• ACS
The Advanced Camera for Surveys (ACS) was installed in Hubble by the crew of the Space Shuttle Columbia (STS-109) during Servicing Mission 3B, which took place March 1 to March 12, 2002. With its wide field of view, superb image quality, and exquisite sensitivity, the Advanced Camera for Surveys (ACS) has 10 times more discovery power than the camera it replaced. In other words, ACS can produce 10 times as many science results in the same amount of time.

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• FGS
The Fine Guidance Sensor (FGS) is an optical sensor used on the Hubble Space Telescope to provide pointing information for the spacecraft and also as a scientific instrument for astrometric science.

A FGS consists of a large structure housing a collection of mirrors, lenses, servos, prisms, beam-splitters and photomultiplier tubes.

There are three fine guidance sensors on Hubble located at 90-degree intervals around the circumference of the telescope. Two FGSs are used to point the telescope at an astronomical target and hold that target in the scientific instrument's field of view. The third FGS can then be used as a scientific instrument for astrometry.

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• NICMOS
The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) provides imaging capabilities in broad, medium, and narrow band filters, broad-band imaging polarimetry, coronographic imaging, and slitless grism spectroscopy, in the wavelength range 0.8-2.5 microns. NICMOS has three adjacent but not contiguous cameras, designed to operate independently, each with a dedicated array at a different magnification scale.

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• STIS
The Space Telescope Imaging Spectrograph is a powerful general-purpose spectrograph that is complementary to - not superseded by - the Cosmic Origins Spectrograph (COS). STIS was installed during Servicing Mission 2 on February 12, 1997. STIS performed on-orbit operations for approximately 65,000 hours (7 years, 171 days) before failing on August 3, 2004. The instrument remains in safe mode.

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• WFPC2
The Wide Fieldand Planetary Camera 2 (WFPC2), a second-generation imaging camera, was installed on Hubbleduring Servicing Mission 1 (SM1) in 1993. WFPC2 is an upgraded version of the telescope's original Wide Fieldand Planetary Camera 1 (WFPC1). WFPC2 significantly improved ultraviolet performance over WFPC1. In addition to having more advanced detectors and more stringent contamination control, it also incorporated built-in corrective optics.

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• Batteries
Each battery consists of 22 cells in series along with heaters, heater controllers, pressure measurement transducers and electronics, and temperature-measuring devices and their associated electronics. Three batteries are packaged into a module measuring roughly 36 by 36 by 10 in. (90 x 90 x 25 cm) and weighing about 475 lb (214 kg). Each module is equipped with two large yellow handles that astronauts use to maneuver the module in and out of the Telescope in space.

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• COSTAR
The Corrective Optical Space Telescope Axial Replacement (COSTAR) was developed as an effective means of countering the effects of the flawed shape of Hubble's mirror. COSTAR is a telephone booth-sized instrument which placed five pairs of corrective mirrors, some as small as a nickel coin, in front of the Faint Object Camera (FOC), the Faint Object Spectrograph (FOS) and the Goddard High Resolution Spectrograph (GHRS).

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• Gyroscopes
The Gyroscopes are part of Hubble's pointing system. They provide a frame of reference for Hubble to determine where it is pointing and how that pointing changes as the telescope moves across the sky. They report any small movements of the spacecraft to Hubble's Pointing and Control System computers. The computers then command the spinning reaction wheels to keep the spacecraft stable or moving at the desired rate. The gyroscopes work by comparing Hubble's motion relative to the axes of the spinning masses inside the gyroscopes. In the absence of external torques, these axes remain stable relative to the fixed stars in the sky. By keeping Hubble fixed relative to these axes, Hubble stays stable relative to the stars.

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• Outer Blanket Layers
Hubble is protected by multi-layer external blankets. They maintain normal operating temperatures of Hubble's electronic equipment. The second servicing mission in 1997 discovered some damage to these external blankets, though limited to the outermost layer and not affecting the insulation's protective function or Hubble's operation. Starting with servicing mission 3A in 1999 astronauts began covering Hubble with permanent sheets called the New Outer Blanket Layer (NOBL). The NOBLs are designed to protect Hubble's external blankets and are specially coated stainless steel foil trimmed to fit each particular door. Each cover is supported by a steel picture-frame structure. Expanding plugs, like common kitchen bottle stoppers, fit into door vent holes to allow quick installation.

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• Solar Arrays
The solar arrays (SA3) are rigid arrays, which do not roll up and therefore are more robust. Although one-third smaller than the first two pair (the original pair and the ones installed during SM1), they produce 30 percent more power. They are less susceptible to extreme temperatures and their smaller size reduces the effects of atmospheric drag on the spacecraft.

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