The Iris Nebula (NGC 7023) is a small reflection nebula located in Cepheus, a northern constellation high in the summertime. Surrounding this object are swaths of dark nebula (heavy dust) which block starlight and appear grey or brown; these nebula extend far beyond the boundaries of this image all over this area of sky. Since dark nebula can be especially difficult to capture from light-polluted skies, I saved this object for a yearly trip that my local astronomy group takes to a nearby dark site.

Acquisition Details:

Telescope: Astrotech AT72ED with Focal Reducer (340mm, f/4.9)

Camera: ZWO ASI1600MM-P

Luminance: 139 x 300” from a Bortle 2 site in August 2018

Color: Red: 60 x 120” | Green: 60 x 120” | Blue: 60 x 120” from my Bortle ~4.5 home in October 2018

Total: 17.5 hours integration

My Goals With This Image

Having seen images of this object online taken from similar cameras and telescopes, I was confident that I could capture the reflection nebula itself, but the real test was how much of the darker background nebula would appear in the final image. Many online images will show the reflection nebula, but the only indication of the surrounding dark nebula are suspicious patterns of absent stars. In some ways this mirrors how clouds appear in locations devoid of light pollution; while near a city clouds will be plainly visible at night since they reflect light pollution, but absent this light they are merely dark voids which slowly move across the sky.

This image is also one of the first shot with my ASI1600MM-P; I was only able to take this camera out for a few total hours for testing prior to making this trip, which was slightly risky considering the lack of any internet connection to download any missing drivers or updates.

My strategy in capturing this object was to use the dark skies to my advantage as much as possible. When shooting an object in LRGB, or Luminance plus color (RGB), the Luminance channel holds all the details. Color does not necessarily require the same quality of skies. For this reason, I focused on shooting with only my Luminance filter while at the dark site. Later when I had returned to Wichita, I shot the color data over a few more nights, completing what I needed to form this photo. Upon later examination of the color image, I found a decent amount of background dust as well, which has since caused me to wonder just how worthwhile it is to pack up all my equipment and drive further out of the city - I may have to redefine what I consider to be a “dark” object to make future trips more worth the time.

The image itself:

The Iris is a reflection nebula, or a collection of gas and dust which reflects nearby starlight. Surrounding this nebula are oceans of thicker dusty regions which do the opposite, entirely blocking out starlight in the visible spectrum. I say visible spectrum because if I had access to an Infrared filter some of the stars within these thicker dark nebula may have become plainly visible (see the image below for a comparison of what i mean)

The Iris is just beyond magnitude 6, putting it probably outside the naked eye range of most people, but this magnitude makes it a fun object to capture in a wide range of focal lengths. The image I captured was at about 340mm, but longer focal lengths can trade the surrounding dust for more detail on the Iris itself, and shorter focal lengths can capture significantly more detail on the wider ocean of dark nebulae surrounding this part of our sky.

At 340mm, the image I shot is just under 6 times the width of the Full Moon in our sky, so this is by no means some obscure corner of the cosmos, instead it fills an area of sky roughly equivalent to what your fist would cover held at arms length.

Challenges with this Image:

The main difficulty I had with shooting this target was the constant problems of weather. Despite being in an area far from the city, the 3 nights of my stay suffered from significant haze which caused ringing or halo effects in the stars of each exposure. The final total of 139 Luminance images came from a larger total of 180, many of which were deleted due to this issue.

Second, my doublet suffered from focus sag. Since the focal reducer is attached on the back end of the focus tube, it must be racked out several inches to reach focus, and the combined weight of the camera and filter wheel beyond this cause enough flexing to distort many of the edge stars. This is not an insurmountable issue since it can be solved by either cropping or partially solved through using shorter exposures, but it is persistent and annoying.

One minor issue was that the Luminance data was slightly overexposed; some of the core detail of the Iris had been lost (probably also due to some hazy subframes which I did not closely examine and delete). Fortunately, once I gathered the necessary color data on this object (which was shot with shorter exposures) I simply extracted a Luminance copy of that color image and used it to compress the dynamic range of the overexposed image

Camera Performance:

The ZWO ASI1600MM-P performed perfectly for what I needed. Since I could not easily bring this camera to a dark site to test the ideal exposure times (to prevent overexposing the Iris), this was done on the fly as I lined up the object during the first night of my trip. A generous amount of online discussions are available on this camera, how long your exposures should be, what gain to use, etc. This topic is complicated and with only 6 hours of useful time per night, I opted to ignore it and simply shoot 5-minute exposures for as long as I could.

The on-board cooler, used to reduce overall camera noise before the image is even saved to my laptop, performed great considering the outdoor conditions of Kansas summer. During the ~25C nights, the camera cooler was quite capable of lowering the sensor temperature to -10C