DIY Equatorial Platform (lower latitudes)

dlavall · September 22, 2023

I'm starting a project to build an equatorial platform for my 10" F5 Dob. At first I was considering using Optical Ed's design process, using a circular section of the polar axis cone for the North bearing. Then somebody suggested a VNS design as documented at www.reinervogel.net. When considering the VNS design at my latitude (28° N) the pivot point for a pure VNS design would extend some 45 inches from the north bearing. This could be accommodated by using a hybrid design where the south bearing is a circular section of the RA cone. However, when I also considered the added complexity of generating the geometry of the VNS sectors accurately I decided to go with Ed's concept. This is a hybrid of Gee-Poncet (for the south bearing, using skateboard bearings) and D'Autume (for the north bearing, possibly using small pillow blocks).

I also very much like Ed's idea of driving the table with a worm sector set-up under the table (see the section on his page about his "Worm Sector Drive").

I initially designed the RA board assuming a board weight of 8 pounds (as Ed does, for his CG calculation) but when I calculated an estimated weight of the result using the known density of 18mm Baltic Birch plywood, I came up with a total weight of 10.135 pounds. I also calculated the initial CG with nothing in the focuser. The weight of my 2" 24mm eyepiece is 1.6 pounds, and I'd like to add a DSLR at some point, so I am considering a redesign with the appropriate weight, but we'll see. It turns out adding weight to the OTA gets the COG closer to the initial designed COG height, so might go with the initial design. Who knows? I might want to add a guide scope at some point! I do want to keep the torque as low as possible, and I am leaning towards a re-design with 1.6 pounds added to the OTA.

Would appreciate any ideas you might have, especially if it comes from experience! This is my first time trying to make one of these. I'm also not sure what kind of motor I'm going to drive it with: AC synchronous, stepper, etc?

I attached a Fusion 360 drawing of the RA table using the assumed weight of 8 pounds. I tried to attach an Excel file that contains my COG calculation as well as calculated torque depending on added weight. The website wouldn't let me to I made it available here. The calculated weight of the platform is also in the spreadsheet, in case I want to change the COG a bit.

I am planning on picking up a sheet of 3/4" baltic birch plywood tomorrow and I have some pillow blocks and stainless shafts coming today that I might use for this project.

Equatorial_plate v22.f3d

Mr H in Yorkshire · September 22, 2023

If you expect to add components at various times, would it help/be too complicated to design to a theoretical maximum, add that weight physically to the moving base (as removeable part-weights) and then you have the flexibility to remove the nearest amount each time you do add a component.

Astronomist · September 22, 2023

I am also currently undertaking an equatorial platform project For my 10" F5. With regards to motors, the usual choice is to use an EQ1 or 2 RA Drive , which are speed adjustable, and very affordable. If you are looking for great accuracy however you must look towards stepper motors and drivers (not sure if onstep can cope with platforms or not). Good luck with your project!

Edited September 22, 2023 by Astronomist

dlavall · September 23, 2023

7 hours ago, Mr H in Yorkshire said:

If you expect to add components at various times, would it help/be too complicated to design to a theoretical maximum, add that weight physically to the moving base (as removeable part-weights) and then you have the flexibility to remove the nearest amount each time you do add a component.

That's a great idea. I decided to change the COG to balance with 1.6 pounds added to the OTA. That way the torque is acceptable from my smallest eyepieces up to about 2.5 pounds. I don't think I'll go much past that unless I add a guidescope. Like you said, If I do I can always add weight to the base of the rocker - it's easy enough to figure out how much is needed to eliminate the additional torque.

5 hours ago, Astronomist said:

I am also currently undertaking an equatorial platform project For my 10" F5. With regards to motors, the usual choice is to use an EQ1 or 2 RA Drive , which are speed adjustable, and very affordable. If you are looking for great accuracy however you must look towards stepper motors and drivers (not sure if onstep can cope with platforms or not). Good luck with your project!

Thanks! I'm sort of leaning towards a stepper, but I'm going to go with whatever I can match up best with off the self parts to start with. A lot is going to depend on what gear ratio I'll need with my sector-worm setup. I'd like to start with something simple and consistent so I can get a good idea how bad the periodic error ends up being.

Mr H in Yorkshire · September 23, 2023

6 hours ago, dlavall said:

Like you said, If I do I can always add weight to the base of the rocker

You may have understood my suggestion. What I had in mind was to add the maximum anticipated weight at the design/construction stage. The torque and power demands are then predictable. Then when observing, as you add or change a component (eyepiece whatever), add back or take away the same weight from the base. I picture the weight as thick slices of lead, say 500 grams each. Is that clearer, does it sound feasible?

dlavall · September 24, 2023

Thanks. II do understand what you're saying. My intention is to design it so that I have acceptible torque within the lower range where I'll be using the scope most of the time. Then, if/when I add heavier compononts (e.g. guidescope, etc) I can then add weight to the base. When I get a chance, I'm going to re-run the COG calculations using variables for weight added to the OTA and weight to compenate for it in the base.

I also decided that I'm going to use a stepper motor for the drive. They're reasonable expens-wise and I can use an arduino for control. I think I could get away with a nema 17 size motore, but I might move up to a nema 23 for the additional torque.

Mr H in Yorkshire · September 24, 2023

That all sounds logical. My son has a 12" Dob, I proposed to make him a platform but he was not particularly interested. I had planned to make the platform upper a replica of the actual supplied Dob base, thus the Alt sides and Az swivel would be integrated into the platform, onto which the scope could be placed.

dlavall · September 24, 2023

4 hours ago, Mr H in Yorkshire said:

I had planned to make the platform upper a replica of the actual supplied Dob base, thus the Alt sides and Az swivel would be integrated into the platform, onto which the scope could be placed.

That's a good idea! I updated my COG calculation file and uploaded it here. I included a new section that incorporates an added weight to the OTA as well as a way to enter an added weight to the base to balance it out. I based it on a average 0.86 pound weight (equal to my 2" 18mm) so it works with all my eyepieces without adding any weight, and I can calculate how much to add to the base in case I end up adding something heavier than 2 pounds.

Now I need to update my RA table again in Fusion 360.. Hopefully just one more time!

It might be overkill, but I might go with a nema 23 stepper to get the additional torque.

Mr H in Yorkshire · September 24, 2023

50 minutes ago, dlavall said:

It might be overkill

Engineers would call it safety margin. Think of the money and time to install a less powerful motor, discover it's limitations then install another one.

I just remembered why I thought of making the platform upper as a a Dob base - it lowers the COG of the whole mass. I never went any further so I don't know how significant that would be in terms of drive torque needs, I just thought it would be a bit more stable.

Edited September 24, 2023 by Mr H in Yorkshire

dlavall · September 25, 2023

21 hours ago, Mr H in Yorkshire said:

Engineers would call it safety margin. Think of the money and time to install a less powerful motor, discover it's limitations then install another one.

I just remembered why I thought of making the platform upper as a a Dob base - it lowers the COG of the whole mass. I never went any further so I don't know how significant that would be in terms of drive torque needs, I just thought it would be a bit more stable.

Yes, After looking at the load, I decided I didn't want to take any chances, so I am going with a NEMA 23 stepper. As for making the platform my Dob base, I'm planning on making 2 platforms, for two different latitudes, and they're for the same scope, otherwise I definitely would go that route.

dlavall · September 25, 2023

I updated my drawing for a CG that best suits the average of my eyepieces. It seems to do ok up to about two pounds being added to the OTA, and if I need to add something heavier,I can adjust the CG by adding weight at the bottom of my rocker.

I also made sure that the clearance for the pillow blocks for the north bearing have enough clearance to allow a 15° total travel. It actually gives me 19°, which gives me some room to play with.

Next step is to figure out the "height" of the worm sector from the bottom of the platform. I plan on having my stepper drive a 1/4-20 threaded shaft.

I think I'm going to get the stepper and associated accessories from Amazon:

Equatorial Platfowm - updated COG Rev2 v4.f3d

Edited September 25, 2023 by dlavall
Updated info

dlavall · September 29, 2023

I'm going to start cutting wood this weekend. I'm not sure how much I'll get done because of all the "must get done" chores that always have to get done on weekends.

I made progress on the design the past few nights. I decided to move forward with the following:

8mm Pillow blocks with stainless shafts for the north bearing (got these from Amazon for pretty cheap)
3 8mm skateboard bearings for the south bearing
NEMA 23 stepper with drive and Arduino nano for control (will order from Amazon)
I'm going to try to make a worm and sector for driving the platform like Ed explains on his web page

I attached the latest revision to my drawing in Fusion 360 (in archive format). I'm probably going to make the south bearing block a little wider for stability. I'm also adding a couple of rendered images for those who don't have Fusion 360. The pillow block assemblies with be on 1" blocks mounted to the base (not yet drawn).

Equatorial Platform Rev 4 v10.f3z

dlavall · October 4, 2023

Update: I received a number of parts that I needed for this projects and finally started cutting some parts for the RA (upper) platform. I finalized the design of that part to match the predicted COG calculated using the volume of the platform, bearings, etc. It took a couple of times to zero in on it because my first designs had more area for the upper platform than what was needed. I also had to account for enough room for the motor, and where I needed to put the drive sector to accommodate that. Darnit it though, I just realized I'm going to have to work out one more iteration of that design to account for the mass of the drive sector.

Here's a parts list for some of the things I already ordered and redeived:

Small (8mm ID) pillow block bearings - https://www.amazon.com/dp/B01MT1MODX?psc=1&ref=ppx_yo2ov_dt_b_product_details
8mm SS shafts - https://www.amazon.com/dp/B08HYJTR54?ref=ppx_yo2ov_dt_b_product_details&th=1
NEMA 23 Stepper motor - https://www.amazon.com/dp/B00PNEPW4C?psc=1&ref=ppx_yo2ov_dt_b_product_details
DM542T stepper motor drive - https://www.a mazon.com/dp/B06bearings and Y5VPSFN?psc=1&ref=ppx_yo2ov_dt_b_product_details
Stepper motor mounting bracket - https://www.amazon.com/dp/B00Q6GIO5K?psc=1&ref=ppx_yo2ov_dt_b_product_details
Some Arduino Nanos - https://www.amazon.com/dp/B07G99NNXL?psc=1&ref=ppx_yo2ov_dt_b_product_details

I still need some items to build my circuit for the motor control, but I have a breadboard I can use to prototype it in the mean-time. I was a little doubtful on the quality of the bearings and the SS shafts, but I was pleasantly surprised! The shafts are round within 10 microns the entire length! I was worried they could add too much error on top of any PE I end up with. Using Fusion 360 for the design of this project has been great. I've been "learning while doing" but the cool thing is that I was able to use it to rotate the RA table in each direction to make sure I maintained clearance during an entire tracking session. So far I have 11.5° each way for a total of 23°. This will allow me to ensure I have enough travel to set up proper limit switches for stopping and recycling the tracking.

I also need to figure out how I'm going to mount the motor and lead screw (SS 1/4-20 threaded rod). It needs to be in an assembly that supports the lead screw on both ends, and I want it to be able to engage/disengage from the Delrin sector. I'm attaching the latest Fusion 360 drawing as well as some captured renders for those who don't have Fusion. Note that I still need to take a little off the corners of the south bearing block so they don't interfere with the travel of +/- 11.5°.

Any and all suggestions are appreciated!

Equatorial Platform Rev 5 v3.f3z Equatorial Platform Rev 5 - rolled west 11.5 v1.f3z Equatorial Platform Rev 5 - rolled east 11.5 v1.f3z