DIY Fork Mount for Widefield Imaging Rig

[Gina]

I prefer the idea of a proper dome with shutter as that would be good to keep the wind off.  I shall be looking at both that and a clamshell design.  Clamshell may be more difficult to seal when closed.

[wimvb]

On 17/12/2019 at 13:45, Dr_Ju_ju said:

Hmmm, methinks the design starts to look like a Mesu , especially if you're planning on automating each axis of movement

The original Mesu design perhaps 😋 :

http://www.astro-imaging.com/Equipment.html

http://www.sternwarte-hattingen.de/hatt/hatt2010.htm

(scroll down to the 10th image)

@Gina: if you put the DEC wheel on the side with the colour camera, won't that give you a better weight distribution? Also, which parts will be in metal and which will be 3D printed? I wonder where material stiffness is most critical.

[Gina]

Um...  what colour camera?  I'll colour the 3D printed parts and post a new screenshot.

[Gina]

OK...  Metal parts are grey, white or black though the astro camera is red and that is metal.  3D printed parts are various colours.

Edited December 19, 2019 by Gina

[wimvb]

40 minutes ago, Gina said:

Um...  what colour camera?

Sorry, I assumed the camera without the filter wheel was an OSC camera.

Edited December 19, 2019 by wimvb

[wimvb]

18 minutes ago, Gina said:

OK...  Metal parts are grey, white or black though the astro camera is red and that is metal.  3D printed parts are various colours.

Is there a risk that the fork itself may flex?

[Gina]

I don't think so.  It's pretty "beefy" and the load is quite low.  It will be printed in PLA which is pretty stiff.  High percentage infill too.

[Gina]

I posted a thread on my Mini Dome Observatory but have had lots of time to think since then.  Started with a small conventional dome with shutter and built a lot of it.  It was too small and I had a problem working out how to control the shutter.  Later I went on to design a clamshell type but I'm not really happy with that either.  Things have moved on since then and I have better CAD software and expect to get my Giant 3D printer working which should be capable of printing nearly 500mm x 500mm x 700mm which is somewhat bigger than the printer I used for the dome I built of 300mm x 300mm x 250mm.

On the whole I prefer a proper dome with shutter as it would keep the wind off and I think it may be easier to weatherproof.

[Gina]

This is a model of the fixed part of the clamshell from my other thread.  The moving part is a matching shape (which I don't seem to have) which pivots on the holes either side.  There is just one moving part.  My main objection to this design is that there is a large overhang of the sphere.  Really just a matter of appearance.  I think it may be alright engineering-wise.  I know one thing - if I go with this design it will be very different and quite unique!!  Also, I think I would need to produce a scale model and see if the weatherproofing is effective.  If it works it will be a very simple thing to make.

First view shows how it will be orientated in use and second how it could be printed, if using PETG with its better overhang printing ability.  This part would fit over a mating part which would be the base and secured with screws.

Edited December 19, 2019 by Gina

[Gina]

Designed the matching moving part.  Not sure about how to print it though...

[Gina]

Assembly of both parts.  Closed and open.

[Gina]

From operating the model, I don't think there's any problem with the weatherproofing.  The south side has a good overlap and the gap could be reduced or draught proofing strip added.  On the north side the two "ledges" meet.  I haven't shown the drive but this would be just a pulley like the RA or DEC drives - possibly bigger. 

[Gina]

These models are smaller than what's needed for the real dome but I can soon draw the full size version.  I'll do that next, I think, and check that it clears all the moving parts.

[lux eterna]

1 hour ago, Gina said:

Please make it yellow, and you will (probably) have the first moving 3D pacman statue 🙂. But maybe it would attract too many tourists, wanting to take a selfie with it...

Seriously, it looks very proffessional. It´s fantastic what is possible these days, with CAD and 3D printers.

Ragnar

[Gina]

Trying the full-size dome on the rig.  It's only barely big enough!  This is just the dome part without the cut-out for the mount.  This is 450mm ID.  I could do a bit bigger on my Giant printer if I can get the whole of the bed for printing.  ATM it's a fair bit less but I'm rebuilding the printer and hoping for a bigger printing area.

Edited December 19, 2019 by Gina

[Gina]

Finished the fixed part of the dome, I think.

[Gina]

I'll draw the moving part tomorrow...  Oh,and the base.  And add the screw holes in the fixed dome part to match the holes I shall put in the base.

[lux eterna]

Have you planned for the remote control system yet ? I guess maybe a remote RPI controlled from your living room via wifi ? Do you have a power outlet nearby, or will this be battery powered ?

Having read some more of the previous posts, with the tracking resolution calculations, I think what will be the limiting factor in this system is not the resolution itself (and not mechanical flex either) but the PE. With multiple small PE´s originating from different parts, these will sometimes cancel out each other but also occasionally sum up. But I am sure this design will be more than good enough for widefield imaging though.

Ragnar

[Gina]

Yes, the system will run from an RPi using WiFi.  The mount slewing and tracking will use the INDI mount driver modified for the different RA and DEC drive ratios.  Camera will use the standard ZWO INDI driver and focus motor modified Astroberry Focuser driver.  PA controls will use a modified Astroberry Board driver.  Remote control will indeed be from my living room using KStars/Ekos running in Linux Mint.  Power will be supplied from my main observatory a few metres away at 13.8v from the main observatory supply with battery back-up.  The dome open/close motor will also be controlled from the RPi, initially from the Astroberry Board driver.  Stepper motors will be driven by A4988 driver modules controlled from the RPi GPIO pins.

[Gina]

I'm hoping that with belt drive the PE will be minimal.  The final drive large pulley will probably be worst offender though I strive to get my 3D printing as accurate as I can.  The other pulleys are commercial aluminium products and should be pretty accurate.  Fit on the axles is very close with little, if any, play, axles are a close fit in the ball bearings too. 

Edited December 20, 2019 by Gina

[Gina]

Looks like there's plenty of room to accommodate a NEMA 17 inside the dome to drive the moving part.  That will make things much simpler.  I think it should be powerful enough for direct drive.

[Gina]

This is the part of the dome that connects with the base and will form the starting point for the base design - maybe the most complicated 3D print of the whole project!  It will need printing in several parts and be bolted together.

The base needs to fit inside this so I need the measurements for the inside.  I have all the measurements from the design except for the one shown below.

If I decide to put that as the bottom of a printed part it will need a flat part.

Edited December 20, 2019 by Gina

[Gina]

View from underneath shows that the rectangular part of the base can go straight down.

[Gina]

This is the view up the RA axis showing the cylindrical part of the cover/base.  An extension (or lump out the side) will be needed to cover the RA drive pulleys and belts.  Once past the RA drive pulley the cover can bend round and down.  I'll show this later.

Edited December 20, 2019 by Gina

[Gina]

This screenshot shows the cylindrical part of the cover.