All About Space Astronomer Book - 2014 UK

example, an image size of 3,504 x 2,336 pixels would contain 8,185,344

pixels or 8.2 Mpx. The number of image sensor photosites is also typically

represented by a megapixel count.

CMOS vs CCD

Digital cameras typically use one of two types of imaging chip – CCD or

CMOS. CCD or Charge Coupled Device sensors use photosites which

convert incoming light into charge. CMOS or Complementary Metal Oxide

Semiconductor sensors have additional circuitry next to each photosite

which converts incoming light to a voltage.

Both types have pros and cons but the main objection to the newer CMOS

technology when used for astrophotography came about because of higher

noise levels. However, as the technology has matured, the gap between the

two types has decreased.

Types of Digital Camera for Astrophotography

So far we have described the fundamental basics of digital sensors, the

device that sits at the heart of a camera. Now we’ll have a look at the

cameras themselves. The number of digital cameras which are able to take

astronomical photographs is quite large and in some cases can actually be

rather surprising.

Photographic Digital Cameras

Photographic digital cameras are cameras which are normally used for

general photographic purposes but which can also be coerced into use for

astrophotography. There are a lot of devices which fall under this banner,

from basic point-and-shoot models through to highly sophisticated digital

single lens reflex cameras (DSLRs) which, as we’ll see later, are capable of

taking over the role that was so well occupied by film cameras not so many

years ago.

FITS (extension .fit, normally uncompressed)

The Flexible Image Transport System, or FITS, defines an information

storage standard commonly used for astronomical images and data.

Cooled astronomical CCD cameras often store their imaged results in FITS

files. The FITS standard, like TIFF, is flexible and extensible. Many dedicated

astronomical image processing programs can read FITS files and many

general image editors can open FITS files via plugins. As well as image data,

a FITS file can contain extra data pertinent to the image itself. For example,

the temperature of the CCD chip can be recorded and then this data is used

by certain programs to match the image with calibration files taken at the

same temperature.

Camera Raw (extension depends on camera manufacturer,

uncompressed)

Many high-end cameras will allow their images to be stored in a RAW

uncompressed file which can be read by proprietary software or possibly

by general image editing programs if the format is from a popular make

of camera. These files are, in theory, untouched by the camera’s internal

tweaking routines and represent the purest form of image that the camera

can deliver. RAW files are popular with deep-sky imagers for this reason.

AVI (extension .avi, compressed or uncompressed)

The Audio Video Interleave format is, as in the case of TIFF files, a transport

framework for video data. As such it can employ a number of different ways

for storing video data, which may be compressed or uncompressed. The

mechanism used for storing the data is defined by the program that encodes

and decodes the video data, known as a codec.

Image Size

The size of a reconstructed image file is often stated in terms of the number

of pixels it contains horizontally and vertically. Multiplying these figures

together gives the total number of pixels in the image and indicates the

"resolution" of the camera's sensor that took the image. This value is often

stated in millions of pixels or mega-pixels, abbreviated to Mp or Mpx. For

[4] A webcam typical of

the type used for lunar

and planetary imaging.

[3] Size comparison between common astronomical camera sensors.

High Frame Rate

Planetary Camera

35mm Full Frame DSLR

Non full-frame

DSLR

(APS-C)

Comparison between common imaging sensor sizes

3

4

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