DIY Fork Mount for Widefield Imaging Rig

[Horwig]

second vote for forward metals, very civilised people to deal with

 

[Gina]

I think I've used them myself via ebay.

Found two more pieces of aluminium both 200mm square, one 4mm and one 3mm thick.

[Gina]

I think I have virtually sorted out the mounting and the PA adjustments.  Here is a cross-section drawing and I'll draw and post some 3D versions shortly.

Edited May 3, 2017 by Gina

[Gina]

First 3D model.  This shows the Alt adjustment.  The orange part will be a 3D printed spur gear and will be matched with a smaller pinion on a stepper motor.  The smaller horizontal plate will be lifted by the weight of the imaging rig by leverage with the pivot being at the left hand end of the plate.  If it isn't lifted I can add a spring. 

The bolt that forms the Alt adjustment could be in a slot in the plate and then the plate can be turned in the horizontal plane, pivoting on the bolt on the left, providing the Az adjustment.  The plate would be turned by an extension on the right with a nut on a threaded rod turned by gears and stepper motor.  I used this method in the past for alignment on my 400mm triple imaging rig.

[Gina]

This photo shows the idea for the Az adjustment.  The smaller plate was one of the ones that carried the ST80 scopes on the 400mm triple rig and that was adjusted in the same two planes, with the Alt being with a screw up through the bottom of the big plate.

[Gina]

I've ordered a pair of 3" stainless steel hinges.  One small step...

[Gina]

I've now ordered some aluminium channel for the fork - 1" x 1" x 1/16", which I intend to bolt together and to the 1" pipe.  Hope this will be rigid enough.

[Gina]

I'm still finding pieces of aluminium but no more of them useful for this project so far.

[Gina]

I'm sure I should have two aluminium discs I got when I used the NEQ6 mount on my concrete pier and replaced with bigger and thicker steel plates when I upgraded to the EQ8.  These plates should be around somewhere and I think would be suitable for mounting the forks onto the aluminium tube.

[Gina]

I was mistaken   No aluminium discs - just the steel ones I'm using now.  Been looking through my observatory build thread.  I started with the 250mm diameter x 10mm thick steel discs and painted them white to match the NEQ6.  A 200mm square x 5mm thick aluminium plate with the corners rounded off was used to make the ledge for the NEQ6, also painted white.  When I got the EQ8 (in black) I repainted the steel discs black to match and didn't need the adapter plate.  That should be around somewhere but probably not much good for the fork mount as the hole in the middle is too big.  No wonder I can't find any aluminium discs!

[Gina]

I have an acrylic disc about 6" diameter and 1" thick (I think) that I can turn on the lathe and use for both the connection of forks to aluminium tube and as the large pulley for the RA drive.

[Gina]

The 500mm x 250mm x 10mm aluminium plate is quite a lot bigger than needed and causing problems with the roof box so I'll sort out something smaller.

The 280mm x 170mm x 10mm to which I was going to attach the pillow block bearings, will be used as the base and the 300mm x 200mm x 4mm sheet will do for the bearings.  To allow for PA adjustments a 200mm x 150mm x 4mm plate will be mounted on top of the base plate and attached to the bearing plate with hinges (as before, not shown here).  A 200mm x 100mm x 4mm ali plate will provide an upright to support the top of the bearing plate.  This can be attached to the moving plate with aluminium angle and to the bearing plate with bolts (not shown).

Edited May 7, 2017 by Gina

[Gina]

The hinges arrived in the post today plus some more aluminium channel and whilst clearing up this afternoon in my "Conquest of the Rats" project, came across some more pieces of aluminium.  A sheet 300mm x 200mm x 3mm and two discs 7" diameter x 3mm and a few other bits that I don't think will help with this project.

[Gina]

Here's a new model of the mount support parts.  This is using the 300mm x 200mm aluminium plate I found this afternoon.  This is actually bigger than the thicker base plate but that doesn't matter.

[Gina]

I've come back to this project having constructed a basic 3D printed dome and needing to see if this dome will work with the mount.  The mount is probably over-engineered for what I'm proposing to put on it but as has been said - not such a bad thing.  Means it's bigger than I would like but I already have all the main parts and most of the design so I don't want to start again.  I think having put it to one side for a while and gained more experience and information it could be improved with a minor change in design.

Here is a rough cross-section diagram of how it currently fits with the dome and a modified widefield imaging rig.

 

Edited June 19, 2017 by Gina

[Gina]

Small change and drive pulleys added.

[Gina]

I was going to use NEMA17 motors for both axes but the ubiquitous 28BYJ-48 stepper motor with gearbox would be powerful enough I believe, particularly for the DEC drive.  The smaller motor could be placed directly on the fork and save complications and weight.  The gearbox has a ratio of 63.68395:1 which could be entered into the driver software/firmware just like the NEQ6 etc. had a peculiar gear ratio.  With a gearbox in the motor casing a much lower ratio would be needed on the belt drive to the DEC axis on the fork.

[Gina]

Fastest the 28BYJ-48 rotates is 10 rpm so if I used a 5:1 reduction on the belt drive it would take about 15s to move the rig 180° - that's fine. 

As for the resolution, half-stepping works out about 4076 steps/rev. which is 360/4076 = 0.0883°.  With 5:1 further reduction angular resolution half-stepping = 360/(4076x5) = 0.0177 or about 18 pixels (0.001° per pixel with current lens and camera).  Or with 16x micro-stepping = 18/8 or about 2 pixels.  This is fine for the DEC axis.

I think it might be reasonable to use the 28BYJ-48 stepper for RA as well but with a 10:1 belt drive ratio to give pixel resolution.  180° rotation would take 30 seconds - again adequate.  This will simplify the construction.

Edited June 20, 2017 by Gina

[Chriske]

Gina,

Will the dome be removed manually or is it a two section dome that will rotate out of the way..?

[Gina]

It's a standard type of dome (though miniature) with a slot and shutter.  See THIS THREAD

[Chriske]

Nicely done...

[Gina]

Thank you

Here's another cross-section showing how I think it might continue from the bottom of the dome down to the wooden support framework (not shown).  Below the dome and glued to it will be a circular ring with inside ledge resting on another circular ring which will be attached to the casing that reaches down to the support framework.  I think the dome can be located by 3 or 4 small spur gears running on the inside of a circular ring gear on the dome.  One of these would be driven by a stepper motor to rotate the dome.

[Gina]

Now modelling the casing - in stages.  A mixture of 2D and 3D as I progress.

 

Edited June 21, 2017 by Gina

[Gina]

The dome and some of the mount is no longer needed to model the casing.

[Dr_Ju_ju]

While at it, don't forget to make provision for a Pi3, hubs, cables etc....