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Astro Projects

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All Sky Camera Mark 7

Gina

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This is my latest generation of all sky cameras and based on the ASI178MM followed by ASI185MC CMOS astro camera and a Fujinon fish-eye lens of 1.4mm focal length.  Although rated at f1.8, this lens lets a lot more light through than this would imply.  Image capture is provided by a Raspberry Pi 3 in conjunction with INDI software.  This is used with KStars/Ekos client software running on a Linux Mint desktop indoors.  Communication is via Wi-Fi.  The Mark 6 ASC has proved inadequate after being in use for some time. 

This blog will describe the problems of the Mark 6 and report my progress in developing this new version.

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I have decided I'm pretty definitely going to combine weather station wind sensors and ASC on the same mast.

5a257b244f829_Diagram03.png.9c65853102387e7881655cc7666c0561.png

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Decided to attach the buck converter to the circuit board with the motor drivers rather than having it separate.  This means some connections could be wired straight across to the power side of the driver modules.  A link from the +13.8v input to the motor power lines and the +13.8v on the 5pin connector.  The power input Gnd connects to the Power Gnd and motors -ve.  Then the +5.1v from the buck converter was taken across to the 5th pin on the 5pin power connector.  The latter will now provide power to the RPi and HAT as Power Gnd, +5.1v and +13.8v.  The +3.3v and Gnd from the HAT connect back to the motor driver board as the +3.3v and Data Gnd.  This grounding arrangement prevents ground loops (often referred to as earth loops in the UK).

5a25dab02f79a_FocusMotorDrivers04.png.c3bf16d63e23b80c3bd6dcec48c15faf.png

Edited by Gina

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Still wiring up and attaching wires to the tiny crimp sockets on the connectors etc.  It's a long and tedious job!

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I think it's about finished - at least it is for tonight :D  I'll give it a careful visual checking over tomorrow and if all seems well I might apply power.  I have yet to finish writing code for the RPi.

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Found one unsoldered joint and a couple of suspect joints which I re-soldered.  Also, when I plugged everything in to connect RPi assembly with motor driver board I found the wiring effectively put the power input at the top instead of bottom so had to remove the buck converter, turn it round and rewire it.  Of course, it would have been better on the other side of the motor drivers but that circuit was already at the edge of the board (I wired up all that before deciding to put the buck converter on the same board).  Not a problem - just longer wires.  Heres the new arrangement.

5a27e24e6668f_FocusMotorDrivers05.thumb.png.e84f5ce788763622cf3eddad6a371db2.png

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Having completed the construction of the arm and casing etc. for my weather station wind instruments, I'm now coming back to the ASC as the casing will contain the Raspberry Pi that will run the weather station wind sensors.  May be the same RPi as used for the ASC but I have yet to confirm that.

I am having a problem with the design of the ASC casing due to the way the lens can move about with the alignment system.

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I really don't like the three stepper motor focussing and alignment system even if I have almost completed it!!  As I posted above it's causing problems with the casing, it takes up a lot of room and will probably be pretty awkward to operate.  I think that's why I haven't felt like doing any more work on the ASC.  I have now decided that I need to find an alternative solution - the present arrangement is not practical.

Edited by Gina

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The main problem with using the threaded C mounting as a helical focuser is the slop in the thread so I'm now wondering if spring loading the thread would cure that problem.  I think it likely that if I can find a way to keep the lens straight while focussing, there will be no need to adjust the alignment.  There is still an offset needed due to the imaging sensor being offset from sensor in the camera.  The offset is only around a mm but with a 5mm sensor height, this is significant in getting full coverage.

Edited by Gina

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First of all I'm printing an adapter to connect the camera to the CS/C adapter.  This includes a 1mm offset which I think is about right.  I can run a test to see and adjust the design if wrong.

5a3ad60c62efd_LensMounting01.png.b8c1ecab2af546ac0dc1b12b1e77671a.png

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Test run completed using SharpCap (haven't got the RPi set up).  Here is a photo of the rig and a screenshot from SharpCap of the image.  The 1mm offset was too much so I turned the plastic part on the camera.  I estimate the required offset is around 0.5mm.

5a3ae3ac0cd94_LensMounting03.thumb.png.c55a5ee112543a08c6b9dd52ef207bb5.png5a3ae689dda60_LensMounting04.png.e691f80360e288f715ccd608513c943f.png

 

Edited by Gina

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Don't know when I shall be able to check this outdoors, the weather forecast is for solid cloud for the foreseeable future :(  Misty too!

I now have just two stepper motors to control - focus and lens cover and I may be able to put both A4988 driver modules on the RPi HAT.  The whole casing is in for a re-design now and this includes the lens cover drive arrangements.  I'm expecting to reduce the size of the casing from the last design.  More thinking needed :D 

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Since the lens now rotates (for focussing) I can't fasten a fixed casing to it.  Instead the lens can have a separate conical shaped part attached with overlaps the main casing.  The lens cover would then cover this.  I have yet to decide how the cover will be controlled.

5a3ba77bc1f3d_LensCover01.png.4474be9f36431e95d6ba79e2936045fb.png

Edited by Gina

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Since the lens rotates, the yellow part in the diagram above will rotate with in and hence the casing will have to be circular in section too but depending on the electronics the casing might be 120mm or 125mm rather than 145mm so a bit smaller.

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I'm straining my brain trying to think up a way to drive the lens cover while keeping the stepper motor dry.  The motor will want to be inside the main casing or possibly in a little pod.  Drive could be gears, axle with bevel gears, timing belt and pulleys or even fishing line cord and drums (like I use for my 3D printers).  Whatever I use must all be below the edge of the lens or it would obstruct the view.  In typing this I have activated my brain a bit :D  An idea has occurred to me - I wonder if I could use direct drive with  little casing containing the stepper motor on a sort of "stalk" :D

Edited by Gina

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This diagram shows how a little stepper motor might be direct coupled to the arm which operates the lens cover.  It would be covered by a small enclosure, of course.  This arrangement might be simpler than drive rods, belts or cord.

5a3c1b68998b7_LensCover02.png.0c79252dc769843903d55ef4635a19d0.png5a3c2a20348df_LensCover05.png.b4a49416eb7fece9219d4f4d636ac2c8.png

Edited by Gina

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