Moravian instruments, Inc., source: https://www.gxccd.com/art?id=579&lang=409, printed: 28.09.2020 0:12:39
|The cooled C2 series global shutter CMOS cameras were developed for imaging under extremely low-light conditions in astronomy, microscopy and similar areas. Mechanical design of this series inherits from earlier CCD-based G2 Mark II cameras, which makes the C2 series fully compatible with vast range of telescope adapters, off-axis guider adapters, internal or external filter wheels, Camera Ethernet adapters, guiding cameras etc.|
C2 cameras are designed to be connected with the host PC through USB 3.0 interface, operating at 5 Gbps. Cameras are also compatible with USB 2.0 port to communicate with a host PC.
Alternatively, it is possible to use the Moravian Camera Ethernet Adapter device. This device can connect up to four Cx (with CMOS sensors) or Gx (with CCD sensors) cameras of any type and offers 1 Gbps and 10/100 Mbps Ethernet interface for direct connection to the host PC. Because the PC then uses TCP/IP protocol to communicate with the cameras, it is possible to insert WiFi adapter or other networking device to the communication path.
Please note that the USB standard allows usage of cable no longer than approx. 5 meters and USB 3.0 cables are even shorter to achieve very fast transfer speeds. On the other side, the TCP/IP communication protocol used to connect the camera over the Ethernet adapter is routable, so the distance between camera setup and the host PC is virtually unlimited.
The C2 cameras need an external 12 V DC power supply to operate. The wall adapter providing proper voltage is shipped with every camera.
Note the camera must be connected to some optical system (e.g. the telescope) to capture images. The camera is capable of long exposures, necessary to acquire the light from faint objects. If you plan to use the camera with the telescope, make sure the whole telescope/mount setup is capable to track the target object smoothly during long exposures.
Please note the camera head is designed to either accept Internal filter wheel or to be able to connect to the External filter wheel, but not both. If the Internal filter wheel variant is used, External filter wheel cannot be attached.
C2 camera models are equipped with Sony IMX global shutter CMOS detectors with 3.45 × 3.45 μm or 4.50 × 4.50 μm square pixels. Individual models differ in resolution only.
All used sensors utilize global electronic shutter. This means every pixel within the image is exposed in the same time, as opposed to rolling shutter sensors, which exposes individual lines one after another. There is no difference for long exposures of static objects, but imaging of moving objects using short exposure time using rolling shutter leads to image shape distortions.
C2 camera models with 3.45 × 3.45 μm pixels and 8- and 12-bit digitization:
C2 camera models with 3.45 × 3.45 μm pixels and 12-bit digitization only:
C2 camera models with 4.50 × 4.50 μm pixels and 12-bit digitization only:
Cameras limited to 12-bit read mode are marked with letter A, following the model number. For instance, if C2-12000 marks camera with both 8- and 12-bit read modes, C2-12000A denotes camera model with only 12-bit read mode. All other parameters (sensor size, pixel resolution) are equal.
CMOS camera electronics primary role, beside the sensor initialization and some auxiliary functions, is to transfer data from the CMOS detector to the host PC for storage and processing. So, as opposite to CCD cameras, CMOS camera design cannot influence number of important camera features, like the dynamic range (bit-depth of the digitized pixels).
The sensors used in C2 cameras shows very good linearity in response to light. This means the camera can be used also for entry-level research projects, like for instance photometry or variable stars etc.
The slow variant of both read modes can be used to slightly lower the amount of heat generated by the sensor, as the communication interface operates at half speed compared to fast mode. Also, when the camera is connected using USB 2.0 interface, fast read mode provides data at higher speed than the USB 2.0 can handle and thus causes more interruptions of image digitization process.
The digitization speeds mentioned above are valid for USB 3.0 connection. Also please note the digitization speeds do not necessarily lead to corresponding FPS, because every image downloaded has to be processed and displayed, which also consumes time. This time is negligible, if slow-scan camera needs many seconds for image download, but in the case of fast CMOS cameras, time for image processing in the PC (e.g. calculation of image standard deviation etc.) can be longer than image download itself.
Despite one byte per pixels is transferred from camera to PC in the 8-bit read mode, many astronomical processing software packages work with 16-bit or 32-bit images only (e.g. SIPS). So, images occupy the same space in the computer memory regardless of the read mode.
Also, standard format for image storage in astronomy is FITS. While this format supports 8-bit per pixel, this variant is rather unusual and 16 or 32-bit integer or 32-bit floating-point pixels are typically stored to disk files to achieve as wide compatibility as possible.
Sensors used in C2 cameras offer programmable gain from 0 to 24 dB, which translates to the output signal multiplication from 1× to 15.9×. Gain can be set with 0.1 dB step.
Note the C2 camera firmware supports only analog gain, which means real amplification of the signal prior to its digitization. The used sensors support also digital gain control, which is only numerical operation, bringing no real benefit for astronomical camera. Any such operation can be performed later during image processing if desired.
Generally, many sensor characteristics depend on the used gain. Hence, we provide two lists of parameters for both minimal and maximal gain.
Camera/sensor parameters for sensors with 3.45 × 3.45 μm pixels:
Camera/sensor parameters for sensors with 4.50 × 4.50 μm pixels:
Please note the values stated above are not published by sensor manufacturer, but determined from acquired images using the SIPS software package. Results may slightly vary depending on the test run, on the particular sensor and other factors (e.g. sensor temperature, sensor illumination conditions etc.), but also on the software used to determine these values, as the method is based on statistical analysis of sensor response to light.
C2 cameras are capable of very short exposures. The shortest exposure time is 125 μs (1/8000 of second). This is also the step, by which the exposure time is expressed. So, the second shortest exposure is 250 μs etc.
Long exposure timing is controlled by the host PC and there is no upper limit on exposure time. In reality the longest exposures are limited by saturation of the sensor either by incoming light or by dark current (see the following sub-chapter).
Regulated thermoelectric cooling is capable to cool the CMOS sensor up to 4 °C below ambient temperature. The Peltier hot side is cooled by fan. The sensor temperature is regulated with +/-0.1 °C precision. High temperature drop and precision regulation ensure very low dark current for long exposures and allow proper image calibration.
The camera head contains two temperature sensors — the first sensor measures directly the temperature of the CMOS sensor package. The second one measures the temperature inside the camera shell.
The cooling performance also depends on the environmental conditions and also on the power supply. If the power supply voltage drops below 12 V, the maximum temperature drop is lower.
The stated values are valid for C2-12000A camera. As noted above, maximum ΔT of lower resolution sensors (C2-5000A, C2-3000A) is higher, but ΔT of corresponding non-A camera versions is lower.
Maximum temperature difference between CMOS sensor and ambient air may be reached when the cooling runs at 100% power. However, temperature cannot be regulated in such case, camera has no room for keeping the sensor temperature when the ambient temperature rises. Typical temperature drop can be achieved with cooling running at approx. 90% power, which provides enough room for regulation.
The 12 V DC power supply enables camera operation from arbitrary power source including batteries, wall adapters etc. Universal 100-240 V AC/50-60 Hz, 60 W brick adapter is supplied with the camera. Although the camera power consumption does not exceed 40 W, the 60 W power supply ensures noise-free operation.
The power connector on the camera head uses center-plus pin. Although all modern power supplies use this configuration, always make sure the polarity is correct if other than the supplied power source is used.
Power consumption is measured on the DC side of the supplied power adapter. Camera consumes more energy from the AC outlet than stated here.
The camera contains its own power supplies inside, so it can be powered by unregulated 12 V DC power source — the input voltage can be anywhere between 10 and 14 V. However, some parameters (like cooling efficiency) can degrade if the supply drops below 12 V.
C2 camera measures its input voltage and provides it to the control software. Input voltage is displayed in the Cooling tab of the Imaging Camera control tool in the SIPS program. This feature is important especially if you power the camera from batteries.
Compact and robust camera head measures only 114 × 114 × 65 mm (approx. 4.5 × 4.5 × 2.6 inches). The head is CNC-machined from high-quality aluminum and black anodized. The head itself contains USB-B (device) connector and 12 V DC power plug. Integrated mechanical shutter allows streak-free image readout, as well as automatic dark frame exposures, which are necessary for unattended, robotic setups.
Camera head with integrated Internal filter wheel is 77.5 mm thick. Filter wheel offers 5 positions for standard 1.25-inch threaded filter cells. A variant of filter wheel with 6 positions for unmounted D26 mm filters is also available.
Back focus distance is measured from the sensor to the base on which adjustable adapters are mounted. Various adapters then provide back focal distance specific for the particular adapter type (e.g. M48 threaded adapter back focal distance is 55 mm).
Stated back focal distance already calculates with glass permanently placed in the optical path (e.g. optical window covering the sensor cold chamber).
When the adjustable adapter base, intended for camera with Internal filter wheel, is mounted on camera without filter wheel, the resulting back focal distance is only 21 mm.
C2 cameras are equipped with mechanical shutter, which is very important feature allowing unattended observations (fully robotic or just remote setups). Without mechanical shutter, it is not possible to acquire dark frames, necessary for proper image calibration.
Mechanical shutter in the C2 cameras is designed to be as reliable as possible, number of open/close cycles is virtually unlimited, because there are no surfaces rubbing against each other. The price for high reliability is slow shutter motion. Luckily, mechanical shuttering is not necessary for exposure control, only for taking dark frames and possibly bias frames — all used CMOS sensors are equipped with electronic shuttering.
Camera firmware optimizes the shutter operation to avoid unnecessary movements. If a series of light images is taken immediately one after another, the shutter remains open not to introduce quite significant delay of the close/open cycle between each pair of subsequent light images. In the case next image has to be dark or bias frame, shutter closes prior to dark frame exposure and vice versa — shutter remains closed if a series of dark frames is acquired and opens only prior to next light frame. If no exposure is taken for approximately 1 second while the shutter is open (this means after a light image exposure), camera firmware closes the shutter to cover the sensor from incoming light.
The S sized External filter wheel diameter is greater (viz. External Filter Wheels), but the back focal distance of all external filter wheels is identical.
If the camera model, intended for usage with External filter wheel, is used without filter wheel at all, two types of adjustable adapter bases can be used.
When a thin adapter base, intended for camera with Internal filter wheel, is used, the back focal distance is only 21 mm.
Thick adapter base has the same thickness like the External filter wheel. This means all adapters, attached to this thick base, keep the same back focal distance like if attached directly to External filter wheel shell or to a camera with Internal filter wheel and “thin” adapter base.
Various accessories are offered with C2 cameras to enhance functionality and help camera integration into imaging setups.
When there is no filter wheel inside the camera head, all electronics and firmware, intended to control it, stays idle. These components can be utilized to control external filter wheel with only little changes. Also the camera front shell can be manufactured thinner, the space for filter wheel is superfluous.
All telescope/lens adapters of the C2 series of cameras can be slightly tilted. This feature is introduced to compensate for possible misalignments in perpendicularity of the telescope optical axis and sensor plane.
The C2 camera telescope adapters are attached using three pulling screws. As the adapter tilt is adjustable, another three pushing screws are intended to fix the adapter after some pulling screw is released to adjust the tilt.
C2 camera can be optionally equipped with Off-Axis Guider Adapter. This adapter contains flat mirror, tilted by 45° to the optical axis. This mirror reflects part of the incoming light into guider camera port. The mirror is located far enough from the optical axis not to block light coming to the main camera sensor, so the optics must be capable to create large enough field of view to illuminate the tilted mirror.
C2-OAG is manufactured in two variants, one with M42 × 0.75 thread (T-thread) and another with M48 × 0.75 thread. Both variants are designed to be compatible with external filter wheels and to preserve 55 mm distance from the sensor.
If the OAG has to be used on camera with internal filter wheel, the OAG is mounted to adapter base like any other adapter. Resulting Back focal distance remains the same.
OAG guider port is compatible with C1 cameras (and also older G0 and G1). It is necessary to replace the CS/1.25” adapter with short, 10 mm variant in the case of C1 cameras. Because C1 cameras follow CS-mount standard, (BFD 12.5 mm), any camera following this standard with 10 mm long 1.25” adapter should work properly with the C2-OAG.
C1 cameras are available with CS-mount adapter as well as with T-thread (M42 × 0.75) adapter. To work properly with C2-OAG, C1 with CS-mount adapter only must be used. Larger T-thread adapter is not mechanically compatible with OAG.
C2 camera heads are equipped with tripod thread (0.25”) on the top side. This thread can be used to attach 1.75 inch dovetail bar (Vixen standard). It is then possible to attach the camera head, e.g. equipped with photographic lens, directly to various telescope mounts supporting this standard.
C2 cameras employ the same desiccant container like the larger C3 and C4 cameras, aw well as CCD based G2, G3 and G4 cameras. The whole container can be unscrewed, so it is possible to exchange silica-gel without the necessity to remove the camera from the telescope.
This is why the container itself does not contain any sealing, which could be damaged by high temperature in the owen. The sealing remains on the sensor cold chamber instead.
New containers have a thin O-ring close to the threaded edge of the container. This O-ring plays no role in sealing the sensor cold chamber itself. It is intended only to hold possible dust particles from entering the front half of the camera head with the sensor chamber optical window, shutter and possibly internal filter wheel. While the O-ring material should sustain the high temperature during silica-gel baking, it is possible to remove it and put it back again prior to threading the contained back to the camera.
Container shipped with the camera by default does not exceed the camera head outline. It is equipped with a slot for tool (a plastic tool is included with every camera, but e.g. some coin can be used, too), allowing releasing and also tightening of the container.
Camera head is available in several color variants of the center plate. Visit manufacturer's web pages for current offering.
The Moravian Camera Ethernet Adapter device allows connection of up to four Cx cameras of any type on one side and 1 Gbps Ethernet interface on the other side. So, this device allows attaching of cameras to virtually unlimited distance using the routable TCP/IP protocol.
Moravian Camera Ethernet Adapter device is described in detail here.
Software and driver support of the Cx series CMOS cameras is as rich as is the case of their Gx series CCD camera siblings.
The SIPS (Scientific Image Processing System) software package version 3.21 or later is necessary to control C2 cameras including latest C2-7000.
Support for CMOS based Cx cameras was gradually added to individual SIPS version. While previous minor SIPS versions could be able to recognize C2 cameras, always use v3.21 or later for reliable camera operation.
Powerful SIPS (Scientific Image Processing System) software, supplied with the camera, allows complete camera control (exposures, cooling, filter selection etc.). Also automatic sequences of images with different filters, different binning etc. are supported. With full ASCOM standard support, SIPS can be also used to control other observatory equipment. Specifically the telescope mounts, but also other devices (focusers, dome or roof controllers, GPS receivers etc.).
SIPS also supports automatic guiding, including image dithering. Both autoguider port hardware interface (6-wire cable) and mount Pulse-Guide API guiding methods are supported. For hi-quality mounts, capable to track without the necessity to guide at last during one exposure, inter-image guiding using the main camera only is available.
But SIPS is capable to do much more than just camera and observatory control. Many tools for image calibration, 16 and 32 bit FITS file handling, image set processing (e.g. median combine), image transformation, image export etc. are available.
As the first S in the abbreviation SIPS means Scientific, the software supports astrometric image reduction as well as photometric processing of image series.
SIPS software package is freely available for download from this www site. All functions are thoroughly described in the SIPS User's Manual, installed with every copy of the software.
Drivers for ASCOM standard as well as native drivers for third-party software are also available (e.g. TheSkyX, AstroArt, etc.). Visit the download page of this web site for current list of available drivers, please.
Also INDI drivers for 32 bit and 64 bit Linux running on x86 and ARM are available. Also drivers for TheSkyX package running on macOS are supplied with the camera.
SIPS software package allows automatic guiding of the astronomical telescope mounts using separate guiding camera. Proper and reliable automatic guiding utilizing the computational power of Personal Computer (e.g. calculation of star centroid allows guiding with sub-pixel precision) is not simple task. Guiding complexity corresponds to number of parameters, which must be entered (or automatically measured).
The Guiding tool allows switching of autoguiding on and off, starting of the automatic calibration procedure and recalculation of autoguiding parameters when the telescope changes declination without the necessity of new calibration. Also swapping of the German Equatorial mount no longer requires new autoguider calibration. There is also a graph showing time history of guide star offsets from reference position in both axes. The length of graph history as well as the graph range can be freely defined, so the graph can be adjusted according to particular mount errors and periodic error period length. Complete log of calibration procedure, detected offsets, correction pulses etc. is also shown in this tool. The log can by anytime saved to log file.
An alternative to classic autoguiding is the inter-image guiding, designed for modern mounts, which are precise enough to keep tracking with sub-pixel precision through the single exposure, and irregularities only appear on the multiple-exposure time-span. Inter-image guiding then performs slight mount position fixes between individual exposures of the main camera, which eliminates traveling of the observed objects through the detector area during observing session. This guiding method uses main imaging camera, it does not use another guiding camera and naturally does not need neither OAG nor separate guiding telescope to feed the light into it.
Color sensors have red, green and blue filters applied directly on individual pixels (so-called Bayer mask).
Every pixel registers light of particular color only (red, green or blue). But color image should contain all three colors for every pixel. So it is necessary to calculate missing information from values of neighboring pixels.
There are many ways how to calculate missing color values — from simple extending of colors to neighboring pixels (this method leads to coarse images with visible color errors) to methods based on bi-linear or bi-cubic interpolation to even more advanced multi-pass methods etc.
Bi-linear interpolation provides significantly better results than simple extending of color information to neighboring pixels and still it is fast enough. But if the telescope/lens resolution is close to the size of individual pixels, color artifacts appear close to fine details, as demonstrated by the image below left.
Multi-pass algorithm is significantly slower compared to single-pass bi-linear interpolation, but the resulting image is much better, especially in fine details. This method allows using of color camera resolution to its limits.
SIPS offers choosing of color image interpolation method in both Image Transform and New Image Transform tools. For fast image previews or if the smallest details are significantly bigger than is the pixel size (be it due to seeing or resolution of the used telescope/lens) the fast bi-linear interpolation is good enough. But the best results can be achieved using multi-pass method.
Example images captured with C2 cameras.
All images published with permission of their respective authors.