Canon 60Da CMOS Sensor Physical Dimensions: 22.3mm x 14.9mm Total Number of Pixels: 18 million Size of the Pixel Array: 5200 x 3462 pixels Individual Pixel Size: 4.3 microns square

Almost all consumer level DSLR cameras these days are being made with CCD or CMOS chips, or variations on these designs.

The chief characteristics listed by manufacturer's marketing departments that define most DSLR cameras are the physical size of the chip and the number of individual pixels on it. The size of the chip is stated in millimeters and the number of pixels is usually given in "megapixels", or millions of pixels.

For astrophotography purposes however, the characteristics we are most interested in are the pixel size, full-well capacity (how many photons can be recorded before a pixel saturates), the sensitivity of the chip, quantum efficiency, and the noise in the camera. These numbers are not usually released by camera manufacturers, but can usually be deduced from user tests. Roger N. Clark has a very informative Digital Camera Sensor Performance Summary with these numbers for various cameras.

The most important thing for astrophotography is how the camera handles the signal in relation to how much noise is present in the camera. The most recent generation cameras from Canon and Nikon perform very well in these regards.

Don't worry too much about the following technical terms now, they will be explained in detail later in Chapter 3.

Important Camera Technical Characteristics

• Quantum efficiency - The percentage of photons that hit a CCD or CMOS sensor that are detected and turned into photoelectrons.

• Pixel size - The physical size of the pixel in the pixel array in the sensor. Usually measured in microns. Larger pixels can gather more photons than a smaller pixel in the same amount of time, and also store more. Smaller pixels yield more resolution.

• Filter Factors The sensor in a DSLR camera has a series of filters, such as Bayer and long-wavelength filters, that light must pass through. The filter transmission characteristics, which wavelengths are passed and in what amount, are important to the efficiency of the photon recording process. Stock cameras, for example, do not pass much of the hydrogen-alpha wavelength through the long-wavelength filter, so it is harder to record red emission nebula.

• Fill Factor - The percentage of a photosite (pixel) that is actually sensitive to light. Pixels have electronics on them, so the total surface area is not gathering photons.

• Noise - Noise is random and non-repeatable signal in an image. Cameras have repeatable signal, such as thermal signal, that can be removed with calibration frames. But every signal also has noise associated with it. Read noise and thermal noise, cannot be removed, and must be dealt with by gathering more signal.

• Dark current - Also called thermal signal or thermal current, is signal that is created from electrons released by the thermal energy in the sensor substrate, even when it is not exposed to light. Dark current can be removed with a dark calibration frame, but its associated dark noise can not. More dark current signal leaves more dark current noise behind after calibration, which is why low dark current in a DSLR is important.

• Full well capacity - The number of electrons converted from photons that can be stored in a pixel's well. The Full-well capacity divided by the noise determines a camera's dynamic range. Dynamic range is the brightness range that can be recorded with detail in both light and dark areas.

• Gain - Gain defines how many electrons are represented by each Analog to Digital Unit (ADU), a number that represents the CCD or CMOS sensor's digital output. DSLR cameras can change ISO by changing the gain.

• Signal-to-noise (S/N) ratio - A measure of the quality of a signal, expressed as the ratio of the signal to the noise present.

• Bit depth - Describes the number of steps of tonal resolution, or brightness levels, into which the dynamic range is divided.

Camera manufacturers use clever technological tricks and sophisticated image data handling inside the camera to produce low-noise cameras. The noise characteristics of the latest generations of Canon and Nikon cameras produced since about 2004 - 2005 have been very good.

Sensitivity, on the other hand, is directly related to quantum efficiency, pixel size and filter factors. The quantum efficiency of DSLR sensors seems to have been staying relatively fixed over the most recent generations of cameras.

For normal daytime photography, where you have a lot of light and signal, smaller pixels are not a problem. Smaller pixels mean more resolution. More resolution means you can make larger prints. But in the real world, you are not going to see much difference in the results from an 8-megapixel camera compared to a 10-megapixel camera.

As camera manufacturers shrink the size of pixels in their sensors, they have come up with some very effective techniques, such as using microlenses, to overcome the inherent problems that smaller pixels create. The total area of an individual pixel is usually not sensitive to light, only a certain percentage is. Some of the space is taken up by electronics on the pixel. One of the best methods is to use a small microlens over each pixel to gather photons from a larger area than the actual light-sensitive area on each pixel. This effectively increases the signal that is gathered for each pixel.

What camera manufacturers seem to be improving in each new generation of DSLR cameras is noise. This is very important for low-light photography. Lowering the camera's readout noise, and sophisticated signal processing to reduce thermal signal really help produce low-noise cameras which are great for astrophotography.

These improvements in camera technology work very well. Every new generation of camera seems to come out with more megapixels because this is a very attractive marketing feature. These smaller pixels have microlenses, and better electronics and better digital image processors, and the overall signal-to-noise ratio of these most recent cameras is very good.

Physical Size of the Sensor

This is the size of the area at the focal plane that the sensor occupies. It is normally about the size of a postage stamp in typical consumer DSLR cameras.

"Full frame" sensors are 36mm x 24mm. This is the same size as traditional 35mm film and are usually found in the top-of-the-line professional model cameras.

Canon sensors come in three basic sizes: approximately 36mm x 24mm, 29mm x 19mm, and 22.5mm x 15mm. Canon's entry level cameras all have 22.5mm x 15mm sensors.

Nikon sensors come in two basic sizes: approximately 24mm x 16mm and 36mm x 24mm.

Field of View and Crop Factor

In the days of film, most photographers considered 35mm cameras as a kind of standard. Many photographers were used to thinking of lens coverage in terms of 35mm film, which was actually 36 mm x 24 mm in rectangular size. When digital cameras came along, their sensor sizes were initially smaller than 35 mm film. When the same lens was used on these digital cameras as on a 35mm film camera, they gave a smaller field of view because of the smaller sensor size. This became popularly referred to as a "multiplier" factor for lens coverage. A camera such as the Nikon D5000 was said to have a 1.5x multiplier factor. Photographers began to think that this was cool, that they were getting something for nothing. For example, a 200mm f/2.8 lens would somehow, magically, become a 300 mm f/2.8 lens! Camera manufacturers didn't exactly discourage this kind of perception. It is, however, incorrect.

A smaller sensor does not multiply anything. What it does is crop the field of view. The image scale is exactly the same. A 200 mm lens still produces exactly the same size image as it did on a 35mm film camera, only the smaller sensor in the digital camera cannot take it all in, so it crops out the difference in size to give the same size field of view as a 300mm lens on 35mm film. If it was film, this would not be a good thing because you would have increased grain and a loss of sharpness because you would have to enlarge the image more. Luckily with the increased resolution and lower noise of digital sensors, this is not as much of a problem.

Photographic sensors in DSLR cameras come in several different crop factors: 1.0x, 1.3x, 1.5x, and 1.6x. The crop factor considers a 35mm negative (36 x 24mm) to be standard, so a 36 x 24mm sensor would have a crop factor of 1.0x. A 23.7 x 15.6 (rounded to 24 x 16) would be a 1.5x crop factor (36/24 = 1.5) and so on.

Canon entry-level and advanced-amateur DSLR cameras have a 1.6x crop factor. Equivalent Nikon models have a 1.5x crop factor. Full-frame 1.0x crop-factor sensors are only found in top-of-the-line professional models.

Pixels and Megapixels

Digital sensors have a certain number of individual pixels in them. Technically they are called photosites, but everyone calls them pixels. The total number of actual pixels determine the true optical resolution of the camera.

The optical resolution is easily determined by counting the number of actual pixels in the sensor by multiplying the number in one dimension of the array by the number on the other side. A checker or chess board has 8 rows by 8 columns of squares, therefore it has 8x8 squares, for a total of 64 squares. Think of pixels the same way.

For example, Canon's 60Da camera has a 22.3 mm x 14.9 mm rectangular sensor that has 5184 pixels on the long side and 3456 pixels on the short side. This makes up a mosaic, or grid of 5184 x 3456 pixels, for a total of 17,915,904 pixels. We round this number and call it 18 million pixels, or 18 megapixels, with mega meaning million.

For most photography, the more pixels you have the better. More pixels mean higher resolution. More pixels also mean that you can make larger prints from the original file.

Don't think that the number of megapixels in a camera sensor is the most important thing to consider in buying a camera. More pixels in the same space mean more resolution, but also require higher quality optics to deliver this resolution.

In the real world many factors conspire to produce an image that does not take advantage of the full resolution of the sensor anyway. For example, for daytime photography, most shots are hand held, which reduces image resolution because of camera shake. For astrophotography, the seeing in long exposures usually reduces resolution.

For very high megapixel cameras, you also need excellent optics and top-of-the-line camera lenses and telescopes to take advantage of their resolution.

Spatial Resolution

The number of pixels define the sensor's spatial resolution. For example, the Canon T2i (550D) has 18 million pixels in an array of 5184 x 3456 pixels in a 22.3 mm x 14.9 mm rectangular space. That means each pixel is 4.3 microns square in size.

Smaller pixels can resolve more detail, such as in planetary photography, if the optics are capable of delivering it.

Dynamic Range

Dynamic range is the amount of difference between the brightest and darkest parts of a scene, or image, that a camera can record.

Although human visual perception can encompass an enormous range of brightness, imaging devices cannot. They can only record detail in the highlights and shadows within a particular brightness range. In photographic terms, for today's high-end consumer DSLRs, this dynamic range is usually about 8 - 10 stops for a raw file.

The dynamic range in a digital camera is defined as the full-well capacity divided by the noise. The full well capacity is the number of electrons that can be stored in a pixel.

Dynamic range usually goes down as higher ISO speeds are used in a digital camera.

Camera Noise

Noise and unwanted signals are some of the biggest problems we have to face with digital cameras, especially for astrophotography. Some cameras are much better than others in controlling noise. This is accomplished by sophisticated chip designs and image-processing algorithms made possible by powerful computer chips inside the camera.

Other strategies, such as in-camera dark-frame subtraction, are also employed by camera manufacturers to deal with long-exposure thermal current created by heat inside of the camera. In this process a "dark frame" (where no light hits the sensor) of equal exposure length to the "light" frame (the actual real exposure of the object of interest) is made immediately after a time exposure. Usually any exposure longer than 1 second can be affected by thermal noise. This dark frame takes a picture of just the thermal current and then it is subtracted from the light frame to remove the thermal signal and improve the image. In-camera long-exposure noise reduction works fairly well for normal photography, but astrophotographers have found it better to make their own separate dark frames and process them and the light frames later. This will be explained in detail in the chapter on how digital cameras work.

ISO and Sensitivity

The ISO rating of a DSLR digital camera usually specifies its sensitivity to light in relation to the level of its digital signal output. ISO can be measured different ways in digital cameras. ISO is an abbreviation for International Organization for Standardization that sets standards and definitions for how ISO sensitivity is measured.

Digital cameras with CCD and CMOS digital sensors really only have one basic level of sensitivity to light. This sensitivity is defined by the sensors quantum efficiency, fill factor, microlens array, Bayer filter transmission characteristics, and other factors. These can't really be changed once a camera is manufactured.

DSLR cameras have the ability to change ISO in the camera, but this is done by changing the gain and electronically amplifying the signal that the sensor produces.

Most digital sensors have a native ISO of around 100 or 200, depending on how it is measured, but the ISO can be set to higher speeds. This, however, decreases the signal-to-noise ratio in the image when the exposure is reduced as the ISO is increased. For a given light level, shorter exposures mean less signal, at any ISO.

Compared to film, when measured at the same signal-to-noise ratio, the CCD and CMOS sensors in DSLR cameras are much more sensitive than film.

Quantum Efficiency

A digital camera's quantum efficiency describes the percentage of photons that hit a CCD or CMOS sensor that are actually detected. If half of the photons are detected, then the quantum efficiency of the sensor is 50 percent. Quantum efficiency varies by the wavelength of light.

A sensor's quantum efficiency is important because a camera with a higher quantum efficiency can produce a better quality image in a given time because it records more photons.

Most one-shot color cameras, either DSLR or specialized astronomical cameras, have similar quantum efficiency.

Monochrome astronomical CCD cameras usually have better quantum efficiency than one-shot color cameras.

Red Sensitivity and Long-wavelength Filters

Normally the CMOS and CCD sensors used in DSLR cameras are very sensitive to red light.

Most DSLRs however, are designed with a built-in, low-pass, long-wavelength cutoff filter in front of the sensor. The low-pass characteristics of the filter are designed to prevent moire and aliasing. The long-wavelength filtration is designed to improve color reproduction and make it more like our human visual perception.

The problem is that the long-wavelength filtration also filters out most of the light at 656.5nm, the wavelength of hydrogen-alpha. This is the wavelength of the red light that is emitted by hydrogen emission nebulae. Filtering out most of this light is a problem for astrophotography if you want to take pictures of these red nebulae, which are some of the largest and most beautiful objects in the night sky.

Canon made the 20Da and 60Da, a DSLR camera body specifically made for astrophotography to address this problem. These two cameras have a special long-wavelength filter that passes more of the hydrogen-alpha wavelength. In the Canon 60Da, the filter passes 3 times more hydrogen-alpha than the normal stock 60D.

It is possible to modify a DSLR camera for astrophotography of emission nebulae by removing or replacing the long-wavelength filter and substituting one that passes almost all of the hydrogen-alpha emission wavelength. A replacement filter of the same thickness maintains the camera's autofocus and manual visual focus capabilities. A replacement filter that filters out ultra-violet and infra-red wavelengths past the hydrogen-alpha wavelength can be very useful in refractive optical systems where color correction is important.

Removing or replacing the long-wavelength filter, of course, invalidates the manufacturer's warranty and runs the risk of ruining the camera if not done correctly. However, astrophotographers have successfully removed the filters in both Nikon and Canon cameras and produced excellent images of objects with red hydrogen-alpha emission wavelengths.

Third-party vendors such as Gary Honis, Hap Griffin, Lifepixel, MaxMax, and Andy Ellis in the UK can modify your camera by removing or replacing the long-wavelength filter for a fee.

Bit Depth and Tonal Resolution

In addition to their spatial resolution and dynamic range, each camera also has a tonal resolution. This means how many different tones, or steps of gray, can be differentiated from the darkest to the brightest tones that the camera can record.

CCDs and CMOS sensors are actually analog sensors. But because we want to work on an image in a computer, we have to digitize it and break the continuous tones up into discrete steps so they can be represented by whole numbers that a computer needs to work with. This is called quantizing the information and it is done by the analog to digital converter in the sensor.

Tonal resolution is described in bit-depth. A "Bit" is a Binary Digit. A bit can either be on or off, a one or a zero.

Perhaps because we have 10 fingers, we normally work with 10 digits (0 through 9) in a numbering system called base 10.

Computers work with only two digits, a base 2 numbering system, because they work with transistors instead of fingers. Transistors used in digital electronics are operated in an on-off mode, so when they represent a number it can only be a one (on) or a zero (off).

Base two numbers are written in binary notation and are usually talked about in terms of "bits". The "bi" in binary meaning two, like the two wheels on a bicycle.

Bit Depth is represented in exponential notation in the form 2^x, or 2 to the Xth power, which means 2 times itself x number of times.

We normally just ignore the exponential notation and say that a tonal depth of 2⁸ is 8 bits. This represents 256 individual steps of tone.

For example:

2² means 2 x 2 which equals 4.

2³ means 2 x 2 x 2 which equals 8.

2⁴ is 2 x 2 x 2 x 2 = 16.

2⁸ is 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 = 256.

Graphically:

8 bits of tonal depth, with 256 steps, is the minimum required to fool the eye into thinking there are continuous tones in an image.

DSLR cameras usually work in 12 bits of tonal depth, or 4096 individual steps of gray. The latest models are now working in 14 bits or 16,384 steps. Dedicated astronomical CCD cameras usually have 16 bits of tonal depth or 65,563 steps.

More bit depth is especially helpful in the shadow areas because detail can be differentiated from noise. Greater bit depth allows better tonal manipulations and improved shadow detail. Two colors or gray tones that may be very close can be distinguished at a higher bit depth. A lower bit depth may record those two different colors or tones as the same.

The more bit depth we have, the closer the approximation to a continuous tone image we have. If major editing must be done to an image, working at a high bit depth is a great advantage. Having more steps between black and white helps when we start manipulating the data to enhance the parts of the image that we are interested in. When we stretch the data in a curves or levels operation, the limited number of steps we start with can become an even smaller number, and posterization can occur. Posterization is when a visually continuous tone breaks up into perceptible steps or banding of tone or color.

Bit Depth and Color

Color images are made up of three individual black and white channels, one each of red, green and blue. Each of these grayscale channels divides the color they represent into the number of steps defined by the bit depth. Because there are three channels, the total number must be multiplied by three to get the bit depth for the color image that these three channels represent. Therefore, 8-bits per 3 color channels equals 24 bits total. Each channel has 256 steps, so 256 x 256 x 256 produces 16,777,216 possible colors.

Even though our cameras may record information in 12 or 14 bits, and our computers can easily work with them, our output devices are almost always limited to 8 bits per channel, or 24 bits total. Monitors and printers work in 24-bit color. Before outputting an original image file, we must convert it to 24 bits for color output. Still, this leaves us with more than 16 million colors to work with, which are usually more than enough.

File sizes

The file sizes that digital cameras create can be confusing for a number of different reasons.

• The Basic Calculation - The basic amount of information stored in an image file is dependant on the number of pixels and the number of steps that the dynamic range of each pixel is broken down into (the bit depth). If we multiply these two factors together, we get the true file size when the image is created in the camera.

  For example, the Canon 20Da has a sensor with 8,185,343 pixels, and it works at tonal depth of 12 bits (4096 steps).

  8,185,344 pixels x 12 bits = 98,224,128 bits.

  Computers store information in bytes of 8 bits, so we divide by 8...

  98,224,128 / 8 = 12,278,016 bytes, or about 12 megabytes. This is the actual data that is created when we take an image.

  Because Canon uses lossless compression, these 12 megabytes can be shrunk down to a smaller file size, depending on the amount of information present in the image. They can usually be shrunk down to about 8.7 megabytes for a single raw file. This is what is stored when it is written to the compact flash memory card as a raw file.

• Bits and Bytes - The file stored on the memory card in the camera is 12 bits. A bit is a binary digit, either a 0 or 1. However, when the image comes out of the camera and goes into a Computer, things change. Computers work with bits and bytes, they can't work with anything in between. A byte is 8 bits. So anything larger than one byte but smaller than two bytes, must be labeled as the next largest full unit, two bytes.

  If we have a camera that is capable of delivering 12 bits, that is larger than a single byte (8 bits) and must be specified as two bytes (16 bits). The 12 bits of real information are specified as a 16-bit number with null bits (zeros) added to pad the number to 16 bits (2 bytes).

• Color In 3 Channels - CCD and CMOS digital sensors are really grayscale devices, they only record black and white and shades of gray. They produce color images by placing color red, green and blue filters over each pixel and synthesizing the color data later. So when the data is stored in the camera as a raw file, it is really grayscale data, that's why it only requires 1 channel. When the file is opened later and the color is created, it requires 3 channels.

• 8 Bits, 12 Bits, 16 Bits - The camera works in 12-bits of tonal depth. The computer works in 16 bits. Most output devices work in 8 bits. File sizes change as we go from one bit depth to another. In a Canon 20Da camera, the original file is about 12 megabytes for one channel of grayscale information. This is padded up to 16 megabytes when the image is opened in the computer. It is also turned into 48 megabytes when the color is created (16 x 3 = 48). When the image is converted down for output to an 8-bit-per-channel device such as a desktop printer or computer monitor, this 48MB file in changed into a 24MB file.

• Computers Count to a Million Differently - Here is the math for the image when opened: 3504 x 2336 pixels = 8,185,344 total pixels. Multiply this by 3 channels and then by 16 bits per channel.

  8,185,344 x 3 x 16 = 392,896,512 bits.

  To turn 8 bits into 1 byte divide by 8. You may calculate (in your head if you are really good) 392,896,512 bits divided by 8 = 49,112,064 bytes. But if you save the image, and look at its properties, it is listed as 46.8 MB (49,127,424 bytes). What happened?

  The original data of 49,112,064 bytes turned into 49,127,424 bytes because there was 15,360 bytes of overhead added to the file for the specific file type, which was TIFF in this case. Basically this overhead is header info that describes the kind of file it is, how the data is organized, and other information.

  The really curious question is why is it listed as 46.8 MB and not 49.1 MB.

  This is because computers count in powers of two and consider 1,048,576 to be a megabyte (a million bytes), not 1,000,000.

  So for the full calculation:

  we divide 392,896,512 by 1,048,576 to get 374.6953125 megabits

  and dividing by 8 to get bytes, we get 46.8369140625 megabytes,

  or rounded off, 46.8 megabytes.

• uncompressed File Sizes Are Different than Compressed File Sizes
  Original files in the camera can be saved in different flavors. Uncompressed raw files are usually the largest, while compressed JPEG files are the smallest.

  Raw files are the largest because they contain the original, raw data from the camera's sensor. Some manufacturers compress this raw data with proprietary lossless file formats that do not throw away any data but save some space.

  JPEG compression on the other hand, is called lossy (as opposed to lossless) because it throws away data to get a smaller file size. JPEG compression does a pretty good job of it, shrinking an 12mb file down to about 5mb at a high quality compression ratio.

  Nevertheless, because we have put so much effort into counting photons from so far away, and because it gives us so much more flexibility in working with the data later, we should always shoot raw format, because this preserves the original data as it came from the sensor. Who knows, in the future, a new way of interpreting this data may be developed that is superior to what we have today. The raw file should be considered a precious original that is archived and safeguarded.

• Different Compression Ratios produce Different JPEG File Sizes - When a file is compressed with a lossy compression algorithm such as JPEG, different quality settings can be chosen that vary the resulting file size. The more information that is thrown away, the greater the reduction in file size. However, the more information that is thrown away, the greater degradation of the image, and the worse it will look.

  Original images should always be archived in the camera's raw file format. Once the file is opened in an image processing program and calibrated, corrected and enhanced, it should be saved in a uncompressed file format such as TIFF, and archived. It should only be saved as a JPEG when reduced in size and displayed on the web or sent in an email.

• Cameras Offer Different Selectable In-Camera Resolutions - Many DSLR cameras are also able to save images in other resolutions than the true optical resolution. To accomplish this an algorithm interpolates the original raw data in the camera. For example, the 3504 x 2336 pixel array in the Canon 20Da can also output files that are 2544 x 1696 or 1728 x 1152 pixels. This however is not true "binning" as understood in the astronomical CCD community.

  True dedicated astronomical CCD cameras can "bin" pixels together, where neighboring individual pixels are combined together to form a sort of super-pixel. These cameras can bin pixels in a 2x2 box or larger combinations. Although this cuts down on the spatial resolution of the camera, it makes the binned pixel much more sensitive to light because it has a larger area which can collect more photons in a given amount of time, and this larger area can store more total photons.

Raw vs JPEG

The raw file in a dedicated astronomical CCD camera is the unmanipulated data straight from the sensor. This is the most pristine data you can get out of the camera. In DSLR cameras, the true raw data is almost always massaged in some way before it is written to the raw file, so in truth, it is not what purists would consider raw. However a DSLR camera raw file is the most basic data we can get to work with that does not have tonal curves, sharpening or color correction applied to it. DSLR camera raw data can be better calibrated with dark, bias and flat-field frames than can JPEG data.

Most of us just shoot JPEG files in the camera for normal daytime images. The JPEG file is processed in the camera according to the parameters we set in the menus for sharpening, contrast and color, to make it look pleasing. The camera will do a very good job of processing the image of a normal daytime scene if it is exposed correctly. It will also do as good a job as it can on an astronomical image, however there is usually much more that we can do to the original raw data to make it look better for astronomical subjects. This is why we should always shoot raw files for astrophotography. The additional bit depth, especially, will help tremendously when we stretch the image and increase its contrast to bring out faint details.

JPEG Quality Settings

DSLR cameras usually offer different JPEG quality settings which vary the amount of compression and the resulting quality of the image. They are usually labeled with such terms as "Fine", "Normal" and "Basic".

Fine would be the highest quality setting, which would give the smallest compression ratio and yield the largest JPEG file size. Basic would give the highest compression ratio and yield the smallest JPEG file size, but would also be the worst quality.

No matter what the JPEG quality setting is, the file, when opened will always open to the true optical resolution of the sensor if that is the resolution that was selected in the camera.

For example, a Normal, medium-quality JPEG image file that was shot in the camera at full optical resolution may be stored on the memory card in the camera as a 3 MB JPEG file. A Fine, high-quality JPEG image file that was shot at the same full optical resolution may be stored on the memory card in the camera as a 5 MB file. They will both open to 23.4 MB files when opened in an image processing program. The Fine-quality JPEG will have much better quality in terms of color and detail and will have less JPEG compression artifacts than the Normal-quality JPEG file.

In-Camera Adjustments

When shooting JPEGs in the camera, the original raw data must be processed to make it look like a normal photograph that is suitable for human visual perception.

Most of this processing is transparent to the user, but several different parameters can be adjusted in the camera by the user:

• White Balance - changes the white point so that colors are adapted for different lighting color temperatures.

  For example, shooting an image with the camera set to daylight white balance when the subject is lit by tungsten light will result in a very red image. Correctly setting the white balance to tungsten for a tungsten illuminated scene will yield correct colors.

  The night sky, however, is not illuminated in the same way as a normal terrestrial scene. The stars, nebulae and galaxies are their own light source. To correctly record their colors, it is usually recommended that a daylight white balance be used because our eyes evolved under daylight and this is what we consider normal. However, the night sky will be colored by natural air glow, and the atmosphere can be contaminated by light pollution. This can give the sky background an unappealing brown color when daylight white balance is used, but, it is actually correct. This can be corrected by adjusting the colors in subsequent image processing.

• Color Adjustments - Some cameras allow further adjustment to the color in addition to the white balance setting. Like using light balancing and color compensating filters with film, you can adjust the color on an amber-blue axis, or green-magenta axis, or combination of both.

• Contrast - Contrast of the processed JPEG image can be increased or decreased to a degree.

• Tonal curve adjustment - Some cameras allow the uploading of a custom tonal curve into the camera for processing of the image.

• Saturation - The color saturation can be increased or decreased.

• Sharpening - Sharpening is usually required because of the low-pass blurring filter in front of the sensor. The amount of sharpening can be controlled, from none to a fairly high amount. In-camera sharpening can easily make high-ISO and thermal noise much worse.

These in-camera settings are only applied to JPEG files, they are not applied to raw files.

DSLR Camera Features

In addition to camera attributes, manufacturers also offer dozens and dozens of camera features. We won't cover all of them here, but we will discuss those of interest to astrophotographers.

• Live View - The most useful feature in new cameras is live-view focus. Once you use this feature, you will never want to go back to your old method of focusing for astrophotography.

• Remote Live View - The live view video that is normally displayed on the LCD on the back of the camera can also be output to a separate monitor via an analog cable. It can also be viewed on your computer if you use the Camera manufacturer's software to control the camera through the USB2 cable.

• Single-Wire Remote Control - with older cameras, two cables were needed to completely control the camera for long-exposure astrophotography with a computer. One USB cable controlled camera functions like ISO. Another serial cable ran to the camera's bulb port for exposures longer than 30 seconds. The latest model Canon cameras can control all functions, including Bulb, as well as giving a remote Live View through a single USB2 cable. Nikon cameras still require two cables.

• Noise Reduction

  • Long-Exposure Noise Reduction - uses an in-camera dark frame taken after the initial exposure and subtracts it to remove the thermal signal from heat inside the sensor.
  • High-ISO Noise Reduction - reduces color noise at high ISO settings.

• Amp Glow Newer cameras also have much reduced, or totally eliminated, amp glow, the red glow on the edges of the frame caused by electroluminescence in the transistors in the readout amplifier.

• Dust reduction - Canon and Nikon use a technology that shake dust off of the filter in front of the sensor through high-frequency vibrations. The ability to take a picture of the dust and then subtract it later is software is also offered.

• Miscellaneous Features

  • Framing Rates: for example, the Canon 40D will shoot at a maximum of 6.5 frames per second (fps) compared to the Canon 30D which could shoot at 5.0 fps.

  • Buffer Size: The Canon 40D could shoot up to 75 JPEG Large/Fine images whereas the Canon 30D could shoot up to 30 JPEG Large/Fine images.

  • Number of Autofocus Points is increased in later model cameras.

  • LCD Size: in the Canon 30D the LDC on the back of the camera is 2.5 inches. On the Canon 40D it is 3.0 inches. However, both still use the same 230,000 pixel display, so although the display is larger, the resolution is the same. Newer cameras, such as the Canon 50D and Nikon D3 and D300 have a very good high-resolution 3-inch display with 920,000 pixels.

  • sRAW is a smaller file-size raw format offered by Canon.

  • Faster In-Camera Processors: Canon's latest cameras use faster and more powerful DIGIC III processors, just as Nikon does with their EXPEED processors.

  • Extended Dynamic Range for Highlights: Canon's Highlight Tone Priority and Nikon's Active D-Lighting give more dynamic range by changing the gain and modifying the tonal curves at the cost of a bit more noise in the shadow areas.

  • Interchangeable focusing screens

  • Memory Card Type: Compact Flash vs Secure Digital cards.

  • Spot metering - useful for shooting the Moon and Sun, but not deepsky.

  • Tiltable LCD - makes it much easier to view the image on the LCD when the scope puts the camera at an awkward angle.

  • Video Capture - allows recording of high-definition movies to the memory card in the camera.

  Many of the features of newer cameras, such as higher framing rates, larger buffers and more auto-focus points, would be of interest to, say, sports photographers, but not of much help to astrophotographers.

High-Definition Video

Some features, like the ability to shoot high-definition video, are not useful for deep-sky astrophotography, but can be a lot of fun for normal daytime photography.

Some astrophotographers ask if high-definition video can be used for high-resolution planetary work. The problem is that to produce the 1920 x 1080 pixels used in the high-definition video, the sensors original native resolution must be downsampled. This is not a good thing for imaging high-resolution planetary detail.

However, Canon's 60D, 60Da and 550D (Digital Rebel T2i) have a special video recording mode called Movie Crop Mode that crops the central 640 x 480 pixel area of the sensor and record VGA video at 640x480 pixels at 60 frames per second. This can be very useful for planetary imaging.

Canon's other cameras, that do not record high-definition video, but that do have Live View, can also be used in another manner for planetary imaging. The analog video signal from the Live View mode can be captured on a computer. When the 5x zoom function is used on the Live View, the resulting video is captured at 1:1, or close to 1:1, to the sensor's native resolution. This video can also be used for high-resolution planetary imaging.

The captured video can then be used in a program like Registax where the software picks out only the best frames for sharpness, and stacks them to improve the signal-to-noise ratio.