Digital Setting Circles for Amateur Telescopes

The why, how, and what of computer aided observing.
by Larry Bogan

Finding An Object in the Sky with No Setting Circles.
The 'traditional' and most enjoyable method of finding an object in the telescope requires learning the sky. Example: The Saturn Nebula in Aquila, NGC7009
Hubble Image

  1. Locate the constellation and the general position of the object in the current night sky.
    Use a Large Scale Finder Chart showing constellations and object location relative to the horizon.
  2. Learn the star pattern around the object required to find it in the finder or binoculars
    Use a Smaller Scale Finder Chart showing the star patterns in a region about 5 degrees in diameter.
  3. Finally learn the location of the object within the small field of view of the telescope.
    This requires a Fine Scale Finder Chart if the object is dim and hidden among stars of similar brightness. That would not be necessary for the Saturn Nebula which is brighter than all the stars in the field of view of the telescope.
  4. If the object is to be imaged using a CCD camera an even a More Detailed Finder Chart is required.

Coordinate Systems and Setting Circles

6 pm 5 November

The positions of celestial objects are specified with celestial coordinates.

REVIEW of Celestial Coordinate System: To Illustrate the difficulties in using the Altitude-Azimuth system the following diagrams show the Horizon System position versus time and date. Position of the Saturn Nebula on:

To illustrate the difficulty of calculating the horizon position,
a page shows the details of the conversion between celestial and horizon coordinate systems

Advantages of Computer Aided Location of Celestial Objects.

A transducer must be used to send the angular position of the axes of the telescope to a computer. The computer then calculates the telescopes position relative to the sky (with input of latitude, longitude, and time). The most accurage transducer is an optical angular position encoder.

Encoders as angular position indicators

How Does It Work:?

Shown in the diagrams are the important parts of a typical optical encoder. A transparent disk with equally spaced dark radial marks is attached to the rotation axis. A fixed light (LED) sent through the series of dark stripes is sensed with a photodetector. The electrical output is high with light and low with no light. As the axis rotates the electrical signal will change in step movement of the movement of the stripes beneath the light sensor. Two light sensor set near each other provide information to determine the difference in rotation direction. These also create a method of quadrupling the resolution of the encoder.
Incremental Encoder Electrical Output: - two channels A and B

360 deg/revolution

Resolutions: Incremental Encoders
Cycles/Revolution Resolution Max. Rotation
8000 Hz read
Max. Rotation
50,000 Hz read
1000 4000 5.4' arc 2 rev/s 12 rev/s
2000 8000 2.7' arc 1 rev/s 6 rev/s

Encoder Interface

A special interface is needed between the encoder and the computer. This has a microprocessor (computer) that

Operations of the Encoder Interface and Computer: (Interface has clock and microprocessor)

Commercial Computer Interfaces and/or Encoder Controllers

(only representative and not a complete list)

The Brooklyn Street Observatory Installed Ditial Setting Circles

An working system showing attachment of the encoders and wiring to the computer-encoder interface. A 33 cm Newtonian on Dobsonian mount using the Outranous interface (no longer available). Page of Details

Acadia University's Celestron 8 with Encoders and Computer Interface

An example of a telescope retrofit with encoders is the one shown below. It allows students not completely familiar with the sky to find celestial object to image with the CCD camera. The environment in which the telescope is used is light polluted which makes it all the more difficult to find celestial objects using the traditional star hopping using star patterns. The computer aided telescope is a big help in this environment.

A couple of Sky Programs that can talk to an encoder interface
L.Bogan Nov 2000