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Stasi
Zeiss
Pentacon
  
GSK   SR-899
Universal low-noise reflex camera

SR-899, also known as GSK, 1 is a low-noise single-lens reflex camera (SLR), developed in 1977 by Pentacon in East Germany (DDR). The camera was intended for covert observations and covert surveillance by the repressive security service of the DDR - Ministerium für Staatssicherheit (MfS), also known as the Stasi. A wide range of options was available for it, such as (remote) control units, concealments and pinhole lenses. The device is also known by its Stasi designator 14101.

The camera – basically a modified Praktica EE2 35 mm SLR 2 – has a standard M42 thread (also known as Praktica-mount) which allows a wide range of standard lenses to be used. In addition, adapter rings were available for use with special surveillance lenses and high-end pinhole lenses.

The image on the right shows the GSK with an f3.5 135 mm lens (SO-3.4) that was developed especially for use in observations and covert surveillance. Lenses with a different focal length were also available. These lenses have an M36 mount, for which an M36/M42 adapter is used.
  
GSK with Pentacon 135 mm lens

The above configuration would normally be operated from inside a concealment, such as a shoulder bag or a piece of furniture. Depending on the circumstances and the expected distance to the target, one of several lenses would be selected from the Stasi's Special Optics (SO) range.

In addition to the regular lenses from the SO-series, the Stasi also used special pinhole lenses that were developed by Carl Zeiss in Jena (DDR). A pinhole lens requires just a tiny little hole – typically 1 to 3 mm – to create a high-quality image, without the risk of being detected.

The image on the right shows a GSK fitted with an SO-3.5.1 pinhole lens, two range extenders (2x and 2.5x) and an M36/M42 adapter ring. For adjustment, a 90° viewfinder is fitted over the regular eyepiece. It has to be removed once the camera lens is adjusted to avoid light-leakage.
  
GSK with pinhole lens and 2.5x and 2x range extenders

The above configuration would usually be hidden behind a wall or inside a concealment, with only the tip of the lens looking through a tiny hole in the wall. The camera could be operated manually – with a handheld shutter release button – or by an automatic control unit at adjustable intervals.

In the latter case, the camera would usually be fitted with the additional film cartridge shown in the image on the right. It replaces the camera's rear door and can hold up to 17 metres of 35 mm film, which is equivalent to 450 exposures.

The camera in the image is fitted with a covert ƒ2.8/50 lens with a fixed No. 8 aperture at the front. This could be replaced by an adjustable diaphragm. It was also possible to fit a data back unit instead of the film cartridge, that recorded the current time and date onto the film via an internal projection system with miniature flash.
  
GSK with covert 50 mm lens and film cartridge for 450 exposures

Development of the camera started in the early 1970s at the request of Department 31 of the OTS of the Stasi, and was carried out by Pentacon under Stasi project number 14101. The aim was to develop a universal low-noise 35 mm SLR camera that was suitable for covert photography [3]. A lot of research had already been done in the mid-1960s at the East-German Science Academy in Berlin — Deutsche Akademie der Wissenschaften (DAW) 3 — for the development of a static semi-transparent foil-based mirror 4 that could replace the noisy mirror of a regular SLR camera [D][E].

The geräuscharme Spiegelreflex Kamera (GSK) was given the model number SR 899, and went into production in 1977 at the Pentacon production facility Objekt 8 in Dresden-Niedersedlitz in the DDR (East-Germany). Although the exact production quantity is unknown, it is estimated from the surviving serial numbers that around 2000 units were manufactured [1][3].

 History of the GSK (off-site)

  1. GSK = Geräuscharme Spiegelreflex Kamera (low-noise reflex camera).
  2. Based on the design of the Praktica EE2.
  3. In 1972 renamed Akademie der Wissenschaften der DDR.
  4. Pioneered by Canon in the Pellix camera in 1965.  Wikipedia

SR-899 (GSK) - top view
GSK with Pentacon 135 mm lens
GSK with f5/17 pinhole lens
GSK with pinhole lens and 2.5x and 2x range extenders
JO=3.5.1 pinhole lens (f5/17) fitted to the GSK
Pinhole lens with aperture cap
Pinhole lens with removed aperture cap
The Crypto Museum workshop seen through a 1 mm hole
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SR-899 (GSK) - top view
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GSK with Pentacon 135 mm lens
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GSK with f5/17 pinhole lens
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GSK with pinhole lens and 2.5x and 2x range extenders
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JO=3.5.1 pinhole lens (f5/17) fitted to the GSK
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Pinhole lens with aperture cap
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Pinhole lens with removed aperture cap
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The Crypto Museum workshop seen through a 1 mm hole

Features
Below is a quick overview of the features on the bare body of the GSK. The camera is based on a standard Pentacon 35 mm SLR camera with a motor winder bolted to the bottom. The camera has been modified to produce as little noise as possible. For this reason the mirror has been replaced by a semi-transparent foil that does not flip out of the light path when taking pictures. To avoid light leaking from the viewfinder to the lens, the viewfinder can be closed with a hinged flap.

Overview of the features of the bare GSK camera unit

The lens port has M42 thread, also known as Praktica-mount, for which a wide variety of lenses was available at the time. For observations, the Stasi had selected four lenses with a focal length of 35, 50, 75 and 135 mm respectively. For covert surveillance, such as in hotel and conference rooms, the Stasi used the SO-3.5.1 pinhole lens, that came with a set of three range extenders.

Side view of the GSK camera with SO-3.5.1 pinhole lens and three range extenders.

The image above shows the GSK camera with the SO-3.5.1 pinhole lens and all three range extenders (2.5x, 2x and 1.5x) bitted between the lens and the camera body. Note the presence of the M36/M42 adapter ring between the lens assembly and the input port of the camera.

 More about the SO-3.5.1 pinhole lens

GSK camera body
Viewfinder with hinged cap
GSK with open film compartment
Exposure counter
GSK camera and accessories
GSK with f5/17 pinhole lens - side view
Pentacon GSK camera with Zeiss pinhole lens, used by the Stasi
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GSK camera body
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Viewfinder with hinged cap
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GSK with open film compartment
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Exposure counter
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GSK camera and accessories
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GSK with f5/17 pinhole lens - side view
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Pentacon GSK camera with Zeiss pinhole lens, used by the Stasi




Click to see more

Parts
CGSK camera system
2.8/35 P-Flektogon lens (M36)
2.8/50 Biotar lens (M36)
3.5/75 Tessar lens (M36)
3.5/135 Sonnar lens (M36)
Adapter for fitting M36 lenses to a camera with M42 mount
High-end pinhole lens kit SO-3.5.1
Thermal radiation detector (200 m)
90° viewfinder (periscope)
Cartride for 17 m film, or 450 frames
Data unit with digital time display
Hand controller to trigger an exposure
Automatic controller
RCU
Power supply unit
PSU
External battery
Power cable (DIN 5)
Camera   SR-899 (GSK) 899 001
The basic SR-899 camera consists of two integrated units: a modified 35 mm camera – based on the design of the Praktica EE2 SLR – and a low-noise motor-drive.

The camera is unique in that the noisy mirror of the SLR is replaced by a static semi-transparent foil, that is not flipped out of the light path when shooting pictures.

 Read the manual
 Take a look at the foil-based mirror

  
SR-899 (GSK)

90° Viewfinder   periscope
This 90° periscope could be attached to the eyepiece of the viewfinder, to aid the adjustment of the camera under difficult conditions, for example when the camera was installed inside a piece of furniture or behind a wall or ceiling.   
90º periscope (viewfinder)

Film cartridge   899 002
To increase the maximum number of exposures — limited to 24 or 36 frames when using regular 35 mm film — this cartridge could hold up to 17 metres of 35 mm film, equivalent to 450 frames.

The unit replaces the camera's rear door and is attached to the camera body by means of two spring-loaded clips. The film transport is driven by a cogwheel at the edge of the motor winder.

Move over the image to see the combination.
  
Film cartridge 17 m (450 frames)

Data back unit   899 013
This unit replaced the rear door of the GSK and allows time and date to be superimposed on the picture. It is based on a converted digital watch and has its own internal miniature flashlight, driven by the flash socket on the camera.

Embedded in the film pressure plate is a miniature lens that projects the image from the watch directly onto the rear side of the film.

Move over the image to see the combination.

 Read the manual

  
Data back unit for SR-899 (GSK)

Note that the 17 m film cartridge and the Data back unit are mutually exclusive. They can not be mounted on the camera body simultaneously, as they each replace the rear door of the camera. There was also a combined cartridge/back unit.
Manual controller   899 006
When taking pictures manually, the exposure button shown in the image on the right must be connected directly to the camera. Each time the red button is pressed, an image will be taken.

The device can also be connected to the Remote Control Unit (RCU), in which case the grey knobs can be used to shoot a series of pictures. In that case, the remote control unit must be connected to the camera using an interconnection cable.

 Pinout of the plug

  
Manual shutter release button

Automatic controller   899 003
For stand-alone (unmanned) observations, this external control unit could be used to (remote) control the camera, for example for taking pictures at configured intervals. In the latter case, the camera would usually be fitted with the the 17 metre film cartridge, so that it would not run out after 24 or 36 exposures.

The unit can read the camera's light meter and can set the exposure time.

 More information

  
GSK control unit (Steuergerät)

Power supply unit   899 005
The GSK camera must be powered external by 9V DC source, such as the battery of a car or the mains power supply unit (PSU) shown in the image on the right. The PSU has a fixed cable at its secondary side, that ends in a 5-pin male DIN socket with the same pinout as the camera.

A simple 1:1 DIN 5-pin male/male cable can be used to connect the PSU to the camera. A suitable cable was supplied with the kit.

 Wiring details

  
Power supply unit

External battery   899 004
For situations where no mains power was available, the camera (or the remote control unit) could be powered by the external battery unit shown in the image on the right.

The case accepts 6 D-size battery cells of 1.5V each, that are series connected to provide the 9V for the camera.

Move over the image to look inside.
  
External battery for GSK

Power cable   899 010
Power is distributed between the various parts of the SR-899 (GSK) system by means of 5-pin DIN 180° male/male cables of various length. The image on the right shows the short version.

Note that only three pins of the 5-pin connector are used, and that a 2-wire cable is sufficient for power distribution.

 Wiring layout

  
Power cable

Thermal radiation detector   T-1.1
The GSK can be further expanded by adding thermal radiation detector T-1.1, shown in the image on the right. It can detect a person up to a distance of 200 m, and can be used to trigger the camera automatically.

Coming soon
  
T-1.1 thermal radiation detector in travel suitcase

SR-899 (GSK)
SR-899 (GSK) - top view
SR-899 (GSK) - bottom view
Adapter M36/M42
Film cartridge with packaging
Film cartridge 17 m (450 frames)
Film cartridge 17 m (450 frames)
Driving gear
Cogwheel for film transport
GSK with covert 50 mm lens and film cartridge for 450 exposures
Rear view of GSK with film cartridge
Frontal view of GSK with film cartridge
Data back unit for SR-899 (GSK)
Data back unit for SR-899 (GSK)
Battery compartment
Digital clock removed from back unit
Digital clock
Data back installed instead of the camera's rear door
GSK with data back
Rear view of GSK with data back
Miniature projection lens embedded in te film pressure plate
90º periscope with packaging
90º periscope (viewfinder)
Power supply unit
Electronic shutter release buttom
Manual shutter release button
GSK control unit (Steuergerät)
GSK control unit (Steuergerät)
Interconnection cable between camera and control unit
External battery for GSK
Interior
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SR-899 (GSK)
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SR-899 (GSK) - top view
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SR-899 (GSK) - bottom view
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Adapter M36/M42
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Film cartridge with packaging
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Film cartridge 17 m (450 frames)
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Film cartridge 17 m (450 frames)
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Driving gear
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Cogwheel for film transport
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GSK with covert 50 mm lens and film cartridge for 450 exposures
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Rear view of GSK with film cartridge
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Frontal view of GSK with film cartridge
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Data back unit for SR-899 (GSK)
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Data back unit for SR-899 (GSK)
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Battery compartment
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Digital clock removed from back unit
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Digital clock
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Data back installed instead of the camera's rear door
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GSK with data back
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Rear view of GSK with data back
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Miniature projection lens embedded in te film pressure plate
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90º periscope with packaging
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90º periscope (viewfinder)
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Power supply unit
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Electronic shutter release buttom
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Manual shutter release button
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GSK control unit (Steuergerät)
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GSK control unit (Steuergerät)
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Interconnection cable between camera and control unit
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External battery for GSK
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Interior

Special lenses   SO
For covert photography from within a (portable) concealment, the Stasi used special lenses: the so-called SO-series, or Sonderoptik (special optic). Each lens was identified by a name (e.g. SO-3.1) and/or a Stasi project number (e.g. 14601). The lenses were designed to be able to shoot pictures through a small hole in a concealment. Each lens came with an adjustable diaphragm that could be placed in front of the lens. In addition, each lens came with a selection of fixed-aperture discs that could be mounted in front of the lens instead of the adjustable diaphragm.

Note that the first three lenses (35, 50 and 75 mm) are so-called front stop lenses [4] of which the (virtual) entrance pupil (EP) [5] is located in front of the first lens element. The EP position coincides with the physical position of the externally fitted diaphragm, the fixed aperture and/or the hole in the concealment through which the subject under surveillance is photographed.

 Original product description


2.8/35 lens   SO-3.1
SO-3.1, or project 14601, was a f2.8/35 mm 60° P-Flektogon lens with M36 mount. It was fitted to the GSK by means of a M36/M42 adapter. Supplied with adjustable and fixed diaphragms.

This lens is a so-called front stop lens in which the (virtual) entrance pupil is located in front of the first lens element. This means that the image is not cropped by the externally fitted aperture.

 Lens specification
  
P-Flektogon lens 2.8/35 (SO-3.1)

This lens is made of Thorium glass and produces radioactive radiation with a strength of ~ 3µS/h.
This is ~ 30 times the background radiation.
2.8/50 lens   SO-3.2
SO-3.2, or project 14603, was a f2.8/50 mm 45° Biotar lens with M36 mount. It was fitted to the GSK by means of a M36/M42 adapter. Supplied with adjustable and fixed diaphragms.

This lens is a so-called front stop lens in which the (virtual) entrance pupil is located in front of the first lens element. This means that the image is not cropped by the externally fitted aperture.
  
Biotar lens 2.8/50 (SO-3.2)

This lens is made of Thorium glass and produces radioactive radiation with a strength of ~ 6µS/h.
This is ~ 60 times the background radiation.
3.5/75 lens   SO-3.3
SO-3.3, or project 14604, was a f3.5/75 mm 32° Tessar lens with M36 mount. It was fitted to the GSK by means of a M36/M42 adapter. Supplied with adjustable and fixed diaphragms.

This lens is a so-called front stop lens in which the (virtual) entrance pupil is located in front of the first lens element. This means that the image is not cropped by the externally fitted aperture.
  
Tessar lens 3.5/75 (SO-3.3)

3.5/135 lens   SO-3.4
SO-3.4, or project 14605, was a f3.5/135 mm 18.5° Sonnar lens with M36 mount. It was fitted to the GSK by means of a M36/M42 adapter. Supplied with adjustable and fixed diaphragms.   
Sonnar lens 3.5/135 (SO-3.4)

Lens adapter   M36/M42
This adapter allows lenses with M36 thread to be mounted to a camera with M42 thread, such as the GSK. M42 thread was widely used on Praktica and Pentacon cameras in the 1970s and 80s.

M42 thread (Praktica mount) is still widely used today, and M42-adapters are available for most modern cameras.
  
Adapter M36/M42

Pinhole lens   SO-3.5.1
For covert surveillance, such as in hotel and conference rooms, the Stasi used the SO-3.5.1 pinhole lens, developed by Carl Zeiss Jena.

This lens has M28 thread, for which an adapter to the camera's M42 thread was supplied. As the lens has a fixed focal length, a set of range extenders was supplied with the kit, providing enlargements of 1.5, 2 and 2.5 respectively.

 More information

  
f5/17 pinhole lens with adjustable diaphragm and pinhole lens cap

SO-3.1 and packaging
P-Flektogon lens 2.8/35 (SO-3.1)
SO-3.2 with packaging
Biotar lens 2.8/50 (SO-3.2)
SO-3.3 with packaging
Tessar lens 3.5/75 (SO-3.3)
SO-3.4 with packaging
Sonnar lens 3.5/135 (SO-3.4)
Fixed aperture (8) lens opening
Adjustable diaphragm
GSK with f5/17 pinhole lens
GSK with pinhole lens and 2.5x range extender
GSK with pinhole lens and 2.5x and 2x range extenders
JO=3.5.1 pinhole lens (f5/17) fitted to the GSK
Pinhole lens with aperture cap
Pinhole lens with removed aperture cap
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SO-3.1 and packaging
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P-Flektogon lens 2.8/35 (SO-3.1)
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SO-3.2 with packaging
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Biotar lens 2.8/50 (SO-3.2)
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SO-3.3 with packaging
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Tessar lens 3.5/75 (SO-3.3)
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SO-3.4 with packaging
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Sonnar lens 3.5/135 (SO-3.4)
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Fixed aperture (8) lens opening
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Adjustable diaphragm
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GSK with f5/17 pinhole lens
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GSK with pinhole lens and 2.5x range extender
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GSK with pinhole lens and 2.5x and 2x range extenders
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JO=3.5.1 pinhole lens (f5/17) fitted to the GSK
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Pinhole lens with aperture cap
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Pinhole lens with removed aperture cap

Click to see more

Interior
The GSK camera is based on the design of the Praktica EE2 35 mm SLR. The enclosure (body) is very similar and a manual (mechanical) shutter release knob is still available at the front. Bolted to the bottom of the body is a relatively large motor winder, which forms a major part of the design.

Apart from a motor-driven winding mechanism, it also contains an electronic shutter-release mechanism that is driven by an electromagnet (solenoid). It is powered by an external 9V DC source that is connected to the 5-pin DIN socket at the side. The shutter can be released externally by a hand unit or by an automatic control unit, connected to the grey 7-pin socket.

One of the most innovative features of the GSK is the replacement of the moving mirror by a semi-transparent foil. 1 Part of the light is deflected to the viewfinder, whilst the rest reaches the film.
  
Semi-transparent foil replacing the flippable mirror

For this reason, the mirror (i.e. the foil) no longer has to be flipped out of the light path when taking a picture, like with most other SLRs. As a result, the camera is much quieter than a regular SLR. The image above shows the foil – placed under a 45° angle – as seen through the lens port. Both the shutter (behind the foil) and the eyepiece (above the foil) are visible simultaneously.

Due to the complex, compact and fragile nature of the motor winder, we have decided not to take detailed pictures of the interior. Instead we are showing several images that were taken by Detlev Vreisleben when reparing a similar unit [1].

  1. Pentacon was not the first to use a static semi-transparent foil instead of the moving mirror. In 1965, Canon introduced the Canon Pellix which had the same feature.  Wikipedia

GSK with open film compartment
Rear door removed from the camera
Semi-transparent foil replacing the flippable mirror
Interior - motor drive unit - Copyright Detlev Vreisleben [1]
Interior - motor drive unit - Copyright Detlev Vreisleben [1]
Interior - motor drive unit - Copyright Detlev Vreisleben [1]
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GSK with open film compartment
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Rear door removed from the camera
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Semi-transparent foil replacing the flippable mirror
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Interior - motor drive unit - Copyright Detlev Vreisleben [1]
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Interior - motor drive unit - Copyright Detlev Vreisleben [1]
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Interior - motor drive unit - Copyright Detlev Vreisleben [1]

Connections
Power supply unit
The output of the power PSU is available on a 5-pin 180° female DIN connector at the end of a fixed cable. Below is the pinout when looking into the connector. This is the same as the pinout of the 5-pin 180° DIN socket on the side of the camera. A suitable 5-pin DIN male-male cable was supplied with the kit.

  1. 0V (connected to 2)
  2. 0V (connected to 1)
  3. not connected
  4. +9V DC
  5. 17m cartridge end 1
    Pinout of the 5-pin DIN power connector of the camera when looking into the socket
  1. Pin 5 is only used when the camera is powered via the (optional) remote control unit. The camera uses it to signal to the remote control unit that the film in the optional 17m cartridge has reached the end of the film.

Remote control
The GSK can be controlled remotely via the grey 7-pin female receptacle at the side of the camera (just below the 5-pin DIN socket). This connector – an RFT NS104 – accepts an NS105 male plug. The female socket has the following pinout when looking into the socket on the camera:

  1. Single exposure (connect to 7)
  2. Light meter output (1)
  3. Light meter ouput (2)
  4. +4.5V for light meter (input)
  5. Shutter open
  6. Shutter close
  7. Single exposure (connect to 1)
    Pinout of the 7-pin RFT NS104 remote connector when looking into the socket
Manual controller
In the simplest configuration, the handheld controller can be connected directly to the camera, in which case only the red button can be used to take a picture. Below is the pinout of the 7-pin male plug at the end of the controller's cable, when looking into the plug.

  1. Common
  2. n.c.
  3. n.c.
  4. Series exposure (connect to 1) 1
  5. n.c.
  6. n.c.
  7. Single exposure (connect to 1)
    Pinout of the 7-pin RFT NS104 remote connector when looking into the socket
  1. Only when connected via the automatic control unit.

Specifications
  • Type
    SLR
  • Purpose
    Surveillance, espionage, daylight, IR photography
  • Manufacturer
    Pentacon
  • Country
    DDR (East Germany)
  • User
    Stasi (MfS)
  • Format
    24 x 36 mm
  • Mirror
    Fixed, semi-transparent foil (low noise)
  • Advance
    Low-noise motor drive
  • Film
    Standard 35 mm
  • Mount
    M42 (Praktica)
  • Quantity
    2000 units (est.)
  • Power
    9V DC
Special Lenses
Model 1 Project ƒ Angle Format Dist. 2 Mount Remark  
SO-3.1 14601 2.8/35 60° 24 x 36 mm 48 mm M36 P-Flektogon  
SO-3.2 14603 2.8/50 45° 24 x 36 mm 48 mm M36 Biotar
SO-3.3 14604 3.5/75 32° 24 x 36 mm 48 mm M36 Tessar
SO-3.4 14605 3.5/135 18.5° 24 x 36 mm 48 mm M36 Sonnar
SO-3.5 14606 5/17 80° 24 mm Ø 45 mm M28 No diaphragm
SO-3.5.1 14606 5/17 80° 24 mm Ø 45 mm M28 With diaphragm
 SO-3.1 to SO-3.4 product description

  1. SO = Sonderoptik (special optics).
  2. Distance between the rear end of the lens and the film (or image sensor).

Part numbers
Concealments
  • Attaché briefcase
  • Travel suitcase
  • Shoulder bag
  • Handbag
  • Shopping bag
  • etc.
Known serial numbers
It is currently unknown how many GSK cameras were manufactured, but from the surviving serial numbers, it is estimated that around 2000 units were made [1]. The serial numbers of the early prototypes (approx. 50 unit) were of the format 50xx, whilst production cameras were given 6xxx and 7xxx serial numbers. Below is a non-exhaustive list of known serial numbers:

  • 5248·
    eBay (November 2021)
  • 5561
    eBay (November 2021)
  • 6537·
    eBay (November 2021)
  • 6877·
    Crypto Museum (Netherlands)
  • 7002
    Private collector (Germany)
  • 7039
    Highest serial number seen so far [1]
  1. A dot behind the serial number (e.g. 6877·) indicates that the device was originally supplied with a control unit (Steuergerät) with the same serial number.

Nomenclature
The GSK camera is known by the following names:

  • GSK
  • Geräuscharme Spiegelreflex Kamera (low-noise reflex camera)
  • SR 899
  • 14101
Glossary
GSK   Geräuscharme Spiegelreflex Kamera
{German) Low-noise reflex camera
JO   Justieroptik
German) Adjustment optics
SO   Sonderoptik
(German) Special optics
Datasheets
  1. Trigger IC MCC102 - notes 1
    Integrierter Schwellenspannungsschaltkreis (German).
    Personal notes of Dr. Zahnert (GSK developer), 1972.

  2. A902 datasheet 1
    Integrierte Schaltungen für Kameras (German).
    Radio Fernsehen Elektronik Nr. 25 1976. pp. 333-335.

  3. Integrated circuits - Application notes 1
    Mikroelektronik Information - Application,
    Heft 9: 30 October 1982.

  4. TCA345 Threshold Switch - datasheet
    TCA345 = MCC102 = A302D. Siemens, August 1990.
  1. Document kindly provided by Detlev Vreisleben [1].

Documentation
  1. Operating instructions for SR-899 (GSK) 1
    Bedienungsanleitung für das Kamerasystem SR 899 (German).

  2. GSK functional description 1
    Funktionsbeschreibung des GSK - Kamerasystem (German).
    Undated. 1 page.

  3. GSK adjustment procedure 1
    Entwurf eines Justierplanes für GSK (Kamerateil).
    14 August 1974. 6 pages.

  4. Semi-transparent foil for cameras - Final Report 1
    Dauerreflexsuchersysteem für photographische Kameras. Abschlußbericht.
    DAW, Berlin-Adlershof, 22 July 1966.

  5. Semi-transparent foil mirror - Final Report 1
    Ausfürhrlicher Abschlußbericht zur Forschungsarbeit
    Dauerreflexsucher (Folienspiegel) - in German language.
    VEB Pentacon, WTZ, 31 December 1966

  6. Manual for the data back and digital clock
    Bedienungsanleiting für die Datenrückwand 899 013
    und für die Datenkassette 899 014 (Digitaluhr).
    Undated. 7 pages.

  7. Overview of internal switch functions in the GSK
    GSK: Übersicht über Schalterfunktion im Kamerateil (German).
    Unknown author, 18 March 1974. 2 pages.

  8. Production quantity 1987 2
    Produktion Kostenträger 107 - Plan jahr: 1987 (German).

  9. Circuit diagram of the Praktica EE2 camera 1
    The GSK is based on this design (German). Undated.

  10. SO-3.x Sonderobjective - product description 1
    Undated.

  11. SO-3.1 (ƒ2.8/35) lens specification 1
    Carl Zeiss Jena, 17 December 1973.
  1. Document kindly provided by Detlev Vreisleben [1].
  2. Document obtained from BStU [2] and kindly supplied by Detlev Vreisleben [1].

References
  1. Detlev Vreisleben, Personal correspondence
    November 2021.

  2. Bundesbeauftragte für die Stasi-Unterlagen (BStU) 1
    Federal Commissioner for the Stasi-Records.

  3. Die geräuscharme Spiegelreflexkamera SR 899 (GSK)
    Dresdner Kameras. Retrieved November 2021.

  4. Eberhard Dietzsch, Front stop photo lenses
    SPIE Vol. 1780 Lens and Optical Systems Design, 1992. pp.721-726.

  5. Wikipedia, Entrance Pupil
    Retrieved November 2021.
  1. Full name: Bundesbeauftragte für die Unterlagen des Staatssicherheitsdienstes der ehemaligen Deutschen Demokratischen Republik (DDR) — Federal Commissioner for the Records of the State Security Service of the former German Democratic Republic (GDR) — officially abbreviated to BStU.
  2. Document obtained from BStU [2] and kindly supplied by Detlev Vreisleben [1].

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