Taking star trails is one of the easiest forms of nighttime photography. it requires less equipment than full out astrophotography, only a camera that can take a long exposure and a tripod. In a pinch you can do without the tripod.In star trail photography a long exposure is used to reveal the scene. With illumination provided by starlight the needed exposure will be minutes long, during which time the rotation of the Earth will cause the stars to trail. Each star will trace a short streak on the camera detector as it moves through the field of view.
For a number of reasons taking one very long exposure is a problem with digital cameras. Without getting into a technical discussion of noise, dark current and hot pixels we will simply advise taking short exposures. You can always try a twenty minute or half hour exposure and see for yourself. Thus the technique is to take a series of short exposures, usually one to five minutes long, and add these together in processing. By taking a series of short exposures, the final exposure length is limited only by the camera battery or the arrival of dawn.
If the camera is sensitive enough, and you have a fast lens, you might try starscape photography, where the stars are not trailed by the motion of the Earth. In contrast, star trail photography can be done by almost any camera that can take a long exposure. The difference is in the length of the exposures, long versus short, star trail or starscape.
The center of Virgo is thick with galaxies. The nearest large super-cluster of galaxies lies a mere 53 million lightyears away, includes upwards of 1300 member galaxies and spans over 8° on the sky. The center of the Virgo Cluster is marked by the large elliptical galaxies M84, M86 and M87.
The Great Nebula of Orion is a beautiful object, the brightest nebulae in the sky. It is also quite easy to photograph, making it somewhat of a standard target. I, like many astrophotographers, use the nebula as something of calibration target to check new equipment and processes.
This shot was taken with a Canon 6D and a TV-76mm telescope, a combination I want to work with this summer. I also changed up my processing flow a bit, re-ordering the steps, to achieve better calibration. The result is a more neutral color tone in the original, I can then saturate the image to taste for display or printing. Th original might be a bit closer to true color. Of course, “true color” is a bit of an illusion in astrophotography, where everything is relative.
The Pleiades were not my primary target for the night, but that was setting and I was in no hurry to shut the gear down when there was a lot of dark remaining in the night. I glanced around the sky looking for a target appropriate for the field of view of the gear and just chanced to look at the star cluster.
For my first pass on the Rosette I used about an hour of data. Ten four minute exposures, ten one minute exposures and ten 15 second exposures. The stack is designed to capture a wide dynamic range by using several different exposure lengths. Multiple images taken at each exposure to reduce the noise inherent in long exposure astrophotgraphy. All of this data is aligned and stacked to produce the final image.When looking at the result it was apparent that I had not been successful in reducing the noise far enough. More exposure time was needed. The shorter exposures were not the problem, I needed more of the longer four minute exposures to reach the faint nebulosity.
The next night I acquired and shot again, another 24 exposures on the Rosette, another 1.6 hours of exposure. A planned astrophoto outing to the MKVIS at 9,200ft elevation offered a chance to get even more high quality exposures. This time I got 30 more exposures, another two hours of total exposure time.
Combining all 3.8 hours of exposure resulted in a notably nicer image. The noise in the fainter sections of the image is pleasantly reduced. It should be with 54 four minute exposures used.
I need to do some sort of systematic comparison of the data gathered in my driveway versus the data obtained at the MKVIS up on the mountain. Having a series of exposures taken with the same gear, on the same target, and at the same time of night just a few days apart should allow a reasonable comparison. I was unable to expose any longer at the VIS despite the colder temperatures (the camera was about 10°C colder). How much better is the VIS? Can I get data nearly as good without packing up the gear and heading up the mountain?
The plan was to do some astrophotography Saturday night. The weather forecast forced a re-schedule, a winter storm arriving Saturday afternoon, Friday it was to be.The timing proved to be excellent, Mauna Kea providing glorious skies Friday night. Clouds hovered over the side of the mountain on arrival, but there was no worry. These were the sort of clouds that would vanish with the daylight. Anxious tourists wanting to see through the telescopes repeated the same question, will the clouds go away? I answered the questions with the relaxed assurance of experience of years on Mauna Kea, and continued to set up the gear. The clouds didn’t even last as long as I expected, dissipating as the sun settled into the horizon.
Setting up at the Mauna Kea VIS means a crowd of people. Hundreds of tourists that have come to enjoy a dark Mauna Kea Sky. I was joined by Raymond, a Hilo amateur also looking to take some photos. We setup side by side in a parking space just off the patio where the VIS was setting up their telescopes for the night. We would be in the center of the crowd for a while.
A result from Saturday night. About an hour of integration all told… Good color, very nice stars, good detail. While encouraging, I really need more time on this one, there is too much noise in the fainter regions of the nebula. I can get more data and add it to the stack to continue to improve the result. From up on the mountain I should be able to use longer exposures with darker skies. Colder conditions will reduce the dark current in the camera, also aiding in longer exposures.Compare the processed shot with the single subframe I published yesterday. The final shot is a stack of 10 x 4min, 10 x 1min, and 10 x 15s frames, 30 frames for just under an hour of exposure. Also needed is another 10 x 240s dark frames and 10 flat field frames to use for calibration. This takes another hour, but was done after the telescope was rolled back into the garage and I went to bed.
The Rosette Nebula is a large star forming region just east of Orion in the constellation of Monocerus. It is quite large, the main body seen in the photograph is well over a degree across. The dark region in the center is about the size of the Full Moon
A good night in the driveway last night.After solving the little equipment issues, waiting out weeks of bad weather and waiting for the Moon to go away, I finally had a nice photographic night. Setting up at sunset I shot until the Moon rose. Most of the targets I have shot before, Orion, Rosette, Markarian’s Chain. I also shot Melotte 111, an object that has always intrigued me. The full frame camera and the TV-76 has a wide enough field to capture this very large and nearby star cluster.
I want to use this combination camera and telescope with a very wide field to work over the dark nebulae of the Summer Milky Way. With 384mm focal length the full frame camera gives me over 5.35° x 3.56° field of view. The beehive also lent itself to the wide field, another large object that was worth a stop to shoot.
Aside from one little equipment issue to start, more an operator education issue, the gear worked great. Note to self… Must turn off camera WiFi before the camera will link to the computer via USB. I am currently using APT to control the camera, a very impressive bit of software. The auto-guider worked perfectly, frame after frame nearly identical. I can slideshow through dozens of sub-frames and not see any difference, no drift!
A single sub-frame of the Rosette Nebula is shown to the right. The Rosette is an emission nebula with most of the light emitted at the wavelength of Hα 656.28 nm. My Canon 6D is stock, no astrophoto modifications. Despite an IR cutoff filter that blocks much of the Hα light the camera captures a fair amount of the nebula. Perhaps I should get a 48mm Hα filter for the camera to shoot from the driveway.
With everything working so well I really need to haul the rig up to Hale Pohaku for a night of imaging under darker skies than I get down at 1000ft. Next weekend is dark-of-the-Moon weekend. Need to plan an outing?
It will take a bit to process the results of the night. As usual, keep an eye here on Darker View for the finished photos.