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
- 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.
- 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.
- 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.
- 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.
- If an Equatorial mounted telescope is used and properly aligned with the sky, the object can be found using its coordinates and the telescope setting circles, [one standard bright star and its celestial coordinates are needed]
- If an Atitude-azimuth mounted telescope is used then a
complicated calculation is needed before setting circles can be used to find an object in the sky. This is not a very practical system without a computer controlled telescope.
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.
- Location of a very dim object among dim stars.
- Location of many objects quickly in a routine survey or research
- Substitution of digital setting circles for mechanical ones.
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
- Count at 1 per step (one cycle per optical mark)
- Count at 4 per step (four cycles per optical mark)
- Determine direction of moment
Resolutions: Incremental Encoders
8000 Hz read
50,000 Hz read
A special interface is needed between the encoder and the computer. This has a microprocessor (computer) that
- (1) keeps track of the encoder position and direction of rotation by polling the optical sensors at a high rate,
- (2) converting the information to numbers and
- (3) communication with the outside world (or other computer).
Operations of the Encoder Interface and Computer: (Interface has clock and microprocessor)
- Clock and Microprocessor Counts and Interprets movement of the Encoder.
- Microprocessor receives commands from RS232 port and sends back status and/or positions of encoder.
- Computer with database of astronomical objects and a control program calculates geometry of the telescope and converts encoder positions to telescope direction angles.
- Sky Programs show where the telescope in pointed in the Sky (When properly aligned!!!).
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.
- Earth Centred Universe 3.0A - Sky Program
- Orion Sky Wizard CTI (9 V battery required)
- 4000 cycles/rev encoders
- Santa Barbara Group ST-7 CCD Camera
- CCDOps DOS software
- Laptop with Windows 98
A couple of Sky Programs that can talk to an encoder interface
L.Bogan Nov 2000