Startrail technique

Startrail technique

This post is also available in: itItaliano

Welcome to this tutorial dedicated to one of the most singular and spectacular techniques of night photography: the startrail.

The term derives from the English star + trail, or “path of the stars”, in practice the luminous path that the star leaves as it passes through the sky.

Startrail photography theory

But how are these trails born?

Some will say that the stars are moving in the sky, but this is true according to our perception of terresti. And so the ancients believed it was. In reality, science has explained to us that the sky is still and it is our Earth that rotates on itself, on its axis.

In fact, the Earth performs a complete rotation within 24 hours, thus originating the cycle of the day. This is why the night sky appears full of stars … that walk.

From now on, let’s say that the sky is rotated, just for the sake of explanation in this tutorial.

So, if this sky “rotates”, it will have to rotate (as it does for all the rotations) around a center, a pivot of rotation. It is called the Earth’s axis and passes – like a hypothetical line – through the poles. In Italy this hypothetical line, if extended indefinitely beyond the North Pole, would end up on a star.

This star is in fact called “Polar Star” and is part of the Ursa Minor constellation . The star, briefly named “Polaris”, has been famous since ancient times precisely because it indicates the North and has therefore always been of enormous help for orientation in travel and navigation in the times when the modern navigation aids did not exist. .

How to find Polaris in the night sky

But even today, even if you are deprived of any electronic instrument or compass, thanks to Polaris you can orient yourself very easily. Look in his direction and spread his arms crosswise: in front of you you will have the North, behind the South, to the right East and to the left West.

But let’s get back to our startrail.

We have said that the stars “rotate” in the sky around the Polar. But how do they rotate? Easy: counter-clockwise, ie they rise to the east and go to the west, just like the Sun and the Moon. But this is obvious, because they too are not really moving in the sky, but they always appear to us because of the Earth’s rotation. It is therefore natural that both the stars, the Sun and the Moon follow the same direction.

From this we understood that if we were able to record in some way the movement of the stars, we will see them draw the curved lines (circles) around the Polar moving from East to West.

But we have the tool and it’s the camera!

fotografia startrail

And here it is startrail.

How to photograph startrail

When using the film it was very easy to make startrail photography: just leave the shutter open for a very long time and the trails were beautiful and ready on slide. Things are very different with digital photography.

If it is true that the stars “move” in the sky, it is also true that they do it with great slowness, and their movement is practically imperceptible to the naked eye. In fact, if you reflect, they must travel the inner circle in 24 hours, so that their angular velocity is very small.

On average, the time to have a decent startrail, that is, with trails long enough to be beautiful, goes from an absolute minimum of 1 hour to the whole night.

The total shooting time depends very much on the lens used. The more you are on the wide-angle lens and the wider the framed sky will be; therefore, the length of the trails must be greater to be evident. From this it follows a very long total shooting time.

Startrail with 12mm super wideangle, 2 hours

If instead you are using a telephoto lens, then the same reasoning as before allows you to get obvious trails in shorter total times. The portion of the framed sky is in fact reduced, so the movement of the stars is more evident and the stretch traveled greater with the same time compared to a wide angle.

Startrail at 70mm, 40 min.

But if with the film was enough to open the shutter in B for 1 or 2 hours, and the development of our startrail was nice ready on the film, we know that with digital cameras we can forget B poses of 1 hour. Both for reasons of noise, and for reasons of sensor cooking.

The method used to obtain such long total times is in a technique, a workflow that combines shot and post production. It splits the shot that should be – for example – 1 hour in many partial shots of 30 seconds, which is the maximum time allowed by the camera.

NOTE – it is essential that the shots are made a continuous sequence, without interruptions between them, as if it were a “burst” in the sports photo.

To make a startrail for 1 hour, we must therefore create a sequence of 120 shots of 30 seconds each.

  • 1 hour = 60 minutes
  • shutter speed = 30 seconds
  • total number of shots = 120

If instead we wanted a total time of 2 hours we should do 240 shots, and so on.

Once we have our sequence of shots the:

  • we import on the PC
  • we develop in LR

At this point we have the complete series of images that make up our startrail. If we open any one we notice that the stars do not appear as points, but rather as dashes. At 30 seconds their movement, even if very slow, begins to be evident.

At this point comes into play a free software (available bilingual English and German) named startrail.exe

http://www.startrails.de/html/software.html

Download it to your PC and launch it.

To use it, just upload the photos of your sequence and tell the software to merge them into a startrail. He thinks about the rest. Once finished you only have to save the final photo obtained from the merger.

If you do not want to use this software you can also use other programs available on the web, such as Starstax. Or the same Photoshop, loading all the photos as layers and activating for all the levels the method of fusion in summation of light called “lighten”. However, this process requires powerful PCs, since PS will have to load at least a hundred levels.

Startrail photography equipment

In theory, a camera with a suitable lens, a tripod and a remote control are enough to get a perfect startrail. In theory, but we know that often the theory then clashes with reality.

So let’s see in detail the individual elements and specifications that must have.

Camera for startrail

Considering that startrail it is night photography, the camera must be able to work well at ISO 6400. Even if in special situations it may happen to work at 3200 or even 1600 ISO, the value of 6400 must be able to be managed in an appropriate manner and without excessive noise.

Lens for startrail

There are no specific indications, you can go from super wide-angle to telephoto, through fisheye, wide-angle and normal. What changes is the total shooting time, which increases as the field width increases.

Tripod for startrail

Stable, only really important consideration.

Remote control for startrail

Fundamental, unless the camera has some function for the sequence of shots. You do not need an interval timer, because – as mentioned before – the sequence of shots of the startrail must always be continuous and without interruptions.

Live Composit for startrail photography?

Lot of guys know the Live Composit feature exhisting on some cameras.

What is it?

An automatic function that adds the light of successive shots directly in camera. In practice, the camera performs a sequence of shots but instead of saving a RAW for each shot, adds the light of each of them on a single shot, which is displayed in real time on the display and then saved as the only RAW.

The temptation to use it for a painless startrail, that does not create a very heavy folder with hundreds of RAW, is so great. But don’t do it.

But not because it doesn’t work, on the contrary, it works very well.

Don’t do because night photography is always full of unknowns, which always creates problems.

  • planes flying in the scene
  • passing car lights that illuminate the ground or trees in the frame
  • friends or other photographers who light up flashlights or cell phones

These are the classic events, very frequent, that ruin one or more shots in the series of photos for night photography.

Now, if you have made a series of single shots it will be easy to edit individually those with errors to bring them back to cleaning and use them to build the startrail.

But if you have used a Live Composit method it will be completely impossible to work on the final shot to remove, for example, the airplane  trails or the friend who lit the torch.

Summary: workflow to create a startrail photograph

  • camera on a tripod
  • frame the image
  • set exposure data adapted to the shooting situation
  • focus on infinity
  • start of the shooting sequence
  • RAW download on PC
  • RAW processing with LR
  • use of startrail.exe or other software