Perseïd Meteors

Perseïds - Composition of 8 meteors (Fujifilm setup)

The Perseïds are perhaps one of the most well-known meteor showers, showing peak activity around August 11-12. The radiant of this meteor shower is in the constellation Perseus, hence the name. The meteors are considered to be related to comet Swift-Turtle, a comet first discovered in 1862. When the earth crosses its orbit, the stream of debris left behind by the commit causes a fairly reliable shower of meteors every year, with peak activity up to 50 visible meteors/hour and sometimes even higher under excellent conditions.

With visual observations, meteors typically show as a short bright flash of white light travelling across the sky. On photographs however, meteors can be very colourful objects. These colours are produced both by metals in the meteorite itself as it burns in the atmosphere, as well as the nitrogen and oxygen in the atmosphere that turns into plasma. The final colours can thus vary dependent on the composition of the meteorite, as well as the speed at which it travels and associated intensity of the plasma colours of the atmosphere.

Planning

The best nights to observe the Perseïds are between August 11-13. Cloud forecasts suggested the first night would be the best. Conditions looked promising with the moon being at only 10% and setting at 22:34h. Astronomical darkness started at 23:51h and at that time, the constellation of Perseus was well above the North-Eastern sky. A clear visibility of the North-Eastern sky could be obtained from an observation location often used by the local astronomy club in Gasteren. That is also a very dark area, further helping optimal conditions.

Conditions

All images were taken from a country-location near Gasteren, The Netherlands (53.03, 6.67), on August 11, 2021. Temperature was around 13 degrees Celsius and almost no wind at 13 km/h. Humidity was quite high, in the 90’s, and fog passed over the fields regularly, having a serious effect on the photographs. Moon was fully absent during the observations

Capturing

The images were captured using regular SLR camera’s with regular wide-angle lenses. Meteors are very unpredictable as to where they pop up. The point they all seem to come from is the radiant, but from there they can show up in all directions. The wider angle-of-view is of the lens used, the more likely it is that meteors are captured on camera. Two camera-setups were used. A Fujifilm GFX100 medium-format camera with 23mm f/4 lens with a horizontal angle of view of 87º. And a Leica SL2 full-frame camera with 21mm f/1.4 lens with a horizontal angle of view of 81º. Each on their own tripod. One with a Rainbow Astro RST-135E mount, allowing to track on one point on the sky. The second one a static tripod, making the sky move slowly across the frame with time. In both setups, the center of the frame was pointed towards the radiant in Perseus.

Technical Details Setup 1

Lens
Mount
Camera

Fujinon GF 23MM F/4.0 R LM WR
Gitzo GT3543XLS tripod with Arca-Swiss Cube C1 geared head
Fujifilm GFX100

Technical Details Setup 2

Leica Summilux-M 21mm f/1.4
Gitzo GT5533S tripod with Rainbow Astro RST-135E tracking mount
Leica SL2

Lens
Mount
Camera

Frames

With the GFX100, 10s and 15s images were taken at f/4 and ISO 1600. The in-camera intervalometer was set to capture pictures continuously with 1s intervals. Focus was achieved by manual focus and zooming in as much as possible into the live-view image, and further verified by the on-screen focus-scale to just hit infinity while rotating the focus-ring on the lens.

With the SL2, 10s and 15s images were taken at f/1.4 and f/2 and ISO800. This makes the Leica images a bit higher exposed than the Fujifilm images. Also here continuous shooting with 1s intervals. An interesting design difference was discovered here. Fujifilm starts counting interval at the end of the previous frame, Leica at the start of the previous frame. So if you want 1s interval between 10s images, on the Leica you set interval to 11s and on the Fujifilm on 1s. The Summilux-M 21mm is a fully manual focus lens and can be focused using indicators on the lens. Long Exposure Noise Reduction was turned off for both cameras.

For the static camera (first the GFX100, later the SL2), exposure-time is limited due to rotation of the sky. The so-called 500-rule is often used. Maximum shutter speed is 500 divided by focal length of the lens. So a 21mm lens should have a maximum shutter speed of 500/21 = 24 secs. In my experience however, this is much too liberal. At 20s and 25s, clear star-trails were visible. Instead, 15s was found to be the maximum. A ‘300-rule’ would be a safer bet. This is consistent with earlier findings when photographing Comet Neowise.

In total 1133 images were shot. The 8 meteors on the GFX100 image were shot between 23:08h and 01:17h. The 5 meteors on the SL2 between 23:27h and 00:04h.

Perseïds - Composition of 5 meteors (Leica setup)

Image

The main reason to take so many images is to increase the chance that a meteor is captured on one of them. In total 5 meteors were captured with the SL2 and 8 meteors were captured with the GFX100. The final images are composites, consisting of a background sky, a foreground and an image for each individual meteor.

Images captured with the GFX100 are 100 Megapixel in size. After stacking and cropping, the final image is 10,676 x 8007 pixels, or 85 Megapixel. The SL2 files are 47 Megapixel each and after stacking and cropping, the final image is 6888 x 4596 pixels, or 32 Megapixel.

Not all photographic lenses work well in astrophotography. Corner softness, coma, astigmatism, vignetting are typical problems seen in even top-line lenses of the main brands. Being a first to bring these lenses out for astrophotography, it was a bit of a test how well they behaved. The Leica Summilux-M 21mm was behaving poorly. Heavy astigmatism and coma throughout most of the frame and heavy vignetting degrade the overall image quality. Granted, this is an f/1.4 lens, which is very fast. Stopping down helped somewhat, but overall this lens is not a great astrophotography lens.

The Fujinon GF 23mm however performed exceptionally well. At f/4.0 not the fastest, but star shapes held well throughout the frame and vignetting was well controlled. The full-frame equivalent of this lens a focal length of 18mm, so quite wide. The aspect ratio of 4:3 further helps creating a large field of view.

Processing

With both cameras, Dark (10) and Flat (10) frames (10) were shot. Culling the frames and finding the meteors was done in Capture One Pro. Meteors can easily be confused with satellite- or plane trails. Differences are that meteors have tapered ends, show various colours and are only visible in one frame. After synching White Balance between frames, RAW images were exported as uncompressed TIF files.

Calibration using darks and flats, as well as alignment of all frames was done using PixInsight. Especially in the case of the GFX100 image, a couple of meteors are at the edge of the frame. Therefore a careful selection of the reference frame during registration was critical to ensure all meteors ‘fitted’ on the final image. Image integration of 20 images used for sky background was done in PixInsight. For stacking, attempts were made to use Starry Sky Stacker, Starry Landscape Stacker, and Affinity Photo. But results were not very encouraging, probably due to user error. Familiarity with PixInsight made it a fast and easy alternative..

The Background stack, and each image with a meteor were brought into Photoshop. The meteor images were fully masked with the exception of the trail of the meteor. Sometimes a small curves adjustment needed to be made on the meteor image to match background signal with the overall background image. Meteor images were slightly enhanced using brightness and saturation. Finally a foreground image was added from one of the single images.

Conclusion

This was a first attempt to photograph meteors and a good learning experience. The chance of actually recording a meteor was lower than expected. With some meteors showing up outside of the frame, the wider the lens the better. Perhaps a 14 or 15mm would be best. The Summilux-M 21mm is not great for astrophotography. The GF23mm with the GFX100 seems to be a winner. It’s not very fast, but when using a tracking mount this is no problem.

Besides all efforts to end up with an image of the meteors, another lesson can be learned here. Relaxing flat on a reclining chair and absorb the experience of meteors flashing along the sky is a great experience in itself. Even though the frequency was a bit lower than expected, the event was a great experience.