Deep space photography skills of SLR camera

Everything is difficult at the beginning, so is astrophotography. The following are the deep space photography skills of SLR cameras that I collected for you, hoping to help you!

There is no doubt that the digital SLR camera is the most comprehensive camera at present. Nowadays, no other camera can be used as widely as it is: it is close enough to shoot children's birthday parties in its own garden; Far enough to record distant galaxies, and there is no modification, just equipped with a telescope. Digital SLR camera makes celestial photography open the door for everyone who is interested in shooting the night sky.

There are several factors that make digital SLR cameras very suitable for astrophotography. In the design of these cameras, most of them use the same lens as the traditional 35mm film era, and their sensor frames are much larger than ordinary instant cameras. Many consumers have installed CMOS sensors in APS C format in their digital SLR cameras, which is about 65% of 35mm film, similar to many mid-range astronomical CCD cameras.

Different from film, CMOS sensor in digital SLR camera will not fail in reciprocity rate (reciprocity rate means that the aperture and shutter time of camera can be matched to the best fixed exposure under normal conditions of film photography. However, when the shutter time is too long or too short, the matching of the best exposure may fail. This is the reciprocal rate fault. It records every photon. Two minutes exposure time is enough to take amazing photos of Orion nebula; And with an ordinary telephoto lens exposure 10 minutes, you can record stars of 16 magnitude. In addition, when shooting the night sky, the digital SLR camera does not need to be connected to a computer.

When buying a digital SLR camera, you have many choices, but Canon's products are the most popular products in astrophotography. Nikon and Pentax's other digital SLR cameras are similar, the main difference lies in the access mode of functions and the format of picture files. In fact, all digital SLR cameras at present shield the far red end of the visible spectrum, but this band is very important in astronomy, because hydrogen will fluoresce here. In order to increase the sensitivity of the camera to the red band, many astrophotographers will calibrate the camera themselves. They will remove the infrared filter from the camera and replace it with a filter that allows more red light from the hydrogen emission line to pass through. Bold photographers who want to improve their cameras can buy modified near-infrared filters and do it themselves. If the camera is modified in this way, it must set a custom color balance before it can be used for daytime shooting.

If you plan to buy a new digital SLR camera for celestial photography, a function worthy of special attention is "real-time display". This function allows you to watch real-time video on the LCD screen on the back of the camera after turning on the sensor. Compared with other methods, you need to adjust the focal length of the lens or telescope slightly. If your camera doesn't have the "real-time display" function, then you need other forms of focusing assistance; Or after a short exposure of 5 seconds, watch the effect quickly on the back screen.

Take pictures of the stars

When you choose a camera, you must prepare some extra equipment before shooting the night sky. First of all, you should have an instrument that allows you to expose for a long time without touching the camera. You can open the camera shutter and expose it for up to 30 seconds; You should also set a delay to wait for a moment before the shutter is opened until the vibration caused by pressing the shutter button stops. For longer exposure, you can use the cable releaser with built-in timer exposure controller, which can set a series of long exposure actions to eliminate the necessary delay between shooting different images. Accessories manufacturers such as PHOTTJX(www.photttix.com) can produce such a cable releaser with a timed exposure controller, which is also easily available on Amazon and Yi Bei. But it should be noted that you must choose the model that matches your camera.

For the first time, you need a tripod to take celestial photography with a digital SLR camera. Even if your main goal is to take close-ups of deep space objects through telescopes, it is helpful to be familiar with the camera functions used to take various deep space photos by putting the camera on a tripod to shoot some simple targets. Tripod can also bring convenience for shooting synthetic images, wide-angle images of the Milky Way, meteor showers and other common targets of astrophotographers.

Try to mount your camera on a tripod at night when there is no moon and the stars are shining. Set the wide-angle lens to the maximum aperture (minimum focal ratio) and manually focus a bright star through real-time focusing. Focus the real-time focused video and adjust it to the clearest focus. The photosensitive speed is set to 1600, and the exposure time is 30 seconds. In this way, you can get a picture of deep space celestial bodies full of starlight, and maybe some bright celestial bodies.

A few nights of practice will make you more familiar with the performance of your camera, such as mirror locking, noise reduction, and exposure procedures of the timing exposure controller, which is of great benefit to celestial photography.

If you really want to deeply expose the nearby stars, you can "carry" the camera on the top of the telescope, track the stars with the telescope, and shoot with the lens of the camera. In this way, you will find that the standard zoom lens with 18 to 55 mm on many digital SLR cameras is not very suitable for celestial photography. Its speed is too slow (generally not more than f/4.5), not as clear as many fixed-focus lenses. In addition, because the telescope tilts in the process of tracking the sky, as a zoom device, the focal length or focal length of the camera zoom lens will be biased.

Fixed focal length lens is more suitable for celestial photography. Of course, you can buy very good telephoto lenses from Canon, Sigma or other manufacturers. Here is a very useful trick: you can use the adapter to transfer the old manually focused Olympus, Nikon, Pentax, Contac/Continental Optics and other miscellaneous lenses to Canon EOS or Nikon digital SLR cameras. Since autofocus cannot be used in deep space celestial photography, an old-fashioned manual focusing lens can be used, which is much cheaper than the latest lens sold in the market. Fotodiox can provide adapters.

If you want to try to shoot celestial bodies with your own telescope, you must have an adapter. This is usually a T-ring and an adapter to fix your camera in the eyepiece of the telescope. With this equipment, you can shoot the moon with a telescope as a camera lens immediately. To take pictures of deep space objects, you must first try to do exposure for 5 seconds or more to check how long your camera bracket tracks before the star becomes an orbit. In fact, even the high-end telescope bracket needs to use automatic star guidance or other specific methods to compensate the errors in the tracking process, which may come from the bracket itself, wind vibration, or other vibrations.

Picture processing

No matter what kind of celestial photography you are shooting, the photos taken directly by the camera are usually not the final images. The images taken by short-time fixed tripod exposure are always too dark, while the exposure of star guide device for 2 to 3 minutes is often too bright because of light pollution. This is a common phenomenon. In the camera, automatic white balance is often not suitable for deep space celestial photography, because the target celestial body to be photographed is too dark for the computer in the camera to judge accurately. But we can adjust all these settings in the image processing software. Although the software that comes with some digital cameras can provide some basic adjustment functions, I strongly recommend installing a special software program suitable for celestial photography with digital SLR cameras. Most software can be tried before purchase, and DeepSkyStacher is free software.

Once you choose the processing software, the next two steps will immediately make your photos more beautiful. The first is black film correction (flat field correction). When you take a long exposure with a digital SLR camera for more than a few seconds, you will see some randomly scattered color pixels, that is, some red, green and blue spots, which means that the sensor has so-called "thermal noise" in these positions.

The best way to avoid thermal noise is to subtract the dark field. The so-called dark field is a photo taken by keeping the sensor out of contact with any light, but keeping everything else unchanged-exposure time, sensitivity and sensor temperature unchanged. Dark fields have the same thermal noise as the captured images, so subtracting them can eliminate the thermal noise. For most digital SLR cameras, turn on the long exposure denoising mode, and the camera will automatically deduct the dark field. At this time, after a long exposure, the camera will quickly shoot another dark field with the shutter closed, and automatically deduct it from the original photo to record the final image. Although this mode is very convenient, it can ensure that all exposure settings in the dark field are the same as the original image, but it is a waste of time and could have been used to take more photos.

Another method is to take one or more photos manually when the lens cover is not open, and finally deduct them from the software.

It's best to take more pictures, at least six, so that the software can average and eliminate random noise. A set of dark fields can be applied to multiple celestial bodies shot at the same night, as long as their exposure time and sensitivity settings are the same.

In addition to dark field correction, the second technique will make your photos smoother, which is to expose your target celestial body several times and then synthesize it. This technique is called "superposition" and it has many advantages. You can do an hour's exposure, but you don't need to guide the stars accurately all the time for an hour. If there is something wrong with the guide star, you can take a lot of exposed images and then discard those tracking images with poor quality. It can also avoid the interference of dust and fog in the sky, because no exposure will last for a long time. The superposition algorithm in the image processing program can automatically discard the large differences between photos such as plane trajectory and random thermal noise when synthesizing photos. When synthesizing multiple exposures, the noise level in the superimposed image will decrease in proportion to the square root of the number of exposures used to synthesize the image.

Of course, this method also has limitations. You can't stack 3.6 million photos with an exposure time of 11000 seconds into an image with a total exposure time of1hour. Every useful photo must have a long enough exposure time, usually 5 to 10 minutes, unless you have to reduce the exposure time because of the limitation of tracking accuracy.

The superposition and subtraction of dark fields are the most suitable methods for raw files, not JPEG files, because the latter has been scaled nonlinearly and a lot of data has been discarded during compression, which makes JPEG files much smaller than raw files. After subtracting the dark field superposition, you can adjust the brightness, contrast and color balance of the image, and you can also sharpen the image when saving the final version.

When you improve the recorded data, you can learn many skills and methods to "squeeze" more information from the photos. The tips introduced in this article will make you walk more smoothly on the road of shooting celestial bodies and take fewer detours.