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 monochrome Planet Cam/Autoguider uses a high quality, low noise, Kodak interline progressive scan true CCD sensor. For a discussion of the interline CCD system, click on the "Interline" icon above.
The Kodak KAI-340 CCD sensor in the ST-i monochrome has a 648 x 484 array of 7.4 micron square pixels and a measured read noise of only 9e-. Anti-blooming is standard. The ST-i uses only the Class 1 version of this CCD, 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 resolution 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 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 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 monochrome 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 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 monochrome 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.
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 monochrome 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 monochrome 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.