Thirty degrees up: altitude is a camera setting

The July Milky Way never comes out the way it does in other people’s pictures. Same phone, same number of frames, a forecast that promised clear skies, and what you get on screen is a grey and orange smear with the galactic centre somewhere inside it. The camera gets blamed first. Then the site, which clearly was not dark enough. The culprit is usually a number nobody writes down: how many degrees above the horizon the target was sitting while the sensor did its work.

Air comes in layers

At the zenith, starlight crosses the thinnest slice of atmosphere available, and that slice is defined as one air mass. Lower your aim and the path lengthens as the inverse sine of the altitude. Thirty degrees above the horizon means two air masses, twenty means nearly three, ten degrees means a shade under six. Down at the true horizon the schoolbook trigonometry gives up, because the Earth curves and so does the air, and the count climbs to around forty.

Every layer costs light. In the best case anyone can imagine, spotlessly clean air at sea level, atmospheric extinction runs to 0.16 magnitudes per air mass, so a star at the zenith has already surrendered fourteen per cent of itself. At thirty degrees the loss doubles to 0.32 magnitudes, at ten degrees it reaches 0.90, and at five degrees Sky & Telescope puts it at a full three magnitudes, which is over ninety per cent of the light gone. Treat those figures as the floor. A humid August night on the plain, or the dust still hanging in the air after a hot southerly, will push the coefficient towards 0.40 magnitudes per air mass, and from there every row of the table gets multiplied by two and a half.

Half the signal, four times the frames

Compare the same star at the zenith and at ten degrees, using the textbook air of the best case: 0.74 magnitudes of difference, which is a little over half the photons. A photometrist shrugs and adds a correction column to the spreadsheet. Anyone shooting with a phone cannot, because unlike a four inch refractor a lens two millimetres wide counts its photons one at a time.

The real bill arrives in the stack. When the dominant noise comes from the sky background, the signal to noise ratio of a sum grows with the square root of the frame count, so halving the target signal while the background stays put means you need four times the frames to land back where you were. Two hundred exposures become eight hundred. One hour becomes four, the battery folds long before that, and meanwhile the target has dropped further. Forty minutes of shooting at twenty five degrees beats two hours spent chasing the same object as it slides towards the lit-up town at the end of the valley.

The background rises as the target falls

There is a second half to the problem, and it works in the opposite direction. A city’s artificial light does not leave in a straight line: the atmosphere scatters it and drops it back on you from every direction, most heavily where the layer of air is thickest. That orange dome you can see over the county town with your naked eye sits low. If your target is at fifteen degrees, it is inside it.

With a phone’s wide angle the whole thing becomes visible within a single frame. A seventy degree field pointed south holds sky at fifty degrees of altitude and sky at five in the same exposure: one and a half air masses at the top, ten at the bottom. The gradient you find after stretching was not invented by the stacking software, and no background subtraction removes it entirely, because that background is neither uniform across the frame nor steady through the session. Read the gradient before you fight it. If it climbs from the bottom of the frame and stops dead where the ridgeline begins, you already know its name.

The sky that never gets high enough

The maximum altitude an object reaches, at the moment it crosses the meridian, takes a second to work out: ninety degrees minus the difference between your latitude and the target’s declination. The galactic centre sits at declination minus twenty nine degrees. From London it culminates at nine and a half degrees, from Edinburgh it barely clears five, and from Flagstaff, Arizona it manages twenty six. That is six air masses from southern England on the best night of the year, and no amount of gear compensates. Those images of the Milky Way core with dark dust lanes and red nebulosity almost always come from Chile, Namibia or Australia, where the same object passes eighty degrees up or better.

That calculation changes how you pick targets in the first place. Nothing culminates higher than the zenith, and below a certain declination nothing is worth chasing at all from where you live. The rest is timing: every object crosses the meridian once a day, and the useful window is the two hours either side of it, while the air mass stays near its minimum. Knowing where to point is half the craft, and knowing when is the other half.

What refraction does, and what it does not do to you

As well as removing light, the air bends the path of the rays and lifts everything by a few arcminutes. At the horizon that comes to 34 arcminutes on average, which is why the US Naval Observatory defines sunrise as the moment the centre of the disc is still fifty arcminutes below the geometric horizon (34 of refraction plus 16 of semi-diameter). Put plainly: the Sun you watch coming up rose a while ago, and for a couple of minutes you are photographing a mirage. A newly risen Moon looks squashed for the same reason, its lower limb lifted more than its upper one.

Two effects of low-altitude air, on the other hand, you can safely ignore on a phone. Refraction depends on colour and separates blue from red, but the instrumentation literature puts that dispersion at a little over one arcsecond at forty five degrees from the zenith, and a pixel on the main wide camera covers about seventy, so you will never see it. Seeing degrades with air mass too, roughly as its 0.6 power, and it stays a row that hardly concerns you as long as the phone is looking at the sky on its own. Both come straight back the moment you rest the lens against a telescope eyepiece.

Shooting it anyway

Planning a session starts with transit times rather than settings. Pick two or three targets that culminate at different hours, shoot each one inside its own window, and accept that the fourth object on the list, the one crossing the meridian at four in the afternoon, is not on this month’s menu. If the target refuses to climb above twenty degrees regardless, choosing a site stops being about the Bortle number and starts being about azimuth: a mediocre sky with a clear southern horizon and no settlement in that direction beats an excellent one with a small town parked exactly where you are looking.

One last thing about the stack. Frames shot at twenty degrees of altitude and frames shot at forty carry different background levels and different gradients, and mixing them into one sum means averaging two skies together. On a long night, split the take into blocks and treat them separately: the block that sank lowest usually gets binned, and binning it improves everything else.

The number you need appears in no camera menu. It sits in your planetarium app under altitude, and it changes on its own while you stare at the screen.