Which Digital Camera to

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Which is the best digital camera to buy? A frequent question but one to which there is not one answer. It is equivalent to asking which telescope is best. Some telescopes are good for the planets, some are best for deep-sky. Others are optimized for portability, while some require an observatory. It is the same for digital cameras and the CCD chip at their heart. There is a further limitation in that the CCD should be optimized not only to the type of objects to be imaged but to the telescope as well. For many starting out in digital imaging who are asked what their target objects are, the answer is everything! That makes choosing a single camera a little difficult, and some compromises will have to be made.

The starting point for choosing a camera should always be the individual pixel (photosite) size. These are usually square, but not always for video-derived chips, and are measured in microns (1/1000 of a millimeter). Common sizes currently range from 6 to 24 microns (see Figure 1.4). As you might have guessed, generally

KAF-4300E 2084 x 2084 50.0 x 50.0 mm 24 micron pixel

KAF-16801E 4096 x4096 36.9 x 36.9 mm 9 micron pixel

CCD42-40 2048 x 2048 27.6 x 27.6 mm 13.5 micron pixel

KAF-1001E SITe S1003 1024x 1024 24.6 x 24.6 mm 24 micron pixel

KAF-4300E 2084 x 2084 50.0 x 50.0 mm 24 micron pixel

KAF-16801E 4096 x4096 36.9 x 36.9 mm 9 micron pixel

CCD42-40 2048 x 2048 27.6 x 27.6 mm 13.5 micron pixel

KAF-6303E 3088 2056 27.8 x 18.5 mm 9 micron pixel

KAF-1001E SITe S1003 1024x 1024 24.6 x 24.6 mm 24 micron pixel

KAF-4202 2048 x 2048 18.4 x 18.4 mm 9 micron pixel

KAF-1301 (L) E KAF-3200E KAF-1602E

1280x 1024 2184x 1472 1536x 1024

16 micron pixel 6.8 micron pixel 9 micron pixel

KAF-6303E 3088 2056 27.8 x 18.5 mm 9 micron pixel

CCD47-10 CCD77

1024x 1024 512x512

13 micron pixel 24 micron pixel

CCD47-10 CCD77

1024x 1024 512x512

13 micron pixel 24 micron pixel

SITe1502 KAF-261E

512x512 S12xS12

24 micron pixel 20 micron pixel

KAF-1401E KAF-401E

1320x 1037 768x521

6.8 micron pixel 9 micron pixel

Figure 1.4. Sizes of common CCDs. (Courtesy Finger Lakes Instrumentation).

the low end ones are best for smaller telescopes and the high end for bigger telescopes. However, there is a bit more to it than that. It comes down to what is referred to as sampling, the actual number of pixels used to resolve detail. We need to be able to resolve the detail that our telescope and location are capable of delivering. Unfortunately, some math is needed to work this out. As a general rule we need to exceed the resolution of our telescope by a factor of at least 2, a bit more helps in the case of the planets.

So what is the resolution of your telescope and location? For long exposure deep-sky imaging from a suburban backyard, where atmospheric turbulence is the norm, it is probably only in the range of 3 to 5 arcseconds. Nothing like the theoretical figures quoted in telescope specifications! To exceed this by 2, we need a plate scale (a term from the old photographic days!) of say 2 arcseconds per pixel. To calculate this, use the formula:

plate scale (arcseconds per pixel) = 206.3 x pixel size (microns) / focal length (mm) or optimum pixel size = plate scale (arcseconds per pixel) x focal length (mm) / 206.3.

For a focal length of 2000mm and pixels of 20 microns, this computes to 2 arc-seconds per pixel - a good match for our telescope at its poor location. If a tele-compressor is used to reduce the focal length to around 1250mm then the result is 3.3 arcseconds per pixel, which is insufficient, and this would be referred to as undersampling. This would be okay for, say, supernova detection but the resulting image would not be very photo-realistic - stars would be blocky and square. Undersampled images cannot be "improved" much by image processing so, generally, it is better to err by over- rather than undersampling if "pretty pictures" are the goal. A better choice for the reduced focal length would be pixels around 9 to 13 microns.

The example given referred to deep-sky imaging. However, it is a somewhat different story for the Moon and planets. Here we will wait for optimum seeing, and exposures are so short that suburban seeing turbulence is much reduced. If we redo the math with the same 2000mm focal length, a typical telescope resolution of around 0.5 arcsecond maximum gives a plate scale of 0.25 arcsecond per pixel. For this we find we need 2.4 micron pixels. Clearly we would need to magnify the image using eyepiece projection or a Barlow lens (i.e., increase the effective focal length) to match the typical pixel sizes available to us. A point to note from this is that for Jupiter, at 50 arcseconds maximum size, we will need a CCD with around 200 to 250 pixels across to resolve all the detail. However, a bit more helps, and oversampling is always better for the planets where we will want to "sharpen" the raw images considerably.

Color

We live in a colorful universe, so why not image it in color? Until recently, CCD imagers have only been monochromatic. What has changed this is the arrival of the X3 sensor from Foveon (strictly it is a CMOS imager not a CCD), which stacks three color-sensitive pixels one on top of another. For all other chips the only way to create a color image is by means of color filters. In a single-shot color imager each photosite has its own color filter built on top of it. So some will have blue, some green and the others red filters (some sensors use the complementary colors). These filters are arranged in a special repeating pattern across the chip and the camera software is able to change the discreet color values into a smooth color image (see Figure 1.5). For the more normal mono CCD we have to use 3 color filters, in turn, in front of the whole chip. The number of images we need to take is immediately tripled. The single-shot color imager might seem the answer to all our needs but the mono imager has the benefit that, when used in mono mode, it has no filters reducing the light reaching the chip or, when used with a filter, all pixels are recording an image. The single-shot color on the other hand, for mono work, is approximately 3 times slower or less efficient - even more when you consider that a typical unfiltered CCD is highly sensitive in the near infrared as well. So if your primary interest is deep-sky then a mono imager plus separate filters makes a lot of sense but if your interest is the planets or the brighter deep-sky objects then a single-shot color imager does make life easy.

Figure 1.5. The color filter array (CFA) over the top of the sensor is needed so that it can create a color image. Without the CFA a sensor could only produce a grayscale image. In this array (Bayer) each sensor now only detects red, green or blue light.

Figure 1.5. The color filter array (CFA) over the top of the sensor is needed so that it can create a color image. Without the CFA a sensor could only produce a grayscale image. In this array (Bayer) each sensor now only detects red, green or blue light.

For general deep-sky images a practical method of color imaging has emerged, known as LRGB (lightness or luminance, red, green, blue). Briefly this comprises adding, to our best monochrome image, the color information from a lower resolution one. This lower resolution image can be taken with the camera set to 2 x 2 binning, which will increase sensitivity but reduce exposures by 4 times. Alternatively, the color information can come from a different color CCD camera (if you have a friend with a Starlight Xpress) or even an old 35mm color slide.

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