Gina

I think that completes the hardware description.

Next to the camera cooling and dew heater control circuits. In both, power Gnd is green and logic/data Gnd is black. The only route from the data Gnd back to the incoming power Gnd is back through the RPi. Isolated grounds avoid a ground loop and subsequent problems. In the camera control, the +5v (actually 5.1v) is connected to the Peltier TEC through a diode to provide the LOW level of cooling when the J438 P channel power MOSFET is off. This occurs when GPIO13 is at logic 0 and the opto-coupler is off, leaving the photo-transistor not conducting. In this state the J438 gate is held at source potential and the MOSFET is OFF. When the GPIO13 line goes high the EL817 turns on, it's photo-transistor turns on and pulls the gate potential to Gnd ie. -13.8v with respect to the source and the J438 turns on providing the full 13.8v to the Peltier TEC. The diode form the +5v rail is reverse biased. The dew heater control starts off similarly but with the EL817 opto-coupler controlled by a different GPIO line. In this case the GPIO line is normally high - logic 1 - +3.3v and the current flowing into the LED switches the EL817 on, the collector connection is pulled down to 0v and the IRLZ44N channel power MOSFET is OFF. This provides the OFF state of the dew heater. When the GPIO line goes low - logic 0 - 0v, the opto-coupler turns off and the collector is no longer pulled to Gnd and +5v is applied to the MOSFET gate, turning it ON. This provides the ON state of the dew heater.

The motor driver module is a standard A4988 type and the connections are pretty much standard. Motor power is provided from the 13.8v main power and logic power from the RPi +3.3v line. The motor side is isolated from the logic/data side and they have separate ground connections. Very basic use of the A4988 module is shown below but in this application the speed control lines (MS1,2,3) are pulled to logic 1 to give 16x microstepping and the ENABLE and RESET lines are also held at logic 1. The SLEEP input is used to control motor power.

Some more details of the control circuitry. As mentioned previously, power from the main observatory supply comes in via the twin core cable. The full voltage is used for the dew heater, for the HIGH level of camera cooling and for the stepper motor, which is controlled by the daughter board where basic controls are converted into drive currents for the two coils of the stepper motor. From the main power supply 5.1v is derived with a buck converter (seen in the left hand of the box in this photo) to supply the Raspberry Pi board and for the LOW level camera cooling. On the power side, the 13.8v has yellow wires and the 5.1v wires are red. Power Gnd is wired in green. The orange wire connects to the dew heater and the blue wire to the Peltier TEC for camera cooling. On the data side, the control form the RPi (green PCB behind the HAT) is connected to the HAT (big red PCB) via a double row connector at the top. On the HAT, the stepper motor is controlled using DIR (direction), STEP turns the motor shaft by one step, and SLEEP (which powers down the motor when not needed). These connections are the white, yellow and blue covered wires to the stepper motor driver module (small daughter board at left of HAT). Power for the stepper driver logic of +3.3v is derived from the RPi and coloured red. The rest of the circuitry on the HAT controls the camera cooler (Peltier TEC) and dew heater. Because the RPi is easily destroyed by input currents into it's data lines (GPIO), opto-couplers are used to separate the data lines of the RPi from the power circuitry that drives the dew hearer and TEC. These opto-couplers can be seen as the small black rectangular components under the orange wire. From these, resistor networks and power MOSFETs control the relatively high currents for the camera cooler TEC and dew heater.

Before continuing with the construction details I'll take a pause to give some results. So far I have found the camera cooling to be quite adequate and I doubt the ambient conditions would get much worse. Last night the breeze had died and there was little if any air movement across the cooler meaning that all the cooling was by convection. Cooling was significantly less than when there was an easterly breeze but still adequate to kill noise and hot pixels. I would imagine the visual results will be a lot better when the weather is cooler and we get real astronomical darkness on a clear night. As for daytime use - no problem even in full sunlight, though the sun does spread somewhat. A couple of images. Firstly taken last night just before midnight UTC (1am BST) - 60s exposure and camera gain of 150. With 4m delay between captures the camera temperature dropped to -9°C and rose to -5.5°C by the end of the exposure. HIGH level cooling with 13.8v on the Peltier TEC. This is a screenshot of the KStars FITS Viewer with cropping and contrast enhancement in GIMP. µ The second image is a daytime shot taken in the morning a few days ago with virtually no cloud - contrails instead. 32µs exposure and gain of 0. LOW level cooling with 5v on the Peltier TEC. This is a full scale image as written to disk, converted from FITS format to PNG in PixInsight with no other processing. This also shows the coverage of the fish-eye lens image on the camera sensor.

I'll have to see if I can find the model number. It's 30mm square and about 5mm thick. Rated at 15v and about 20W so relatively low power. I bought it from Farnell element14 and I'm pretty sure it's this model :- MCPE-127-10-25 - Peltier Element, Thermoelectric Cooler, 19.6 W, 6.9 ohm, 2 A, 15.7 V, 75 °C I tried various Chinese Peltier TECs but found they were very inefficient - a lot of power input for little actual cooling. Cheap but not very cheerful!!

So far I haven't found any need for better camera cooling and I doubt the ambient conditions would get much worse. Last night the breeze had died and there was little if any air movement across the cooler meaning that all the cooling was by convection. Cooling was significantly less than when there was an easterly breeze but still adequate to kill noise and hot pixels.

If you provide adequate ventilation a wooden observatory won't get as hot. Even in this very hot weather in full very strong sunshine my observatory has been quite acceptable to work in. And I can't stand much heat.

I thought of PVC cladding which I had used on a new back porch for the house but the cost for all 4 walls of a 5m x 2.5m x 2m (more in places) structure was prohibitive. Also, at that time I could only find it in white which would have made the observatory stand out like a sore thumb in the green environment! Good quality redwood shiplap for the local timber merchant, though costing more than standard shiplap, was very much cheaper than PVC. After several years the redwood shiplap is fine. And yes, I did coat it both sides with good quality wood preserver. IMO it paid to go for the best in timber option.

I used shiplap on mine. Still fine after several years but could do with a coat of wood preserver now I think.

Recommended.

Yes that's what i did on my observatory. The nailer has been very useful generally. A good buy.

Actually, I wouldn't mind spending up to £100 for decent 3D CAD software.

"OAP needs free software for designing 3D printed parts preferably running under Linux"

I've used SketchUp for several years to produce designs for 3D printing but it has its difficulties. I'd be interested in trying other CAD software so I'll look at OnShape - thank you . A we bit later... OUCH!! Can't afford that ?

No, don't go and hide - your contributions to SGL are much appreciated

Yes.

They both seem to be thermometers but what for? Seems a strange place to put a thermometer. I have seen dials on focuser knobs before but thought they indicated focuser position.

I also have some other failed prints that look quite artistic ?

Take care James

Had a thought about how I could make a waterproof cooling fan if I want more cooling. Got the idea from a submersible water pump for fountain or aquarium. Make the motor waterproof and drive the fan by a magnet. The fan could use a nylon bearing which isn't affected by damp or wet.

New Blog started :-

I made sure I built mine far enough from the fence to get a step ladder in so that I could work on the two sides next to my boundary. Mind you I wasn't short of space.

Actually, the plywood stood my weight alright when I was laying the rubber roofing.

Why?