Hardware Test Last revised: 02/17/03 Pietro.org Astronomy

 C5+ Drive Base & Computer Control Circuit Board 

 

Introduction

This hardware report takes us inside the drive base of a Celestron C5+ SCTs. This is the mechanical and electronic "heart" of the C5+ system: sequestered in this compartment is the clock drive, power connections, hand controller connection, and the computer circuit board that makes it all happen. Most of what is here will probably be relevant to the base C5 model (if anyone has images of the equivalent internals of the C5 -- please email them for inclusion in this report). 

 

The long-running Declination Drive thread on the message board is the impetus for this article. A simple question about a replacement Declination motor evolved into a discussion on the functioning of the hand controller, Pulse Width Modulation, and features of the controlling chip. The Celestron 5" Usage Notes section on the Hand Controller summarizes these functions and is a must read to understand all the tracking features (some not documented) available on your model C5+.  

 

The end of this report contains oscilloscope traces of the digital signals sent to the motor drives.

 

Disassembly Images

Clicking the black and white images below will open a pop-up with an enlarged color version of the image. Highlighting the black and white photos will display a negative image that may show more detail on your screen.  

 

C5+ Drive Base Cover Panel

Drive Base Cover Panel

 

Panel Items From Left to Right 

9-volt battery compartment (large rectangle)

Hand controller plug (telephone jack)

Declination drive power output (mini-jack)

12-volt alternate power input (mini-jack)

Master power switch

Power-on, red LED above power switch.

 

Note that you may want to explicitly label the 12-volt alternative power mini-jack on your cover plate as input. The existing "12VDC" label is insufficient, since it is not unheard of for a telescope base to have an accessory power output

 

C5+ Drive Base Panel Removed

Drive Base Panel Removed 

 

Note that the 4-screws at the extreme corners of the cover plate were removed to lift-off the cover plate. The 4 interior screws visible on the base's cover plate (previous image) hold the circuit board in place. 

 

This exposes the Right Ascension clock drive in the base compartment and computer circuit board on the back (right side) of the Base Panel. The exposed compartment is mostly empty. 

 

Items of Note 

The metallic cylindrical item on the left side of the base compartment is the Right Ascension clock drive. 
The black telephone cord from this drive leads back to the base panel and terminates at a plug on the computer circuit board. 
Power wires from a circular hole in the 9-volt battery compartment are visible on the left side of the base panel. They connect to and power the computer circuit board and clock drive.

 

C5+ "Front" of Computer Circuit Card

"Front" of Computer Circuit Card

 

Note that what I call the "front" of the circuit card is aimed toward the ground when installed on the telescope base. I state this because it is important not to mix-up the Declination drive power output and 12-volt alternative power input mini-jacks -- they look the same and are unlabeled on the circuit board.

 

Items of Interest

The master power switch and red LED are visible on one side. 

Along one edge of the card (opposite the big Tangent chip) are the power mini-jacks. 

The face-on telephone jack is for the hand controller. 

The telephone jack with the wire prongs showing is the connection to the Right Ascension motor drive (see prior image). 

The cards is dominated by the large Tangent chip. Based on the state of the buttons on the hand controller, this determines the power pulses going to the Right Ascension and optional Declination drives. Different versions of this chip give the C5-series telescope different abilities -- not always stated in the telescope instruction booklet.

 

C5+ "Back" of Computer Circuit Card

"Back" of Computer Circuit Card

 

Note that what I call the "back" of the computer circuit card is the side first visible when the Drive Base Cover Plate is removed and flipped over -- see "Drive Base Panel Removed" image.

 

Items of Interest

Black circular pairs of split-prongs anchor the two telephone jack plugs -- for the hand controller and Right Ascension drive.

The parallel wire loops on one side of the card are from the 9-volt battery power supply.

Various manufacturing serial numbers are visible on the card.

 

C5+ Computer Circuit Card "Back Flip" View

Computer Circuit Card "Back Flip" View

 

This image of the "back" side of the computer circuit card is flipped to show the circuit patterns as they would appear looking through the front of the card -- as if you had Superman's X-Ray vision!

 

This allows easy comparison of the components on the "front" view with the circuit pattern on the back.

 

Click the image on the left to bring up the enlarged color version of the image. Moving the mouse over and away from the image will "blink" the opposite sides of the circuit card. 

 

Pulsed Power Traces

Below are some graphs of the pulsed signal sent to the Right Ascension and Declination motors. Voltage is on the vertical axis and time on the horizontal. These oscilloscope generated drive wave-forms were kindly sent in by Kim Thomas -- the originator of the Declination Drive thread. They were made with an older model C5+. The hand controller only allowed for the most basic motor controls.

 

Pulses to Declination Drive 

 

The optional Declination drive is normally called on to correct small drifts in Declination when image guiding. It is either positively or negatively tracking in Declination -- as selected by the North/South Hand Controller buttons. 

 

Declination Pulses for Tracking South (-) Declination Pulses for Tracking North (+)

 

Oscilloscope Notes

Signals pulse from 0 to 5-volts. The sign of the voltage gain depends on the tracking direction.

According to the oscilloscope readout, a pulse cycle takes about 1.5-milliseconds -- so there are 666 pulse cycles in one second of time -- a cyclic rate of 666Hz. Kim notes that this is "way too fast for any conventional motor" (this writer doesn't like it because it's the "number of the beast"). It may indicate a problem with the controller chip, since Kim was having difficulty getting a Declination motor to work with the setup. (I'd be interested in seeing the trace-out from a working Declination drive setup.)

The voltage increase lasts 1/3 of a cycle (0.5-milliseconds) and the remaining 2/3 of the cycle (1-millisecond) shows the voltage as off. That's why the readout show skinny rectangles. 

 

Pulses to Right Ascension Drive

 

This Right Ascension drive turns on with the Master Power Switch. It is fed pulsed power to allow it to track the stars at the sidereal rate. This rate can be sped-up (positive, west) or slowed down (negative, east) for fine image tracking. 

 

Oscilloscope Notes

In normal tracking the signals pulse from 0 to 1.3-volts. The cycles last about 14-milliseconds -- about 70Hz. This allows the drive to track at the sidereal rate to follow the stars.

Pressing the east button on the hand controller retards the drive by generating a 0.9-volt signal with a 21-millesecond cycle -- about 14Hz.

To increase tracking speed in the direction of sidereal motion, the west button is pressed. This generates a 1.9-volt signal with a cycle of 10-milliseconds -- about 100Hz.

Right Ascension Drive Normal Tracking
  
R.A.. Drive Eastward Drift Tracking (-) R.A.. Drive Westward Drift Tracking (+)
 

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