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Gina

Gina's All Sky Camera - Mk 5 - with ASI178MM, Peltier TEC Cooling & RPi

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This is an upgrade of my Mark 4 with different camera and several additions including Peltier TEC cooling to reduce noise and using the Raspberry Pi for image capture and control.  In addition I shall be using WiFi to connect from a desktop computer for control and image transfer.  I'm currently learning how to use the RPi almost from scratch - I have used Linux in the past.

Edited by Gina
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Still researching what I can do with the RPi.  For this all sky cam I shall want image capture and camera control - maybe oaCapture - and also remote focussing, for which I shall use a small stepper motor, plus Peltier TEC cooling of the imaging camera which will require power control and temperature sensing.  I also expect to need a dew heater for the dome which also wants power control and temperature sensing.

I was half expecting to need an Arduino to read the temperature data from the DS18B20 digital thermometer chip but I've found that the RPi can do this directly.  Driving stepper motors is no problem.  Not sure yet if the RPi had PWM capability but this can always be programmed so I'm not worried about that.

DS18B20+ One Wire Digital Temperature Sensor and the Raspberry Pi

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OK...  That's temperature reading, now to the other things I want to do with the GPIO pins of the RPi. viz. PWM and stepper motor control.

Using PWM in RPi.GPIO
RPi.GPIO 0.5.2a now has software PWM – How to use it
PWM via DMA for the Raspberry Pi

Stepper Motor Control with the Raspberry Pi (YouTube)
How to connect Stepper motors to a Raspberry Pi
Stepper Motor Control In Python
Control Stepper Motors With Raspberry Pi: Tutorials and Resources

The stepper motor control is exactly the same way as I do it with the Arduino.

All this means that I can do all I want to with the RPi so no need for an Arduino or AVR chip.  This can also apply to other astro projects and should considerably reduce complication :)

Edited by Gina
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I am now working with the RPi 3 with the INDI control system and KStars/Ekos client controlling the RPi over WiFi LAN and using the INDI server and drivers.  Image capture is provided with this system and a separate imaging app is not needed.  This will also control a stepper motor for focussing and on/off switching for a dew heater.  I haven't looked into whether there's an INDI driver for the 1-wire DS18B20 digital thermometer - I only know that the RPi can handle 1-wire.

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Can't find any reference to "1-wire" INDI drivers so I won't have any temperature readings at first and I'll just use manual control of the dew heater for now via. Ekos/INDI.

For focussing I have modified a 28BYJ-48 stepper motor to bipolar connection to use with a Pololu A4988 stepper driver module.  There is an INDI driver that can be used with this.  Another INDI driver controls 4 of the GPIO pins and one of these can switch the dew heater.  Initially I was going to use a power MOSFET for switching but these need 5v logic input and the RPi output is only 3.3v logic so a transistor will be needed as level converter.  I find I have some BC517 darlington transistors that may be able to switch the dew heater directly.

The BC517 is rated at 625mW power dissipation and the CE saturation voltage is 1v so this would limit the collector current to 625mA.  Derating this a bit to 500mA and using a 12v supply the heater power limit would be 11v at 500mA = 5.5W.  This should be more than enough and I'll probably try with less.

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I was going to use nichrome wire as in earlier versions but I think I might go over to resistors.  The original heater showed burnt marks where the ring of nichrome wire was mounted and tied.  I used heat-shrink sleeving on the bare wire with larger short pieces of heat-shrink to hold the six wires together.  I see from one of my earlier posts that I was using 0.7A but I think that was a lot too much.  The power supply for the final position will be my 13.8v main observatory battery backed supply and I think 0.5A at 12v ie. 6W will be more than adequate for heating.

I could provide 2 levels of heating by using 2 heaters and 2 drive transistors from 2 of the 4 GPIO lines controlled by the INDI driver.  TBH I think 2w should be plenty.  I believe the RPI will do PWM but I haven't found an INDI driver for that as yet so I think I'll go for 2 or 3 fixed levels but starting with just the one.

Cheapest and easiest resistors to find are ¼W but even a 2W heater would need 8 so I think I'll go for ½W or 1W.

 

Edited by Gina

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Hi Gina,

what length exposures are you planning? Unless you are pushing up beyond 1 minute I'm not sure that cooling will gain you much with the 178 sensor. This is the dark frame histogram of my QHY178C at 50% gain level, 30s exposure, room temperature

Capture.PNG

There are some pixels up through all the different values (see the plot below with logarithmic vertical scale), but not really a significant number out of a 6 megapixel sensor. Also this would be lower at outside temperatures.

Capture2.PNG

The Sony sensors (178, 224, 174, etc) do have an amp-glow like issue which *is not* helped by cooling. QHY have a tweak for this that I haven't seen from any other manufacturer. They won't say exactly what it is they are doing, but in my experience it does seem to be effective.

cheers,

Robin

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Thanks Robin :)  I'm using one minute exposures and found the cooling made a significant difference.

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Yeah, 1 minute is where it starts to make a big difference, 30s or less is marginal, 15s or less is probably not worth it.

Have you found a wide angle lens that will illuminate the whole sensor - that must be more of a challenge with the bigger sensor on the 178...

cheers,

Robin

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I'm using the supplied fish-eye lens with which the sensor covers most of the FOV.  I get a bit clipped off top and bottom.  I would have preferred a slightly shorter focal length which would cover more of the sky.

Screenshot from video 2016-07-25a.jpg

Edited by Gina
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I've ordered some 1W 10Ω resistors for the dew heater.  4 of these in series giving 40Ω would give 3.6W from 12v with a load current of 330mA - well within the capability of the BC517.  Or maybe 6 in series for 2.4W and current of 200mA.

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An alternative might be to have 8 resistors and connect in series for 2W heating or a series/parallel arrangement for 8W with possible an intermediate 4W with 4 resistors in series.  Probably not worth making it too complicated though.

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Decided to use either 7 or 8 10Ω  resistors around the edge of the dome depending on how many fit conveniently, running off the 12v supply and switched with a BC517.  I'll see how well this works and add further heating if required.

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Gradually making progress on the RPi HAT and connections.

RPI HAT 03.jpg

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Just about finished now and tested including the camera cooling and dew heater though the dew heater is only showing 6v rather than 12v so it looks like the switching transistor is dropping 7v (supply voltage of 13v).  Dew heater resistance is 70Ω so current is 6/70 = 0.086A and power of just over 0.5W the power dissipation in the transistor will be 0.068x7 = 0.6W which is bang on the limit.  It should be switching hard on and be dropping about 1v.  Something I shall need to look into.

Here are some photos.

Base part showing RPi plus HAT and camera with lens mounted on top of a round aluminium pole mounting with a Peltier TEC between camera and aluminium heat sink.  This and the RPi are powered from a drop-down buck converter supplied from 13.8v in the observatory. 
ASC Mark 5 RPi 01.jpg

Inside of top showing the focussing stepper motor and dew heater connections with their wires and plugs to plug into the RPi HAT.
ASC Mark 5 RPi 02.jpg

Camera connected via USB cable with cooler and dew heater plugged in.
ASC Mark 5 RPi 03.jpg

Top of ASC showing lens, camera, focus gearing to the lens focus ring and dew heater resistors.  Outside that is the ring that holds the clear acrylic dome.
ASC Mark 5 RPi 04.jpg

Dome added
ASC Mark 5 RPi 05.jpg

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ASC set up on pole outside front window and a few feet above the bungalow roof.  Here is the Preview image on the client laptop running KStars/Ekos.  Haven't tried resetting focus for diffraction of the dome yet as the view is mainly cloud.  I could try manual focus on the trees but I think I'll wait until night and just run the auto-focus as I did very successfully last night before the dew spoilt the image.

2016-10-20T1620.JPG

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This is the control panel that I use to turn the dew heater on/off.  It uses the Astroberry Board INDI driver and takes the "Line A" GPIO pin to switch the dew heater using a Darlington transistor (BC517).

Astroberry board Ekos control panel 01.JPG

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Found one little deficiency in the system for controlling the exposure of the ASI178MM camera.  The minimum camera exposure is 32µs which is useful during daylight hours in bright conditions but the KStars/Ekos/INDI driver system doesn't work below 1ms.  Whether this is a limitation of KStars/Ekos/INDI or the indi_asi_ccd driver I can't tell but my feeling is the driver as it seems that the drivers determine pretty much everything.  I might look at the source code of the driver (if I can find it) and see if there's anything easily altered to fix the problem). 

The above doesn't apply to night viewing, of course, and I might set up another ASC for daytime, with a colour camera to show the difference between blue sky and plain grey cloud.  A colour camera can also capture some pretty impressive sunsets too :)  This could be a possible project for the future - I want to get may main imaging system working under Linux and the RPi first.

Edited by Gina

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If I can get the 120MC working you can pick them up quite cheaply.

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I have an ASI185MC that'll do nicely :)

The dome is causing some image aberration around the edge of the FOV.  This is after using auto-focus on the street lights.

Auto-Focus 02.JPGAuto-Focus 03.JPG

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The sky is beginning to clear - the clouds are going :)

2016-10-20T1927.JPG

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Increased exposure to 20s.

2016-10-20T1932.JPG

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Applied auto-focus to a star.  Here's the result.

2016-10-20T2016.JPGAuto-Focus 04.JPG

 

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Exposure increased to 60s.

2016-10-20T1952.JPG

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30s exposure, gamma 20, brightness 10 and Preview auto-stretched.

2016-10-20T2006.JPG

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