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.

The C3 cameras are designed to work in cooperation with a host Personal Computer (PC). As opposite to digital still cameras, which are operated independently on the computer, the scientific cooled cameras usually require computer for operation control, image download, processing and storage etc. To operate the camera, you need a computer which:

1. Is compatible with a PC standard and runs modern 32 or 64-bit Windows operating system.

2. Is an x86 or ARM based computer and runs 32 or 64-bit Linux operating system.

   Remark:

   Drivers for 32-bit and 64-bit Linux systems are provided, but the SIPS camera control and image processing software, supplied with the camera, requires Windows operating system.

3. Support for x64 based Apple Macintosh computers is also included.

   Remark:

   Only certain software packages are currently supported on Mac.

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.

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 Overview

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.

First, there are variants differing in the cooling performance:

• Standard cooling

• Enhanced cooling (11 mm thicker due to increased heat sink)

Second, there are variants differing in filter wheel control:

• Camera with Internal filter wheel.

• Camera with control port for External filter wheel. This model allows attachment of several variants of external filter wheels with various number of filter positions and sizes.

C3 camera model with Internal filter wheel accepts two sizes of filters:

• Filter wheel with 5 positions for unmounted D50 mm (or 2”) filters or filters in 2” threaded cells.

• Filter wheel with 7 positions for unmounted D36 mm filters.

There are three sizes of the External filter wheels, capable to accept various sizes of filters, available for the C3 cameras:

• Small “S” size wheel for 10 unmounted filters D36 mm.

• Small “S” size wheel for 7 unmounted filters D50 mm or filters in 2” threaded cells.

• Small “S” size wheel for 5 unmounted square filters 50 × 50 mm.

• Middle “M” size wheel for 10 unmounted filters D36 mm.

• Middle “M” size wheel for 7 unmounted filters D50 mm or filters in 2” threaded cells.

• Middle “M” size wheel for 5 unmounted square filters 50 × 50 mm.

• Large “L” size wheel for 9 unmounted D50 mm or filters in 2” threaded cells.

• Large “L” size wheel for 7 unmounted square filters 50 × 50 mm.

And third, there are two sizes of adjustable adapters, which can be used with C3 cameras:

• Small “S” adapters, compatible with C2 cameras, are used for e.g. M48 × 0.75 and M42 × 0.75 threaded adapters, Nikon bayonet adapter, 2” barrel adapter etc.

• Large “L” adapters, compatible with C4 cameras, intended for large diameter attachments between camera and telescope, e.g. M68 × 1 threaded adapter or G3-OAG, which is also equipped with M68 × 1 thread.

Adjustable adapters are mounted on adapter base when camera with internal filter wheel or camera without any filter wheel is used or directly on the external filter wheel front surface. This means both “S” and “L” adapter bases can be mounted on any camera, but external filter wheels are made for one particular adapter size only:

• “S” external filter wheels are compatible with “S” adapters

• “M” and “L” external filter wheels are compatible with “L” adapters

Camera head and numerous accessories comprise imaging system, capable to be tailored for many applications.

Components of C3 Camera system include:

1. C3 camera head with Internal Filter Wheel

2. C3 camera head with Internal Filter Wheel and Enhanced Cooling option

3. C3 camera head capable to control External Filter Wheel

4. C3 camera head capable to control External Filter Wheel with Enhanced Cooling option

5. 1.75” dovetail rail for C3 camera head

6. Moravian Camera Ethernet Adapter (x86 CPU)

7. Moravian Camera Ethernet Adapter (ARM CPU)

   Remark:

   The 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 unlimited distance.

8. 7-positions internal filter wheel for unmounted D36 mm filters

9. 5-positions internal filter wheel for 2”/D50 mm filters

10. 10-positions filter wheel for “S” or “M” housing for unmounted D36 mm filters

11. 7-positions filter wheel for “S” or “M” housing for 2”/D50 mm filters

12. 5-positions filter wheel for “S” or “M” housing for 50 × 50 mm square filters

13. External Filter Wheel “S” size (5, 7 or 10 positions)

14. M42 × 0.75 (T-thread) or M48 × 0.75 threaded “S” size adapters, 55 mm BFD

15. Canon EOS bayonet lens “S” size adapter

16. Nikon bayonet lens “S” size adapter

17. External Filter Wheel “M” size (5, 7 or 10 positions)

18. External Filter Wheel “L” size (7 or 9 positions)

19. 9-positions filter wheel for “L” housing for 2”/D50 mm filters

20. 7-positions filter wheel for “L” housing for 50 × 50 mm square filters

21. C1 auto-guiding camera

    Remark:

    The C1 cameras are completely independent devices with their own USB connection to the host PC. They can be used either on C3 OAG or on standalone guiding telescope.

    The C1 camera can share the Moravian Camera Ethernet Adapter with up to 3 other Cx cameras to be accessed over network.

22. M68 × 1 threaded “L” size adapter, 47.5 mm BFD

23. Canon EOS bayonet lens “L” size adapter

24. Off-Axis Guider with M68 × 1 thread, 61.5 mm BFD

CMOS Sensor and Camera Electronics

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.

Both IMX571 (used in C3-26000) and IMX455 (used in C3-61000) sensors are supplied in two variants:

• Consumer grade sensors. The sensor manufacturer (Sony Semiconductor Solutions Corporation) limits their usage to consumer still cameras only with operation time max. 300 hours per year.

• Industrial grade sensors, intended for devices operating 24/7.

All sensor characteristics (resolution, dynamic range, …) are equal, sensors differ only in target applications and usage time. C3 is technically digital still camera, only specialized for astronomy. If it is also “consumer” camera strongly depends on users. Cameras used for causal imaging (when weather permits) only rarely exceeds 300 hours of observing time per year. Cameras permanently installed on observatories, utilizing every clear night and possibly located on mountain sites with lots of clear nights exceed the 300 hours/year within a couple of months. This is why C3 cameras are offered in two variants:

• C3-26000 and C3-61000 with consumer grade sensors, intended for max. 300 hours a year operation.

• C3-26000 PRO and C3-61000 PRO with industrial grade sensors.

C3 camera models with consumer-grade sensors include:

C3 camera models with industrial-grade sensors include:

Camera Electronics

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).

Sensor linearity

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.

Download speed

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.

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.

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.

Camera gain

Sensors used in C3 cameras offer programmable gain from 0 to 36 dB, which translates to the output signal multiplication from 1× to 63×.

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:

Conversion factors and read noise

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.

Sensor dynamic range, defined as full well capacity divided by read noise, is greatest when using gain 0, despite somewhat higher read noise:

• At gain = 0, dynamic range is 52,800 / 3.51 = 15,043×

• At gain = 2750, dynamic range is 16,900 / 1.46 = 11,575×

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.

So, which gain settings is the best? This depends on the particular task.

• Gain set to 2750 can be utilized if imaging through narrow-band filter with appropriately short exposures, so the background noise does not exceed the read noise. This is typical for aesthetic astro-photography, where the lowered full well capacity does not negatively influence the result quality.

  But even without narrow-band filters, the extremely low read noise allows stacking of many short exposures without unacceptable increase of the stacked image background noise, caused by accumulation of high read noise of individual exposures.

• Gain set to 0 offers maximum full well capacity and the greatest sensor dynamic range, which is appreciated mainly in research applications. Pass-bands of filters used for photometry are relatively wide and dominant source of noise is the sky glow.

  But also for RGB images, used for aesthetic astro-photography, higher dynamic range allows longer exposures while the bright portions of the nebulae and galaxies still remain under saturation and thus can be properly processed.

Binning

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.

Hardware binning

C3 camera implements 2 × 2 binning mode in hardware in addition to normal 1 × 1 binning.

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

When the HWBinning parameter is set to true, the in-camera hardware binning is used. This mode brings faster download time, but also introduces several restrictions:

1. Maximal binning is limited to 2 × 2, higher binning modes are not available.

2. Asymmetrical binning modes (1 × 2, 2 × 1, ...) are not allowed.

Adding vs. averaging pixels

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.

In theory, the resulting S/N ratio of binned pixel remains the same regardless if we add or average them. Let's take for example 2 × 2 binning:

• If we add 4 pixels, signal increases 4-times and noise increases 2-times — three additive operations increase noise by √((√2)^2×(√2)^2 ). Resulting S/N increases 2-times, but only until the sum of all pixels is lower than the pixel capacity.

• If we average 4 pixels, signal remains the same but the noise is lowered to 1/2 as noise is averaged √((√2)^2×(√2)^2 )/4. Resulting S/N also increases 2-times, but only until the noise decreases to lowest possible 1-bit of dynamic 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.

Binning in photometry

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.

Both above mentioned configuration parameters require at last the software/drivers version:

• SIPS version 3.33

• Moravian Camera SDK version 4.11

• ASCOM drivers version 5.13

• Linux INDI drivers version 1.9-2

• Linux libraries version 0.7.1

• macOS libraries version 0.6.1

• TheSkyX Windows/Linux/macOS version 3.4

• AstroArt drivers version 4.3

If the camera is used through the Moravian Camera Ethernet Adapter, it’s firmware must be updated to version 53 or newer.

Exposure control

The shortest theoretical exposure time of the C3 cameras depends on the used sensor type:

• C3-26000 shortest exposure is 139 μs

• C3-61000 shortest exposure is 156 μs

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.

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).

Mechanical shutter

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.

GPS exposure timing

C3 cameras can be equipped with GPS receiver module (see the Optional Accessories chapter). The primary purpose of the GPS receiver is to provide precise times of exposures taken with the camera, which is required by applications dealing with astrometry of fast-moving objects (fast moving asteroids, satellites, and space debris on Earth orbit, …).

The GPS module needs to locate at last 5 satellites to provide exposure timing information. Geographic data are available if only 3 satellites are visible, but especially the mean sea level precision suffers if less than 4 satellites are used.

The camera SDK provides functions, allowing users to access precision exposure times as well as geographics location. The SIPS software package main imaging camera control tool window contains the “GPS” tab, which shows the state of the GPS fix.

Determination of exact exposure time is quite complicated because of the rolling-shutter nature of the used sensors. Camera driver does all the calculations and returns the time of the start of exposure of the first line of the image. Still, users interested in precise exposure timing need to include several corrections into their calculations:

• Individual image lines are exposed sequentially. The time difference between start of exposure of two subsequent lines is fixed for every sensor type:

  • C3-26000 line exposure takes 34.667 μs

  • C3-61000 line exposure takes 39.028 μs

• If the image is binned, single line of resulting image contains signal from multiple added (or averaged) lines, each with different exposure time start. The exposure start of individual lines of the binned images differs by the single line time difference, multiplied by the vertical binning factor.

• If only a sub-frame is read, it must be considered that the sensor imposes some restrictions to the sub-frame coordinates. If the required sub-frame coordinates violate the sensor-imposed rules, camera driver enlarges the sub-frame region to fully contain desired sub-frame and then crops it by software. The provided start exposure time then concerns the first line actually read from the camera, not the first line of the resulting (software cropped) image.

  For instance, the y-coordinate of the sub-frame must not be lower than 25 lines. If a sub-frame with lower y-coordinate is asked by the user, whole frame is read and cropped by software. Note the camera SDK offers function AdjustSubFrame, which returns the smallest sub-frame, fully containing the requested sub-frame, but also fulfilling the sensor-imposed sub-frame coordinate restriction. If adjusted sub-frame is read, no software cropping occurs and image exposure time concerns the first line of the image. The SIPS software offers the “Adjust Frame” button, which adjusts defined sub-frame.

Always use the latest camera drivers (ASCOM or Camera SDK DLLs in Windows, INDI or libraries in Linux) available on the web. Also, always update the firmware in the Moravian Camera Ethernet Adapter if the camera is connected over Ethernet.

Cooling and power supply

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.

C3 cameras are available in two variants, differing in the cooling performance:

• Standard cooling cameras achieve regulated temperature difference up to 40 °C under environment temperature.

• Enhanced cooling cameras can regulate temperature up to 45 °C under environment temperature. Compared to standard variant, enhanced cooling cameras are somewhat bulkier due to bigger heat sink, slightly heavier and somewhat noisier because of more powerful fans.

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.

Overheating protection

The C3 cameras are equipped with an overheating protection in their firmware. This protection is designed to prevent the Peltier hot side to reach temperatures above ~50°C sensor cooling is turned off to stop heat generation by the hot side of the Peltier TEC modules.

Turning the overheating protection on results in a drop in cooling power, a decrease in the internal temperature of the camera and an increase in the temperature of the sensor. However, when the camera cools its internals down below the limit, cooling is turned on again. If the environment temperature is still high, camera internal temperature rises above the limit an overheating protection becomes active again.

Power supply

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.

Mechanical Specifications

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.

Camera front view

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.

Camera without filter wheel

Camera with Internal Filter Wheel

Camera with the “S” size External filter wheel

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.

Back focal distance

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.

There are two groups of the telescope and lens adapters, differing in back focal distance definition:

• Adapters without strictly defined BFD. These adapters are designed to provide as low BFD as possible.

• Adapter with defined BFD. These adapters are typically intended for optical correctors (field flatteners, coma correctors, ...) and also for photographic lenses. Keeping the defined BFD is necessary to ensure proper functionality of correctors or to be able to achieve focus with photographic lenses.

  Remark:

  Special variant of adapter with defined BFD is Off-Axis Guider adapter. Keeping its distance from the sensor is then necessary to achieve focus with guiding camera when the main camera is focused.

Adapters without back focal distance defined

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.

Adapters with defined back focal distance

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.

Optional accessories

Various accessories are offered with C3 cameras to enhance functionality and help camera integration into imaging setups.

External filter wheels

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.

Telescope adapters

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.

Adjustable telescope/lens adapters are attached slightly differently depending if the External filter wheel is used or not:

• If no External filter wheel is used, C3 adapters are not mounted directly on the camera head. Instead, a tilting adapter base, holding the circular spring, is always used.

• If the External filter wheel is used, the adapted base is not necessary, as the Mark II External filter wheel front plate is already designed to hold the spring and it also contains threads to fix respective adapters.

C3 cameras are offered with two sizes of adjustable adapter base:

• Small “S” adapters (also used with C2 cameras)

• Large “L” adapters (also used with C4 cameras)

Adjustable adapters are mounted on adapter base when camera with internal filter wheel or camera without any filter wheel is used or directly on the external filter wheel front surface. This means both “S” and “L” adapter bases can be mounted on any camera, but external filter wheels are made for one particular adapter size only:

• “S” external filter wheels are compatible with “S” adapters

• “M” and “L” external filter wheels are compatible with “L” adapters

Small “S” size adapters:

• 2-inch barrel — adapter for standard 2" focusers.

• T-thread short — M42 × 0.75 inner thread adapter.

• T-thread with 55 mm BFD — M42 × 0.75 inner thread adapter, preserves 55 mm back focal distance.

• M48 × 0.75 short — adapter with inner thread M48 × 0.75.

• M48 × 0.75 with 55 mm BFD — adapter with inner thread M48 × 0.75, preserves 55 mm back focal distance.

• Canon EOS bayonet — standard Canon EOS lens adapter (“S” size). Adapter preserves 44 mm back focal distance.

• Nikon F bayonet — standard Nikon F lens adapter (“S” size), preserves 46.5 mm back focal distance.

Large “L” size adapters:

• M68 × 1 — adapter with M68 × 1 inner thread and 47.5 mm back focal distance.

• Canon EOS bayonet — standard Canon EOS lens adapter (“L” size). Adapter preserves 44 mm back focal distance.

• Nikon F bayonet — standard Nikon F lens adapter (“L” size), preserves 46.5 mm back focal distance.

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.

Off-Axis Guider adapter

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.

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.

GPS receiver module

The C3 cameras can be equipped with an optional GPS receiver module, which allows very precise timing of the exposure times. Geographic location data are also available to the control software through specific commands.

The used GPS receiver is compatible with GPS, GLONASS, Galileo and BeiDou satellites.

The GPS receiver can be attached to the back side of the camera head. If the GPS module is removed, the GPS port is covered with a flat black cover.

Attaching camera head to telescope mount

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.

Spare desiccant containers

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).

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 color variants

Camera head is available in several color variants of the center plate. Visit manufacturer's web pages for current offering.

Moravian Camera Ethernet Adapter

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.

Software Support

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.

Automatic guiding

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.

Advanced reconstruction of color information of single-shot-color cameras

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.

Shipping and Packaging

C3 cameras are supplied in the foam-filled, hard carrying case containing:

• Camera body with a user-chosen telescope adapter. If ordered, the filter wheel is already mounted inside the camera head and filters are threaded into place (if ordered).

• A 100-240 V AC input, 12 V DC output “brick” adapter with 1.8 m long power cable.

• 2 m long USB 3.0 A-B cable for connecting camera to host PC.

• USB Flash Drive with camera drivers, SIPS software package with electronic documentation and PDF version of User's Manual.

• A printed copy of camera User's Manual

Image Gallery

Example images captured with C3 and C1× cameras.