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The following are definitions which are accessed throughout this web site.

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BBox Compatibles

The Software Bisque BBox is a battery-operated decoder designed to monitor the positions of two incremental encoders, and provide this information to a host computer via its RS-232C communications port.   Encoders of 2048- to 8192-tic resolution—post quadrature—are typically used.

Since we introduced the BBox in 1989, several other manufacturers have offered BBox-compatible CPS decoders.  While these decoders come in a variety of shapes and features, their means of interfacing with TheSky is similar.  To simplify, we sometimes refer to them all generically as "BBox."  So that you may determine if yours is one of these, here is a listing of BBox-compatible commercial CPS decoders:

NOTE: There may exist BBox-compatible devices not listed here. Please consult your CPS manufacturer and Software Bisque to determine compatibility.

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A decoder is the component of a CPS which monitors the encoders and makes positional information available to the user through either an on-board display, a host computer, or both.

The decoder usually takes one of four forms:

The autonomous (or stand-alone) unit will offer some type of human-readable, alphanumeric display—usually the LCD (liquid crystal display) or LED (light emitting diode) varieties.  This type of decoder cannot be interfaced with a host computer (see hybrid autonomous/serial unit below).  JMI's NGC-miniMAX is an example of this type of decoder.

The external serial unit has no human-readable display, but instead relies upon a host computer—usually running TheSky software—to present the user with an intelligible representation of the telescope's position.  Software Bisque's own BBox is an example of this type of decoder.

The hybrid autonomous/serial unit offers both a human-readable display and a serial connection to a host computer, thus allowing the choice of stand-alone or host computer operation, or even both simultaneously.  JMI's NGC-MAX is an example of this type of decoder.

The internal PC bus card, like the external serial unit, has no human-readable display and relies upon a host computer—usually running TheSky software—to present the user with an intelligible representation of the telescope's position.  Unlike the external serial unit, however, this type of decoder is inserted into a bus slot inside the host computer, and is therefore incompatible with most portable computers.  The C-Link Card (available from Software Bisque) is an example of this type of decoder.

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An encoder is the component of a CPS which measures the movement of a telescope mount's axis.  To be useful, an encoder must be monitored by a decoder.

An incremental shaft encoder is most commonly used. This type of encoder uses a transparent disc, around the perimeter of which is a pattern of radiating opaque lines with transparent gaps (of the same width) between them. Each of these opaque/transparent line pairs defines an encoder cycle, therefore the total number of line pairs is equal to the encoder's cycles-per-revolution rating.

By placing this disc between an optical emitter-detector pair, an electrical signal is generated whenever a transparent line is between them (assuming the encoder is powered). The signal is broken when an opaque line is between the emitter-detector pair, and it is the monitoring of this on-off pattern which allows the movement to be measured. And because each cycle of the encoder disc is divided into an on and an off signal, we can discern a movement as small as one-half-cycle.


The problem with a single emmitter-detector pair's electrical on-off pattern is that the direction of movement is not measureable—a clockwise movement appears no different from a counter-clockwise one. To solve this, a second emitter-detector pair is introduced—the first designated Channel A, the second Channel B. Channel B is positioned such that it is one-quarter of a cycle (or 90) out-of-phase with the Channel A. In other words, when Channel A is at the beginning of a cycle (the leading edge of an opaque line is directly in its beam), Channel B is three-quarters into a cycle (the middle of a transparent line is directly in its beam).

If you were to record the signal changes from both channels during one cycle, it would look like this:

cycle: 0/4 1/4 2/4 3/4
Channel A OFF1 OFF2 ON1 ON2
Channel B ON2 OFF1 OFF2 ON1

Notice that Channel A's reading at 1/4 cycle is the same as Channel B's at 2/4 cycle (we've added the '1' and '2' designations to make the pattern clearer). Thus, the two channels are one-quarter cycle (90) out-of-phase.

If you were to record the signal changes while rotating the encoder the opposite direction, it would look like this:

cycle: 0/4 1/4 2/4 3/4
Channel A OFF1 ON2 ON1 OFF2
Channel B ON2 ON1 OFF2 OFF1

Notice that the pattern of readings runs opposite (left-to-right) that of the first example. Because of this, from one reading to the next, we can determine which direction the encoder has moved!

There's another important benefit of this dual-channel operation. While each channel only resolves an encoder cycle into two parts (OFF and ON), by combining both channels, we can actually resolve a cycle into four parts (OFF/ON, OFF/OFF, ON/OFF and ON/ON)—doubling the resolution!


The process of resolving each encoder cycle into four parts (as described above) is called quadrature, and one quarter-cycle is called a tic. The number of tics-per-revolution is equal to four times the number of cycles-per-revolution. Encoder manufacturers commonly express their encoder resolutions in cycles-per-revolution, but the astronomical community most commonly uses tics-per-revolution.

Additionally, an encoder can be externally geared to increase (or decrease) a system's effective resolution. To find the effective resolution, multiply the encoder's resolution by the number of encoder revolutions per revolution of the mount's axis.

Example 1: A 2160 tic-per-revolution encoder has a 58-tooth gear on it; the mount's axis has a 116-tooth gear.
Solution: The encoder will turn twice per mount axis rotation (116 / 58 = 2), therefore the effective resolution will be 2 x 2160, or 4320 tics-per-revolution.

Example 2: A 4000 tic-per-revolution encoder has a 32-tooth timing pulley on it; the mount's axis has a 75-tooth timing pulley, with a 200-tooth timing belt between them.
Solution: The encoder will turn 2.34375 times per mount axis rotation (75 / 32 = 2.34375), therefore the effective resolution will be 2.34375 x 4000, or 9375 tics-per-revolution. Note that the belt length is not a factor in this calculation—it only determines the pulley-to-pulley distance.

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CPS (Computerized Pointing System)

A CPS, or Computerized Pointing System, can be categorized into two main types: active and passive. Both types provide the computer with information which allows TheSky to plot an indication of the telescope's current position (where it is aimed) against the artificial star field. The difference between the types is in how they move the telescope to a new position.

An Active CPS is one which can be commanded to automatically move the telescope to a desired position, without user assistance. This form of movement is referred to as slewing, and is usually accomplished with high-speed motors which move the telescope at one- to eight-degrees-per-second. Examples of the active CPS include Meade's LX200, Celestron's Ultima 2000, and Software Bisque's Paramount.

Some active CPS protocols currently supported by TheSky:

Coordinate 1
Ultima 2000

A Passive CPS is one which requires that the user move the telescope to the desired position. When used with a passive CPS, TheSky helps the user find the desired position by dynamically indicating how far, and in which direction, to move the telescope. Examples of the passive CPS include JMI's NGC-MAX, Celestron's Advanced AstroMaster, and Software Bisque's SGT system.

Some passive CPS protocols currently supported by TheSky:

C-Link Card
Sky Commander

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CPS Command Protocols

A protocol is the "language" used for communications between a personal computer and a peripheral device. The following is general information regarding some popular CPS command protocols. Software Bisque is not authorized to provide specific details about most of these, but wherever possible, contact information is provided.

ACL (Astronomy Command Language)

CPS Type: Active
Developed by: Merlin Controls
Comments: This protocol has been widely promoted as a standard for active CPSs. It is most commonly found on research-grade telescope mounts, but has also been popular with advanced do-it-yourself types.
ACL is a complex protocol which incorporates routable packetization, error-checking, command-extension and more.
Used on: Merlin ArchImage, DFM Manufacturing mounts, and several others. Also used for the Mt. Wilson TIE 61cm instrument.
For more info: Please ask Software Bisque for contact details.

LX200 Protocol

CPS Type: Active
Developed by: Meade Instruments Corp.
Comments: Used by Meade Instruments for their popular LX200-series of slewing telescopes, this protocol is probably the most widely imitated by do-it-yourself types. This protocol is relatively simple, and therefore most-easily implemented in a custom system.
Due to both its popularity and simplicity, several software titles on the market support only this protocol.
Used on: Meade LX200, LX600, LX650, LX700 and LX750 mounts. A subset of the protocol is used on the Magellan I and II.
For more info: Please contact Meade Instruments in Irvine, CA, USA.

BBox Protocol

CPS Type: Passive
Developed by: Software Bisque, Inc.
Comments: Introduced in 1989, this protocol has since been emulated by many manufacturers of passive CPSs. It has also been widely used by do-it-yourself types. This protocol is relatively simple, and therefore easily implemented in custom systems.
Due to both its popularity and simplicity, several software titles on the market support this protocol.
Used on: Software Bisque's BBox and compatibles.
For more info: Please contact Software Bisque.
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CPS Pointing Accuracy

Pointing accuracy is a measure of the difference between the intended and actual positions when pointing a telescope mount at a desired target. The degree of accuracy you require depends primarily upon the apparent angular size of your field of view, be it visual, photographic, or otherwise. A discussion of each of the following factors which affect pointing accuracy follows:

Positional Resolution

Positional resolution is the precision to which the telescope's position can be measured. In a CPS, this is normally determined by the resolution of encoders, or the step-size of stepper motors.

Alignment Calibration

Alignment calibration is the initial process of determining the telescope's orientation within the celestial sphere. This determination can be detrimentally affected by erroneous system data (including some, or all of: hardware setup, Earth-location, date/time, etc.), inaccurate fulfillment of pointing requirements, user inexperience, and more.

Atmospheric Refraction

Atmospheric refraction is the bending of light rays as they pass through the atmosphere at an oblique angle. The closer a celestial target appears to the local horizon, the more atmosphere its light passes through before reaching the observer (or telescope), and therefore the more its path is bent. This bending results in an apparent displacement of the target from its no-atmosphere position. This displacement is as great as one-half-degree at the horizon. Air temperature, humidity and pressure also affect the amount of refraction.

Mechanical Distortions

All mounts suffer from some degree of mechanical distortion, such as non-orthogonality (axes not perfectly perpendicular), flexure, slop, etc. This reduces the accuracy of the derived position, by resulting in non-linear movement of the telescope throughout its range of motion.

System Integrity

The system integrity can be compromised by many factors, including loose motor and/or encoder fittings, unmeasured motor rate-fluctuations, electrical signal failures, inaccurate time measurement, and more.

Target Data

While probably the least significant of all these factors, an error in the target's assumed celestial position can be affected by inaccurate catalog data, unaccounted precession, and—for Solar System objects—orbital elements, telescope's Earth-location, date/time, etc.



Due to all of the above factors, and more, a telescope rarely points exactly at the desired target. However, measures can be taken to reduce the magnitude of some of these factors, and most can be compensated for by sufficiently sophisticated software. The best example of such software is the world-renowned TPoint, which is used by nearly every major observatory in the world to achieve extremely accurate pointing.

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NGC-MAX Compatibles

The NGC-MAX (and its compatibles) is a stand-alone CPS decoder which has a built-in LED display providing telescope position, object information, and more.   Later models (ca. 1992 to present) also feature an RS-232C serial port which can be used to interface them with TheSky software.

Because a single manufacturer provides this decoder to several OEM companies, it is known by many names*.  To simplify, we sometimes refer to them all generically as "NGC-MAX."  So that you may determine if yours is one of these, here is a listing of NGC-MAX-compatible CPS decoders:

*For the record, these companies each create their own encoder installation kits, and specifiy features and databases used. While TheSky treats them similarly, they do, in fact, have minor (and in some cases major) differences.

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Last modified: Friday December 13, 2002.