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Astronomical Camera using the PiXi-200


This is the project concept that led the development of the PiXi-200 in the first place, a programmable slow-scan CCD camera readout controller & data acquisition system designed specifically for astro-photography.


The original project used a Hitachi 8-bit processor card running a Fourth interpreter. I think it ran at 4MHz and no, I’ve not missed any zeros off that figure. Ok it was slow but it did the job at the time. After running several discrete logic based digital sequencers to control the readout from the CCD the final scheme did in fact use an FPGA however this was a 2,000 gate one-time programmable Actel ACT1020 that did the job quite well. But the processor was still somewhat slow. So the plan was to upgrade the project using the processing power of the Raspberry Pi and the digital processing & I/O capabilities of a 200,000 gate Xilinx FPGA. The following project concept goes some way to describe how it would be possible to use the PiXi-200 to upgrade the project and bring a 19 year old camera back to life…


Summary of PiXi-200 functions used in this project:


GPIO1 (24 x 3.3v digital I/O):

11 x I/O used as ADC control & data interface;

13 x I/O spare;


GPIO2 (16 x open-collector / open-drain outputs):

8 spare high-current output;

8 spare low-current outputs;


GPIO3 (16 x 5v I/O):

6 x I/O used as the 4-phase vertical transfer clock controls & 2-phase readout controls;

2 x I/O used as the 2-phase horizontal transfer clock controls;

1 x I/O used as the output amplifier pre-charge clock control;

1 x I/O used as the substrate clock control;

6 x I/O spare;

1 x I2C port spare;


Analogue:

1 x ADC input used to sample the pixel data;

1 x ADC input used to sample the temperature of the CCD;

1 x ADC input used to sample the external temperature;

1 x ADC input used to sample the -9V power supply status;

1 x ADC input used to sample the +15V power supply status;

3 x ADC input spare;

1 x DAC output to trim the -9V power supply;

1 x DAC output to trim the 15V power supply;

1 x DAC output to control the peltier cooler drive current;

1 x DAC outputs spare.


RS232 Serial Interface:

Could be used as a camera control port, possibly linked to a telescope computer or observatory control system co control the camera, taking pictures, controlling the length of the exposure etc..


FPGA:

Implements a programmable sequence controller that controls the readout of the CCD. Since it’s programmable the readout sequence can be easily tweaked or even completely re-designed to optimise the readout sequence of the CCD or even allow it to be changed to use a completely different CCD.


External Hardware:

The original project comprised three single Euro-card sized boards mounted in a frame which also included two multi-output power supply modules and the Fourth interpreter based processor. There was also a separate power supply to drive the Peltier effect coolers used to lower the CCD operating temperature. All in all it was a little on the large side… The plan would be to shrink this down to one or two boards mounted within the camera head.


CCD power supply, biasing & gate drivers;

CCD output amplifier biasing, correlated double sampling


Please see the photos’s in the AstroCam gallery for some pictures of the original AstroCam project.


A draft schematic of a possible implementation of the PiXi as a CCD imager readout controller can be found here.


Got any thoughts about this project or want some advice? Feel free to email us at pixi@astro-designs.com.