Author: T. F. Droege Date: 970513 Revision: #0 970513 Revision: #1 970815 Key Words: documentation, instrumentation, ADC
This note will discuss the over all system design of the Mark IVA
camera system. I hope it will be a useful note for those that plan to
use it. The goal is to build a complete package. Just like buying a
LX-200 from the store. I expect you to just be able to open the box,
plug it together, and have everything you need to make it work. As
always, some of you will be planning on different uses than the
standard package. This should help you plan how to make use of all
the stuff designed into the Mark IVA.
Figure 1
is the block diagram
of the system. It shows the major parts and how they are cabled. The
Mark IVA is designed to answer the objections to the Mark IV system
which would have required a PC for each camera head.
2.0 Telescope System
As designed for my use, the Mark IV will again be a triplet. I plan to run with V, R, and I filters. There will be a barn door mount which has a resolution of about 5" of arc per step in RA. There will be declination motion of a few arc minutes per step. The RA drive is run by a precision, temperature compensated (by software), voltage controlled oscillator. Its center frequency can be changed about +/- 1% in 0.01% steps. The declination drive is intended for positioning only, there is no provision to move it during an exposure. All three cameras will rest on the same "barn door" frame.
The first camera head design contains the CCD, switches to drive the clock lines, and a two channel pre-amplifier. Only one channel can be used at a time and is selected by a hardware jumper. The package is 4 1/2" in diameter, and about 2" thick. It has a "servo" mount groove and several available screw mount positions. All connections are made to the back of the camera head. There are three connectors, two water tubes, and a desiccant connection. The connectors extend about 3" before the cables can be bent, less if the cable shield enclosure is removed. This is a lot of stuff coming off the back of the camera head, but little sticks out beyond the 4 1/2" diameter. This should allow it's use in moderate sized Schmidt cameras.
Some care has been taken to protect the camera from electrical transients. All leads to the camera contain some type of protection, either diode clamps for fast signals or RC filters for slow ones. One layer of protection is provided by the metal case of the telescope which contains the camera heads. The second layer is the completely shield enclosed camera head. The last line of defense is the protection on the individual lines. At least for my installation, I plan to locate lightning rods around the camera which connect to the 4" diameter, 24' tall mounting post. I will ground the post with a ground rod. I will also try to put a second layer of electrostatic shield around the control cables. Still, a direct hit will probably kill everything.
One of the cables connected to the camera head contains the clock levels and clock pulses. The scheme is to deliver DAC generated DC clock levels and pulses to the camera head where fast switches controlled by the TTL clock pulses generate the actual clocks.
A second cable to the camera head brings power to the thermo-electric cooler. The pre-amplified camera analog signal is now brought out on a BNC connector to reduce possible pick up from the clock lines.
A camera control cable brings signals to the telescope enclosure for performing the operating functions. It contains drivers for two stepping motors for RA and declination. There are also drivers for two model aircraft servo motors. There are several limit sensors, and several extra signals which might be used to open covers, etc..
One of the stepping motor drives is intended for the RA drive and will produce one step for each 5" of arc. The design has been changed so that there is now independent control of high/low speed and direction. The usual mode will be to step forward in the RA direction at the sideral rate, and step back at 64 or 128 times sideral rate. Limit switches can be used to sense one absolute position at the limit to about 10" of arc.
A second stepper motor drive is under program control. It is intended to move the camera in declination. It can move a specified number of steps in either direction. Again, a limit switch is used to sense the absolute position at the limit switch.
Two pulse width controlled TTL signals are provided to drive model airplane servo motors. These will generate torques of order 20 in oz and can be positioned over 90 degrees to a precision of about 1/2 degree. One is planned as a focus control, the other will drive the shutter. Time for a 90 degree move is of order 0.15 second.
Several extra signals from the Stamp computer will allow a variety of
controll functions, like LED flashing, to be accomplished.
3.0 Electronics
The control electronics will be housed in a 12" x 12" x 8" metal box. There are four electronics cards in the box. Power supplies in the box will be powered from the mains. Power requirements are modest and were provided from the PC in the prototype. The switching power supplies used were quite noisy, and we have switched to linear supplies. A variety of DB series connectors, all different, are used to prevent mis-mating.
The Control Board contains the miscellaneous electronics needed to control a camera set.
3.1.1 Stamp Computer
The control electronics contains a BASIC Stamp computer which communicates with the controlling PC over an RS-232 interface. Eight character strings are sent to the Stamp which interprets them into various control functions. Only the 4 low order bits in each ASCII character are interpreted to try to eliminate problems with operating systems that do not like to see full random bytes.
3.1.2 ADC Section
The control electronics contains a 16 bit (slow, 10 us) ADC that can measure 32 different signals. Some of these are listed below:
Vertical Clock High Level Vertical Clock Low Level Horizontal Clock High Level Horizontal Clock Low Lwvel Reset Clock High Level Reset Clock Low Level Spare Clock Level 0 Spare Clock Level 1 Channel 0 CCD Temperature Channel 1 CCD Temperature Channel 2 CCD Temperature Channel 3 CCD Temperature Electronics Temperature Air Temperature Water Temperature Channel 0 TEC Current Channel 1 TEC Current Channel 2 TEC Current Channel 3 TEC Current Channel 0 Temperature Command Channel 1 Temperature Command Channel 2 Temperature Command Channel 3 Temperature Command + 15 Volts - 15 Volts + 5 Volts - 5 Volts + 12 Volt TEC power
A measurement is made by sending an 8 character string to the stamp containing a request to read an ADC channel and the channel number. Returned is a 8 byte byte string with 4 bits of the measurement in each of four bytes. Each operation takes about 0.3 seconds.
3.1.3 Pulse Generation Section
By sending a command string, 16 different pulses can be generated. These are used for purposes like starting the read out, triggering the ADC, generating a variable pulse width to drive a model airplane servo, etc..
3.1.4 Level Sense Section
A command string can interrogate 8 different logic level generating devices. These are typically used to sense the condition of limit switches.
3.1.6 DAC Section
The control card contains 16 ea. 8 bit DACs. These are loaded by a command string that specifies the channel and the value. They are scaled approximately +/-12 volts, or about 100 mv per bit. Their primary purpose is to provide the eight clock levels for the CCD. One channel controls a trim on the RA drive, four set the TEC temperature.
3.1.7 RA Stepping Motor
The control card contains a high precision VCO. There is a thermometer located next to the VCO that will allow an additional level of compensation under computer control. The VCO is counted down to drive the 4 wire stepping motor drive electronics. There are two control signals. On/Off determines if the motor is moving. When On it moves. When Off, the motor current is reduced to zero to conserve power and to remove a noise source. Forward/Reverse determines the direction. A high/low speed bit determines if it steps at sideral rate or 128 times it.
The drive electronics is 4 wire, current controlled. With the standard current sensing resistors it drives about 300 ma per winding. The current controlled means that there are no conventional stepping motor current limiting resistors. The electronics puts full (12 volts) across the motor winding until the current gets up to the preset value. It then goes into a switching mode that greatly reduces the power input by using the L of the winding to hold up the current. This type of drive is much faster and more efficient than the common 6 wire drives. It is possible to drive a six wire motor with this drive.
For the Mark IVA, each step moves a lead screw 0.0005". The "barn door" for the first design is 21" long. This gives a 4.3" angular step for each pulse.
3.1.8 Declination Stepping Motor
A second stepping motor is intended to drive the declination mount. This can be commanded a direction and a number of steps. It will also be able to accept a command to go to a limit and stop. This motor full steps and moves 0.001" per step.
3.1.9 Control Connectors
Two DB37 connectors are provides to control external devices. These control connectors include the 4 bit stamp bus, several spare stamp control lines, spare ADC inputs and spare DAC outputs. They also contain the stepping motor and servo motor drive signals, and the power supply lines. There is enough stuff (we hope) that the cable can be used for most dome control functions.
The scan engine card contains the logic that controls the horizontal and vertical CCD scan. There are two scan engines.
3.2.1 Vertiacl Scan Engine
The vertical scan engine is controlled by a 128k byte EPROM and a clock. A pulse from the control card can start and stop the vertical scan engine. Until stopped, it clocks out the EPROM contents on 8 control wires. Three of these are used to generate the vertical clock phases. Another can cause the scan to pause. Still another can stop the process. Another can start the horizontal scan engine. The 128k EPROM will allow a wide variety of clocking schemes. The present program has 14 micro steps for each vertical shift. This uses about 22% of the program space. Only about 10 micro steps are necessary allowing control of much larger CCDs when they become available. The normal procedure is for the vertical scan engine to generate the vertical sequence, then put itself in pause while starting the horizontal scan engine. When the horizontal scan is complete, the horizontal scan is stopped, and a continue is sent to the vertical scan engine.
3.2.2 Horizontal Scan Engine
The horizontal scan engine is similar to the vertical scan engine except that it contains two 128k by 8 bit EPROMs which provide 16 control lines. The extra lines are used for the CCD clocks as well as control of ADC start, byte selection, dual slope integration etc..
A third electronics card contains four linear power amplifiers. These compare a thermister at the back of the CCD with a command voltage and drives the TEC with the resulting error signal. This should be a better thermal design than the Mark III, but the DAC is not as stable so it will probably have similar characteristics, about 0.01 C stability.
A fourth electronics card contains four fast ADC channels. The camera signals arrive on isolated ground BNC connectors and are received differentially. The signals are integrated at one polarity during the reset time and at the opposite polarity during signal time. The accumulated signal is sent to the ADC. All the ADC channels send their data a byte at a time to the same memory card. The scanner can have two programs. The camera heads can be individually selected to receive clocks. This allows alternating acquisition with read out for many camera combinations.
The Mark IV presently uses the ADS7805 which results in a 40 second
read out time. We are considering the ADS7815 which would give a 10
second read out time. New devices are becoming available. We have
designed this board so that it can be replaced to upgrade the
conversion speed when suitable devices become available.
4.0 Power Control
The power control box is yet to be designed. The intent is to provide
a single spot where all power cords plug into the mains. This control
box will contain the master power on/off switch. It will also be
controlled by the Stamp computer which will perform a "dead man"
function. The stamp must receive periodic messages from the main
computer over an RS-232 connection. If it does not receive messages
at appropriate intervals, it will take action. Such action might
consist of shutting down everything, or of rebooting the computer.
5.0 Computer
We envision using a standard PC. The computer will have bi- directional communication with the Stamp computer on the electronics care over an RS-232 connection. The computer receives information from the cameras over a fast byte parallel cable to the resident memory card. Only slow functions are processed over the RS-232 connection. The fast data path is only from the cameras to the memory card.
The Memory Card is now 16/32/64 Mbytes. It receives bytes in parallel with a clock from the ADC on the Scan Engine card. Depending on the cable length, we should be able to accept a 20 Mbyte or faster data rate. The card is simple in concept. For each clock it receives, it writes a byte in the next available memory location. The sending end can test to see if the memory is available (memory address register set to zero) and can send a "done" signal to the memory when the CCD is unloaded. The memory can interrupt when it receives a "done".
A single serial port is required to control a set of four cameras.
6.0 TEC Power Supply
The camera thermoelectric coolers require about 12 volts at 3 amps for
each camera head.
7.0 Pump and Cooling Water System
A circulating water pump and cooling water capacity sufficient to absorb about 50 watts for each camera is needed.
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