C3 cameras employ the latest generation of Sony IMX CMOS sensors, offering exceptional quantum efficiency thanks to back-illuminated design and very low dark current. Despite relatively small pixels, full-well capacity exceeding 50 ke-. Combined with full 16 bit digitization and perfectly linear response to light make these cameras suitable for both aesthetic astro-photography as well as astronomical research.
|C3 cameras employ the latest generation of Sony IMX CMOS sensors, offering exceptional quantum efficiency thanks to back-illuminated design and very low dark current. Despite relatively small pixels, full-well capacity exceeds 50 ke-. Combined with full 16 bit digitization, perfectly linear response to light and exceptionally low read noise, these cameras are suitable for both aesthetic astro-photography as well as astronomical research. Sensor formats from APS to photographic full-frame (24 × 36 mm) ensure wide field of view and optimally utilize capabilities of the optical systems most commonly used by amateur astronomers.|
Mechanical design of the C3 series of astronomical CMOS cameras inherits from earlier CCD-based G3 Mark II cameras, which makes the C3 camera line fully compatible with vast range of telescope adapters, off-axis guider adapters, filter wheels, Ethernet adapters, guiding cameras etc.
Rich software and driver support allow usage of C3 camera without a 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 C3 camera with broad variety of camera control programs.
C3 cameras are designed to be attached to host PC through very fast USB 3.0 port. While C3 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 C3, but also C1, C2 and C4) 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 C3 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. C3 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.
C3 camera head is designed to be easily used with a set of accessories to fulfill various observing needs. The camera head itself is manufactured in several variants.
Both Internal and External filter wheels for D36 mm filters can be used with C3 camera equipped with APS size sensors only. Cameras with “Full-frame” sensors (24 × 36mm) cannot use such small filters.
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.
Note the “S” and “M” filter wheels are of very similar dimensions and hold the same number of the same filters. They differ in the adjustable adapter size only.
Camera head and numerous accessories comprise imaging system, capable to be tailored for many applications.
C3 cameras are equipped with Sony IMX rolling shutter back-illuminated CMOS detectors with 3.76 × 3.76 μm square pixels. Despite the relatively small pixel size, the full-well capacity over 50 ke- rivals the full-well capacity of competing CMOS sensors with much greater pixels and even exceeds the full-well capacity of CCD sensors with comparable pixel size.
The used Sony sensors are equipped with 16-bit ADCs (Analog to Digital Converters). 16-bit digitization ensures enough resolution to completely cover the sensor exceptional dynamic range.
While the used sensors offer also lower dynamic resolution (12 and 14 bit), C3 cameras do no utilize these modes. Astronomical images always use 2 bytes for a pixel, so lowering the dynamic resolution to 14 or 12 bits brings no advantage beside the slightly faster download. But cooled astronomical cameras are intended for very long exposures and a fraction of second saved on image download is negligible compared to huge benefits of 16-bit digitization.
C3 camera models with consumer-grade sensors include:
C3 camera models with industrial-grade sensors include:
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 C3 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.
C3 cameras are equipped with on-board RAM, capable to hold several 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 digitize and download single full frame depends on USB connection type.
If only a sub-frame is read, time needed to digitize and download image is naturally lower. However, the download time is not cut proportionally to number of pixels thanks to some fixed overhead time, independent on the sub-frame dimensions.
Download times stated above are valid for cameras with firmware version 3.3 and later. Older firmware download times were approximately 30% longer.
The driver is sometimes forced to read bigger portions of the sensor than the user defined because of a sub-frame position and dimension limitations imposed by the sensor hardware. Sometimes it is even necessary to read the whole sensor.
It is recommended to click the Adjust Frame button in the Frame tab of the SIPS camera control tool. The selected frame dimensions are then adjusted according to sensor limitations. Adjusted frame is then read from the sensor, without a necessity to read a bigger portions or even whole sensor and crop image in firmware.
C3 camera electronics supports in-camera 2 × 2 binning. If this binning mode is used, download speed increases because of less amount of data read from camera.
The in-camera binning is supported by firmware version 3.3 and later.
Download speed when using the Moravian Camera Ethernet Adapter depends if the 100 Mbps or 1 Gbps Ethernet is used, if USB 2 or USB 3 is used to connect camera to Ethernet Adapter device, but also depends on the particular network utilization etc. When the camera is connected to the Ethernet Adapter using USB 3 and 1 Gbps Ethernet is directly connected to the host PC, download time of the C3-61000 full frame is approx. 2.5 s.
Sensors used in C3 cameras offer programmable gain from 0 to 36 dB, which translates to the output signal multiplication from 1× to 63×.
Note the C3 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.
Camera driver accepts gain as a number in the range 0 to 4030, which corresponds directly to sensor register value. This number does not represent gain in dB nor it is an exact gain multiply. However, the driver offers a function, which transforms the gain numerical value to gain expressed in dB as well as multiply. Some selected values are shown in the table:
Generally, many sensor characteristics depend on the used gain. Also, the used sensors employ two conversion paths. One path offers very low read noise, but cannot utilize full sensor dynamic range. Another conversion path offers maximum pixel capacity, but at the price of higher read noise. The cross point is set to gain 3× (approx. 10 dB), where the full well capacity drops from more than 50 ke- to ~17 ke-. The read noise then drops from ~3.2 e- RMS to ~1.5 e- RMS.
Also, it is worth noting that in reality the noise floor is not always defined by read noise. Unless the camera is used with very narrow narrow-band filter (with FWHM only a few nm) and under very dark sky, the dominant source of noise is the sky glow. When the noise generated by sky glow exceeds approximately 4 e- RMS, extremely low read noise associated with gain set to 2750 or more is not utilized and dynamic range is unnecessarily limited by the lowered full well capacity.
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.
The camera driver and user’s applications offer wide variety of binning modes up to 4 × 4 pixels as well as all combinations of asymmetrical binning modes 1 × 2, 1 × 3, 1 × 4, 2 × 4 etc. To allow such flexibility, binning is performed only in the camera driver (software binning) and does not rely on the limited capabilities of the hardware binning.
The negative side of software binning is the same download time like in the case of full-resolution 1 × 1 mode. For typical astronomy usage, the small fraction of second download time is irrelevant, but for applications sensitive to download time, the hardware 2 × 2 binning can be useful.
C3 camera implements 2 × 2 binning mode in hardware in addition to normal 1 × 1 binning.
Hardware binning is supported by camera firmware version 3.3 and later. The Windows SDK supports the hardware binning from version 4.11 and the SIPS software package from version 3.33.
Hardware binning can be turned on and off using the parameter HWBinning in the 'cXusb.ini' configuration file, located in the same directory like the 'cXusb.dll' driver DLL file itself.
[driver] HWBinning = true
Despite the number of pixels in the 2 × 2 binned image is 1/4 of the full resolution image, the download time is not four-times lower.
The traditional meaning of pixel binning implies adding of binned pixels. This originated in CCD sensors, where pixel charges were literally poured together within the sensor horizontal register and/or the output node.
For CMOS sensors with full 16-bit dynamic resolution, the negative side of binning is limiting of the sensor dynamic range, as for instance only 1/4 of maximum charge in each of the 2 × 2 binned pixels leads to saturation of resulting pixel. CCDs eliminated this effect to some extend by increasing of the charge capacity of the output node and also by decreasing of the conversion factor in binned modes. But such possibilities are not available in CMOS detectors.
CMOS sensors with less than 16-bit precision often just add binned pixels to fulfill the available resolution of 16-bit pixels. For instance, camera with 12-bit dynamic range can sum up to 4 × 4 pixels and still the resulting binned pixels will not overflow the 16-bit range.
As the C3 camera read noise in the maximum dynamic range (gain 0) is around 3.5 ADU, halving it in 2 × 2 binning mode still keeps the read noise above the lower 1-bit limit and at the same time binned pixel will not saturate. For higher binning modes, the noise approaches lower limit, but averaging pixels still protects from pixel saturation, which is more important than limiting of S/N.
If we take into account that the image background noise is only rarely defined by the read noise of the sensor, as the noise caused by background sky glow is typically much higher, for 16-bit camera averaging pixels is definitely the better way to bin pixels compared to just adding them. This is why both software and hardware binning modes in the C3 cameras are by default implemented as averaging of pixels, not summing.
However, both software and hardware binning modes can be switched to sum binned pixels instead of average them by the BinningSum parameter in the 'cXusb.ini' configuration file:
[driver] BinningSum = true
Let’s note there is one more possibility to bin pixels — in the application software. This time binning is not performed in camera hardware nor in the camera driver. Full resolution 1 × 1 image is downloaded from the camera and software itself then performs binning. The SIPS software adds pixels instead of averaging them, but at the same time SIPS converts images from 16-bit to 32-bit dynamic range. This means S/N of the binned images always increases, pixels never saturate and read noise newer approaches lower limit. The negative side of this option is two-time bigger images.
Saturated pixels within bright stars are no issue for aesthetic astro-photography, but photometry measurement is invalid if any pixel within the measured object reaches maximum value, because it is not possible to determine the amount of lost flux. Software performing photometry (e.g. the SIPS Photometry tool) should detect saturation value and invalidate entire photometric point not to introduce errors.
But binning efficiently obliterates the fact that any of the binned pixels saturated (with the exception of all binned pixels reached saturation value). So, using of binning modes for research applications (photometry and astrometry) can lead to errors caused by lost flux in saturated pixels, which cannot be detected by the processing software due to binning.
This is why the behavior of both software and hardware binning modes is user-configurable through the BinningSaturate parameter in the 'cXusb.ini' configuration file:
[driver] BinningSaturate = true
If the BinningSaturate parameter is set to true, resulting binned pixel is set to saturation value if any of the source pixels is saturated. For aesthetic astro-photography, keeping this parameter false could result into slightly better representation of bright star images, but for research applications, this parameter should always be set to true.
Note the BinningSum and BinningSaturate parameters have any effect if the camera firmware version is 5.5 or later. Prior firmware versions just averaged binned pixels and the pixel saturation was not taken into account when hardware (in camera) binning was used.
The earlier camera drivers, performing software binning, also used pixel averaging for binning, but handled the saturated pixels like the BinningSaturate parameter is true.
If the camera is used through the Moravian Camera Ethernet Adapter, it’s firmware must be updated to version 53 or newer.
However, such short exposures have no practical application, especially in astronomy. The camera firmware rounds exposure time to a multiply of 100 μs intervals, so in reality the shortest exposure time of both camera models is 200 μs.
Note the individual lines are not exposed at the same time, regardless of how short the exposure is, because of the rolling-shutter nature of the used sensors. The difference between the first and last line exposure start time is 0.15 s for the C3-26000 and 0.25 s for the C3-61000 camera.
There is no theoretical 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).
Please note the short exposure timing is properly handled in the camera firmware version 6.5 and later.
C3 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 C3 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 a few seconds while the shutter is open (this means after a light image exposure), camera firmware closes the shutter to cover the sensor from incoming light.
Regulated thermoelectric cooling is capable to cool the CMOS sensor from 40 to 45 °C below ambient temperature, depending on the camera type. The Peltier hot side is cooled by fans. 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 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 the 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 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.
Power consumption is measured on the 12 V DC side. Power consumption on the AC side of the supplied AC/DC power brick is higher.
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.
C3 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, except a “brick” power supply, are necessary. Another connector allows control of optional external filter wheel. Integrated mechanical shutter allows 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).
C3 camera front cross-section is the same for cameras with Internal Filter Wheel or without filter wheel, as well as for variants with standard and enhanced cooling.
The “M” and “L” sized External Filter Wheels diameter is greater (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.
C3 cameras are manufactured in many variants and can be connected with various accessories, which leads to many possible back focal distance values.
Most commonly used adapters without strictly prescribed back focal distance are M48 × 0.75 thread for C3 cameras with the “S” adapter base or the “S” sized External Filter Wheel and M68 × 1 thread for C3 cameras with the “L” adapter base or the “M” and “L” sized External Filter Wheel.
Let us note the M48 × 0.75 threaded adapter is sometimes used with 55 mm BFD, e.g. when used with optical correctors. This is why two models of this adapters are available — “short” variant with as low BFD as possible and “long” variant, which preserves the 55 mm BFD.
There are three basic variants of C3 camera, differing with back focal distance of the camera head front shell — camera without internal filter wheel, with Internal Filter Wheel with External Filter Wheel. But adapters preserving back focal distance are always designed with the same thickness. Their dimension counts with the BFD of the tiltable adapter base 33.5 mm, which corresponds with BFD of the camera with External Filter Wheel.
However, adapters not mounted on the External Filter Wheel tiltable base must be mounted on standalone tiltable adapter base attached to the camera head. Such adapter base is designed to provide exactly the same 33.5 mm BFD when mounted on camera with Internal Filter Wheel.
If a camera without filter wheel is to be used with adapter preserving the defined BFD, it is necessary to use a thick tiltable adapter base, which also provides the 33.5 mm BFD. Thickness of this adapter base equals the thickness of the External Filter Wheel shell.
Various accessories are offered with C3 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.
Various telescope and lens adapters for the C3 cameras are offered. Users can choose any adapter according to their needs and other adapters can be ordered separately.
All telescope/lens adapters of the C3 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.
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.
C3 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 C3-OAG offers the M68 × 1 thread on the telescope side. The back focal distance is 61.5 mm.
Note the C3-OAG is manufactured for “L” size adapter base, so it is compatible with “M” and “L” external filter wheels only.
While C2-OAG (with M48 × 0.75 or M42 × 0.75 inner thread) for “S” size adapter base can be technically mounted to “S” size external filter wheel, the mirror is so close to optical axis, that it partially shields sensors used in C3 cameras and C2-OAG is not recommended for C3-61000 camera.
When used on camera with Internal filter wheel, thin adapter base is used.
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 C0 and C1 cameras with CS-mount adapter. It is necessary to replace the CS/1.25” adapter with a short, 10 mm variant in the case of C1 cameras. Because C0 and 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 C3-OAG.
C3 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 C3 cameras are supplied with silicagel container, intended to dry the sensor cold chamber. This container can be unscrewed and desiccant inside can be dried in the oven (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 oven. 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 (or for just a coin), 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.
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.