Dont' be afraid of CCD

The Digital revolution (I)

Amateurs are sometimes relictant of using new technologies. For years however many of us are using computers and electronic digital devices to develop their hobby, from image intensifiers, photometers to full robotized telescopes sometimes remotely controlled. But prior investing in a CCD, we all have had a question in mind : how to work with a CCD and optionally with what image processing software ?  I intend to explain you this in the following pages.

Since the introduction of CCD cameras (Couple Charged Device) by SBIG in the amateur community late 80's, most daring amateur astronomers are used to work with them to record the faint light falling down the stars and deep sky objects (DSO).

One of the numerous reasons of this passion is the drastic size reduction of electronic components. The second one is the price drop which exhibits a 50% drop each 2 years (Moore's Law). Then because in the century of Information Technology an electronic device can be computer controlled.

CCD cameras are light sensitive devices with this plus to be designed to be driven by an instrument thanks to a built-in microcontroller. Their images can also be download in a computer for a later processing.

In view of their size and weight, CCD cameras support with difficulties light installations at a few hundreds euros which weight does not exceed 1 kg; the camera is heavier that the all installation ! We can conversely mount them without problem on small scopes from 80 to 127 mm of aperture at the condition that they be supported by a sturdy mount and equipped with the appropriated photo adapter.

A CCD detector looks like a small solar cell which photosensitive array covers often less than 50 mm² encapsuled in a electronic circuit.

The detector is fixed in a frame that looks like to a chip and placed in a house with various input-outputs for the cooling system, the eyepiece, the filters wheel, the connexion to the computer and the power line.

The chip of a CCD camera (as well as the one of a webcam or a digital camcorder) is constituted of rows and columns made of photosensitive cells called pixels (picture elements), although this term is faulty as technically speaking pixels are the components of the resulting image. We will however continue to use this term as it is entered in the every days language.

As all high-tech electronic accessories, CCD chips used to work with the physics vocabulary. Therefore we will not be amazed if even a newby in this field borrows the vocabulary to quantum physics and electronic to explain his work. Like in radioastronomy "integration time" means "exposure", "dark current" means "noise" or "parasitic signal", "blooming" means "overexposition", "binning" means "larger pixel", etc. But don't worry, after one hour of reading you will be ready to enter this exciting field.

CCD performances

CCDs have a high quantum efficiency; 60% of the photons striking the chip are recorded, on par and sometimes even better than some professional systems. They have an excellent linearity (the output signal is nearly proportional to the number of incident photons), without reciprocity failure in long exposures as we know using argentic emulsions when trying to catch faint DSOs. Even the famous hypersensitized Kodak TP2415 cannot compete against the extreme high response time and resolution of a CCD chip. The difference is of at least a factor 10 to 1 in favor of the CCD. 

Now if I tell you that you can get in 2 minutes of exposure what you tediously got in around half an hour standing in the cold using an argentic emulsion, you quickly realize what is your benefit using a digital camera ! Yes, CCD is a true revolution... This remarkable technology allows you to reach magnitude 15 in one second of exposure and magnitude 19 in only one minute at the prime focus of a 8" f/10 telescope ! These CCD cameras have a sensitivity equivalent higher than a 20,000 ISO film, without the grainy !

CCD cameras performances differ from each other in several significant aspects. The first is the number of pixels contained in the chip, the product of the number of rows and the number of columns; one speak of a few millions pixels on an amateur detector to about 150 millions pixels on a professional CCD detector. The second aspect is the  physical size of each pixel (usually between  9 and 24 microns) and whether it is square or rectangular. The third aspect is called the pixel depth; it determines how many bits can be stored to code the brightness level of the pixel. Finally the last aspect is the wavelength sensitivity of the chip.

MEADE PICTOR 416 APOGEE AP-7 SBIG ST-7EA 675x510 pixels of 9m 512x512 pixels of 24m 768x510 pixels of 9m

Take for example the CCD camera Apogee AP-9, one of the most performing on the place. It uses a KAF-1600 chip containing 3072 columns and 2048 rows, so 6.3 millions pixels, exceeding by far the resolution of most digicams. Each pixel measures 9 microns (0.009 mm) on a side, so the entire chip is only 27.6x18.4 mm, not larger than a piece of sugar.16 bits are used to digitize the brightness of each pixel, resulting in 2¹⁶ or 65536 gray levels. As it takes 2 bytes to code 16 bits, each image takes about 12 MB on disk ! As for one of the smallest CCD camera, the SpectraSource Lynxx 2000, its chip offering 320x240 pixels and 4096 levels of grays, its files do no exceed 160 KB in size. As most CCD detectors, their spectral response is quite high in the red and infrared part of the spectrum up to about 1.1 microns but drops to very low values to the shorter wavelengths in the near ultraviolet, in the same way as the response curve of photoamplifiers.

Check the specifications of CCDs

Currently, one of the most performing CCD available to (fortunate) amateurs is SBIG KAI-11000M of... 11 megapixels (4008x2672 pixels). Its price is as much "mega" : between 8 and 9000$... A color model is available as well. Here is what it gives in LRGB imaging in the hand of the expert Jon Christensen.

Some CCD incorporate also features that automate guiding, long integration times and antiblooming (pixels saturation). But you have to know that trying to capture a galaxy of 21st magnitude per square arc-second with a chip having 1 arc-second pixel is like trying to capture a 21st magnitude star ! You now understand why we have to find the optimal correlation between the scope focal ratio and the pixel size to get the best sensibility. 

A good compromise will give an image presenting a high signal-to-noise ratio and a smooth appearance without reducing too much the resolution. One trick : for stars the CCDs optimum detectability is reached when the star's image is about twice the pixel size (around 20-25 microns) but a too large pixel will also increase noise. On the other side, the larger pixel has a greater sensitivity to nebulosity but smaller pixels record easier faint stars... At last for DSOs you need the largest aperture and the fastest f/ratio in order to produce well defined star images across the CCD chip. So what to choose ? Here comes your skills !

To check : Wodaski Ron's Magic CCD Camera Calculator

To help you, we can say that the resolution has to be pretty much fixed for your subject; in taking as a rule the Nyquist sampling theorem, it states that he pixel size (photosite) should be one-half the size of the Airy diffraction disk. Given this, it is easy to find the focal ratio needed to get this optimum resolution :

f = 2 x Photosite size / l

With a 10 microns pixel size and a sensitivity peak at l = 0.7 microns, we get a focal ratio of about  f/29 in planetary astrophotography. This corresponds to a resolution of about 0.25 arc-sec/pixels (206 * Pixel size / focal length), a value rarely found in planetary imaging where the resolution is more often near 0.7"/pixel (for a 12" f/10 scope, 0.7 arc-sec/pixel corresponds to a resolution of about 1.5 km on the moon surface, enough to picture small features like domes or rilles in high resolution). On deep sky objects, due to the decreasing of the focal ratio by a factor 5 or higher, the resolution exceeds rarely 2 arc-sec/pixel.

Choosing a higher resolution to record galaxies should produce surely amazing pictures but will request also local seeing conditions rarely observed, the resolution being often not better than 1.5" during long exposures. Without to forget that your field of view should be small. For information, a CCD chip like KAF-0400 used in the SBIG ST-7 camera covers around 18.5x12.4 arc-minutes on a 8" f/6.3 SCT (6 times less than a 24/36) and several other chips are even below these values. 

Next chapter

Integration times and common problems