The cooled G3 series Mark II CCD cameras were developed for imaging under extremely low-light conditions in astronomy, microscopy and similar areas. Design of this series inherits from earlier G3 Mark I cameras but brings some significant enhancements. G3 cameras employ precise electronics providing uniform frames and extremely low read noise limited only by CCD detector itself.

Modular mechanical construction allows various camera variants to be combined with rich set of accessories, including telescope adapters, off-axis guider adapters, internal or external filter wheels, Ethernet adapters, guiding cameras etc.

Included free SIPS software package allows usage of G3 camera without necessity to invest into any 3rd party software package. However, rich software and driver support (e.g. ASCOM for Windows, INDI for Linux etc.), shipped with the camera, provide the way to integrate G3 camera with vast variety of camera control programs.

G3 Mark II Camera Overview

G3 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.

G3 camera model with Internal filter wheel contains 5 positions for unmounted D50 mm filters or filters in standard 2” threaded cells.

There are three sizes of the External filter wheels, capable to accept various sizes of filters, available for the G3 cameras:

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

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

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

And third, there are two sizes of adjustable adapters, which can be used with G3 cameras:

• Small “S” adapters, compatible with G2 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 G4 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 G3 Mark II Camera system include:

1. G3 camera head with Internal Filter Wheel (5 positions)

2. G3 camera head with Internal Filter Wheel (5 positions) and Enhanced Cooling option

3. G3 camera head capable to control External Filter Wheel

4. G3 camera head capable to control External Filter Wheel and Enhanced Cooling option

5. 1.75” dovetail rail for G3 camera head

6. Gx Camera Ethernet Adapter (x86 CPU)

7. Gx Camera Ethernet Adapter (ARM CPU)

   Remark:

   Camera Ethernet Adapter allows connection of up to 4 Gx cameras of any type on the one side and 1 Gbps Ethernet on the other side. This adapter allows access to connected Gx cameras using routable TCP/IP protocol over unlimited distance.

8. 5-positions internal filter wheel for 2”/D50 mm filters

9. 5-positions filter wheel for “S” or “M” housing for 50 × 50 mm filters

10. 7-positions filter wheel for “S” or “M” housing for 2”/D50 mm filters

11. 9-positions filter wheel for “L” housing for 2”/D50 mm filters

12. 7-positions filter wheel for “L” housing for 50 × 50 mm filters

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

14. External Filter Wheel “M” size (7 or 5 positions)

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

16. M42 × 0.75 (T-thread) or M48 × 0.75 threaded adapters, 55 mm BFD

17. Canon EOS bayonet lens “S” size adapter

18. Nikon bayonet lens adapter

19. M68 × 1 threaded adapter, 47.5 mm BFD

20. Canon EOS bayonet lens “L” size adapter

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

22. G0 guider camera

23. G1 guider camera

    Remark:

    G0 and G1 cameras are completely independent devices with their own USB connection to the host PC. They can be used either on G3 OAG or on standalone guiding telescope.

    Both G0 and G1 camera can share the Gx Camera Ethernet Adapter with up to 3 other Gx cameras to be accessed over network.

G3 cameras require at last one free USB 2.0 port to communicate with a host PC.

Alternatively, it is possible to use the Gx Camera Ethernet Adapter device. This device can connect up to four Gx cameras of any type (not only G3, but also G0, G1, G2 and G4) 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.

G3 Mark II Camera Models

G3 series contains the following camera models:

CCD detector

G3 Mark II series of CCD cameras are manufactured with two kinds of CCD detectors:

• G3 cameras with OnSemi KAF Full Frame (FF) CCD architecture. Almost all Full Frame CCD detector area is exposed to light. This is why these detectors provide very high quantum efficiency. FF CCD detectors, intended for research applications, are not equipped with so-called Anti Blooming Gate (ABG – a gate, which prohibits blooming of the charge to neighboring pixels when image is over-exposed) to ensure linear response to light through the whole dynamic range. FF CCD detectors used for astrophotography are equipped with ABG to eliminate disrupting blooming streaks within field of view.

  

  “Full Frame” CCD schematic diagram

  Cameras with Full Frame, non-ABG detectors are suitable for scientific applications, where linear response is necessary for photometric applications in astronomy, microscopy etc. High quantum efficiency could be used also for narrow-band imaging, where overexposure is a rare exception, and for imaging of small objects without a bright star in the field of view.

• G3 cameras with OnSemi KAI Interline Transfer (IT) architecture. There is a shielded column of pixels just beside each column of active pixels on these detectors. The shielded columns are called Vertical registers. One pulse moves charge from exposed pixels to shielded pixels on the end of each exposure. The the charge is moved from vertical registers to horizontal register and digitized in the same way like in the case of Full Frame detectors. This mechanism is also known as “electronic shuttering” because it allows very short exposures and also digitization of the image without mechanically shielding of the detector from incoming light.

  

  “Interline Transfer” CCD schematic diagram

  Remark:

  Also, G3 cameras with IT CCDs are equipped with mechanical shutter, because electronic shutter does not allow dark-frame exposures, necessary for proper image calibration etc.

  The price for electronic shutter if lower quantum efficiency (sensitivity) of IT detectors compared to FF ones. Also, all IT detectors are equipped with ABG, so they can acquire images of very bright objects without charge blooming to neighboring pixels.

Model G3-1000

G3-1000 model uses 1 MPx OnSemi KAF-1001E CCD.

Model G3-6300

G3-6300 model uses 6 MPx OnSemi KAF-6303E CCD.

Model G3-16200

G3-16200 uses 16 MPx OnSemi KAF-16200 Class 1 or 2 CCD with APS-H format.

Model G3-11000

G3-11000 uses 11 MPx OnSemi KAI-11002 Class 1 or 2 CCD.

Camera Electronics

16-bit A/D converter with correlated double sampling ensures high dynamic range and CCD chip-limited readout noise. Fast USB interface ensures image download time within seconds.

Maximum length of single USB cable is approx. 5 m. This length can be extended to 10 m or 15 m by using single USB hub or active USB extender cable. Up to 5 hubs or active extenders can be used in one connection.

Gx Camera Ethernet Adapter device allows connection of up to four Gx cameras of any type through Ethernet interface and TCP/IP network. Because TCP/IP protocol can be routed, the distance between camera and host PC can be virtually unlimited.

Image download time depends on the CCD chip used in particular camera model. Also the read noise depends on the chip as well as on the read mode.

• Preview read mode provides system read noise approx. 1 or 2 e^- above CCD chip read noise.

• Low Noise read mode is somewhat slower, but ensures system read noise roughly equal to the manufacturer-specified chip read noise.

Model G3-1000

Model G3-6300

Model G3-16200

Model G3-11000

Notes:

1. Binning can be combined independently on both axes.

2. Stated read noise is measured on particular CCD sensor, evaluated during camera design. Actual read noise of different sensors varies among various manufacturing batches, but also within single manufacturing batch. The camera read noise is determined by the sensor itself and the camera manufacturer cannot affect it.

Cooling and power supply

Regulated thermoelectric cooling is capable to cool the CCD chip from 45 to 50 °C below ambient temperature, depending on the camera type. The Peltier hot side is cooled by a fans. The CCD chip 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.

G3 cameras are available in two variants, differing in the cooling performance:

• Standard cooling cameras achieve regulated temperature difference up to 45 °C under environment temperature.

• Enhanced cooling cameras can regulate temperature up to 50 °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 camera head contains two temperature sensors — the first sensor measures directly the temperature of the CCD chip package. 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.

Camera construction does not allow usage of both air and liquid cooling. Combined cooling (air with the liquid cooling option) is not available, because such cooling does not work effectively enough with air only nor with water only.

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 40 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 (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.

Camera with Internal Filter Wheel

Enhanced cooling variant

Camera with “S” 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.

Enhanced cooling with External filter wheel variant

Optional accessories

Various accessories are offered with G3 Mark II 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 G3 Mark II 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 adapter is attached directly to the camera head (e.g. when camera is equipped with internal filter wheel) or to the External filter wheel case.

• G3 Mark II 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.

G3 Mark II cameras are offered with two sizes of adjustable adapter base:

• Small “S” adapters (also used with G2 cameras)

• Large “L” adapters (also used with G4 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, 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.

All telescope/lens adapters of the G3 Mark II series of cameras can be slightly tilted. This feature is introduced to compensate for possible misalignments in perpendicularity of the telescope optical axis and sensor plane.

The Mark II camera telescope adapters are attached using three “pulling” screws. As the adapter tilt is adjustable, another three “pushing” screws are intended to fix the adapter after some pulling screw is released to adjust the tilt.

Off-Axis Guider adapter

G3 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 G3-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 G0 and G1 cameras. It is necessary to replace the CS/1.25” adapter with short, 10 mm variant in the case of G1 cameras. Because G1 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 G3-OAG.

Attaching camera head to telescope mount

G3 Mark II cameras are equipped with two “tripod” 0.250-20UNC threads on the top side of the camera head. This thread can be used to attach 1.75 inch “dovetail bar” (Vixen standard). It is then possible to attach the camera head, e.g. equipped with photographic lens, directly to various telescope mounts supporting this standard.

Spare desiccant containers

The G3 Mark II cameras are supplied with silicagel container, intended to dry the CCD cold chamber. This container can be unscrewed and desiccant inside can be dried in the owen (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 (of 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.

Gx Camera Ethernet Adapter

Gx Camera Ethernet Adapter allows connection of up to 4 Gx cameras of any type on the one side and 1 Gbps Ethernet on the other side. This adapter allows access to connected Gx cameras using routable TCP/IP protocol over practically unlimited distance.

Gx 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 CCD detectors 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

G3 Mark II 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.

• 5 m long USB 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 G3 cameras.

All images published with permission of their respective authors.