May closes with a New Moon (moonless sky), which deep sky astrophysicists have always been looking forward to. This month brings a potential bonus of a violent meteor explosion, perfectly timed for North American astrophysicists. In fact, it’s so perfectly timed (for US residents) that it falls on the Monday evening of the three-day US holiday weekend.
The usual annual meteor showers (like the summer Perseid showers, for example), we’re used to tending to bring more drizzle than showers. A rate of 70 to 100 meteors per hour is typical and looks good on paper, but this is an optimistic estimate known as the zenithal hourly rate (ZHR) based on what an observer would see if radiation of the meteor shower directly overhead. Factors such as lower radiation in the sky and light pollution reduce this rate.
In the case of (hopefully) meteor explosion, we could see a ZHR of hundreds or a thousand per hour. This often unnoticed meteor shower, known as the Tau Herculids, is likely increased in activity due to the breakup of Comet 73P/Schwassmann-Wachmann. The symbol ‘P’ indicates a periodic comet, which in this case has an orbital period of 5.44 years.
Comets are usually the cause of meteor showers because each close pass of the Sun causes their loose aggregation and boils off some of the rock, but comet 73P was observed in 2005. 1995 has completely disintegrated, presumably due to the additional stress of passing Jupiter.
Although there is considerable uncertainty in predicting the behavior of comets and their accompanying meteor showers, there is still a high chance of activity. The Meteor Society of America lists three patterns that indicate a peak at around 05:05 UTC on May 31, which is 22:05 PDT on May 30 for the west coast of the United States. ZHR cannot be reliably predicted so if the model’s inputs and assumptions are incorrect, a crash could be a possibility! More information can be found in this article earthsky.org.
Shooting Meteor Shower
Given the convergence of time, even if meteor shower activity is so uncertain, it’s an opportunity we shouldn’t miss, especially since meteor shower photography is one of the simplest things to do. do. It just takes perseverance!
In its simplest form, all you need is a DSLR or mirrorless camera that fits on a tripod and a wide-angle lens. The camera should be operated in manual mode with wide lens, fast ISO and long exposure. Normally, I use my Nikon D600 or D850 with one Sigma 15mm . fisheye lens, captures 20-second frames at ISO 1600. Both cameras are full-frame cameras, which maximizes sky coverage. These cameras also have an internal distance measurement function so no additional equipment is required. If your camera is not capable of measuring the internal distance, you will need one outside.
As the evening begins, be sure to manually take a few frames to verify focus and gauge the brightness of the sky. The image of the sky should not be too bright to wash out potential meteors, but also not so dim that the background of the sky is clipped in the dark. If the background sky is not optimal (use your camera’s histogram), adjust the ISO or the length of the exposure to achieve an acceptable image level.
If you’re going to shoot all night like me, it’s best to buy a battery eliminator powered by a large external battery or AC power source. Otherwise, you will need to have some charged batteries and exchange them every few hours.
Another note to remember is to use a memory card with as large a capacity as possible. An eight-hour session at three frames per minute yields 1,440 frames. At 50 MB per frame for high-end cameras, you get 72 GB! In addition, a fast memory card is desirable to minimize downtime between frames. A two-second pause between frames to save images results in a downtime of 10%, and Murphy’s law dictates that the best meteors come out during that time!
An optional piece of equipment to use is follow the star or astronomical mount. In this way, line stars can be eliminated, and complications such as lens distortion can be largely avoided if the frames will be stacked afterwards.
Consequences the next day
Luckily for US residents, it’s Memorial Day weekend, so maybe we can spend another day getting our frames sorted! My standard procedure is to import an entire batch of frames into Lightroom and adjust the profile picture for color and contrast, then export a set of jpeg frames into a subfolder with the output image in high quality. highest quality at the highest resolution of my monitor (2K). Then I used Windows’ native JPEG image viewer in fullscreen mode to cycle through the frames, noting the ones with the meteors. The reason for the reduction in resolution in this set of JPEG images is to be able to cycle through them with minimal delay between frames. While faint meteors may be missed during this period, the finest and brightest will still be clear.
Identify Meteors
While it may seem easy to identify meteorites, doing a proper job requires care, as satellites and aircraft (even fireflies!) can be a problem. In general, I look for the following features:
- Colorful streaks: meteorites often ionize atmospheric gases into red/yellow/green colors.
- Colorless (white) trails could mean you’re looking at a satellite trail.
- Small line through two or more consecutive frames indicates satellite or aircraft. Meteors are often too fast to be captured in multiple frames.
- The red, green or white dots on the path are from the aircraft’s navigation lights.
- The curve (ignoring lens distortion) is the plane.
- Despite the lens distortion, meteor trails from a particular meteor shower will focus on the radiation. It is also possible to capture a random (considered sporadic) meteor from two radiations if two meteor showers are taking place.
Once I identified the frames with the meteors, I went back to Lightroom and marked them (e.g. with a red marker).
Composite frame
Unless the shower is very intense, it’s unlikely that only a single meteor will enter the frame, so you should stitch some or all of the meteors into a single image. If the frames were shot on a tracker, combining multiple frames shouldn’t be an issue as the background stars should mostly align properly. Exceptions may be stars that are close to the horizon in some frames. Atmospheric refraction will tend to come into play at low altitudes (~30 degrees above the horizon).
If unmarked frames have been shot, aligning the stars between frames is a much bigger problem. Distortion in wide-angle lenses makes it impossible to just use translation and rotation to match stars in frames that are far apart in time. In this case, I use a specialized program called Registration Authority, can recognize and correct distortion even in distorted frames. The original use of this program is to align digitized film images that may be distorted due to film shrinking or stretching. The program also nicely overcomes the problem of atmospheric refraction at low altitudes. It’s not free, but works great for astrophotography where star alignment is required and doesn’t require manually specifying waypoints.
The alternative to using Registar is to manually align the frames in Photoshop, focusing only on the areas right around the meteor trail. All but the longest meteor trails can reasonably be summed up when everything else in the frame is obscured.
When combining frames, one of the frames should be used as the base frame of reference. Ideally, this would be halfway between the first and last frames with the meteors, but you may want to consider composition, such as placing the Milky Way in an easy location. bear.
Once the base frame has been defined, Photoshop can be used to load all the frames into layers and mask all but the meteor in all but the base frame. Note that the sky background may have to be adjusted for each meteor frame to match the base frame background color.
Time-lapse article processing option
Another option for post-processing a night’s collection of frames is to assemble them into a time-lapse movie. This is especially effective if heavy meteor activity or sporadic clouds interfere with your evening. To begin with the movie set, once the frames have been batch processed in Lightroom for color and contrast balance, they can be exported as sequentially numbered JPEG files in a subfolder. If the frame number from the files is not wrapped around, the camera filename can be used directly. Otherwise, use Lightroom’s ability to rename the output set with sequentially numbered frames.
Before exporting a frame from Lightroom, use Lightroom to crop the frame for the right video aspect ratio (e.g. 16:9) and export the frame at a resolution that matches the final video file (e.g. 16:9) : 1,920 x 1,080 for HD video). This will minimize the further processing Photoshop will have to do to convert the video.
The next step is to open the first file in the set in Photoshop, paying attention to check the box that says it’s the first file in a series. Photoshop then automatically stitches the frames together in a video sequence that can be enhanced with titles or other labels before outputting to standard video files such as H.264 .mp4 files.
Go out and get the data!
Whether your goal is to get a single frame with a meteor, create a compositing, or assemble a time-lapse video, the key step is to go out and get the data. If a meteor storm occurs, you don’t want to kick yourself for not making an effort. Good luck to all of us!
