If you are like me, you spend most of your time photographing deep-sky objects from your backyard. While I love a good dark sky trip, the backyard is where the majority of my astrophotography projects take place.
The backyard is a convenient option for an impulsive night of astrophotography, but what are we missing out on when we set up our telescopes at home? How much better could the images be if they were taken under a dark sky?
In this article, I’m going to show you a head-to-head comparison of the same nebula taken under two very different skies. One of them was captured from a dark sky site (Bortle 3), and the other one was taken from my backyard (Bortle 6).
For a true head-to-head comparison, I used the same telescope, the same camera, and the same settings to capture the object. Everything was the same for each image, except the sky quality.
How Dark is Your Backyard?
Do you know how dark your backyard is? The most common way of describing the light pollution levels of a location is to use the Bortle Scale.
According to the light pollution measuring tools I use, my backyard is considered to be a Bortle Scale Class 6. This appears as a yellow zone on the light pollution map.
While my backyard appears to be rural on camera, our house is located at the edge of town of a medium-sized city. It’s light-polluted, but it could be a lot worse.
You can use the light pollution map to see the light pollution levels in your area, or even measure the brightness yourself using a Sky Quality Meter.
We measured the brightness of our backyard sky using a Uranus Meteo Sensor.
The Bortle Scale
Most people I talk to who do astrophotography from their backyard are in the Bortle 6-8 range. This is a very common level of light pollution for people living in the city.
If you live in a large city or are close to downtown, the light pollution may be even worse. I created this graphic to show what the Orion constellation looks like in the sky from a Bortle 1-8.
If you’re lucky enough to live somewhere dark (Bortle 4 and below), you get to experience nighttime. You get to see the Milky Way and a sky full of stars, from the comfort of home.
If you live somewhere like this, I hope you appreciate how rare it is, and do your part to make sure it stays that way by using responsible outdoor lighting.
While a bright backyard sky is unfortunate, there are still plenty of great projects you can take on under skies like this, so don’t let it get you down.
My backyard is a Class 6 on the Bortle Scale (Suburban City Sky)
Using Light Pollution Filters
You can use filters to ignore a lot of the light pollution around us. There are many great options available now, including multi-bandpass narrowband filters that isolate all the good bits of most nebulae.
One of my favorite light pollution filters is the 2″ Optolong L-eXtreme filter. This filter allows wavelengths of light in two important 7nm bandpasses to reach the sensor, H-Alpha and OIII.
This is a great choice for astrophotographers in the city who want to capture emission nebulae and supernova remnants. The image data is punchy, with a distinct separation from the object and a city sky.
But what about when you want to capture a natural-looking image of a target without using harsh filters? Many broadband deep-sky objects (such as galaxies and reflection nebulae) can not utilize dual-band filters for a better image.
Shooting without any filters from a light-polluted backyard is challenging, but it can be done. In fact, I prefer to capture certain targets this way, and to deal with the light pollution during the image processing stages.
A Head-to-Head Comparison
The deep-sky object I used for the comparison is the jaw-dropping Horsehead Nebula in the constellation Orion. This area of the sky includes a variety of deep-sky nebulae types including dark nebulae and reflection nebulae.
I used the exact same camera and telescope Ashley used to take her photo from a Bortle Scale Class 3 site last month. It’s an ultra-portable, wide-field astrophotography kit that can be used for a wide variety of projects.
While I did my best to ensure that this is an ‘apples-to-apples’ test, some slight variables may have affected the differences in quality between the two datasets.
Matched Settings
- Sub-Exposure Time (3-minutes)
- Gain (158)
- Total Integration (3 hours)
- Moonless Night
Variables:
I’ll compare the same exposure times and a matching overall integration to truly reveal the differences in sky quality. While the object was a little higher in the sky from the dark sky site (which can lead to a clearer image) both image examples were captured on a moonless night.
Normally, I would use different camera settings from the city than I would at a dark sky site, but for comparison’s sake, I kept them the same.
The Astrophotography Kit
The setup I used included a William Optics RedCat 51 III refractor (WIFD) and a ZWO ASI294MC Pro. This wide-field deep-sky astrophotography setup captures images at a focal length of 250mm.
The camera and telescope were mounted to a ZWO AM3 strainwave drive mount, essentially a smaller ZWO AM5. Just like Ashley did at the Winter Star Party, I used an ASIAIR Plus to control the camera and automate the imaging session.
The camera and telescope used for the project. RedCat 51 III and ZWO ASI2400MC Pro.
Equipment Used:
Results
It should be no surprise that the dark sky data was much better overall, pixel for pixel. However, it may shock you to see the massive improvement only 3 Bortle Scale classes make.
In the graphic below, you can see how much more detail was captured in a single sub-exposure. Both images were 3 minutes long, without the use of a filter.
The images are single, uncalibrated exposures with an auto stretch applied to them using the Screen Transfer Function in PixInsight.
Single Image Comparison. Bortle 6 vs. Bortle 3.
If we take a closer look at the data, we can see just how much more detail is collected near the reflection nebula known as NGC 2023. Although a single exposure is still noisy, there is much more signal present.
Detail in the NGC 2023 Reflection Nebula below the Horsehead.
To really appreciate the difference dark skies make to the data, have a look at the stacked data. Both images include exactly 3 hours of total integrated exposure time.
Each image set was stacked, registered, and stacked using the Weighted Batch Pre-Processing script in PixInisght. Once again, only an auto stretch was applied for a better look at the underlying data.
Difference between a 3-hour integration of the Horsehead Nebula Bortle 6 vs. Bortle 3.
Dark Sky Places
Dark Sky International (formally International Dark Sky Association) protects ecologically sensitive areas, and our universal heritage, with their certified places program.
The International Dark Sky Places (IDSP) program certifies communities, parks, and protected areas worldwide that preserve and protect dark sites through responsible lighting policies and public education.
Milky Way taken from Cherry Springs State Park which is a Dark Sky Park
These places are required to use quality outdoor lighting, effective policies to reduce light pollution, ongoing stewardship practices, and more.
There are 5 categories for DarkSky Places:
- International Dark Sky Communities: cities and towns with outdoor lighting ordinances that educate residents about the importance of dark skies.
- International Dark Sky Parks: public or private conservation areas that implement good outdoor lighting and provide dark sky programs.
- Urban Night Sky Places: urban sites that promote an authentic nighttime experience
- International Dark Sky Reserves: dark ‘core’ zones surrounded by a populated periphery
- International Dark Sky Sanctuaries: the most remote (and often darkest) places in the world
All certified Dark Sky Places provide a quality nighttime experience that will allow you to connect with nature at night.
When you visit a nearby International Dark Sky Place, not only will you have a breathtaking experience, but by visiting these locations, your tourism dollars help sustain and protect these rare and fragile areas for the benefit of future generations.
Trevor Jones
is a deep-sky astrophotographer and a valued member of the RASC. His passion is to inspire others to start their astrophotography journey on his YouTube Channel, so they can appreciate the night sky as much as he does. His images have been featured in astronomy books, and online publications including the NASA Astronomy Picture of the Day (APOD).
