DIY Fork Mount for Widefield Imaging Rig

[Gina]

This mount will have remote PA as mentioned elsewhere so I can set up PA from indoors.  The mount and imaging rig will be contained in a miniature observatory with no room for anything but the equipment.  I haven't finally decided whether this will be dome or clamshell type - it won't be a ROR shed, I'm pretty sure.  I think a clamshell would be easiest to implement but that's another project.

Altitude PA control can be a NEMA 17 stepper motor with single-start trapezoidal screw as used in 3D printers for the Z drive.  A similar system could be used for the azimuth with a lever to translate the linear movement to a small rotation.  This is easier than timing belt or gearing for the very small angle required for PA adjustment.

I already have the design for the RA axis drive using multiple timing belts and pulleys running off a NEMA17 stepper motor and will look at a similar system for DEC having rejected the tiny 28BYJ-48 stepper motor.

Having designed a complete precision 3D printer in CAD this will be the next complicated thing to design, I think.  I don't think imaging rigs will need this level of intricacy apart from the design of brackets and focus gears etc.

[Gina]

Largest diameter pulley I can 3D print ATM is 290mm OD though will be more when I have my Giant printer working again if it's accurate enough.  For the moment though I'll work with 280mm OD to give some leeway.

Looking again at the calculations for the RA drive :-

1. Final drive large pulley = 280mm OD.
2. Final drive small timing pulley will have 20 teeth with a GT2 timing belt.
3. Circumference of large pulley = π x D = 879.64mm
4. Timing belt tooth spacing = 2mm so teeth per 360° = 439.82.  Near enough 440.
5. This give a final drive ratio of 440:20 = 22:1
6. Primary and intermediate reductions will have 120t and 15t timing pulleys giving a reduction ratio of 8:1 each.
7. Overall reduction ratio from motor to RA axis will be 22 x 8 x 8 = 1408.
8. NEMA17 stepper motors have either 200 or 400 full steps per revolution.  16x microstepping can be used giving 3200 or 6400 µsteps/rev.
9. Taking the standard 200steps/rev gives 3200 x 1408 µsteps per 360° = 3200 x 1408 / 360 µsteps per degree

Enough brain straining for the moment...

Edited December 12, 2019 by Gina

[Gina]

Looking now at the angular resolution required to give pixel accuracy :-

1. Pixel size for the ASI1600MM-Cool camera is 3.8µm.
2. Focal length of longest lens I have that I could use in a dual imaging rig is 200mm.
3. By geometry, 1px subtends an angle given by tan(A) = 1px and for these very small angles tan approximates to the angle in radians.
4. So angle in radians = 3.8µm / 200mm = 3.8 / 0.2 x 10^-6 = 19 x 10^-6
5. A radian = 2 x π degrees so angle in degrees = angle in radians x 2 x π
6. Angular resolution required = 19 x 10^-6 x 2 x π = 119.4 x 10^-6° or 1.194 x 10^-4.  Near enough 1 x 10^-4 degrees.

Returning now to the resolution we should get with the drive system above :-

1. 3200 x 1408 / 360 µsteps of a standard NEMA17 motor turns the RA axis by one degree. 
2. Thus each µstep turns the RA axis by 360 / (3200 x 1480) ≈ 0.8 x 10^-4 degrees.   Just about enough though I might prefer to use the 0.9° step size motor and get 0.4 of a pixel resolution.

[Gina]

Now to the DEC drive and a new design having decided against the 28BHY-48 stepper motor with the fragile plastic gearbox.  Instead I plan to use a NEMA series stepper motor, maybe one of the smaller ones such as a NEMA11.  There are two options for the DEC drive depending on whether I want to allow for guiding.  Now whether this mount will need guiding will depend on how well I can engineer it.  It would seem prudent to allow for guiding, in which case the DEC drive would need the same resolution as the RA drive.  I was originally planning to try without guiding and hence a lower resolution and step-down ratio.

[Gina]

MotionCo do a wide range of MXL size timing pulleys and belts as posted above and I can use the same system for the DEC drive as the RA, just that the whole DEC drive system needs to rotate on the RA axis and the final drive pulley has to be smaller.  Smallest timing pulley with 5mm bore to fit stepper motors is 15t and the largest is 120t giving 8:1 as in the RA drive.  With the RA drive and 280mm final drive pulley a 3 stage reduction gave 0.8 pixel resolution.  To get the same for DEC means either a 4 stage reduction or large final drive pulley.  I don't think I can decide this without drawing a CAD model of the whole mount complete with imaging rig.

Edited December 15, 2019 by Gina

[Gina]

If I were to use MXL belt instead of GT2 I could use a 15t MXL pulley and the equivalent final drive pulley OD would be 210mm which seems well do-able.  I don't have MXL belt in stock (only GT2 from 3D printers) but the appropriate length would only be a fiver.

Edited December 15, 2019 by Gina

[Gina]

One thing I can check before I start CAD is the PA drives.  The threaded rod is single start 8mm and pitch of 1mm ie. one turn moves the nut 1mm.  I'll take as a starting point an offset of 200mm (nut from altitude pivot at right-angles to screw) and that a NEMA17 stepper motor drives the screw directly, without any reduction and calculate the number of motor steps per minute of arc PA adjustment.

1. One minute of arc = 1/60 degree or 1/(60 x 2π) = 0.0053 radians
2. Looking at the geometry, this is a distance D at the nut given by tan(A) = D/200.  Where A is in radians and D is in mm.
3. For these very small angles tan approximates to the angle in radians, so D/200 = 0.0053 or D = 0.0053x200 = 1.06
4. This is just over one thread pitch = one revolution of the threaded rod .  So one full step of the stepper motor corresponds to 1/200 minute.

Thus this arrangement gives very precise PA adjustment.  Just waiting for someone to tell me I've got this wrong!  🤣

[Gina]

This CAD model shows the principle of the PA adjustments.  The threaded rod will be much shorter and not go through the altitude plate, which will carry the fork system.  The top horizontal plate will rotate on the lower, thicker plate to provide azimuth adjustment.

[Gina]

PA Elevation adjustment.

[Gina]

PA Azimuth adjustment added.

[Gina]

RA axle and bearings added.

[Gina]

Fork added.  Now I have to decide whether the large final RA drive pulley goes beneath the fork or on the bottom of the axle.  I think the lower position will make for easier belt drive.

[Gina]

Decided to put the pulley on the bottom of the RA axle.  Being aluminium tube the axle should transmit the rotation fine - I can drill holes and put pins or bolts through.  This means a change in fork.

[Gina]

Designed and added the big RA pulley but I think I might make it smaller since I can get sufficient resolution by going for MXL timing pulley and belt rather than GT2.  This means around 200mm would give similar resolution to the current almost 300mm.

[Gina]

Designed the dual imaging rig and added it.  This is probably not the final version.

[Gina]

Added DEC pulley.

[Gina]

This design of imaging rig will be alright for the smaller, lighter lenses but will be considerable front heavy for the bigger ones such as the 200mm f4 and could do with the DEC axis more towards the lenses.  These aren't your modern plastic but proper glass and metal and pretty heavy.

[Gina]

A couple more screenshots of the model.  Pier included now.

Edited December 16, 2019 by Gina

[Gina]

I think the DEC drive pulleys and belts may fit on the side of the fork.

[Gina]

This model reminds me of 5-Alive in that robot film... 🤣

[DaveS]

There looks to be quite a lot of weight on one side of the fork, I would be concerned about out-of-balance forces around the PA. The remote PA looks brilliant BTW. Worth patenting and licensing?

[Gina]

I think the off-balance weight of the EFW and filters may be balanced by the weight of the pulleys and stepper motor on the other side once I've added them.

[Gina]

Been thinking...  I've been designing in terms of having guiding yet I haven't arranged for a guide camera.  OAG is not an option with lenses so it would need a guide scope/lens.  This is getting bigger and bigger at this rate!!  Also, I made a decision when I was looking into this project earlier to go for a single imaging rig as a starting point.  I've been going for an "all singing, all dancing" imaging system lately and I'm wondering if this is such a good idea.  I could start simple and refine things later if the project takes off.  After all, it's only a matter of CAD followed by mainly 3D printing the parts.  I might manage something relatively simple whereas a bigger project may never get off the drawing board.

SO...  Am I designing a complicated "fantasy" setup or shall I go for something more practical?

[Dr_Ju_ju]

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

[Gina]

As a lover of everything different, I feel I need to go for this as a practical project.  With the weather as it is, standard imaging does not produce much joy.