Telescope Equatorial Mount part 1

I picked up a Celestron 4″ (102mm) refractor telescope optical tube assembly that was being clearanced out at OPT Telescopes.  This is my first refractor other than one my dad and I used when I was younger.  Having just the tube does me little good though without a way to hold it.  In a nut shell there are two kinds of telescope mounts: Altazimuth and Equatorial.  An Altazimuth mount moves in altitude, up and down from the ground, and azimuth, left and right along the ground. Following an object that appears to move through the sky due to the earth’s rotation requires you to move the scope on both axes.  In other words, you might be moving the scope up and over to track something.   An Equatorial mount moves in right ascension, along the East West direction on the celestial sphere, and declination, along the North South direction on the celestial sphere.   An equatorial mount only requires the scope to move about one axis to track an object.  A small step from here is to put a slow motor on one axis to track an object automatically.  The equatorial mount still rotates about two axes but one is aligned with the celestial north pole.

Here’s the scope that I picked up.  The lens in it is 4″ in diameter and it is about 39″ long to give you a sense of size.  I added the finder scope (the mini telescope on top)  and diagonal (the triangle shaped thing on the left end of the scope) to it.   Celestron used the tiny dovetail (black rectangle in the center) to mount the scope but it seems small and I’m not a fan of cantilever mounted scopes for vibrational issues.


I’m going to break the mount down into two pieces: the head and tripod.  The head will be the rotating bit that holds the scope while the tripod will hold all of that up.  This post will cover the equatorial head.  I’m going to use iron pipe to make the rotating parts of my mount.  This has been a popular way of mounting scopes for decades with amateur astronomers.  It’s cheap, easy, and pretty simple.  It should also make my dad smile as it is a unique use of water pipe!  Check out this site for an article and diagram of a pipe mount: Lake County Astro.  The pipe mount can be used for altazimuth and equatorial mounts.  It just depends on how you hold it up.

Seen in the picture below is the assortment of pipes that’ll make up my mount.  There are three pipe nipples, short length of pipe threaded at both ends, a flange, T, and reducer.  The larger pipe is 1-1/4″ in diameter and the smaller is 3/4″.  The threads on the pipe are the bearings which allow the mount to move.  The largest pipe will point at the celestial north pole and allow the scope to rotate in right ascension.  The 2″ nipple between the flange and T allows the scope to rotate in declination.  The smaller length of pipe will support counter weights used to balance the scope.


Once the scope is balanced with counter weight it won’t move on its own.  Still, I’d like to have a way of adding friction to an axis if needed or a way to lock the scope in position.  To do this, I’ll use a couple nylon bolts to put pressure on the rotating threads of the pipe.  The first step is to mark off the location where the nylon bolts will go and center punch them.  Next, using the appropriate tap drill for a 1/4-20 thread, two holes are drilled into the T.


Both these holes have threads cut into them with tapping fluid and a thread tap.  Care must be exercised to hold the tap vertical to the surface so that the bolt isn’t crooked.


With the pipe portion of the head completed, I turned my attention to how the scope should be held on the mount.  A pair of rings are typically used which I set about fabricating.  I measured the diameter of the scope tube and then added a little extra to account for felt that I’ll use between the rings and scope tube.  I laid out the rings on a piece of plywood with a compass and set about cutting on the tablesaw.


Next, the rings were split so they can fit around the scope tube.  The pieces were marked to avoid mixing the pieces up.


To clamp the rings to the scope tube I plan to use a couple 1/4″ diameter bolts.  It’s much easier to drill straight while the pieces are still square.  It’s absolutely critical that the table is square to your drill press quill during the drilling to avoid the bit coming out off center on the bottom.  I don’t have a drill bit that is long enough to drill both halves at once so I drilled the top first. (Aside:  A super long bit would probably flex and wander during drilling anyways.)  Drilling into the side of plywood can be troublesome because the bit may wander due to the layers of wood.  Using a center punch to mark the spot you want to drill will help the bit stay where you want it to.


I then clamped both halves together and used the top half as a guide for the drill bit.  Once shallow holes were drilled in the lower half, the pieces were taken apart and the lower half was drilled out.


The bandsaw makes quick work of cutting out the rings.


Over to the bench drill press with a spindle sander to clean up the inside of the rings.  Yes, I need an oscillating spindle sander.


The mini-belt sander is used to clean up the outside of the rings.  The cut outs in the rings allow the bolt heads and washers to sit flat on the surface.


A base is needed to connect the rings to the pipe flange.  I used another piece of plywood 12″ long for this.  The lower rings halves and a center transfer punch were used to locate the holes for the rings.  The flange was used as a template to lay out the holes for it.  Off to the drill press again…which I will spare you pics of.


Since I want my mount to be equatorial, one of the axes must be aligned with the celestial north pole.  To do this I need a wedge that will hold it at the proper angle to the ground.  The correct angle is the local latitude. For me this is 30 degrees.  Luckily, 30 degrees is a common angle used in woodworking so most of the saws and guides have it already marked out.  I came up with a simple design for my wedge and set about cutting it out on the table saw.  In the picture below, I’m about to turn the rectangular piece into another side of my wedge like the one on the right.  The rest of the wedge consisted of two more rectangular pieces 1-1/4″ thick.  The pieces were clamped, drilled, and bolted together.


Now I’m ready to start putting everything together.  The lower halves of the rings were attached to the plate using a couple screws.  I didn’t glue them together though because I may want to use another set of rings one day.  The flange was attached with more 1/4″ bolts.


Shown below is the equatorial head in its current form.  The mount is connected to the wedge with a couple of pipe straps…bolted down of course.  I couldn’t find any nylon bolts and instead got some nylon machine screws.  They work pretty well.  I can add friction to the system and hold the pieces in place from moving on their own.  I can overpower them pretty easily which was expected.  All of the pipes were tightened down except the threads near the nylon bolts so they can turn.  I have about four complete revolutions before the parts fall apart so I don’t have any fears of it falling apart in use.  I do plan to do some more sanding and finish the parts but want to try it out before I do so I can make changes if need be.


Up next I’ll post about the tripod that I’m in the process of making.  So far it’s looking pretty good.

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