Camera head data sheet and selection guide


Introduction


PixCellent provides a broad range of camera heads for use with its cooled CCD camera systems that are designed to suit the needs of a variety of imaging applications. Each type of camera head consists of a CCD, cooled either thermoelectrically or by liquid nitrogen. The camera heads are provided with integral shutters for thermoelectrically cooled heads or removable shutter plates for liquid-nitrogen cooled camera heads. The camera heads can also be provided with standard lens fixing mounts. Thermoelectrically cooled camera heads offer the combination of a conveniently-sized package with extremely high performance in terms of both sensitivity and dynamic range.
The CCDs used in PixCellent's systems are cooled in order to reduce the amount of thermally generated dark current. Accumulation of dark current severely limits the performance of an uncooled system. By lowering the temperature of the CCD, the dark current is reduced by a factor of about 10 for every 20oC drop in CCD temperature. Consequently, sensitivity and dynamic range are greatly extended. Cooling below about -140oC gives no further advantage because the CCD ceases to function properly.


Cooling method

The most appropriate method of cooling depends on the application and conditions of use. There are six types of cooled camera heads available:


Thermoelectric cooling

These heads utilise the Peltier effect to cool the CCD, hence they are often referred to as "Peltier cooled" heads. The Peltier cooler consists of a number of junctions between two dissimilar materials connected together. When current is passed in the correct direction through these junctions, localised cooling occurs. This has the effect of lowering the temperature of the CCD mounted on the cooler. The heat generated by this process is dissipated either by an air cooled heat sink attached to the cooler, or by the circulation of water in a jacket around the device. All thermoelectrically cooled camera heads consist of a Peltier cooled CCD mounted in a sealed compartment. This compartment contains a desiccant in order to absorb moisture from the enclosed air and prevent any subsequent condensation or icing which would occur on cooling. This design assures that the camera head provides many years of maintenance-free operation.

Air cooled heads - TE/A

Air cooled heads are useful in applications where access to a water supply or circulator is difficult, or where the presence of water in the vicinity of equipment is undesirable. It also makes the system easier to transport.
A rear mounted heat sink dissipates heat from the cooling process. This heat sink is in turn cooled by a fan which passes ambient air over the elements. The specifications given for these types of camera heads assume an ambient air temperature of 25oC or lower. Differences in the ambient air temperature have a reduced effect on the CCD temperature.
In cases where the use of a fan is not desired, simple convection cooling may be used at the expense of a somewhat higher CCD operating temperature.

The compact head for small CCDs is called the TE3, and a drawaing is available here:

The larger head for larger CCDs is called the TE4, and a drawaing is available for the version for the Antares controller here: A detailed assmebly drawing is available here.

The larger head for larger CCDs is called the TE4, and a drawaing is available for the version for the Capella controller here: A detailed assmebly drawing is available here.


Liquid cooled heads - TE/W

Liquid (usually water) cooled heads give enhanced performance over the air cooled versions, due to more effective dissipation of the heat from the Peltier cooler. This enables lower CCD temperatures to be reached, with a corresponding reduction in the dark current.
Liquid cooled heads are more suitable than air cooled heads in applications where increased sensitivity and dynamic range are required and where a water supply is readily accessible. It is also physically smaller which may be of additional benefit to some users.


Liquid nitrogen cooling

Liquid nitrogen cooled heads offer the ultimate performance in terms of sensitivity and dynamic range. Dark current from the chip is reduced to less than one electron per pixel per hour in some cases by cooling the CCD to approximately -140oC, even for non-MPP CCDs. This allows a dynamic range of 100,000:1 within a single exposure. Images can be obtained which exhibit extremely large relative variations in intensity without saturation of the CCD, whilst still resolving details in the faintest areas. Some CCDs (such as those from SITe) are operated at -90oC.
The liquid nitrogen cooled heads contain a CCD mounted in a vacuum-sealed housing to minimise heat loss. The CCD is connected by a cold "finger" to a container filled with liquid nitrogen. This effectively cools the CCD to a user selectable and accurately stabilised temperature from -60oC to -140oC., reducing dark current to the absolute minimum. Periodic re-filling with liquid nitrogen is necessary and the heads may require re-pumping with a rotary/diffusion or turbo vacuum pump about once every few months, depending on usage.

High-Performance - LN/K

The high-performance version of the liquid-nitrogen head benefits from significantly longer liquid-nitrogen hold times than the standard unit. It has been designed for applications where space is limited and it is available in a sideways as well as a downward looking configuration. Additional fill tube inserts are available to allow inverted operation (with slightly reduced liquid nitrogen hold times). Drawing is available here.

Standard - LN/S

The standard liquid nitrogen head has been ruggedised and adapted for more universal use than the compact unit. It has a number of mounting points built into the structure and can be configured to be used in upwards, downwards or sideways orientation.Drawing is available here.



Performance characteristics

The performance of the camera heads varies with the type of cooled CCD system they are being used with. These differences are given in the tables below.

Table 1: Thermoelectric air-cooled camera head performance characteristics

TE3/A TE4/A
Cooling method Peltier/Air Peltier/Air
Limiting luminance (lux) 5 x 10-9 (depends on choice of CCD) 5 x 10-9 (depends on choice of CCD)
Dark counts with MPP CCD 2 - 0.01 electrons/pixel/sec 2 - 0.01 electrons/pixel/sec
Weight (kg) 2 4
Height x Width x Depth (mm) 120 x105 x 214 125 x 140 x 187
Optical distance of CCD from front face (mm) 17.5 30
Mounting points available on front face (See below for location of mounting points)

- C-mount thread

- 3 holes tapped M4 equispaced on 67 PCD for extension tube, Pentax K-mount or Nikon bayonet fittings.

- 4 holes tapped M10 10mm from each edge.

Pentax K-mount or Nikon bayonet fittings.

- 4 holes tapped M4 equispaced on 67 PCD for extension tube, Pentax K-mount or Nikon bayonet fittings.

- 4 holes tapped M4 equispaced on 100 PCD.

Tripod mount Tripod thread (1/4" - 20 UNC) Tripod thread (1/4" - 20 UNC)

Table 2: Thermoelectric liquid-cooled camera head performance characteristics

TE3/W TE4/W
Cooling method Peltier / Liquid Peltier / Liquid
Limiting luminance (lux) 1 x 10-9 (depends on choice of CCD) 1 x 10-9 (depends on choice of CCD)
Dark counts with MPP CCD 0.5 - 0.002 electrons/pixel/sec 0.5 - 0.002 electrons/pixel/sec
Weight (kg) 1.4 4.5
Height x Width x Depth (mm) 120 x 105 x 214 125 x 140 x 187
Optical distance of CCD from front face (mm) 17.5 30
Mounting points available on front face(See below for location of mounting points)

- C-mount thread

- 3 holes tapped M4 equispaced on 67 PCD for extension tube, Pentax K-mount or Nikon bayonet fittings.

- 4 holes tapped M10 10mm from each edge.

-Pentax K-mount or Nikon bayonet fittings.

- 4 holes tapped M4 equispaced on 67 PCD for extension tube, Pentax K-mount or Nikon bayonet fittings.

- 4 holes tapped M4 equispaced on 100 PCD.

Tripod mount Tripod thread (1/4" - 20 UNC) Tripod thread (1/4" - 20 UNC)

Table 3: Liquid nitrogen cooled camera head performance characteristics

High-Performance LN/K Standard LN/S
Cooling method Liquid nitrogen Liquid nitrogen
Limiting luminance (lux) 1 x 10-11 (depends on choice of CCD) 1 x 10-11 (depends on choice of CCD)
Dark counts with standard CCD 1 electron/pixel/hour at -140oC 1electron/pixel/hour at -140oC
Weight (kg) 4.5 (empty) 9.0 (empty)
Height x Diameter (mm) 267mm long (body) plus >120 mm for LN fill access , front plate 122 mm diameter. Side access for electrical connectors and for re-pumping. Rear access space required for re-pumping and re-filling of the Dewar 297mm long (body) plus >120 mm for LN fill and vac valve access , 215 mm OD, front plate 236 mm diameter. Electrical connector side access near front plate.Rear access space required for re-pumping and re-filling of the Dewar
Optical distance of CCD from front face without shutter plate fitted (mm) 23.0 incl shutter housing. 12.5
Mounting points available on front face C-mount, K-mount. Other mounts can be provided on request. C-mount, K-mount. Other mounts can be provided on request.
Mounting points available on side of head 12 holes on circle on shutter plate M6 holes on 50 PCD. M6 tapped holes.
Nominal CCD temperature -60oC. to -140oC. -60oC. to -140oC.

 

Drawings are available as follows:

TE/3

TE/4 for Antares

TE/4 for Capella

TE/4 Assembly details

LN/K high performance dewar

LN/S standard dewar

 

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Site last updated: 15 April, 1999.