Homemade Equatorial Mount

Homemade Equatorial Mount

posted in: Homemade Stuff | 2

Take a look at how I designed and built a homemade equatorial mount!

Welcome to my homemade equatorial mount blog!  For a long time now I have wanted to share this epic journey with the Astronomy community, so finally I’m pleased to be able to say “here it is!”

Hopefully you will find this article interesting and inspiring.  So find a comfy chair and I hope you enjoy!

Some of the links on this page may be affiliate links where I make a small amount of money from any purchases you may make after clicking one of those links.  You won’t be paying any extra for any purchases you make as the prices all stay the same.  This is only to earn a few pennies to help go towards the fees to keep this site up and running.

A number of years ago, as you may have already read on my homemade telescope blog, I built my own 10 inch Dobsonian telescope.  While carrying out all the research at the time before I built it, I realised that one day I would eventually want to mount this scope onto something that would enable me to do some imaging.

I looked into all the options, including a platform or ‘Poncet platform’ to just sit the whole Dob assembly on.  Basically for those who don’t already know, this is a low level table which is driven by a small motor and gearbox arrangement.  Then after the Dob has gone through a polar alignment process, it would track the sky accurately.  This option looked pretty favourable for a while as it would have certainly been more straightforward to make, but eventually after reading further I realised that platforms were not the way I wanted to go.  The main reason was because I also wanted the option to mount other scopes I may acquire later on in my Astronomy career.  Therefore I decided on the most popular and most reliable mount design, the German Equatorial Mount!

I thought I could design a good heavy duty mount by taking advantage of all the resources I had to hand at the time.  It would have to be real heavy duty to carry the scope I built which was knocking on the door of around 25kg!  I knew the mount would need to be big n heavy, so I had to ensure it was designed in a way that it could be broken down into manageable sections I could easily lift and transport when required.  Also it would not be a permanent feature in the back yard so it would need to be stored away when not in use.

The company I worked for at the time had a joinery shop and a basic metalwork shop.  Even though I had the luxury of these facilities available to me, they were limited to pretty basic engineering processes, for example they had no milling machines or lathes worth mentioning, so part of the challenge was to come up with a design that would not need too many machined parts which I couldn’t have made in the factory.

I quite liked the idea of making it out of plywood and steel, as I could then easily have these kind of parts made in the factory at work during the slack times.  I knew I could probably get things like lazercut metal parts and a small amount of turned part made as favours from a couple of friendly suppliers we used, but obviously this would need to be kept to a minimum as favours would only go so far.

My main objectives were as follows:

  • To end up with a mount that could be made using the resources to hand.
  • Not to have to spend a small fortune on any of the parts for it.
  • Would be rock solid/heavy duty, but can be dismantled into manageable parts for storage and transportation.
  • Work well enough to eventually get some decent Astro images out of it.

This last requirement was the exciting part for me really, as I didn’t really have clue as to how well it would end up working.  I was always going to be restricted by the resources open to me, so the only thing I could do was ensure measures were taken to maximise my chances within the design and production of all the parts.

Click to enlarge image

I started off with just sketching out ideas to get an idea of what sort of height/size it would need to be to ensure the large scope could easily be manoeuvred to point around all parts of the sky.  Here’s a sketch of one of the first stages.  Once I was happy with the general proportions I started to think about how each of the parts would be constructed. The only real advice I took notice of from the web, was to make it beefy and strong to ensure minimum flexure. I decided to go for a 2” solid steel bar for both of the axis shafts. Probably over kill, they would never bend, even if an elephant sat on the mount! Then I had to ensure they were being held within a construction man enough not to take away the advantage of using such strong and heavy shafts. I came up with a simple design where I could support the bearings without any movement. Basically a simple mdf box construction with loads of support web plates inside, removing any potential twisting etc. The bearings I decided to use where the type that would not have any end or side shift. The type that are adjusted to take out all movement. These are the “Angular contact ball bearing” type. This type of bearing will take both axial and radial loads. I used these at both ends of both shafts arranged in a way that when they were pushed together with a pressure plate at both ends, all the end float and side shift was removed whilst still having a smooth riding shaft. I did have to play around with adjusting the bearing support plates, as they needed to be accurately aligned to enable the bearings to be perfectly square to each other, otherwise they would not be smooth running. Once they were all set up correctly, the result was impressive, zero radial and axial movement while still having a smooth rotation! Just as a side note, the shaft was ground precisely to size to the specified size for the particular bearings used. This was probably one of the most expensive things I had to pay for, as our factory didn’t have the machine for this, so I had to PAY someone real money to do this!!

The rest of the parts of the mount were designed in a way to also accommodate a sturdy structure, also using a mixture of ply wood and steel.  All of the ply sections used to build the mount had been multiple layers bonded together in the board press.  The thickest parts were the sections that slid over the ends of the shafts to connect either the DEC assembly or the telescope.  The reason for this was to ensure a sturdy 90 degree fixing point, as this is critical when connecting the DEC axis to the RA axis.  Before the holes were bored, the plate was clamped to the machine bed, then the top surface was skimmed level.  Once this was done, the plate was turned around so the levelled surface was sitting onto the machine bed, then the new top surface was skimmed level.  Finally, the hole was bored into the centre.  Before this could happen, the machine was double checked to ensure the hole would be bored exactly at 90 degrees to the surface.  The hole-size was machined as an interference fit to the shaft to ensure zero clearance and movement when fitted.  The following are just a few examples of the drawings I did and photos of some of the components during the assembly process.

You can see in the photos that there are three struts linking the legs to the vertical column.  I decided to design these so they are tightened in a way so they push rather than pull, like many commercial mounts I have seen using cables.  To me it seemed both an easier and more sensible solution to suit the leg design I went with.

Here’s a slide show of a few images taken in the various stages of assembly which takes approx 5 minutes to put together.

The altitude and azimuth adjustments are very simple.  The Azimuth bearing design is just a machined section of ply with a textured laminate bonded to the underside which just sits directly onto the powder coated steel plate at the top of the column.  I originally planned to use PTFE blocks for the laminated surface to glide on, but after trying without, it worked fine.  Plus once everything was aligned and tightened up, there would be no additional movement as both surfaces were solid and flat.  The altitude adjustment is done by simply winding a large hand wheel in or out of the steel support frame using a spanner, then tightening the hand wheels on the side of the mount.   The drawings and photos show all the mechanical components, and I guess you’re thinking “what about the drive system?”  I wanted to go for something that I could have made using the resources and materials around me at minimal cost.  I initially looked at a worm wheel drive design, but quickly realised this would work out too expensive.  I then thought about a friction drive.  The issue here was that there would need to be a number of accurately machined and mounted wheels involved to step the drive speed down to the desired level. Meaning a high risk of drive errors due to more moving parts involved.  In the end I decided to opt for a mixture of the two to both keep costs down and limit any potential errors in the drive system.  I chose a 500:1 gear box which would drive a 2” wheel, which in turn would drive a 10” wheel.  The 500:1 gear box is driven by a stepper motor.  At the time of designing the mount, I had no clue as to even how a stepper motor worked and had no knowledge of electronics at all!  I was lucky enough to meet a now very good friend called Tim Duke through the Castle Point Astronomy Club I had then been a member of since I build the Dobsonian.  He was very interested in helping me develop the drive system and get everything working.  Basically……I designed all the mechanical elements of the mount and Tim concentrated on all the electronic bits.  Originally Tim designed and built a box of tricks that just attached to the mount which would send the correct commands to the stepper motors with a choice of two speeds (lunar and Celestial mode).  Also there was a hand controller which had two speeds.  One for aligning objects into the field of view and another to manually make corrections when imaging.  Over the years, I have made a number of modifications to the mount, mainly mechanically, but also Tim has vastly improved the electronics.  I will go through a few of them next:

Originally I designed the mount to have a magnetic slip clutch so I could push both axis around while the RA motor was still driving it.  This eventually would enable me to add either encoders or setting circles and once they were set up would always be calibrated without having to re set when moving from one object to another.  This way of thinking soon changed when I started to have issues.  One night I was manually guiding on an object while imaging when suddenly the star I was guiding on suddenly shot out of sight!  This was because the slightly unbalanced axis outweighed the amount of grip the magnets had which was much reduced by the metal clutch plates getting wet and slippery in the heavy dewy conditions!  I tried adding more powerful magnets, but this then made the load too high when manually pushing, which then gave me problems with the friction drive slipping!!  I then decided to go down the route of fixing the shaft directly to the drive wheel, removing the magnets all together, this also eliminated any clearances that were originally needed for the large drive wheel to rotate around the drive shaft, which was a plus.  The only thing I then needed to do was work out a way in which to disengage the drive wheel from the driven wheel on the friction drive, as now the system would be rigidly fixed.  So I introduced a sprung loaded manually controlled device so I could have a completely free running Dec and RA axis for positioning etc.  This also enabled me to balance the weights of the scope and camera set up more accurately too, which was another plus!

I played around with a couple of different designs for the friction wheel surfaces to get the best grip.  Originally I used a car timing belt forced onto the 10” wheel against the metal 2” wheel surface.  This caused two major issues.  One was the amount of bounce magnified to the end of the scope due the rubber material of the belt and the other was the fact the 2” drive wheel would push quite a way into the rubber thickness producing indentations which would cause variations in the drive speeds.  Obviously this was not going to be anywhere near good enough for imaging, so I changed over to gluing very fine wet and dry paper to both the 10” and 2” wheels.  This gave good gripping properties whilst also maintaining a hard surface.  This took out a large amount of the movement, but not all of it!  This was all down to the 500:1 gearbox, as the backlash was quite high in the particular gearboxes I used.  Currently I am getting round this issue by overbalancing the axis, so the motor is always driving in one direction and not having to reverse and bring the backlash into play.  The electronics have been set up in a way that when the RA is tracking too fast, the motor will just stop for a split second rather than reverse.  Having to correct in the other direction would just mean speeding the motor up, which isn’t an issue as the gearbox is already wound against any backlash.  The issue is the DEC, as this motor is not continually being driven like the RA motor is, there could be situations where the motor will need to be reversed to correct in the opposite direction.  I have been experimenting with setting the mount up so the DEC always needs correcting in one direction so I can balance the DEC axis in the same way as the RA so the motor is always winding the gearbox up under load and not having to reverse.

The most recent change and the most significant one of all is enabling auto guiding!!  This (as all you imagers out there will well know is critical if you want to capture exposure times of 10, 20 minutes etc).  A little while ago I purchased a great bit of kit called the GPUSB from the Shoestring Astronomy Store.  This is a bit of hardware that plugs into both the laptop and the hand pad controller port on the mount.  The idea is that once a guide camera is connected to the laptop, any drift of a guide star within the guide software running on the laptop will then be converted by the GPUSB and sent to the mount as a command to correct either the DEC or RA motor one way or the other.  Tim has now completely re vamped the electronics hardware to include the following:

  • GPUSB interface integrated
  • Lunar speed and Celestial speed
  • Two speed motor manipulation via hand pad (one for auto correcting and another for framing objects)
  • Multiple USB ports for connecting the imaging camera/guide camera/spare port for the future it required.
  • Set of dip switches to give fine adjustment to RA motor speed
  • One USB port connecting to laptop to carry two way signals from the guiding camera to the laptop and from the laptop to the mount.  Also to send and receive data between the imaging camera and the laptop.
M31
Click to enlarge

I’m at a point where I’m pretty happy with the auto guiding accuracy, but still need to run more tests to see where I can make any improvements.  As you can see in this image (the very first image I took since auto guiding), the stars are staying pretty round.  The exposure times where 10 minutes long.  zooming into the image shows a very slight ovalling of the stars at the centre of the image.  There is significant ovalling of stars from the corners towards the centre of the image.  I am in need of a field flattener!!  The current guiding accuracy is not too bad if only working with fairly short focal lengths, the image below was taken using a 525mm focal length scope, but anything longer than that will need an improvement in the guiding.  It’s still early days, in fact I have only used the auto guiding over two nights so far!

I’m planning to add more posts detailing exactly the process I use to get everything set up and ready for imaging, so watch this space!

I’m currently looking for a decent waterproof cover so I can leave it in the yard for a few weeks at a time. This is proving to be more difficult than expected, as I can’t find anything suitable for the size and shape. I’m probably going to buy some waterproof fabric and have it machined up in the local dressmakers. The ultimate goal is to build an observatory in the yard with a slide off roof! Got to get a load of DIY sorted in the house before I can even think of this project, unless I want to end up living in the garden!

Thanks for reading through my article and hope you enjoyed it!

2 Responses

  1. Excellent work! Always great to see clean and well made equipment! A real art!