Star Trails

Few weeks back, I was at Maini’s Hill Cottages, NataDol with a friend. Nice and cool night with no moon in sight. Once the sky was almost dark, we planned on capturing some night shots of the cottages itself. Switched on all the lights and went out to the far end of the driveway… tripod set, camera set, exposure settings checked… and we started capturing the scene. These were to be used for marketing purposes.

The roof was turning out to be particularly dark. I was not in a mood to ‘light-paint’ it. Easy way out – do a longer exposure and later burn down the brighter areas during post-processing, maybe create a HDR. I went ahead with a slightly longer exposure. The result was a nice capture of the cottages which could be used for marketing purposes.

 

Maini's Hill Cottages

(Night capture of Maini’s Hill Cottages. Everything turned out fine. The enlarged photograph looks beautiful. The only minor thing that I noticed – star trails were starting to develop. I had crossed the so called 600 rule)

 

Star-Trails Basics

There is no rocket science in capturing star trails. In fact, it is one of the easiest things to do which can garner instant wows.  All you have to do is to get the basics right.

  • Use a sturdy tripod. The camera should also be mounted in a manner that there is no movement. Recheck all the knobs and locks.
  • Choose a location which is relatively free of light pollution. Street lights, building lights, traffic… all these cause so much of light pollution that trees and birds are harmed, photography is a small thing in comparison. The light-pollution spoils the star-trail photographs too.
  • Choose a night with no moon and clear atmosphere. There should not be any clouds too. Summer day after a dust-storm is big NO.  It is simple common sense, the clearer the sky, the more visible will be the celestial bodies. Larger number of them will give more trails…. a fuller photograph.
  • For lenses, I recommend using a wide-angle to normal lens. This range will provide some foreground too and hopefully hide the movement of foreground elements to some extent.
  • Use ‘T’ mode and not ‘B’. The timed mode permits user to open the shutter once and then with another press close the shutter again. Bulb mode keeps the shutter open as long as it is pressed. This is fine as long as you are using a remote release with a lock, but on a new DSLR, opt for T mode.
  • Have a foreground where possible. It’ll add interest to the overall composition.
  • Most importantly – click raw. I’ll explain this point later on when I discuss about ISO to use.

 

What was that rule of 600?

This was made a long time back to avoid star trails. Dividing 600 by focal length of the lens being used gave an approximate idea beyond which stars will start creating star-trails in night photographs. Actually, the rule was meant to prevent star-trails by keeping the exposure below a certain time limit.

With increase in camera resolution, this rule is almost outdated and the exposure time needs to be further reduced so as to prevent star trails. I use an old camera so this rule works for me.

Anyway, we are discussing about creating star-trails and  not preventing them, so let’s go ahead with that.

 

Exposure Time required for Star-Trails

The sky rotates 360 degrees in 24 hours, or 0.0042 arc degrees per second. Find out about the angle of view of the lens that you are planning to use. A 50mm lens has an angle of view of around 45-60 degrees. To capture star trails the length of one quarter of the frame on a full-frame camera, that translates to an exposure of about 1.5 hour or so, depending on the exact angle of view. Try to figure out the calculation yourself using the first fact in this paragraph.

Longer the focal length, lesser is the time required to create star trails.

The easy way out, just note the location of one particular star and start the exposure, enjoy a beer with your friends while the camera records the movement. Come back when the beer gets over or when that particular star seems to have moved considerably.

When we are trying to photograph, we are actually interested in the linear movement that takes place across our sensor or film frame. Even though the angular movement remains the same, the linear movement captured by the camera will vary. At the celestial north (or close to north star/ Polaris), the movement will not be recorded on camera. So for good star trails, consider this fact too.

Star Trails around North Star

(The bright dot on the left is Polaris or the North Star. Notice how the movement is concentric to it and the outermost trails on the right are considerably longer than the ones closer to Polaris)

 

Noise – Enemy of Star-Trails

Since most of the background is dark, noise turns out to be the biggest enemy of star-trails. Here are some basics that I wrote about noise earlier – Remove Noise (Add Grain?). The two most important sources of noise that we are concerned about in Star-trails are – 1. due to sensor heat and 2. due to irregular amount of light hitting the sensor (shot-noise).

So, our aim is to keep the sensor as cool as possible and to ensure that the amount of light reaching the sensor is in sufficient amount to not cause shot-noise. Capturing less of noise makes the job easier while post-processing.

 

Noise Reduction

There are various methods which astrophotographers use for reducing noise. Yes, that fancy term is for photographers who indulge in photography of celestial bodies, including creation of star-trails.

In old cameras, use the native ISO of the camera, and then run noise reduction algorithms. This is by far the commonest and easiest methods to do. The only negative is the time taken by long-exposure noise reduction. It takes as much time as the initial exposure itself, forcing the photographer to loose precious time. Decision is yours. Remember that this is one of those settings that becomes a part of raw file. (Raw files – what affects them). Noise reduction will also be required during post-processing.

A slightly better technique is to wide-open the lens and let in more light than required, practically exposing the image to the right of the histogram (ETTR).

Keep the camera cool by clicking on chilling winters. This also ensures a clearer sky (provided there is no fog).

Click raw and post-process. Use noise reduction available in your software. Just note one small factor – luminance noise reduction may reduce the number of star-trails by wiping out the dull trails, so use it after you have used everything else.

Spotting or cloning is useful to hide the hot pixels, which are quite common in such long exposures.

 

Star Trails

( A 22 minute exposure on native ISO, post-processed on Affinity Photo)

 

Native ISO or Not?

Some photographers using new cameras, swear by using a high ISO like 1600 or 3200 (depending on the camera model) and overexposing the raw file. This is then corrected during post-processing. So much of light getting captured tends to reduce the noise. This is a hard fact to digest but using a high ISO actually reduces noise in such cameras.

Try experimenting with your own camera in a dark room. Set it on a tripod and manual mode. Capture an image of any dark object at a fixed combination of aperture, shutter speed and focus. Only vary the ISO across in full stops. During post-processing, change the exposure compensation to exactly the same across all images. See which ISO works best for you. (I don’t have to remind you that this works only in raw files.)

The lower ISOs have the benefit of greater dynamic range so consider this also when you decide your ISO for star-trails.

The reason is that the sensors are ISO-invariant or in laymen terms, sensor does not bother about ISO. It is the camera software that alters it. Get directly in touch with me if you want to know more about it.

 

Stacking

This is the new entrant in the world of star-trails.

When you see a person standing at one place and then after a few seconds you look at him again and the person is standing at a different place. What happened? Easy, the person moved. He moved from the first point to the second point. Our mind subconsciously told that to us.

In stacking for star-trails, something similar happens. Multiple photographs are captured. Software calculates the star trails from these multiple shots and presents a single star-trail photograph with good contrast and less of noise.

One such easy to use software – StarStaX. There are many other options too, all you have to do is search on internet. For star-trails using stacking, I recommend using one of these dedicated softwares rather than using the do-it-all common photo-editors.

 

Star Trail with Noise

(Just a simple star trail captured from my balcony. Using it as a filler to break the monotony of text)

 

… and mistakes do happen. Few nights back, after a booze party, I went ahead for some star-trails. I set the exposure to ‘X’ rather than ‘T’ (Camera modes (PSAM and more)). Pressed the shutter release and went away for a long duration. Came back and pressed the shutter release again to supposedly end the exposure. Result – two dark shots taken a long duration apart! I burst out laughing on checking the preview. Yes, stuff happens!

Star-trails are easy to create. All they require is a sturdy camera with full battery power, lots of patience and some clarity on how to reduce noise. Compose anything simple with some foreground elements and you’ll have a keeper. For me, apart from being slow-paced, star-trails also give lots of time to introspect, turn philosophical or just have good time with some like-minded friends.

 

 

2 thoughts on “Star Trails

  1. Nice, full of knowledge article-This site should be a Bible for all prospective photographers who want to hone up their skills.

    Like

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