Celestron "Pixcel 255" CCD Imaging Camera System

Please refer to our SBIG Model ST-237 for information about a similar item under the SBIG trademark, or to our SBIG CCD products section for upcoming announcements about other systems which will also be compatible with the Celestron telescopes.

Illustration at Right: Celestron "Pixcel 255" Camera Head and CPU with an optional personal portable computer.

In 1996 Celestron and the California based "Santa Barbara Instruments Group" (SBIG) company announced their first joint effort: the "Pixcel 255" CCD Imaging Camera system. This camera was conceived and then put together with the novice in mind - from the ease of set up and operation, to convenience of waterproof carrying case - the average beginner can be set up and taking their first image in less than 30 minutes! A claim proven in testing, and now by our customers.

SBIG has been a pioneer in developing CCD imaging camera technology for the university, research, and for the amateur astronomy communities. They are known for delivering on their promises, and for treating customers and distributors with an unusually high degree of dignity (for the industry).

SBIG has continuously set new standards in this field such as with their introduction of the Model ST-4 system. The ST-4 is not only a CCD camera, but also the first working affordable auto guider for astrophotography; indeed it is used on a 9" f15 refractor telescope to guide the 48" astrographic telescope at Mt. Palomar! The ST-4 continues to dominate its niche as the best, and proven performer of its type. This was followed by numerous innovative systems including the original model ST-6 camera. The ST-6 introduced a unique concept (Patented) for astronomical imaging "Track and Accumulate". The ST-6 provided incredible, here to fore unrealized imaging capabilities at an price attainable by institutions, and the demanding amateur. A magazine review of the ST-6 camera suggested that affordable imaging had finally arrived. This was followed by the introduction of Tricolor CCD imaging (to this day in the undisputed lead), then by the only dual chip cameras (capable of full time imaging and guiding simultaneously) the ST-7 and ST-8 systems. In 1997 their pioneering continued with the unrivaled Adaptive Optics Technology for use with the dual chip cameras. SBIG alliances include those companies with the most advanced software writers include the "Software Bisque" company.

The CCD ("charge coupled device") cameras employ an array made up of thousands of photo sensitive pixels. The array is mounted into a "camera head" which is put in place of an eyepiece on a telescope, or in place of a film camera onto a camera lens. The light striking a pixel is registered in terms of intensity and shade of gray; the first common CCD cameras were 8-bit for 8 shades of gray per pixel, the better cameras of today are 16-bit. The data is downloaded through an analog to digital converter which in better cameras handles data in a 16 bit stream, and then passed through control circuits in the CCD camera Central Processing Unit (CPU) and out through a cable to either a Serial, Parallel, or SCSI port of a personal computer. The personal computer runs the operating or control software for the CCD camera, and the software may also provide an ability to manipulate an image to better reveal the desired features, or otherwise arrange the image to be more pleasing and or useful to the end user. Because of their ability to "integrate" an image over time (this length of time depends on the camera and environmental conditions) the CCD cameras are the most practical means to image bright objects such as planets or the moon, or very faint extended objects such as galaxies, even from the relatively light polluted regions. Since the imaging can be done and manipulated on a PC, then there is no additional cost or delay in waiting for results from a dark room as there is with film. Furthermore, several images taken through color filters can be combined (registered) and color balanced to reveal much information about the object, or to render film-like aesthetically pleasing images.

The advantages of film remain:

• a CCD array is relatively small in area compared to that of even a 35mm camera film, and so to image large objects one must employ very short focal length or restrict imaging to smaller objects.

• a color or black and white print can be made from a personal computer, however the resolution of the CCD array (even if it were of comparable size to a film) is not as good as film. So when making relatively large prints those made from film can be enlarged to produce a larger image.

• large CCD arrays (such as those presently offered on the Model ST-8, and larger systems) produce images that require more and more personal computer hard disk space as the image sizes can become very large, and

• larger CCD images require more capable personal computers; typically at least 64 megabytes of Random Access Memory (RAM) and at least a fast "Pentium" or "Power PC" processing power to integrate and manipulate.

The Pixcel 255 camera even when operating with the optional color filter wheel is compatible with systems as fast as f1.8. With 10 micron square pixels it is suitable for systems from 2" to 200" focal length! It will reveal spiral structure in hundreds of galaxies with 1 to 5 minute exposures at the prime focus of an 8" f10 telescope. With such a telescope, stellar photometry down to 18th magnitude is attainable. Planetary Nebulae, Comets, asteroids, become easy targets. Many of these images can be obtained from the comfort of a suburban back yard.

This system permits:

• 1. When used with a personal computer (MS-DOS or Windows) the camera can function as an integrating, cooled CCD camera to capture monochromatic images of very faint objects. It was carefully designed for easy astronomical and scientific imaging.
  

Illustration 1. Relative Size of Common Astronomical Imaging CCD's

To predict actual field of view in degrees (horizontal or vertical) when using the Pixcel 255 CCD system
with a telescope or a camera lens, then employ the following formula:

Horizontal: 57.3/Focal Length of Objective (mm) * 3.2


Vertical: 57.3/Focal Length of Objective (mm) * 2.4


• 2. When used with the optional Color Filter Wheel Magazine (that easily slips into the camera head) then the camera can function
  to produce tricolor images; these can then be combined (indexed and balanced) to produce color images.
  

• 3. When the camera (controlled by a personal computer) is installed on a photoguide telescope,
  or at the guide port of an off axis guiding device can detect any deviations in tracking of a telescope
  (in both axes), and then send correction signals via a relay cable to a telescope drive corrector controller;
  all of this is done with a better frequency, higher accuracy and over a longer period of time than most
  humans can perform. All of this resulting in professional quality astrophotographs limited only
  by the quality of the telescope, and atmosphere.
  

THE CAMERA HEAD AND CPU:

The camera incorporates a thermoelectric cooler. The operating temperature range is user selectable (via the controlling computer keyboard) and it can be regulated to 0.5 degree precision. A regulating thermistor stabilizes the temperature thereby allowing exposure times of up to one hour with very low dark current.

The compact camera head (3.25" x 3" with a weight of less than two pounds) has a cable attached that connects into the camera CPU. This cable is attached at the side of the camera head to eliminate any risk of binding the cable against the base of fork mounted systems when imaging near Zenith. The control signals of an optional Color Filter Wheel are transmitted from the CPU to the camera through this cable.

The compact external CPU (approximately 8" x 10" x 2") can be placed up to 15 feet from the head. A PC notebook computer can be placed on top of this CPU. The CPU has an "On-Off" switch", a connector for Power in, and another for Relay outputs (to control a telescope drive corrector). A cable to connect the "TIC" (telescope interface cable) CPU relay control to most common telescope drive correctors is furnished, as is a CPU to PC fast parallel port cable. The CPU has a direct printer port supporting popular laser and dye sublimation devices. The A.C. power supply is also furnished; this plugs into 115V and provides 12 V.D.C.. It is possible to operate the camera from a 12 V.D.C. power source in the field too.

THE Pixcel 255 FUNCTIONS:

THE BASIC STAR TRACKER FUNCTION:

FOCUS:We suggest a parfocal eyepiece (such as the "IFocus" ocular by Software Bisque) be employed to visually find (or use digital or mechanical setting circles), center and focus (or nearly focus) the CCD onto the area to be imaged. Then the eyepiece is removed and replaced by the CCD head. Now select to enter the "Focus" mode. Set the exposure to 5 seconds. Select the 1 out of 4 SUBFRAME mode and center the object using the telescope's hand control or from the computer keyboard. As the telescope focuser is adjusted to improve focus, more light from the star will be focused onto a smaller area of the CCD, eventually possibly on to only one pixel thereby increasing the intensity which is indicated on the display as a higher value; this is an aid to obtaining precise focus of the star.

In this mode, the camera will continue to take short exposures of from 30 seconds for a full frame - which is not necessary to as fast as every 3 seconds for a 1/16th frame setting. The display screen will automatically refresh and the values adjust accordingly. Once the system is focused onto the star field, the user can select to view the entire field of view of the CCD and then select any particular star to be that which the CCD will use as a reference for guiding.

The CCD camera interface to the telescope consists of a cable from the normally closed/open connector of the camera CPU to the telescope drive corrector or hand control. The camera will produce drive adjustments usually commanded by the telescope drive corrector push button switches. A manual override on the computer keyboard allows the corrections to be made by the user.

By making fine adjustments to the telescope via the computer keyboard, a guide star can be moved onto the detector, or even onto any particular pixel. However, a guide star does not have to be centered onto the CCD detector for the system to function properly. The user can select the length of the star tracker exposure times from 0.1 or more (default is 1 second); so stars ranging over 10 magnitudes in brightness can be tracked without adding optional filters. The area of pixels to be used (guide box) can also be adjusted to accommodate very long focal length guide telescopes, or mediocre atmospheric seeing conditions.

THE "TRACK AND ACCUMULATE" FUNCTION:

AUTOMATIC DRIVE CALIBRATION: Once an acceptable star is found, and when focus is achieved, the operator will select "Calibrate Drive" function so the CCD system will "learn" the characteristics of the telescope drive. The software sequentially activates relays via the camera head output which send a user selectable 1 to 10 second signal to the telescope drive corrector to move the telescope North-South and back, then East and West and back; the telescope will end up returned to the point at which it started. This function "teaches" the CCD system what commands will cause the telescope to move in what direction, and how fast. This calibration feature is invaluable when using a German equatorial mount, or an off axis guider, where the proper direction can be very difficult to guess.

THE TRACK MODE: After calibration, then the operator will select "Track"; any drifting motion of a star across the CCD causes it to appear at a different pixel at each following exposure. The computer controlling the CCD will then calculate how far the star has drifted and generates a control signal, transmitted through the relays to correct the position. The control signal, and its duration are a function of the star's position error. The camera system can take an exposure (integration), read out all the pixel values, and then calculate and transmit the necessary telescope correction in less than one second.

In the "Track" mode the camera acquires a fresh star image, centers the image on a pixel, and holds that star in position by constantly monitoring it - sending correction signals to the telescope drive immediately after the exposure. The CCD is thermoelectrically cooled to enhance its sensitivity to dim stars. This extreme sensitivity enables guide stars as faint as 8th magnitude to be tracked utilizing as small as a 60mm guide telescope. The rapid calculating power of the computer enables the guide star location to be determined within a fraction of a pixel, enabling better than 1 arc second tracking accuracy.

CAPTURE AND DISPLAY THE IMAGE:

The image can be immediately displayed on the computer monitor. Auto contrast can be selected, with computers background and range values, or the background and range values may be manually entered to bring out specific features of interest.

OPTIMIZE AND ANALYZE THE IMAGE:

Entering the CROSSHAIR MODE enables functions such as stellar and diffuse magnitude measurement, and photometric measurements of stellar positions and angles. The 16 bit accuracy (1 part in 65536) allows very accurate brightness measurements to be made. With appropriate filters, stellar temperature can be measured!

The potential applications of this technology are still being revealed by innovative operators, sometime with software specifically developed for their tasks!

SPECIFICATIONS OF MODEL Pixcel 255 CCD SYSTEM DETECTOR

Imaging Resolution:	320 x 240 Pixels, TI 255 CCD
Imaging Pixel Dimensions:	10 x 10 microns
Imaging Array Dimensions:	3.2mm x 2.4mm, 4.4mm Diagonal
Anti-Blooming	Selectable

SPECIFICATIONS OF Pixcel 255 ELECTRONICS

Binning	Variable/Software Selectable
Full Well Unbinned (in Ke-):	33,000
Readout Noise (in e-):	28 RMS Double Correlated Sampling
Dark Current	<5e-/pixel/sec @-5 degrees C

SPECIFICATIONS OF Pixcel 255 HARDWARE

Desiccant:	Yes, internal
Standard Control Software:	PC DOS, Windows or Win95
Communications Port:	Fast Parallel
Analog/Digital Converter:	16 Bit
Digitization Rate, Full Frame:	4 sec. w/o Double Correlated Sampling
Digitization Rate, Full Frame:	8 sec. w/Double Correlated Sampling
Shutter:	Electro-Mechanical
Fastest Shutter Speed:	0.1 Sec.
Longest Shutter Speed:	1 Hour
Cooling From Ambient:	-35 Degrees C
Power Requirements:	115 V.A.C. Provided (12 v.d.c. possible)

POPULAR AVAILABLE OPTIONS FOR THE Pixcel 255 Camera:

• Company Seven can provide wiring diagrams of most common production telescopes to illustrate
  to the user how to wire the provided relay cable into the telescope.
  
• Pixcel 255 Color Filter Wheel Magazine
  
• A variety of nosepieces to fit microscopes or other devices
  
• Image Processing Software
  
• Parfocal Eyepiece to help to find, center, and focus an object
  
• Camera Lens Adapter