|Digital Camera Settings for Astrophotography
|Canon 20Da controls and settings with backlight for viewing in the dark.
These settings are usually set with controls on the top or back of the camera, but depending on the model or manufacturer, may be set in menus.
- Program Mode – Set to Manual.
- Autofocus – Turn off, or set to Manual.
- White Balance – Set to Daylight or use a custom white balance (especially if your camera is modified).
- Drive – Set to One Shot.
- ISO – This is discussed in detail in a following section, but generally should be set to 1600 or 800 for long-exposure deep-sky astrophotos.
- Metering – Generally doesn’t matter, but you can try setting it to Spot if you are shooting the Moon or Sun (with proper filtration)
- Exposure Compensation – Set to Zero, no exposure compensation. Doesn’t really matter because you are not using autoexposure.
- Shutter Speed – Set to the exposure you want, up to 30 seconds can usually be dialed in directly. For longer than 30 seconds, set to Bulb. On some cameras, Bulb may be a separate exposure mode setting, or it may be accessed on the shutter speed dial past the 30 second setting.
- Self Timer – If you don’t have a remote release, you can use the camera’s self-timer to trip the shutter so you don’t have to touch the camera, which will help reduce vibrations and possible star trailing.
- Mirror Lock-Up – For long exposures with a very solid mounting, it probably is not necessary to lock the mirror up before an exposure. For short exposures for high-resolution planetary or double-star work, it is a good idea to lock the mirror up before the exposure to reduce camera movement and vibration caused by mirror slap. Some cameras access this setting with a control on top of the camera and some through a custom setting in a menu. Some cameras like the 20Da require the shutter to be pressed once to lock the mirror up, and then be pressed again to actually open the shutter. Take care to learn exactly how your camera works for this feature, because you could press the shutter thinking you have opened it for a long exposure and go off to do something else, and then come back only to find that all you had done was lock the mirror up and that no exposure had been taken. Some cameras do not offer a mirror lockup up at all, but they may move the mirror up out of the way as the first thing when using the self timer. Read the camera manual to learn how your particular model works.
Other Settings – usually available through menu settings:
- Quality: Raw vs JPEG
|Canon 20Da Menu Settings
Most DSLR cameras come with the file format that images are stored in set to JPEG as the default. Most high-end consumer digital cameras also allow images to be saved in a proprietary “raw” file format that offers the potential for higher quality.
Raw file format preserves all of the original data as it comes from the sensor in the camera. This is, unquestionably, the best file format to shoot. The raw format saves the data in the high bit depth (usually 12 bits) that comes out of the analog-digital converter as linear data. This is extremely important for astronomical images. High-bit linear data allows the most control over how the data is calibrated, manipulated and enhanced to produce the best images.
That being said, if you don’t want to go to the trouble of calibration, and just want a quick image that you can show, you can certainly shoot JPEG formatted files. The camera will turn the linear file into a recognizable photograph, and you can touch it up later in an image editing program if you want to adjust the contrast and color balance. In fact, some new low-noise DSLR cameras as so good, that with brighter objects like M17, you can shoot a single 3 minute exposure in JPEG format and still get a good picture!
If you are a beginner, this may be the approach you want to take. Shoot JPEGs to get started and learn your way around the scope, camera, and astrophotography. Then as you advance to the next level, you’ll want to start shooting raw file format.
Most cameras also allow a choice of image resolution. These settings can be called by different names, but usually go by something like “Large”, “Medium” and “Small”, or “High”, “Medium” and “Low”. This setting lets you pick between the highest, true optical resolution from the sensor, and a lower resolution that is created by in-camera interpolation. You should always pick the highest resolution and quality setting. For example, the Canon 20Da offers 3504 x 2336 pixels of true optical resolution in addition to 2544 x 1696 and 1728 x 1152 lower resolution options.
JPEG – DSLR Cameras also offer different JPEG compression settings, sometimes also called “quality” settings. Higher quality means less compression which results in a larger file size. To fit more images on a card, for normal daytime snapshot photography, some people shoot with a lower quality setting that compresses the JPEG files more. Of course, you lose information when you use JPEG because that is how it achieves such high compression ratios, it throws information away.
Raw and JPEG – If you have a camera that allows you to shoot raw file format and JPEG file format at the same time, you should try this. You can do this with the 20Da. That way the camera applies whatever in-camera settings you have in terms of contrast and sharpening to the JEPG only, and it gives you a chance to take a quick peek at what the image looks like without having to process the raw file.
The in-camera settings for contrast, saturation, color balance and sharpening are not applied to the raw file.
The raw file preserves all of the original data as it comes from the sensor in the camera.
Some cameras offer an in-camera compression for their raw file formats that is different than the compression used with JPEG files. This type of raw compression does not throw information away.
Nikon Raw Format – Nikon apparently applies a mathematical median blurring filter to their images (in addition to the low-pass filter in front of the sensor) after the in-camera dark frame subtraction for built in noise-reduction. This occurs even before the raw image is written to the file. To work around this and get a true raw file, it is necessary to turn the camera off during the in-camera dark frame acquisition after the light file is taken, but before the raw file is written to the disk. The real raw file will then be written to the disk, even though you have turn the camera off. This is true for the Nikon D70, D50, and D200, all of Nikon’s latest generation of low-noise cameras.
- Native Optical Resolution – Many cameras offer different resolution settings. The camera’s native optical resolution outputs the same number of pixels as the image is taken with. This is the setting you want to use. For example, the Canon 20Da offers a native optical resolution of 3504 x 2336 pixels, but also offers lower, interpolated resolutions of 2544 x 1696 pixels and 1728 x 1152 pixels.
- White Balance – If you want to shoot JPEG, the common wisdom is to shoot daylight color balance because this is supposed to give correct color for everything based on the color temp of white from a Sun-like star. You will, however, find that the color of the sky background is not going to be neutral or very pleasing, even when you shoot at sites with no light pollution. For sites with light pollution, its going to be really ugly.If you are shooting raw and JPEG concurrently, there is no reason not to experiment with different white balance settings since it doesn’t affect the raw file. Try Tungsten for sites with a lot of red light pollution. Or try a custom white balance, Or dial in a custom color temperature. There are a lot of ways to do it. A custom white balance on the sky itself can produce a quick fix for light pollution.
- Sharpening – Different cameras apply a different default amount of sharpening, and this amount may even change from subsequent camera model generation to generation. For example the Canon 1D applied more default sharpening to JPEG files than its replacement model 1D Mark II. Initially this made many photographers think that the camera was not as “sharp”, even though it had higher resolution. Because almost every DSLR has a low-pass filter in front of the sensor to deal with problems of anti-aliasing artifacts, the images they produce must be sharpened, either in-camera for JPEGs, or later in an image processing program. Sharpening should be applied in two distinct steps. A general low-level amount of sharpening is applied first to regain sharpness lost to the low-pass filter. A Photoshop Unsharp mask of 100 – 150 %, radius of 0.3 to 0.5 pixels and 0 – 2 levels can be used for this initial sharpening. The image should be given a larger amount of sharpening later if it is output for printing, with the amount based on the specific output type.In-camera sharpening levels for JPEGs should be determined by trial and error for astrophotography. Note that the appearance of noise present in the image will be made worse by sharpening. When done in-camera, any sharpening setting is a compromise between making the stars look better while at the same time making the noise look worse. That is why sharpening later in the work flow in an image processing program can produce better results. Masks can be made for shadow areas with the worse noise where there is no significant image detail, and highlights can be sharpened separately without aggravating the noise.
I now advise imagers to just turn sharpening off in the camera for astrophotos of star fields. You can sharpen later in processing.
- Saturation – Some cameras allow adjustment of the color saturation of the JPEG image in the camera. In general, this should be left to a “normal” setting. Increased saturation will make color artifacts from thermal noise look worse. Note that in some cameras, such as the 20Da, the default setting is for increased saturation. This can be set to normal in the menu settings.
- Color Space – This is a tricky one. In a perfect world, you would want to use the largest color space you have available, which in most high-end consumer digital cameras is Adobe RGB. However, if you do not have a full understanding of color spaces, it is very easily possible that you will not have your color management set up correctly, and using Adobe RGB incorrectly will cause you to have inaccurate color.The safer bet is to use the default sRGB color space. You can do less damage to the images in this color space. If you really know what you are doing, use Adobe RGB. If you don’t know what a color space is, use sRGB.
- Built-in Flash – Turn off, because it won’t reach to infinity. It won’t even reach to the Moon.
- Red-Eye Reduction – Turn off.
- Long-Exposure Noise Reduction – If you are going to shoot just a couple of long-exposure frames in JPEG format, then you should experiment with using the in-camera long-exposure noise reduction that is available in most new DSLR cameras. This function is slightly mis-named as it does not really reduce “noise”, but rather thermal signal.In-camera noise reduction will double the total time it takes you to take your images, but it will help by reducing the thermal signal in the images. A few cameras have such low noise that it may not even be necessary to use long-exposure noise reduction. It depends on the length of the exposure, the ambient temperature, and the camera’s ability to deal with thermal signal. Do a test and take two exposures at the length you plan to use, one with in-camera noise reduction on, and one with it off. Examine them and see how much of a difference it makes.
You should note that in-camera long-exposure noise reduction will essentially double the time it takes to take a picture. Say you want to take a one minute exposure of a subject. The shutter opens, and the one-minute exposure, called the “light frame” begins. It’s called the “light frame” because photons of light are hitting the sensor in the camera and are being counted. This exposure ends, but the image is not immediately downloaded to the memory card in the camera. Instead, another exposure begins, of the same length as the light frame, but this time with the shutter closed. This exposure is called the “dark frame”, because the camera is taking a picture of the dark, with no photons hitting the sensor from the subject. Instead it is taking a picture of the thermal signal in the camera. Because the thermal signal is consistent from frame to frame, if the frames are taken at the same temperature, the processor in the camera can then subtract the thermal signal in the dark frame from the light frame, thereby improving it.
Many people are often frustrated and confused when they take a long time-exposure, and then it ends, but the busy light stays lit on the back of the camera for what seems like way too long. They think that surely it can’t takethat long for the image to be written to a card! It never takes that long during normal daytime photos. Well, that’s right. It doesn’t take that long for normal short exposures. The in-camera long-exposure noise reduction only works for long exposures. It is taking that long because the camera is making a dark frame of the same length as the light frame and processing it in the camera before it is written to the memory card.
Once you get to a more advanced level in your astrophotography experience, you will definitely want to turn in-camera long-exposure noise reduction off. You will do better by shooting a series of dark-frame exposures yourself that you can use later in a more sophisticated way in calibrating the light-frame images.
- High ISO Noise Reduction – Again, like long-exposure noise reduction, if you are just going to shoot a couple of JPEG frames and use them straight out of the camera, you can turn it on. For more sophisticated imaging where you are going to shoot multiple frames and stack them after calibrating with darks, you should turn this setting off.
|Cameras Settings Summary
- Set Program Mode to Manual Exposure.
- Set Drive to Single Shot.
- Set ISO to 400, 800 or 1,600 based on ambient temperature.
- Turn Image Review OFF.
- Turn Auto-focus Off.
- Set White Balance to Daylight
- Set File Format to Raw (or Raw + JPEG).
- Set Optical Resolution to the highest (native) setting.
- Turn in-camera long-exposure noise reduction Off if you are planning on shooting separate dark frames. Turn On if you are not going to shoot separate dark frames.
- Turn off in-camera sharpening if you are shooting JPEGs at a high ISO.
- Set contrast and color saturation to normal if shooting JPEGs.
- Set color space to sRGB. Set to Adobe RGB if you are extremely knowledgeable about color spaces.
- Turn off flash.
- Set exposure to Bulb.
- Put Compact Flash Card in camera.
- Use a remote release to open the shutter.
- Mirror lockup is not necessary for long-exposure astrophotography with a decent mount. Turn mirror lockup on for high-resolution planetary work.