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Beginning Astrophotography:
Star Trails to Nightscape Photography

by Roger N. Clark

Astrophotography is the photography of the night sky, or detailed images of the Sun. This article is to get you started in night sky photography using simple methods, lenses and cameras. Start here if you are new to this type of astrophotography.


The Night Photography Series:


Contents

Introduction
Different Types of Astrophotography Basic Settings and Choices

Focus
White Balance
Star Trails
Exposure Times to Keep Star Images Round (The 200 Rule NOT the 500 Rule)
Light Collection
Star Fields from a Fixed Tripod
Stacking, Tracking and Mosaics
Exposures at Night on Land versus Sky
Safety
Discussion and Conclusions
References and Further Reading

All images, text and data on this site are copyrighted.
They may not be used except by written permission from Roger N. Clark.
All rights reserved.

If you find the information on this site useful, please support Clarkvision and make a donation (link below).


Introduction

Using any camera, film or digital, you can make simple exposures of the night sky to produce pretty images. It is really very simple, but a lot of online guides have misinformation and misconceptions. In this article, I'll try and clarify the misconceptions and get you started to help you become more advanced.


Figure 1. Star trails and beautiful acacia trees, Tanzania, Africa, on the edge of the Serengeti. The trees were illuminated by low-wattage incandescent lights from nearby cabins. The north celestial pole is off the left frame edge, in this view looking northeast. A stock 8-megapixel camera 1.3x crop digital camera from 2007 was used, with sensitivity well below more modern entry levels DSLRs and Mirrorless cameras of today. On the camera was a consumer zoom lens, a Canon 28-135 mm IS lens at 28 mm, f/3.5. ISO 800. Fourteen 1-minute exposures were added together to show the star trails over 1/4 hour.

Different Types of Astrophotography

There are many different kinds of astrophotography. Here is a partial list in order of simple to advanced. This beginner's article will cover star trails and nightscapes. Constellations make up the star fields in a nightscape. The moon is covered by examples in my Moon Gallery, the sun and deep sky photography are covered in detail in this series. Photographing the sun requires special filters. See How to Photograph the Sun. Sections 3 and 4 cover deep sky astrophotography. Photographing planets will be a future article.

Basic Settings and Choices

Night sky photography of constellations, stars, or the Milky Way, is a low light situation that requires specific methods for best results.

Film versus digital. Digital cameras are typically 20 to 50 times more sensitive than film cameras, even high speed film. Film works great for star trails, but is more challenging to record faint details in the night sky. The discussion below is mainly for digital cameras but film will be included in the star trails section.

You will need:

Given the above, here are simple settings to start making some night sky images. We will start with star trails, which are the easiest images to make.

Focus

My method of focusing is to find a set of stars from bright to dim that are easily seen on the camera LCD in live view. Place the stars about half way from center to edge. Zoom in with live view to the maximum magnification you can get. Focus so that there are no color fringing around the bright star and the faint stars show well. Boost your ISO if your aperture is small in diameter. I sometimes use ISO 6400 to 12800. Don't forget to change the ISO back to a lower value, like ISO 1600.

To focus:


Figure 2. Images of a star field on the back of a camera LCD screen (Canon 6D Mark II) with a Sigma Art 35 mm f/1.4 lens at f/1.4. The stars were placed about half way from center to left edge. Avoid focusing on objects in the center, The live view image was magnified 10x and I examined the image with a 3x magnifier. Note the fainter stars marked S1 and S2 as focus changes they show brightness changes better than the brighter stars. Move focus on each side of optimum to understand the aberrations better. Avoid color halos like that in panel h. The sequence a-h moves from one side of focus to the other, with d, e, and f in good focus. Panel e shows very slightly better focus than d and f.

White Balance

Daylight White Balance. Why daylight? After all, it is night. First, we evolved with the sun as our light source, so daylight white balance is natural. When we view other planets in the Solar System, as well as our Moon, they are illuminated by sunlight. The colors we see on these bright objects are the same ones recorded by a digital camera with daylight white balance. So too are stars: solar type stars come out white to yellow-white, hotter stars are bluish, just like we see them, and cooler stars than our sun show yellow, orange and red colors just like we see them with unaided eyes, through binoculars, and through telescopes. Brighter nebulae also show colors in good sized telescopes: pink/magenta from hydrogen emission, to green from oxygen emission, to blue from scattered light (like our daytime skies are blue from scattered sunlight). The night sky is full of wonderful colors. See the series on color starting here: Part 2a) The Color of the Night Sky.

Light pollution and airglow needs to be subtracted. Sites that teach changing white balance are not describing how to remove Light pollution and airglow--it only change the color. The when contrast is boosted, the result is fake colors produced in post processing and variable white balance with scene intensity. See Parts 2d, 2e and 3 for more details on post processing to maintain natural color. Any image you see with a blue Milky Way or blue fringes to the Milky Way are not real colors. If you like such colors, that is OK, just realize they are not natural.

Star trails

Figures 1, 3, 4 and 5 illustrate the huge flexibility in exposure strategies for making star trail images. The film based image in Figure 3 was a single 3.6 hour exposure at ISO 100. Such long exposures may not work with digital cameras, and are not necessary. Multiple shorter exposure enables one to choose the frames that go into the total. That enables you to exclude one or more frames in case of a problem, like someone shined a light on the camera, so only one frame is lost not the entire multiple hour exposure. The star trail image in Figure 4 was made with short exposures of 20 seconds each, and the images added in an image editor by choosing the maximum value from each frame. The advantage of this strategy is also that each frame had minimal star movement so the star field could be shown in one image or a stack.


Figure 3. A star trail image made with ISO 100 (Fujichrome Velvia) film with a 24 mm (35 mm full frame equivalent lens). Exposure was 3.6 hours at f/4.5 (a single exposure). (The camera was a 4x5 view camera with a 90 mm lens and while this is an old film camera and lens that can be bought today for a couple of hundred dollars, a modern consumer digital camera can make similar or better images with consumer lenses in the f/4 range.) This image was made near solar minimum when airglow was very low. The sky shows as blue and green. The blue is from starting the exposure in deep twilight. The green may be due to airglow or film reciprocity failure. Fujichrome Velvia is daylight white balance and saturated colors. Note the different colors of the star trails.


Figure 4. Star trail image made with a Canon 7D DSLR, a 1.6x crop camera, with a Sigma 15 mm f/2.8 lens on a fixed tripod during the August 12, 2018 meteor shower. A total of 306 frames, each 20 seconds long were made at ISO 3200, f/2.8, over a 2.1-hour period.


Figure 5. Star trails from the Park Avenue viewpoint, Arches National Park, Utah. The image was made with a Canon 7D Mark II 20-megapixel digital camera, a 1.6x crop camera, with a Sigma 15 mm f/2.8 lens on a fixed tripod. Thirty three 2-minute exposures made at ISO 800, f/3.5, over about 68 minutes and combined for this image. Note the varied star colors. This is a natural color image. There was no added light on the landscape, only that from the night sky. Frames that had some light on the landscape from passing cars were rejected. Larger gallery image with more details.

Exposure Times to Keep Star Images Round (The 200 Rule NOT the 500 Rule)

The challenge in night sky photography from a fixed tripod when you want sharp stars is the keep exposures short enough to keep stars round. You will read online about the "500 rule" which states that the exposure time = 500 / focal length in mm. For example, for a 50 mm lens, that would be 500 /50 = 10 seconds. But the 500 rule was made in the area of high speed grainy low resolution film. It works fine if you only want web sized images, but not if you want larger sharp images. Examine the images in Figure 6 which shows various exposure times. The so-called 500 rule results in short star trails, which also blurs image detail. Note the pinkish nebula next to the brightest star--it is blurred with little detail with 400, 500 and greater "rules." For crisp round stars, a "200" rule is needed. This rule is not affected by format size; rather the detail is affected more by lens sharpness and pixel size than format size. (Smaller format cameras sometimes but not always have smaller pixels.)


Figure 6. Demonstration of how long an exposure time can be when it shows trailing for a star field near the celestial equator. These are out of camera jpegs with no sharpening, except the last panel, which was produced from a raw file. For a camera with 4 micron pixels, times would have to be about (6.6/4 micron pixels=) 1.6 times shorter for the same length trails in pixels. Note, it is pixel size and lens focal length that determines trailing, not sensor size.

Light Collection

You may read online to use a short focal length lens to maximize exposure time from a fixed tripod. But exposure time is only part of the equation of light collection. The other and more significant part of the equation is lens aperture area. Light collection from an object in the scene, e.g. a star, a galaxy, a Milky Way star cloud, a distant mountain, a tree, or any other object is the product of exposure time times lens aperture area. Not f-ratio. The f-ratio tells light density, like photons per square mm per second, but not total light received from an object. See Part 1c) Characteristics of Best Digital Cameras and Lenses for Nightscape and Astro Photography for more detail. The bottom line is choose the focal length that best frames your intended subject for a good composition, just like you would in a daytime landscape scene. Then for that focal length, use the largest aperture you have that will give acceptable image quality.

Keys to lens selections in low light photography:

Star Fields from a Fixed Tripod

Figure 7 illustrates that a single short exposure can record many stars and a lot of detail in the Milky Way. Such simple methods can make excellent web-sized images. But with so little light collection, large prints or presentation on a large monitor (like a 4K or 8K monitor) will show a lot of noise. To reduce noise, one must collect more light. The can be done with existing lenses by tracking the stars, or taking multiple short exposures from a fixed tripod as discussed in the next section.

Choice of lens field of view is a personal compositional choice. The advantage of a shorter focal length lens is that more of the Milky Way may be in view, but the disadvantage is that those elements, from Milky Way star clouds to foreground landscape objects will be smaller in the frame. Follow the above settings summarized below.

Summary setting for fixed tripod night sky photography.


Figure 7. A single 10-second exposure at ISO 1600, with a 24 mm f/1.4 lens at f/2, 1.6x crop field of view. The image is from an out-of-camera jpeg with a small contrast boost with the curves tool in a photo editor. The white balance is daylight for natural color. A 10-second exposure with this lens is a "240" rule.

Stacking, Tracking and Mosaics

Stacking refers to adding two or more exposures together to reduce noise. Noise will be reduced as the square root of the number of frames averaged. Tracking allows one to make longer exposures with no star trailing. There is little difference between the two methods given the same total exposure time. For example, two 10-second exposures will show almost the same amount of noise as one 20-second exposure.

An advanced solution for keeping stars round using a fixed tripod is to take 4 or so images with a "200" rule, align the images and average them. That will produce essentially the same lower noise result as a long exposure but without the blur from the Earth's rotation. Note that with wide angle lenses, alignment is difficult after a minute or so of exposures because the distortion changes over time--distortions from mapping the spherical sky onto the flat sensor plane. That is called projection distortion and is not fixed by lens corrections.

The solution to the above projection distortion problem is to use a tracker. A simple hand-cranked tracker can be made for a few dollars that works for 50 mm and wider focal lengths and is described in Part 5) Nightscape Photography with a Barn Door Tracking Mount. A low cost motorized mount like the iOptron Skytracker Pro is used by many nightscape photographers and works well with lenses up to about 135 mm. More sophisticated mounts are described in Part 1f) A Very Portable Astrophotography, Landscape and Wildlife Photography Setup.

Mosaics or stitching. Another way to collect more light is to use a longer focal length lens with a larger physical aperture. For example, a 15 mm f/2.8 lens has a 5.36 mm diameter aperture. A 35 mm f/2 lens has a 15.5 mm diameter aperture. The 35 mm lens collects (17.5 / 5.36)2 = 10.6 times more light. From a fixed tripod, the exposure time with the 35 mm lens would have to be shorter, (15/35) = 0.43 times as long, thus the net gain in light collection would be 10.6 * 0.43 = 4.5 times. That is very significant. One would need to do about a 3x3 frame mosaic to cover the same field of view. The result would be an image with many more pixels and for the same sized print, smaller stars, finer detail, and less apparent noise. (This improved image quality works for daytime photography too.) The disadvantage is more post processing, including learning mosaicking software.

The disadvantage of stacking, tracking, mosaics or all three combined for nightscapes is that while the stars move, the land stays fixed from our perspective. Thus, one needs to separate land and sky and treat them separately in a photo editor. This is advanced image processing, and is not for a beginner unless you are an expert with your photo editor (e.g. photoshop or gimp).

The advantage of tracking, stacking and mosaics is better image quality. Compare the first and last panels (tracked) in Figure 6 to the other panels (no tracking). Another advantage is that by combining with a clipping method for outliers, passing airplanes and satellites can be automatically rejected. See Stacking Methods Compared for more information. Figure 8 is a mosaic and shows pinpoint stars in the center as well as in the corners.

Exposures at Night on Land versus Sky

The night sky is brighter than the landscape illuminated by the sky. An example is Figure 7, which shows distant mountains very dark compared to the sky. One alternative is the illuminate the landscape, called light painting. Figure 1 is an example, though the light was from a nearby lodge so I did not add any light myself. Note the distant trees in Figure 1 are dark--a telltale sign of a local artificial light is illuminating the scene.

To record the landscape in natural light requires an exposure typically 4 to 6 times longer than exposures than for the sky (Example in Figure 8).


Figure 8. Summer Milky Way Nightscape with the San Juan Mountains of Colorado in natural color. A mosaic of 30-second tracked exposures on the sky and 2-minute exposures on the land were combined. The red and green sky is from airglow: oxygen molecules emitting light in the upper atmosphere. See gallery image for more detail.

Safety

AFRICA WARNING!

The image in Figure 1 was taken in east Africa in 2007. In February 2013 I was photographing at the same location. We got a lesson in how dangerous such imaging can be. One morning a colleague in the next room went out at 5:15 am to do some nightscapes, and I was going to follow a minute later. As I was about to open the door, when I heard a fast scramble and someone banging into a door. It was my colleague running to get back in. I went out to see what the problem was, and he yelled at me get back in, leopard! A leopard started stalking and then was in the process of charging at him and he then ran back to the room. If he had been a few more steps from his room, he might not have made it to safety.

The lesson is whenever you are in a region that is potentially unsafe, including bears, lions, or other wild animals, hire guides to watch and protect you and pay them well. A photo is not worth your life. A photo is also not worth the life of an animal.

Discussion and Conclusions

Night sky photography can be relatively simple, for example, star trails or a single short exposure. A single short exposure can make interesting web sized images. Light levels of the night sky away from cities are so low that noise from collecting little light usually dominates the image. The light signal itself has noise, even with perfect cameras, and such noise is called photon noise or shot noise. The only way to reduce this apparent noise is to collect more light. To collect more light requires tracking, stacking, mosaics, larger lenses, or combinations of all of these. The resulting sets of images requires post processing to put them all together. It is best in my opinion to work up to this slowly. Begin simple, and work up in complexity to get better quality as your experience and skill grows.

Start with star trails and simple single frame images with the foreground in silhouette. Try light painting, but see the section
Part 001) ETHICS in Night Photography first.

For more details on ISO, collecting light, effect of pixel size and aperture, see
Part 1a) Nightscape Photography with Digital Cameras and
Part 1c) Characteristics of Best Digital Cameras and Lenses for Nightscape and Astro Photography.

For information on planning, see:
Part 1b) Planning Nightscape Photography.

Once you have collected your images see
Part 3a1) Nightscape and Astrophotography Image Processing Basic Work Flow for details on processing.
For help removing light pollution, see
Part 3c) Astrophotography Image Processing with Images Made in Moderate Light Pollution.


If you find the information on this site useful, please support Clarkvision and make a donation (link below).


References and Further Reading

Clarkvision.com Nightscapes Gallery.


The Night Photography Series:


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http://www.clarkvision.com/articles/beginning-astrophotography/

First Published May 31, 2019
Last updated May 31, 2019