The past several weeks have presented opportunities to photograph objects in the night sky. Exceptionally clear skies and dark nights allowed me to capture some long exposures of portions of the Milky Way. Other nights had interesting alignments of the moon with one of more planets.
Early in September, Venus and Jupiter aligned with the Moon in a nearly straight line in the western sky just after sunset. Compare this with an image taken a month earlier. In the course of a month, Mercury has dropped below the horizon while Venus and Jupiter have switched locations with Venus rising higher in the sky as Jupiter dips lower.
Winter is slowly coming to an end and we are about to transition into the warmer months of spring and summer. During these upcoming months, the center of our galaxy—The Milky Way—will be rising earlier each evening.
Milky Way rises above Cathedral Rock in Sedona, Arizona.
For photographers—and just about everyone else, too—staring up on a clear, moonless night with the Milky Way glowing above can be a magical experience. For those who live in brightly-lit cities, however, the Milky Way can be difficult or even impossible to see. From an article at PBS:
Light pollution — the needless shining of bright lights into the night sky — has robbed whole generations of the chance to see nature on its largest scale. It is estimated that as many as eighty percent of all the people alive today have never even glimpsed the Milky Way. (When a massive power outage struck southern California in the 1990s, Los Angeles residents reportedly called 911 to express alarm about strange clouds hovering overhead; they were seeing the Milky Way for the first time.)
With the rapid advancement of digital cameras in the past decade the ability to take images of the night sky has become remarkably easier. Not easy—just easier. Cameras can now take long exposures at high ISO settings to reveal details of the night sky not easily visible to the unaided eye. This has resulted in magnificent photographs of the Milky Way but also other objects such as comets and Deep Sky Objects (DSO).
Still, long exposures of the night sky can result in the stars leaving streaks (i.e., “star trails”) across the image. This is the result not of the stars moving, of course, but the earth’s rotation. Typical wide-angle lenses used for photographing the Milky Way are limited to about 15 to 30 seconds before trails become obvious. In order to capture enough night-sky light at these exposures requires high ISO settings which can add considerable noise to the image. Of course, sometimes star trails are desired as seen in the image below:
Star trails above Cathedral Rock in Sedona, Arizona.
Another option is to use a tracking device that follows the motion of the stars (or, more correctly, counteracts the rotational motion of the earth) allowing the camera to take very long exposures without star trails. The downside of this technique is while the stars remain pin points of light, the ground is blurred as the camera slowly moves during the exposure.
Long exposure without tracking (left) and with tracking (right).
The solution requires taking multiple images: one of the stars with the star tracker on and a second image of the ground with the tracker turned off.
The image shown at the top of this post is a composite to two images: one of the stars and one of the ground.
The star image was taken using the iOptron Skytracker, a relatively inexpensive tracker. The image was shot using a low sensitivity (ISO 400) to minimize sensor noise. The lens was an ultra-wide 16mm shot at f/4 and the duration of the exposure was 534 seconds (~9 minutes). The exposure for the foreground was shorter in duration (4 minutes) and at a higher sensitivity (ISO 1600).
The two images were combined as layers in Photoshop. Masks were applied to each of the images and then blended so that the pin-point stars on one image and the sharp foreground of the other image remained.
It turns out that taking the images was the easy and fun part. Standing around in the middle of the night watching stars, meteors, and satellites cross the sky can be very enjoyable. Not surprisingly, the blending of the images took many attempts and much time.
The Perseid meteor shower of 2015 was better than average because it occurred one day before the new moon. The absence of moonlight meant that many fainter meteors were visible.
Although the shower peaked on the night of 12–13 August, meteors could be seen for several nights before and after. I went out for several hours photographing the shower on two different evenings and was happy to report that clear and dark skies produced a great event.
On the night of 12–13 August, I set up the camera on my equatorial mount so that I could track the radiant across the sky for several hours. I then combined the images with meteors into a single image that clearly shows the radiant. The result was pretty good although I’m certain I saw more meteors than the camera was able to capture.
Perseid meteor shower. Andromeda Galaxy (M31) is also visible in the upper right.
In addition to the meteors, there was a prominent set of waves in the airglow visible low on the horizon. From wikipedia:
Airglow is caused by various processes in the upper atmosphere, such as the recombination of atoms, which were photoionized by the sun during the day, luminescence caused by cosmic rays striking the upper atmosphere and chemiluminescence caused mainly by oxygen and nitrogen reacting with hydroxyl ions at heights of a few hundred kilometres.
Here is a short time-lapse video of the multitude of waves present in the airglow that evening. (A higher quality version of the video can be found here.)
The following night also had meteors but at probably half the hourly rate as the previous evening. Once again I mounted the camera on the equatorial mount but this time I pointed the camera towards the southwest and aligned it with the Milky Way. I managed to capture two very bright meteors as they streaked from overhead down towards the horizon.
Perseid meteor shower.
The last time the Perseids peaked at about the same time as the new moon was in 2007. In August 2018, the moon will be only 4% illuminated providing another dark sky opportunity.
With a very clear night and the moon rise not expected for several hours, I set up the iOptron SkyTracker to capture images of the Milky Way and Cathedral Rock in Sedona. The star image was 4 minutes in length and shot at ISO 1600. The SkyTracker works well enough that the stars still appear as points with this long-exposure image. A second image was then taken using the same exposure settings but with the tracker turned off in order to get a sharp foreground.
The Milky Way rises above Cathedral Rock in Sedona.
Taking the images was easy. So was the post processing. It turned out that combining and blending the two images was harder than I thought. But the result was worth it.
With the moon well past full combined with very clear and dark skies it was time again to do some night sky astrophotography. On an earlier outing, I had taken a few “exploratory” images of potential targets. It was now time to take some longer exposures.
My first target was Rho Ophiuchi cloud complex, a dark nebula of gas and dust that is close to the star ρ Ophiuchi of the constellation Ophiuchus (and located adjacent to the better known constellation Scorpius).
Rho Ophiuchi cloud complex. The planet Saturn is in Scorpio and is located in the upper right of the image.
My tools for the night were a Nikon D700 DSLR (fairly old camera technology by today’s standards), a Nikon 85mm f/1.8 lens (a short telephoto lens that works well for astrophotography), a tripod, and an iOptron SkyTracker equatorial mount for tracking the stars on long exposures.
I took 10 exposures each of 4-minutes duration and then stacked them using the (free) Deep Sky Stacker application. The resulting image was post processed in Photoshop CS6 using Astronomy Tools v1.6.
The result isn’t bad considering I’m still pretty much an amateur at this astrophotography thing. For comparison, check out this amazing version of the Rho Ophiuchi cloud complex at the APOD site.
The next night was almost as clear so I returned again to my viewing location near Mormon Lake. This time, I used an ultra-wide angle lens (16mm). Here you can see the Milky Way rising in the east with Saturn in Scorpio and visible in the center right part of the image. The Rho Ophiuchi cloud complex is also visible to the right of the Milky Way. The glow in the lower right is from the city of Phoenix—locateded over 150 kilometers to the south.
Milky Way rising.
While capturing these images I was treated to the yipping of coyotes, hooting of owls, and could hear a small herd of elk grazing in a nearby meadow.
