Moravian instruments, Inc., source: https://www.gxccd.com/art?id=607&lang=409, printed: 02.08.2021 8:10:33
|The C4-16000 cooled scientific CMOS camera sensors offer the same geometry like the CCDs in the famous G4-16000 cameras — sensor size 37 × 37 mm, 9 μm pixels and 16 MPx (4k × 4k) resolution. Also the mechanical design of C4 cameras inherits from G4 Mark II cameras, which makes the C4 camera line fully compatible with vast range of telescope adapters, off-axis guider adapters, filter wheels, Camera Ethernet adapters, guiding cameras etc.|
Mechanical design of the C4 series of astronomical CMOS cameras inherits from earlier CCD-based G4 Mark II cameras, which makes the C4 camera line fully compatible with vast range of telescope adapters, off-axis guider adapters, filter wheels, Camera Ethernet adapters, guiding cameras etc.
Rich software and driver support allow usage of C4 camera without necessity to invest into any 3rd party software package thanks to included free SIPS software package. However, ASCOM (for Windows) and INDI (for Linux) drivers and Linux driver libraries are shipped with the camera, provide the way to integrate C4 camera with broad variety of camera control programs.
C4 cameras are designed to be attached to host PC through very fast USB 3.0 port. While C4 cameras remain compatible with older (and slower) USB 2.0 interface, image download time is significantly longer.
Alternatively, it is possible to use the Moravian Camera Ethernet Adapter device. This device can connect up to four Cx (and CCD based Gx) cameras of any type (not only C4, but also C1, C2 and C3) 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.
Download speed is naturally significantly slower when camera is attached over Ethernet adapter, especially when compared with direct USB 3 connection.
The C4 cameras need an external power supply to operate. It is not possible to run the camera from the power lines provided by the USB cable, which is common for simple imagers. C4 cameras integrate highly efficient CMOS sensor cooling, shutter and possibly filter wheel, so their power requirements significantly exceed USB line power capabilities. On the other side separate power source eliminates problems with voltage drop on long USB cables or with drawing of laptop batteries etc.
Also note the camera must be connected to some optical system (e.g. the telescope) to capture images. The camera is designed for 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.
C4 camera head is designed to be easily used with a set of accessories to fulfill various observing needs.
As opposed to smaller C2 and C3 camera models, which offer an option to integrate filter wheel into camera shell, large C4 camera sensors need square 50×50 mm filters, too big to fit into Internal filter wheel. So, using of External filter wheel is the only option for C4 cameras.
C4 cameras are equipped with Gpixel GSENSE4040 CMOS detectors with resolution 4096 × 4096 pixels. Pixel size is 9 × 9 μm, which leads to almost 37 × 37 mm light sensitive area.
The GSENSE4040 sensor is equipped with 12-bit ADCs (Analog to Digital Converters) only. However, there are two sets of ADCs inside the sensor, each capable to digitize the image with different gain — one set of ADCs uses low-gain channel, while the second set uses high-gain channel. Both 12-bit outputs of each ADC set can be combined to single image with true 16-bit dynamic range (such combined image is often called 16-bit HDR for High Dynamic Range).
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 C4 cameras show very good linearity in response to light. This means the camera can be used for advanced research projects, like the photometry of variable stars and transiting exoplanets etc.
Combination of both low-gain and high-gain digitization channels into single 16-bit HDR image is designed to carefully preserve linear response to light. What's more, resulting 16-bit image does not combine full dynamic range of both low-gain and high-gain channels, but takes only the perfectly linear portions of both channels. So, the linearity of the resulting 16-bit image is perfect within the full dynamic range.
C4 camera is equipped with on-board RAM, capable to hold multiple full-resolution frames. Downloading of the image to the host computer thus does not influence image digitization process, as the download only transfers already digitized images from camera memory.
Time needed to download single frame depends on the read mode and also whether fast USB3 or slower USB2 is used:
C4 cameras do not offer the users to set gain, beside the two fixed low-gain and high-gain channels. 16-bit HDR image covers whole sensor dynamic range and manipulating with gain would bring no additional benefits.
Please note the values stated above are 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.
C4 cameras are capable of very short exposures, the shortest exposure time is approx. 21 μs. However, the sensor employs so-called rolling shutter. This means the exposure does not start over the whole sensor at once, but exposure of subsequent lines begins with 21 μs delay and the whole sensor is illuminated 8.6 ms after exposure starts. Similarly, end of exposure and pixel digitization is performed line by line with the same delay between lines.
There is no practical limit on maximal exposure length, but in reality, the longest exposures are limited by saturation of the sensor either by incoming light or by dark current (see the following chapter about sensor cooling).
C4-16000 cameras are supplied with Class 1 sensors. The light gathering area of the GSENSE4040 sensors is divided into 4 quadrants, slightly differing in bias levels. This division may remain visible as slightly different background levels, especially when the overall scene illumination is low. Such uneven background typically does not harm scientific measurements, as the differences are well beyond background noise. But aesthetic astro-photography can be negatively influenced if these differences are not removed during image processing.
The GSENSE4040 sensor contains shielded pixels, returning black-level signal (also known as overscan area) in addition to normal illuminated pixels. There are 64 black-level pixels in each of 4096 rows. The C4-16000 camera includes these pixels into each image, so the resulting image width is 4096 + 64 = 4160 pixels. The 64 pixels wide stripe of black-level pixels is also visible on the image above.
However, the camera driver by default removes the overscan area and returns only illuminated area of 4096 × 4096 pixels to the user. This function is controlled by a parameter ‘C4Overscan’ in the section ‘[driver]’ of the driver configuration file ‘cXusb.ini’, located in the same directory like the ‘cXusb.dll’ driver DLL file itself.
[driver] C4Overscan = false
When the parameter value is modified to ‘true’, image returned from camera will include the 64 pixels wide black-level overscan area to the right of the image.
Regulated thermoelectric cooling is capable to cool the CMOS sensor up to 35 °C below ambient temperature. The Peltier hot side is cooled by a fans. The sensor temperature is regulated with ±0.1 °C precision. Cooling and precision regulation ensure the dark current does not ruin 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. The second one measures the temperature inside the camera shell.
The cooling performance 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.
Maximum temperature difference between 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 lowering 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 50 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.
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.
C4 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 154 × 154 × 65 mm (approx. 6 × 6 × 2.6 inches) for the model with standard cooling. Enhanced cooling increases camera depth by 11 mm.
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, no other parts (CPU box, USB interface, etc.), except a brick power supply, are necessary. Another connector allows control of optional external filter wheel. Integrated mechanical shutter allows streak-free image readout, as well as automatic dark frame exposures, which are necessary for unattended, robotic setups.
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. Canon EOS bayonet adapter back focal distance is 44 mm).
Stated back focal distance already calculates with glass permanently placed in the optical path (e.g. optical window covering the sensor cold chamber).
C4 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 automatically acquire dark frames, necessary for proper image calibration etc.
Mechanical shutter in the C4 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 M sized External Filter Wheel diameter is smaller (see External Filter Wheel User's Guide), but the back focal distance of all external filter wheels is identical.
The stated back focal distances (BFD) include corrections for all optical elements in the light path (cold chamber optical window, sensor cover glass, ...), fixed in the camera body. So, stated values are not mechanical, but optical back focal distances. However, no corrections for filters are included, as the thicknesses of various filters are very different.
Please note when the OAG has to be used with a camera without filter wheel, it is necessary to use a thick adapter base of the same thickness like the External filter wheel to keep the distance from OAG to sensor constant. Otherwise, the guiding camera cannot reach focus.
Various accessories are offered with C4 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 C4 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.
C4 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.
The C4-OAG offers the M68 × 1 thread on the telescope side. The back focal distance is 61.5 mm.
If the OAG is used on camera without filter wheel, thicker adapter base must be used to keep the Back focal distance and to allow the guiding camera to reach focus.
OAG guider port is compatible with C1 cameras. It is necessary to replace the CS/1.25” adapter with short, 10 mm variant. 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 C4-OAG.
C4 cameras are equipped with two tripod 0.250-20UNC threads on the top side of the camera head, as well as four metric M4 threaded holes.
These threaded holes 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.
The C4 cameras are supplied with silica-gel container, intended to dry the sensor cold chamber. This container can be unscrewed and desiccant inside can be dried in the owen (see the camera User's Manual).
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. Containers intended for enhanced cooling cameras are prolonged as the camera thickness is greater in the case of this variant.
It is possible to order spare container, which makes desiccant replacement easier and faster. It is possible to dry the spare container with silicagel and then only to replace it on the camera. Spare container is supplied including the air-tight cap.
Spare container can be supplied also in a variant that allows manipulation without tools. But this container is longer and exceeds camera outline. If the space behind the camera is not critical, this container can make desiccant exchange even easier.
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 allows connection of up to 4 Cx cameras of any type on the one side and 1 Gbps Ethernet on the other side. This adapter allows access to connected Cx cameras using routable TCP/IP protocol over practically unlimited distance.
Moravian Camera Ethernet Adapter devices are described in detail here.
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, MaxIm DL, 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.
Example images captured with G4 cameras.
Jmages published with permission of their respective authors.