Astrophotography is an amazing art that allows us to have a glimpse of the depths of the universe. Through it, we are able to capture wonderful celestial images that we wouldn’t see with the naked eye.
However, there are many factors and parameters that affect the quality of the images we capture through astrophotography. One of these factors is exposure time.
In this article, I will be discussing all about Exposure Time for Astrophotography and how it affects the quality of images you get.
I will go deeper and look at the significance of exposure time and the role it plays in capturing stunning celestial images. I will also tell you what factors affect exposure time and how to find the perfect exposure time for your camera. Let’s go ahead and get started.
What is Exposure Time, and Why Does It Matter?
Also referred to as the shutter speed, exposure time refers to the duration the sensors or film of your camera is exposed to light. It is how long the camera’s shutter is open to allow light to reach the sensors so you can capture images.
Exposure time is a critical factor in astrophotography because it determines how much light your camera is going to collect from the night sky. Unlike in daytime photography, where you can easily capture images because of the available light, astrophotographers need to get the right exposure time to capture the perfect images because of the limited lighting.
Now, there are a few reasons why exposure time in astrophotography matters a lot. They include:
The Choice of Exposure Time Affects Image Quality
The exposure time you choose will significantly affect the quality of images you will get at any given time. Often, shorter exposure time will lead to pinpoint stars. I find such exposure time ideal for capturing detailed and sharp images without getting any apparent star movement.
Longer exposure time will allow you to gather more light, hence more data of the deep night sky, creating mesmerizing images.
However, longer exposure time can lead to star trails.
A star trail happens when the rotation of the Earth makes the stars appear as streaks across the sky. Astrophotographers often use such techniques as stacking and dark frames to combat this problem while still retaining longer exposures.
Signal-to-Noise Ratio
Exposure time also affects the signal-to-noise (SNR) ratio of your images. SNR is simply the ratio of the signal of the image you get to the power of background noise.
To better understand this parameter, say you collect 64 photons in an image, and you have a background noise count of 8. So here, the signal-to-noise ratio is 64:8, which may also be expressed as 64 over 8, which is 8. That’s too much noise. Now, say you collect 6400 photons, and the background noise is at 80. So, the SNR is now 80.
This example illustrated the significance of longer exposure time. The higher the exposure time, the more light you gather. This also means the SNR count will increase, and therefore, you are likely to get better images.
Choosing the Right Exposure Time for Astrophotography
From the foregoing, it is clear that choosing the right exposure time plays a key role in the quality of the images you capture. But how do you ensure you get it right?
There are various elements that you must consider. Here are a few of them:
The Object(s) You are Photographing
Different celestial objects require different exposure times. Smaller and fainter objects may require a shorter exposure time than brighter and larger objects.
For instance, if you are imaging the Andromeda Galaxy, you may need longer exposure time than you would need when capturing the moon.
The Equipment You Use
Another key element is the equipment you are using. The features of the camera you are using such as the sensor and aperture of the lens, will affect the exposure time.
Cameras with higher ISO sensor settings and wider apertures will capture more light, so you may need shorter exposure times.
Sky Conditions
Atmospheric conditions, the phase of the moon, and light pollution are some of the sky conditions that may determine the exposure time to use. For instance, when there is a full moon, you may need to use a shorter exposure time, and higher levels of light pollution may necessitate a longer exposure time.
Personal Preference
The exposure time you choose will also be affected by your personal preference as well as the outcome you need. Some astrophotographers prefer longer exposure times so they can collect as much data as possible.
Others prefer shorter exposure times to capture pinpoint stars.
Exposure Time in Astrophotography: The 500 Rule
Among the easiest ways to avoid star-trailing and capture clear and sharp images on a stationary tripod mount is using the 500 rule.
The rule is used when measuring the maximum exposure time required to make a shot before a star trail appears or before the object you are photographing becomes blurry. Having a longer exposure time than the 500 rule allows will often lead to appear blurry.
This rule works best when you are using a fixed tripod. It works for focal lengths of up to 200mm and is especially effective when imaging the Milk Way using a wide-angle camera lens.
There are a few reasons why many astrophotographers use the 500 rule. First, it tells you the maximum exposure time you are going to need before whatever you are photographing becomes blurry.
Second, the rule is very easy to use, and third, it actually works. You, however, need basic math knowledge to use this rule.
That said, there are various online Exposure Time Calculators that use the rule to calculate the exposure time for you.
The 500 rule is:
Exposure Time = 500/ Focal Length x Crop Factor.
To get the maximum exposure time, take the focal length (in millimeters) of your camera and multiply it by the crop factor. You then divide 500 by the product. What you get is the maximum exposure time.
The crop factor is typically 1 for any full-frame camera, 1.5 for any APS-C camera, 2 for micro 4/3 cameras, and 2.7 for a compact camera with a 1-inch type sensor.
For instance, if we are using a Canon EOS 60DA camera (APSC-C sensor) with a 50mm focal length, then our maximum exposure time will be:
500/ (50mm x 1.5) = 6.67 seconds
This rule works because it gives you the ideal shutter speed or exposure time that you will need for different types of cameras so you can get the clearest images.
It serves as a point of reference for the duration you should expose your images using the camera’s system. While it is not an exact science, many astrophotographers have used it, and it works.
Using the Exposure Time Calculator for Astrophotography
You do not need to do any manual calculations to get the required maximum exposure time. The exposure time calculator will solve that for you.
This tool will give you the exact exposure time you need to get clear and star-trail-free images. But how do you use it?
- You first need to know the focal length of your camera or telescope. You can get it in the telescope’s specifications. Enter this figure in millimeters.
- Next, get the crop factor. The crop factor will depend on the type of camera you are using.
- You then need to indicate whether you are using a tracking mount.
- Lastly, click “calculate exposure time,” and voila, the tool will give you the exposure time in seconds.
It is important to note that the 500 rule is a general guideline. All things considered, it should give you the right exposure time for you to get the clearest images possible.
However, real-world factors such as light pollution and atmospheric conditions will have an impact on the results.
Exposure Time Calculator for Astrophotography
Long vs. Short Exposure Time in Astrophotography
Depending on your preferences and the factors discussed above, you are going to either choose a longer or shorter exposure time for astrophotography. Both have pros and cons.
Long exposure time ranges from a few seconds to several minutes or even hours. It provides you with dynamic and magnificent images of the sky as it allows your camera sensor more time to capture light.
Longer exposure time also helps to limit noise, improving the quality of the images you capture. It makes it easier for you to discern between various features in the image.
However, I must mention that longer exposure may also lead to star trailing. You can minimize star trails by using tracking mounts to easily follow the movement of the stars.
Short exposure time entails shooting images within a duration of less than a few seconds. This exposure time is most useful when imaging bright objects such as planets and the moon. It produces clear and detailed images.
Shorter exposure time also helps avoid star trails. It is, therefore, perfect when photographing static celestial objects such as constellations.
One of the downsides of a shorter exposure time is that it is not appropriate for capturing faint objects. It is also prone to higher noise counts, which manifest as color variations and random speckles.
Techniques for Long Exposure Time in Astrophotography
If you like capturing faint objects in the night sky, then you are going to need longer exposure times to get the best shots. However, it is not always easy to get the best images, even when using long exposures.
I put together a list of various techniques I frequently use to achieve cleaner and more detailed images.
Try Star Trailing
While this has previously been discussed as a downside of long exposure time, you can actually get mesmerizing images with this technique. Star trails add a sense of drama and motion to your images, creating an artistic and striking composition.
You are, however, going to need a stable tripod as well as a precise camera location to get the best star trail images.
Panoramic Astrophotography
Here, you will be aiming at getting a wide-angle view of the night sky. I find this technique a great way to capture a huge scale of particular celestial objects such as a galaxy or nebula.
Star Stacking
This is when you combine multiple shorter exposure photos to form a final image with more clarity and greatly reduced background noise. This technique is most effective when imaging faint objects like galaxies that you would otherwise not see with a single short exposure.
Time-lapse Astrophotography
This refers to capturing many shots at predetermined intervals. The shots are taken over a particular period and then assembled to create a time-lapse video.
I use this technique to capture the movement of celestial objects such as stars over time.
Exposure Time for Astrophotography: FAQs
Are longer exposures better in astrophotography?
Longer exposure time is suitable when capturing faint celestial objects. It allows more time for camera sensors to capture light, and therefore, the images are clearer and sharper. However, a longer exposure time can also lead to star trails.
Is a shorter exposure time better than a longer exposure time in astrophotography?
A short exposure time is perfect when imaging brighter celestial objects such as planets and the moon. It can also produce detailed and sharp images. The only downside is that it is prone to higher background noise.
How long should my exposure be for astrophotography?
How long your exposure time should be will depend on various factors, such as the object you are photographing, sky conditions, and the equipment you are using. However, you can use the exposure time calculator to get the maximum exposure time you will need.
Final Thoughts
Exposure time is a crucial factor you cannot underestimate when it comes to astrophotography. Knowing the right exposure time to use gives you a better chance of getting the best and clearest images possible.
The tips and techniques discussed in this article should get you started with getting the right exposure time for your equipment.
