How I Plan Every Astrophotography Session Using Stellarium

The best nights are decided indoors

There are a handful of things worth knowing before the gear comes out. How high your target will climb. How much of the frame it will fill. When the sky goes properly dark, and where the Moon will be while you are working.

Every one of those is knowable in advance, and checking them takes minutes. That is the whole trick. It is not a skill you build at the telescope, it is a habit you build at a desk, and it pays back on the first clear night you use it.

I open Stellarium before every session. Not sometimes. Before every one, whether I am shooting from the balcony in Dubai or driving out to Al Quaa. It is the first thing I open and it is the reason I know what the night is going to look like before I have set anything up.

Seeing exactly what will fit

The software can draw your actual camera and telescope onto the sky as a rectangle. Give it your focal length and your sensor size and it works out the field of view for that pairing, then shows you, to scale, precisely what will land on the chip. No arithmetic, no comparing angular sizes on a web page and hoping.

The picture at the top of this page is that tool doing its job. The red rectangle is my frame at 80mm f/4.8, dropped over the Flame Nebula. The Horsehead is in there too, the dark notch to the right of the bright star. Knowing both would fit in one exposure took about four seconds, and this is how the photograph came out.

One frame is useful. Nine is a decision. Every combination I own, laid over the Flame and Horsehead in turn, which have to be kept together because they are a pair. Same target, same evening, so the only thing that really changes is the rectangle. The 135mm view is pulled back, because that frame covers far too much sky to show at the same zoom as the rest.

Ordered widest to tightest. It cycles on its own, and stops as soon as you pick a frame. Tap the picture to see it full size.

135mm f/2 S30 Pro 60mm f/4.5 S30 60mm f/6 80mm f/4.8 60mm f/7.4 80mm f/6.25 80mm f/7.5

Samyang 135mm f/2 on the Sony A7 III

Almost the whole constellation. Betelgeuse sits off one corner, Rigel off the other, and the Flame is a speck beside the Belt. This is the frame for Orion as a subject in its own right, or for the enormous faint arcs that run across it. It is not the frame for the Horsehead.

Seestar S30 Pro

Wide enough to hold the whole southern end of the Belt. The Flame and the Horsehead sit well inside, and the Orion Nebula crowds the far corner. A good frame when the subject is the region rather than one object.

Askar V, 60mm with the reducer (270mm)

The widest of the Askar frames. Both nebulae sit together with space on every side and Alnitak comfortably inside. This is the forgiving one: room for a framing error, and room for the fainter dust that surrounds the pair.

Seestar S30

Almost the same coverage as the 270mm frame. The whole region fits with a little to spare, which is a large part of why the S30 is such a good grab-and-go for this target.

Askar V, 60mm native (360mm)

Tighter. The pair still fits, but the surrounding space has gone. Worth choosing when the tracking is behaving and the framing is deliberate.

Askar V, 80mm with the reducer (384mm)

Near enough the same coverage as the 360mm, but at f/4.8 rather than f/6, so it gathers light faster. When two configurations frame a target the same way, the faster one wins. This is the one I took out, and this is how it came out.

Askar V, 60mm with the extender (444mm)

The pair now runs corner to corner. Nothing is cut off, but there is no margin left, so the centring has to be right on the night.

Askar V, 80mm native (500mm)

Both are still in, just. Over a few hours any drift starts pushing the Horsehead towards the edge, so this one asks more of the mount than it gives back on this particular target.

Askar V, 80mm with the extender (600mm)

The close-up. The Flame fills the frame and the Horsehead is pushed to the edge. This is the right choice for the Flame on its own, and the wrong one for the pair.

Watch them cycle and the decision makes itself. The 135mm is a different picture altogether, one of Orion rather than of the nebulae. The 600mm is a portrait of the Flame with the Horsehead half out of shot. Somewhere in between sit three or four frames that all hold the pair properly, and at that point the tie is broken on speed: the 80mm at f/4.8 covers the same sky as the 60mm at f/6 and collects light faster, so that is the one that goes out.

What it actually is

That is the payoff. The tool behind it is simpler than it looks.

Stellarium is planetarium software. You tell it where you are, you give it a date and a time, and it draws the sky exactly as it will appear above you. Not an artist's impression. The real positions.

The part that makes it a planning tool rather than a pretty picture is that you can move time. Wind forward an hour and watch your target climb. Wind forward to next Tuesday. Wind forward to February and see what winter brings round. The sky in the software behaves the way the real one does, so anything you can work out in it will hold up outside.

Start with the free web version

If you have never used it, do not install anything yet. Open Stellarium Web in a browser, put in your town, and have a look around. It costs nothing and there is nothing to set up.

Even now I use it for quick questions. It answers the ordinary ones perfectly well:

• What is up tonight?
• When does Orion clear the buildings?
• Where is Saturn?
• Which constellations belong to this month?
• When does proper darkness begin?

That last one is worth knowing properly. Sunset is not darkness. There is a long gap afterwards while the sky is still bright enough to spoil an exposure, and astronomical darkness, the point where it stops mattering, arrives later than most people assume. How much later changes through the year and with how far north or south you are.

The five things I check before anything is packed

This is the whole routine, and it takes about ten minutes.

What is visible. Not what is technically above the horizon, but what is high enough to be worth pointing at.

How high it gets. The altitude a target reaches decides how much atmosphere you are shooting through. Low down you are looking through a much thicker, dirtier slice of air, and from a city that slice is full of light.

When it is highest. Every object reaches a highest point and then starts back down. That moment is culmination, and it is the middle of your best window.

What the Moon is doing. Phase and position, both. A bright Moon on the far side of the sky leaves you plenty to work with. Once it is within forty degrees of your target you will want a narrowband filter, which is a whole subject of its own that I have written about in the piece on Bortle skies and narrowband filters.

How long I realistically have. Between the sky going properly dark, the target clearing the obstruction, and the target dropping too low again, there is a window. Sometimes it is five hours. From a balcony with a building on one side, sometimes it is ninety minutes, and knowing that changes what is worth attempting.

Framing is the part worth paying for

Everything else in this article is free. The framing rectangles at the top of this page are the one part that can cost money, and they are the part I would pay for again without thinking about it.

Before this I framed by trial and error. Now the composition is settled before I leave the flat, which counts for a lot on a target that only clears the rooftops for a couple of hours.

Worth being clear about what costs money, because the naming is confusing. Stellarium on a computer is free and open source, and it has this feature built in. The phone app is a different product with a free tier, and the framing tool is one of the things behind the paid Plus unlock. I use the phone version, so I paid. If you work at a desk, you do not have to.

Build the equipment library once

The setup is the boring bit and it is worth doing properly. Sit down for half an hour and enter every combination you own. After that it is permanent.

Mine currently holds nine. The Askar V is a modular scope, so it accounts for six of them on its own: the reducer, native and extender configurations, each at 60mm and 80mm of aperture. Then the two Seestar units, and the Samyang 135mm for wide work. The six Askar frames are drawn for the ASI585MC Air, the two Seestars carry their own sensors, and the Samyang is paired with the full-frame Sony A7 III. The frame is always a product of the optics and the chip together, so changing the camera changes every rectangle that uses it.

Once they exist, the question changes. It stops being "will this fit" and becomes "which of these nine frames it best", and that is a question you can answer by tapping through them while the kettle boils.

Let the target choose the telescope

Deep-sky objects vary in apparent size far more than people expect. Andromeda is six times the width of the full Moon. The Ring Nebula is about a twentieth of that width. No single focal length suits both, which is exactly why it is worth matching the scope to the target before you set up.

The same process works in reverse when a scope is already set up and I want to know what suits it tonight. Pick the frame, then scroll the sky for something shaped to fit it.

Planning around a balcony

City imaging adds a constraint the software handles better than I expected. My balcony faces one way. There is a building to one side and a wall on the other. Plenty of the sky simply is not available to me, and plenty more is only available between certain hours.

Because Stellarium knows my latitude and can run time forward, I can find the exact window when something crosses the gap I can actually shoot through. That turns a frustrating site into a workable one. It is also how I know when a target is worth waiting three weeks for, because three weeks earlier in the evening puts it somewhere different.

For the dark site trips out to Al Quaa it does a different job. Those nights are long and there is no obstruction, so the plan becomes a running order: this one while it is high, that one after it culminates, the wide field once the Milky Way core is up. Everything is decided in the warm, in advance, so the night itself is just working through the list.

It teaches you the sky along the way

The bonus I did not plan for is that I now know where things are. Not from memorising a chart, but from watching the same sky wheel round the same way a few hundred times at ten thousand times normal speed.

I know Orion is a winter target and that Cygnus owns the summer. I know roughly where to look for Andromeda without checking. That knowledge makes everything downstream faster, and it is the sort of thing that is much easier to absorb by playing with a model than by reading about it. If you want the underlying framework rather than the software, I have written about how the sky is laid out and why it moves the way it does.

Is the paid version worth it

For learning the sky and for casual use, no. The free options are complete and I would happily recommend them to anyone who never intends to photograph anything.

For photography, the framing tool alone repaid the cost on the first night it stopped me packing the wrong scope. That is a low bar for a piece of software that costs less than a filter, and it is worth checking before you spend, because on a computer that feature is free.

The summary is that Stellarium has changed my hit rate more than any single piece of hardware I have bought. Not because it makes the pictures better, but because it puts me on targets that were always going to work. Given how few genuinely good nights there are in a year here, that turns out to matter more than almost anything else in the rest of the setup.