ST-I single-shot color Planet Cam and Autoguider

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Single-Shot Color
Individual pixels in CCD cameras record only light and dark, black and white. They don’t see color. To produce a color image requires taking three separate monochrome (black and white) images though individual red, green, and blue filters. These three black and white images, each representing a single color of light (red, green, or blue), are then combined in your computer to produce the final full-color image.

Most CCD cameras take the three filtered images sequentially and store them in the computer for later processing, with the operator changing color filters between each exposure. However, several CCD manufacturers offer single-shot color cameras that record all three color images at the same time, in a single exposure. These cameras are also available in conventional monochrome versions. The single-shot color CCD cameras are essentially identical to their monochrome counterparts with the exception of the addition of a permanent color filter matrix over the pixels that lets them take all three color images simultaneously, as explained below.


The images in the box above show the basic structure of the pixels on a Kodak CCD detector, such as used on high-end SBIG and Finger Lakes Instrumentation single-shot color cameras. The top row shows a monochrome detector, the bottom row shows a single-shot color detector. The center image in each row is an actual photograph of the surface of the CCD showing a small section of the pixel array. The drawings at the right depict a side view of an individual pixel.

As you can see from the bottom row of images, the CCD structure for the single-shot color version is the same as the monochrome version except for the red, green, and blue pattern of filters over the pixels. The arrangement of colored filters over the pixels in a single-shot color camera is a repeating square of RGGB known as a Bayer pattern. This repeating pattern of RGGB pixels allows the separate red, green and blue data to be collected in a single monochrome exposure and electronically separated into the three monochrome images your computer needs to reconstruct a full-color image. Every fourth pixel sees red, every fourth pixel sees blue and every other pixel sees green. Special software extrapolates the RGB color data for each individual pixel in the frame from the color information in the adjacent colored pixels.

Many of the more economical cameras from Celestron, Meade, and Orion use Sony CCD detectors primarily designed for general use in camcorders and other consumer electronics, rather than the more-specialized detectors from Kodak. The Sony detectors use a color filter matrix of yellow, cyan, magenta, and green filters in a repeating sequence to generate the full color spectrum using sophisticated addition and subtraction algorithms to generate the desired RBG signal. The Sony filter matrix pattern is shown below.

What are the differences between taking three separate exposures versus one? Primarily it is a trade-off between greater complexity, sensitivity, and flexibility at a higher cost for the monochrome camera versus the single-shot color camera’s simplicity, ease of use, and lower overall cost for color imaging. A single-shot color camera needs only one image to do the job of the three needed by a monochrome camera/color filter wheel system. While this is simpler and less time-consuming, it results in a difference in the amount and quality of data recorded by each camera. The final image from a single-shot color camera has the same number of total pixels as a color image created by a monochrome camera and external filters, but it is created from less original data than the three discrete images of a monochrome camera. In addition, only one-third of the color information for each pixel is unique to that pixel and measured directly. The other two color values are approximations, derived from adjacent pixels.

In the case of a monochrome camera, the external color filters can be designed specifically for astronomical use, with high light transmission, precisely tailored response curves, and with better control of the color balance between the emission line and continuum light for different deep space objects. There is no way to tailor the sensitivity and spectral response of each color filter in the matrix to match the emissions of the object you are imaging, or to use special purpose narrowband filters, such as Oxygen III, SII ionized sulfur, H-alpha, etc. The matrix filters are general purpose red, green, and blue filters only.

As far as sensitivity is concerned, the monochrome camera is somewhat more sensitive due mainly to the nature of the external filters compared to the micro-filters placed over each pixel in the single-shot color camera. The monochrome camera requires more work to take a tri-color image, however, and the addition of the required filters and color filter wheel makes it more expensive.

The effective QE (quantum efficiency) of the monochrome camera with external filters is slightly higher than the single-shot color camera based on the filter transmission characteristics. But remember, the monochrome camera must take three frames versus the single-shot color camera's single frame. So for a proper comparison, a monochrome camera taking a 20 minute image through each of the three filters should be compared to a single-shot color camera taking a single 60 minute image. In this case, the single-shot color camera compares very well to its monochrome counterpart. Moreover, self-guiding the single-shot color camera is easier due to the fact that the separate built-in guider detector is never covered by a filter which can affect the tracking performance of the guider. Where a monochrome camera shines is in taking a grayscale image, or in taking narrow band monochrome or tri-color images of emission line objects. But for simple color images, single-shot color cameras are very capable.

Interline CCD
An interline-transfer CCD detector has a parallel register consisting of columns of sensors (photosites or pixels) separated by opaque strips (interline masks). The photons of the image accumulate in the exposed sensor area of the CCD detector.

Unlike conventional CCD cameras, which use a mechanical shutter to keep light from falling on the detector while the accumulated charge is being read out sequentially from the detector to your computer, the interline detector uses an electronic “shutter.” During CCD readout the entire image is first electronically shifted from the sensor columns into shift register columns hidden under the interline masks between each row of pixels. All of the columns shift simultaneously from sensors to shift registers, rather than transferring sequentially, as with a conventional CCD. Readout then proceeds from the hidden shift register columns sequentially to your computer in normal CCD fashion while the now-empty sensor areas start to accumulate more photons.

Since the signal is transferred in microseconds, electronic pixel smearing during download (from photons continuing to be recorded while the pixel is being read) is undetectable for typical exposures. The rapid transfer also allows the interline CCD to act as an electronic shutter to permit very short, very accurate exposures for lunar and planetary imaging.

A drawback to interline-transfer CCDs has been their relatively poor sensitivity to photons, since a large portion of each pixel is covered by the opaque interline mask. Kodak interline CCDs use a microlens assembly over the pixel array to direct the light from a larger area down to each photosite to focus more of the incoming light on the individual pixels.

Rather than using a low cost/high noise CMOS sensor as other planet cams do, the ST-i single-shot color Planet Cam/Autoguider uses a high quality, low noise, Kodak interline progressive scan true CCD sensor. The Kodak KAI-340 single-shot color CCD sensor in the ST-i has a 648 x 484 array of 7.4 micron square pixels and a measured read noise of only 9e-. Anti-blooming is standard. For descriptions of the single-shot color and interline concepts, click on the “Single Shot” and “Interline” icons above.

The ST-i uses the Class 1 version of this color Kodak sensor exclusively, with zero column defects, zero cluster defects, zero dead pixels, and zero saturated pixels. On-chip binning is available for 14.8 micron square pixels if desired, and various sub-frame modes may be used to speed-up the focus and download rates.

A full frame high res image will update at the rate of approximately 5 frames per second. In addition to full-frame, half-frame, and quarter-frame modes, the user may select a sub-frame region of interest (ROI) of any size located anywhere on the CCD. In focus mode, using a 20 x 20 pixel box, the update rate is greater than 20 frames per second.

While some inexpensive guiders save cost by using low cost 8 bit or 10 bit electronics, the single-shot color ST-i uses true high quality 16-bit A/D conversion for superior resolution of the full dynamic range of the CCD, which is greater than 70 dB.

The ST-i body weighs only 2.2 ounces and is no larger than many 1.25" eyepieces, measuring only 1.25" in diameter and 3.5" in length. The camera receives both control signals and power from the USB 2.0 port of your computer, so only one thin USB cable is required for using the camera as an imager.

The ST-i color has an electronic shutter allowing exposure times as short as 0.001 seconds. Unlike most other eyepiece-sized cameras, however, the ST-i also has a built-in mechanical shutter that lets the camera automatically take dark frames. This dramatically improves the performance of the ST-i, particularly when used as an autoguider. The ability to automatically take and subtract a dark frame results in a very smooth background against which detection of dim stars is more reliable.

The ST-i single-shot color comes with a front plate that is threaded for standard 1.25" filters; a 15' (4.6 meter) USB cable; a tracking cable; CCDOPS software with PlanetMasterTM, CCDSoftV5 software, and manuals on a CD-ROM. An optional 1.25" UV/IR blocking filter is recommended for imaging with this color camera.

The Planet Master software will take the sharpest image of the Moon and planets for you automatically. Typically, lunar/planetary imagers take a series of short exposures, knowing that throughout the sequence brief stable periods of seeing will give clear images. The Planet Master takes a sequence of short exposures, grades them for sharpness, and keeps the sharpest one.

Planet Master displays the images it is taking on your computer monitor in a split screen format. The left half of the screen displays the current image. The right side shows the sharpest image taken. The Planet Master command beeps twice each time a sharper image is acquired and updates the right hand image. When you are satisfied with the sharpness of the best image, hit the Done key and the Sharpest image will be shown.

Many planetary imagers like to average several of their best exposures, rather than just accept the single sharpest image. This requires taking a large number of images in a short time and examining each image to determine the best few of the lot. Planet Master makes this easier by automatically examining a series of images for you and assigning a Figure of Merit for sharpness to each one. Then, only the best of the lot can be selected for averaging, without having to tedious examine and judge each image manually.

For guiding purposes, the ST-i can typically guide on a star as faint as the 11th magnitude, using a 60mm guidescope and two second exposures. The guiding output port on the end of the ST-i is opto-isolated so it can be used with any mount having an autoguider input that conforms to the universally-accepted ST-4 standard.

The ST-i single-shot color Planet Cam/Autoguider has both 32 bit and 64 bit drivers and can be used with all Windows versions (including Windows 7) that support USB 2.0, as well as Mac and third Party Linux operating systems.

Pixel Array:
648 x 484
Pixel Size:
7.4 x 7.4 microns
Warranty:
1 year
2 x 2 Binning:
Yes
3 x 3 Binning:
No
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1. Bjorn on 5/24/2013, said: AstronomicsAstronomicsAstronomicsAstronomicsAstronomics
I purchased this camera with the optional guide-scope package. I did choose to go with the color version so that I could try some planetary imaging.

Setup was extremely easy. Once I installed PHD, the camera was recognized right away and it began communicating with my Meade mount.

I also tried the camera very quickly with an imaging program. It worked instantly. I'm very pleased with my purchase. Thanks.
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STI-C CCD Camera Information
Binning Options for the STI-C:
Pixel Size
  Width: 7.4 microns
  Height: 7.4 microns
Chip Size
  Width: 648 pixels
  Height: 484 pixels
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STI-C compared to 35mm film (24x36mm)
SBIG - ST-I single-shot color Planet Cam and Autoguider

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SBIG - ST-I single-shot color Planet Cam and AutoguiderImage showing how the USB 2.0 cable and autoguider cable connect to the end of the ST-iC single-shot color Planet Cam/Autoguider.Image showing the connectors at the end of the STi-C single-shot color Planet Cam/Autoguider.Image showing the size of the ST-iC single-shot color Planet Cam/Autoguider compared to a typical 1.25" eyepiece.
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Our Product #: STI-C
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The SBIG ST-iC is a lightweight eyepiece-sized single-shot color CCD camera that is both an excellent autoguider and a high performance lunar/planetary imager with features and performance unavailable in any other camera in this size and price range . . .





. . . our 34th year