, then confirms when you are on it. This is the fastest route to something specific, and it is also how you learn the sky: after being walked to Vega four times, you stop needing to be.

Paths shows the target's arc across the next twelve hours with wall-clock marks along it, which answers the question that actually matters — not "where is it" but "when is it high enough to be worth photographing".

When the compass lies

This is the single most common reason a sky app appears to be wrong, and it is almost never the astronomy. Phone magnetometers are cheap, and they are thrown off by anything magnetic: car bodies, speaker magnets, magnetic phone cases and mounts, steel railings, reinforced concrete, and the magnets inside MagSafe accessories.

Errors of 10–30° are routine. At that scale, the object you are looking for is simply not where the phone says it is.

Verify Compass

Rather than asking you to trust a figure-of-eight wave, Sidereo lets you correct against the sky itself. Point at a bright object you can genuinely see — the Moon, Jupiter, Sirius — confirm it, and the app measures how far off the compass is and applies that correction from then on.

This is the astronomer's method: you calibrate an instrument against a known reference, not against a hope. It takes ten seconds and it is the difference between the app being useful and the app being annoying.

Take your phone out of a magnetic case before calibrating, and step away from the car. A correction measured next to a car door is a correction that will be wrong everywhere else.

Photographing the sky

Phone astrophotography is genuinely hard for reasons that are nobody's fault: the sensor is small, the sky is faint, and the Earth is turning. Everything below follows from those three facts.

Focus is the shot-killer

Autofocus has nothing to lock onto in a dark sky, so it hunts and lands somewhere wrong. Every star in the frame becomes a small soft disc, and no amount of editing recovers it. Focus must be locked at infinity before you start. Sidereo's Astro Camera does this for you the moment you choose a preset.

Why you cannot just use a long exposure

The Earth rotates 15 arcseconds per second of time. Leave the shutter open too long and every star becomes a short streak. How long is "too long" depends on your focal length, and there are two rules for it.

The 500 Rule — quick, crude, good enough to start:

seconds = 500 ÷ (focal length × crop factor)

An iPhone main camera is about 24 mm full-frame equivalent, giving roughly 20 seconds. That sounds generous until you learn that most iPhones cap a single exposure at about 1 second — which is why stacking exists.

The NPF Rule — Frédéric Michaud's version, far more honest, because it accounts for how small your pixels actually are:

seconds = (35 × aperture + 30 × pixel pitch in µm) ÷ focal length

On a phone, with its tiny pixels, NPF typically gives a number several times shorter than the 500 Rule. The 500 Rule was written for film. Both calculators are under Plan → Calculators, and they will disagree — trust NPF.

Stacking: the way around the limit

If you cannot take one long exposure, take many short ones and add them together. Noise is random and signal is not, so combining N frames improves the signal-to-noise ratio by roughly √N:

The returns diminish, which is why going from 4 to 16 frames is transformative and going from 64 to 100 is barely visible. Astro Camera stacks for you on the faint presets.

Hold still

All of this assumes the phone is not moving. Handheld, at one second, you are photographing your own pulse. A small tripod is the single cheapest upgrade to your astrophotography, and it beats any amount of software. Use the self-timer so your finger is not on the phone when the shutter opens.

Framing

Sky photographs almost always want to be vertical, with the horizon low and the sky given most of the frame. Astro Camera frames at 9:16 by default and crops the saved file to match, so what you shoot is what you keep — no cropping afterwards, and it is already the right shape to share.

Leave something on the ground in the frame. A tree, a ridge, a building. A photograph of nothing but sky has no scale and no story; a photograph of a hill under the Milky Way has both.

Recipes by target

These are the settings Astro Camera applies for each preset, and why. You can override any of them.

The Moon

ISO 100 · 1/250 s · focus ∞

The mistake everyone makes is treating the Moon as a night subject. It is a sunlit rock in full daylight, and it is bright. Expose it like a landscape at noon and you get craters; expose it like a star field and you get a white blob. If your Moon photos are featureless discs, this is why.

Planets

ISO 640 · 1/30 s · focus ∞

Be realistic: a phone will show Jupiter as a bright point, and on a very good night its four Galilean moons as specks beside it. Saturn's rings need a telescope. What a phone can do well is a conjunction — two planets close together, with a landscape.

Stars and constellations

ISO 3200 · 1 s · stacked · focus ∞

The workhorse. Enough sensitivity to record the constellation shapes, short enough not to trail. Stack 4–16 frames and the noise drops away.

The Milky Way

ISO 6400 · 1 s · stacked · focus ∞

The hardest target and the most rewarding. It needs everything to line up: no Moon, Bortle 4 or darker, the galactic core actually above the horizon, and a stable phone. In the northern hemisphere the core is a summer object, low in the south — check Best Nights before driving anywhere.

If the Milky Way is not visible to your own eye at the site, your phone will struggle too. Go somewhere darker; there is no setting that substitutes for a dark sky.

Planning a shoot

Photo Planner

Pick a target and see exactly when and where it is visible — position, rise and set, and what the Moon is doing at the same time. This is where you decide whether a night is worth it before you commit to it.

Frame the Shot

A live camera overlay showing where your target will sit in the actual frame, from where you are standing. It answers the question a planner cannot: will the tree be in the way?

Horizon Scan

Record your real skyline — the buildings, the ridge, the trees — and Sidereo uses it to refine visibility. An object at 8° altitude is above the mathematical horizon and behind your neighbour's roof. Doing this once for a site you use often pays for itself repeatedly.

My Gear

Saved camera and lens profiles, so the calculators use your actual focal lengths, apertures and pixel pitch instead of generic numbers. NPF in particular is only as good as the pixel pitch you give it.

The calculators

• Exposure — 500 Rule and NPF side by side.
• Depth of field — hyperfocal distance and near/far limits.
• Field of view — what actually fits in the frame at a given focal length.
• Star trails — for when trailing is the point rather than the problem.
• Angular separation — how far apart two objects are, for conjunctions.

In the dark

Dark adaptation is real, and fragile

Your eyes take 20–30 minutes to adapt to darkness, and roughly two seconds of white phone screen to undo it. This is not a small effect — fully dark-adapted, you can see stars several magnitudes fainter than when you arrived.

Red light preserves adaptation because the rod cells that give you night vision are far less sensitive to long wavelengths. Sidereo's red-light mode tints the entire app, not one screen, because a single bright screen anywhere undoes the whole thing.

Field Mode

An outdoor dashboard with everything you need while actually observing: a compass pointer to your current target, sky-quality estimate, a session timer, and large controls you can hit with cold hands. It is designed to be readable at arm's length in the dark, which is a different problem from being readable indoors.

The sky-quality figure is estimated from Sun and Moon position and your Bortle setting. It is a good guide, not a calibrated Sky Quality Meter reading, and the app says so where it shows it.

Keeping a record

Observing is more rewarding when you keep score, and a log is genuinely useful — it tells you what you have seen, from where, and under what conditions.

• My Sky — your life list. Mark objects as found; the app tracks them by category.
• Sessions — start one when you go out, and it records what you observed and when.
• Sky Passport — achievements for milestones worth reaching, not for opening the app.
• Quests — generated suggestions for what to look for next, based on what is actually up tonight.

All of it is stored on the device with Apple's SwiftData. There is no account and nothing to sync, which also means: if you delete the app, the log goes with it.

Glossary

Altitude

Angle above the horizon, 0° to 90°. Anything under about 20° is looking through a lot of atmosphere.

Azimuth

Compass direction, 0° at north increasing eastward. 180° is due south.

Magnitude

Brightness, backwards: smaller is brighter. Sirius is −1.5, the faintest naked-eye star about 6.5, each step 2.5× in brightness.

Bortle

Light-pollution scale, 1 (pristine) to 9 (inner city).

Elongation

Angle between an object and the Sun. Small elongation means lost in the glare, however bright.

Conjunction

Two objects appearing close together. The most photographable events for a phone.

Opposition

A planet opposite the Sun from us — closest, brightest, and up all night. The best time to look.

Averted vision

Looking slightly to one side of a faint object makes it brighter, because the edge of your retina is more sensitive. Genuinely works.

Seeing

Atmospheric steadiness. Distinct from transparency: a clear sky can have terrible seeing, and stars twinkle hard.

NPF

An exposure rule accounting for aperture and pixel size. More accurate than the 500 Rule, especially on phones.