NGC 7023 – The Iris Nebula
The Iris Nebula is one of many nebulae in the night sky with a common name associated with a flower. This dramatic reflection nebula resides in the constellation Cepheus and includes plenty of dark interstellar dust in the area.
NGC 7023 is relatively bright when compared to other reflection nebulae at magnitude 6.8 (the Witch Head Nebula is magnitude 13, by comparison). With apparent dimensions of 18′ x 18′, the Iris Nebula is a suitable astrophotography target for a small refractor telescope.
The Iris Nebula in Cepheus. Photo by Trevor Jones.
NGC 7023 is actually the star cluster within the nebula, but the cluster and the reflection nebula are one in the same. The nebula gets its trademark glowing blue appearance thanks to SAO 19158, the star responsible for lighting it up.
The Iris Nebula
Photographing the Iris Nebula using my camera and telescope in the backyard.
An interesting characteristic of this object is the intense interstellar dust that surrounds NGC 7023, clearly blocking out the stars behind it. You can find this deep-sky treasure in your telescope Southeast of Alfirk, and North of Alderamin.
The Iris Nebula can be fully appreciated through astrophotography, as seen in the long exposure image below. The dominant color of this reflection nebula is blue, with interstellar dust lit up by a central young star. I have photographed the Iris Nebula through a variety of telescopes and cameras over the years, and it never disappoints.
The following photo was captured from my Bortle Scale class 6 backyard in the city, using a UV/IR cut filter on my camera. The focal length of the telescope used was 550mm, which is wide enough to reveal much of the surrounding dust in Cepheus.
The Iris Nebula captured using a one-shot-color camera.
The image above was made using 12 x 4-minute exposures (44 minutes total). When photographing the Iris Nebula, it is important to record the soft dusty areas that block starlight in the area. For this reason, attention to star color and size should also be a priority.
Iris Nebula Details:
- Common Name: Iris Nebula
- Cataloged: NGC 7023, Caldwell 4
- Object Type: Reflection Nebula
- Distance from Earth: 1,300 light-years
- Constellation: Cepheus
- Magnitude: 6.8
- Apparent Dimensions: 18′ x 18′ arc mins
Below, you’ll see a picture of the astrophotography kit used to photograph this object. This should be considered to be an intermediate-level deep-sky imaging rig and is quite beginner-friendly in many ways. The ZWO ASIair WiFi device is used to automate the imaging sequence and perform helpful tasks such as focus and star alignment.
The deep sky astrophotography setup used for my image of the Iris Nebula.
Location and when to photograph it
From the northern hemisphere (My backyard is at 43° north), the best time to photograph the Iris Nebula is May-July. This will, of course, depend on the obstructions in your yard, as it sits lower in the sky in April and May.
You’ll need an unobstructed view to the north. From my latitude, the Iris Nebula reaches an altitude of 30° just after midnight by May. Because the Iris Nebula lies so close to the north celestial pole, it spends long periods of time in the same general vicinity of the sky.
The location of the Iris Nebula in the night sky (Sky and telescope).
From latitudes north of 22°, the Iris Nebula is a circumpolar object, which means that is visible all year round. At 43° north, my backyard fits into this category, although it is too low in the sky to photograph “properly” during the winter season. Objects that sit low on the horizon suffer from poor atmospheric seeing and refraction.
From my location, in southern Ontario, I have found May-July to be the best time to take pictures of the Iris Nebula, as it rises high above the horizon to over 60° in apparent altitude.
The position of the Iris Nebula at dusk from my backyard in early May.
Which Camera/Filter to Use
The Iris is a reflection nebula, which means that its light comes from a bright star that reflects nearby dust. This type of deep-sky object does not need a modified astro-camera to record NGC 7023 in detail. Also, narrowband filters that isolate specific wavelengths of light have no benefit on a reflection nebula.
A one-shot-color camera such as the ZWO ASI294MC Pro, or a DSLR camera are great options to consider when photographing the Iris Nebula. As you’ll see in my image below, a modest DSLR camera like the Canon EOS Rebel Xsi is capable of collecting incredible images of this nebula.
I’ve captured the Iris Nebula using a stock DSLR camera, and a color dedicated astronomy camera.
The benefit of using a dedicated astronomy camera like the ASI294MC Pro is the ability to cool the CMOS sensor well below the ambient temperature. Along with dark frame subtraction, this can significantly reduce the amount of noise present in your images.
For my image of the Iris Nebula at the top of this article, I used a simple UV/IR cut filter to record an image in true-color, broadband RGB. The UV/IR cut filter is needed to correctly focus the light through my telescope onto my image sensor.
The filter I used was the Astromania UV/IR (luminance) filter. I like the 2″ round-mounted model because it includes threads on both sides to connect to my field corrector and camera.
The camera filter used for my image of the Iris Nebula (Astromania Luminance Filter).
As is the case for all astrophotography images, the image processing stage is arguably the most important step of the process. For an overview of how I process my deep sky astrophotography images, please see my deep sky image processing tutorial video on YouTube.
To better illustrate my point, have a look at the different post-processing made to my image of the Iris Nebula, before and after. The before image is typical of any photo I take, and I need to stretch the data to reveal the true colors and beauty of the target.
Image Processing Notes
To me, one of the most important features of the Iris Nebula to record are the areas of interstellar dust that block starlight. Because these dramatic areas put added attention on the field of stars surrounding the object, it is wise to make star color and size a priority.
For my image, I ran several iterations of star reduction actions. This included the manual method of select color range > modify > expand > modify > feather > filter > other minimum. You can also run a star reduction action in the Astronomy Tools Action Set, or Annie’s Astro Actions.
Use a curves adjustment layer to bring the brightness of the brown interstellar dust forward. These areas are much more interesting when we showcase them in lighter browns and grays than simply flat black. The level on intensity here will rely on the amount of signal you have collected, so don’t push it too far.
How aggressive you pull the data is up to you. A stronger signal-to-noise ratio will give you more flexibility.
Previous Imaging Sessions on NGC 7023
The image below is from the Summer of 2012. I used a stock Canon Xsi DSLR camera with an Explore Scientific ED80 to photograph the Iris Nebula.
While processing the image in Photoshop, I tried to achieve a pleasing balance between the brightness of that gorgeous interstellar dust, and the background of space and stars. The dusty areas block out starlight behind them, and I absolutely love this image because of that feature. However, by pulling out those faint details, background noise is introduced.
I used selective processing to separate the nebula from the background, so I could process them independently. Honestly, I think 2 more hours worth of light on this subject are needed to properly process this image to its true potential.
Approximately 3 hours of total integrated exposure time on the Iris Nebula with a stock DSLR camera.
More Reflection Nebulae
Reflection nebulae are some of the most enchanting astrophotography targets in the night sky, due to their ghostly, glowing appearances. The Iris Nebula is one of many amazing reflection nebula targets in the night sky. Here are a few more:
- The M78 Reflection Nebula in Orion
- The Witch Head Nebula in Orion
- The Pleiades Star Cluster in Perseus