Sunday, June 12, 2022

Making an Equatorial Telescope Mount

I have been toying with this idea for several reasons:

1. I would like a very portable equatorial mount that is not Chinese and not several thousand dollars.  The Losmandy GM8 is a fine mount but still pretty heavy and pricy for something that needs to hold a 10 pound scope.  The other non-PRC choices are Takahashi and Astro-Physics, both top notch and priced accordingly; not very portable either.

2. It is an interesting design and machining project.  To quote from a Brit who rolled his own:

"On the downside, everybody will know you are completely mad. There is a depressing number of modern astrophotographers who look upon DIY and custom modifications as a bit odd and think everything should just work out of the box like a washing machine. These people tend to buy lots of equipment, complain loudly when it doesn't work and produce mediocre results at best. We don't like these people much."

I would likely look for a really simple solution that drives stepper motors for both axes.  If I get in deep enough I might go through the gearing pain of a traditional (1960s) worm gear driven RA drive, where you gear down 3600 rpm to 1 rev/day.

This is mostly notes for myself and a chance to watch my brain at demented work.

I have built a basic German equatorial mount before using pillow blocks mounted on 1/4" x 1" plywood and 1/2" shafts.  Everything is exposed and looks ugly.  I can buy 1/2" bore roller bearings that are 1 1/8" OD.  Slide them perhaps with some pressure into the two ends of a 1 1/8" ID 1/4" wall aluminum tube.
The shafts are obviously stainless steel...or are they?  If aluminum shafts were available they would reduce weight and ease drilling and tapping holes; the loads involved are not enormous. I could buy aluminum rod and turn it down on the lathe until I could use a heat gun on the bearings and dry ice on the shafts to get a press fit.

Secure the bearings in place with bolts into very shallow tapped holes in the outer wall of the bearing or perhaps just held in by clamping pressure.  1/2" shaft through the bearings.   

Machine circles that press into the ends of the tube over the shaft.  Leave enough margin around the shaft to on occasion add some lubricating oil down the shaft.

Where the north end of the polar shaft meets the declination axis tube, drill a hole through one side of the declination axis tube that allows the polar axis to meet a plug on the other side where four bolts screw into the end of the polar axis.

The construction of the RA and declination axis assemblies is nearly identical.   The "bottom" cover in both cases is thicker to support the worm gear or stepper motor attachment.   The tolerance between shaft and hole is much finer to reduce oil leakage.   It will still be occasionally necessary to remove the lower cover to remove any oil build-up.

A lesson learned from the Cave Optical mount, gone 15 years now, is a Teflon washer between the declination axis and the cover at top of polar axis housing.

The saddle uses a similar design to the declination axis attachment.   The declination axis goes into a blind hole with four bolt holes in the saddle to bind the saddle to the axis's tapped holes.  The saddle can be a standard Vixen style saddle or perhaps make my own.  It is a pretty simple piece of work.

Counterweights: I am not sure if there are barbell weights with 1/2" bore but if not drilling 1/2" hole through cast iron cylinders of 3" length is simple enough.  The locking thumbscrew is the hardest part; you need a 1/4"-20 thumbscrew that is at least 1.5" long to screw into a tapped hole in the side of the counterweight.   Stainless steel counterweights might be nicer from an appearance standpoint and stainless steel is slightly denser than cast iron.  Drilling stainless steel is no fun..

Obviously all the aluminum tubes and axes will be polished down to 1000 grit both for appearance and to go into the bearings without too much force.

The polar axis housing needs to mate to a cap that goes on a tripod or column.
Traditional technique is a tab on the bottom of the housing that fits between two "ears" just slightly wider than the tab.  A bolt hole through the left ear and the hole in the tab and the right ear is a tapped hole.  A bolt through the left ear hole and tab hole tightens down to prevent unwanted polar axis movement.   (The polar axis points at celestial north, generally Polaris for my lifetime.)  

On the north side of the ears is a fence with a tapped hole.  A thumbscrew (and a pretty big one at that) pushes the tab when the bolt is not locking the ears onto the tab until you get the desired latitude.   (The tab has to be an interior cylinder so I guess that program needs to work.) Alternatively, I machine the tab with a 90 degree angle in the top so that bolts through the legs go into tapped holes in the polar axis housing at the tangents.  Either way, I would probably use four 1/4"-20 bolts in tapped holes to hold the tab to the polar axis housing. 

The tab needs degree markings which means use of the rotating table to mark one degree marks.  I am not sure that I can engrave readable numbers next to the lines, but I can make the 10 degree and five degree lines different lengths and perhaps put three, four, and five depressions next to 30, 40, and 50 degree lines.

The cap is 4.5" diameter with the bottom 0.5" turned down to 4" so it slides into a 4" ID 1/4" wall tube.  There are three 1/4"-20 tapped holes in the 4" diameter part of the cap.  

The tube is 40" long with three 1/4" holes around the top in which thumbscrews insert to lock into the tapped holes in the bottom of the end cap.  A dust cap is bolted into the bottom of the column to keep spiders and other creepy crawlies out.  

The legs are an interesting problem.  The simple solution is use 36" long 1/2" wide rectangles held into the interior of the tube just below the dust cover.  I tap two 1/4"-20 holes in the end of each leg and 1/4" holes in the bottom of the column. Thumbscrews hold the legs in place while allowing quick disassembly for trunk transport.  (This is another Cave Optical design.)  Round lightening holes in the legs also give it a 1950s spaceship legs look.  

I think the entire mount, column, and legs could be 15 to 20 pounds.  (You take the counterweights and OTA off to move it.)

I probably want the CNC rotating table before I make this; my inner cylinder excavating program; and a lot of detailed drawings before I start.

Yes I am enjoying this back of the envelope designing way more than drawing it all out and machining it will be.

As should be obvious from the time, I cannot fall asleep without thinking about this.  The declination axis carries the counterweight but most modern mounts make the counterweight shaft non-integral with the axis so you can reduce the transport size of the mount.  At the point where declination mount exits the housing drill and tap a 3/8"-16 hole.  The counterweight shaft gets threaded 3/8"-16 male.   You can now unscrew the counterweight shaft.  You can also adjust the length for whatever OTA you are using.  You could carried away and make a two or three part counterweight shaft, each threaded into the next, depending on how many counterweights you need.

Setting circles are needed if this does not end up with digital controls.  This is one of pet peeves.  Traditional mount setting circles are coarse: 1 degree markings in declination: 5 minutes in right ascension.   

The setting circles will be cut from six inch diameter aluminum discs, 2" thick.  Once mounted in a CNC rotating table, the program uses a center drill to cut short lines every .5 degrees and every 4 minutes.  Longer lines at 5 degrees and 20 minutes.  At hour and 10 degree lines (even longer) the center drill scratches numbers.  There is software to cut in whatever font you want but it is not good for really small fonts so use right angle numbers sort of like old Nixie tube displays.  0 is square, 1 is a single line, 2 is a straight line then a diagonal then another straight line.  Pretty? No but plenty readable.  

These are a loose fit on both axes with a .5" extension in which a set screw is placed.  The declination set screw is locked down because this setting circle does not ordinarily move.  The right ascension set screw is very lightly touching perhaps with a small piece of Teflon between set screw and axis to allow drag but also ready rotation with slight effort.  (Another Cave Optical design.)

Lots of good suggestions in the comments. Tubes for the axes give most of the stiffness of a solid cylinder and save some weight.  Of course with 1/2" OD, you need a pretty thick wall to avoid deformation by the counterweight clamps.  I think 1/8" wall would be sufficient but this saves very little weight.  If it does not seem much of a gain go to 1/2" solid.  For larger axes such as 1" OD, a 1/4" wall is much stiffer and a bigger weight reduction but this seems bigger than needed for a tiny mount.

Another possibility: stainless tube 1/2" OD 1/16" wall.  I believe this would be lighter than 1/2" solid and pretty darn rigid.

Because the declination axis is only going to the end of the housing, the counterweight shaft can be 1" OD to allow use of 1" ID barbell weights.   They still need to be tapped for locking screws but they do not need boring.  They might not be as pretty as stainless steel.


  1. If you do a continuous worm gear drive on the RA axis, consider using a simple clutch to engage / disengage the drive from the shaft (even something as simple as a driven collar with a set screw).

    This could let you 'slip' the RA axis to align with a star, then simply re-engage the drive to track...

  2. why not use thick-walled aluminum tube for the axes as well?
    Save 3-40% with no loss of strength The center does little for the rigidity.

    Also, put a decent spring load somewhere to prevent lash.

  3. I suggest you consider something other than drilling/tapping a bearing race. One, they tend to be much thinner than you would think from looking at the externals, and they are hardened. I'm not sure drills or taps would survive the attempt. Perhaps a clamp around the bearing (split housing), or a snug fit with some variation of loktite to fixture it.

    One potential mistake designers make is using too course a thread in aluminum.
    Also, steel only needs about one diameter of depth when threading, but aluminum should have at least 1.5, and preferably 2.0 dia depth. Sometimes a jam nut on the other side of the threaded part is sufficient to add strength when it can't be thicker.

    Set screws can be found with all manner of tips: nylon, brass, teflon, steel ball (some with spring), toothed, flat, radiused, and more. They can also have plastic type side inserts to resist position changes.

    1. Good idea. Three set screws to hold it in place. I picked 1/4" wall because that gives 5 turns.

  4. For oil leak control, consider using o-rings or actual oil seals. They both come in standardized dimensions, and there is an o-ring kit that uses rolls of material that you cut to length and glue the ends together.
    Along with that, there is oil absorbent pads or batting that could be wrapped around the shaft to suck up loose oil.
    I rebuilt a Porsche heater-A/C fan that used that to keep the bushings lubed in the electric motor. OEM design for many decades! (a new assembly was ~$1000 8-)

    1. Good suggestions. I see self-lubricating bearing as well.