Do you have back and forth rock in your rack and pinion focuser? I did and I fixed it with about $15 worth of parts. The illustrated story, below. At the bottom I have a parts list and step-by-step instructions if you want to do this on your own.
My Celestron C6-NGT is not a big light bucket but it is reasonably portable and that means I can plant my eyeball on her at a moment’s notice. I don’t need to invest a whole evening to enjoy a cup or two of coffee while gazing; I just drag the gear out to the back patio and viola, I’m entertained! The simple design of this Newtonian with a rack and pinion focuser makes the gear accessible, unintimidating and a big hit at parties. At our annual Christmas blowout in 2011 we had a line of people 10 deep for 4 hours waiting to check out Jupiter. For some, this was their first time looking through a “real” telescope.
While my scope gets a lot of use, Celestron did not count on me abusing this equipment when they designed the focuser. After about 10 sessions connecting a digital camera to my scope I began to notice rocker in the focuser draw tube. I must admit that I hung the afocal adapter off my Baader-Hyperion glass hand grenades, wrecking this focuser pretty good, as you will see below. I could calm the movement somewhat by clamping down on the focuser lock but that created more problems. First, it only stopped the movement in the N/S direction. E/W was still full of play unless I really tightened the lock like an animal. Second, this worsening condition effectively meant I could not collimate the scope. There was enough play in the assembly that the centerline of the draw tube moved each and every time I turned the knob. When fully extended the eyepiece was wiggling at the top almost an eighth of an inch from side to side! After 3 nights of blurry vision I decided to fix the focuser myself. Let me take you through the process. If your rack and pinion focuser is anything like mine you should be able to breathe new life into your eye for the sky.
Figure 1: The Culprit, two screws removed
Figure 1 shows the focuser almost fully extended and with 2 of 4 screws holding the rack and pinion assembly removed. The silver piece is the draw tube with the rack (the gear teeth screwed to the tube). To remove the draw tube I need to remove those last 2 screws and the knob shaft with the pinion gear attached. That will allow the draw tube to slide out completely. Before we do that, let’s take a peek at just how much play this draw tube has inside the housing. I wouldn’t want you to think I was exaggerating:
Figure 2: .020″ Shims fit loosely in the gap, but there is even more gap below the housing rim
My concern at this point is growing exponentially. This is a serious problem and I am worried there is just too much damage I cannot see to attempt a repair. The aluminum tubing shows no sign of wear from the outside and the gap at the top of the assembly is a mirage. The “lip” at the top of the housing is about 1/16″ deep and thicker than the rest, meaning the gap is actually wider below the surface than what you can see here. So how the heck did Celestron fit these parts snugly at the factory? To find out I removed the remaining screws, then carefully separated and placed all of the parts in a dust free container. The tricky part was avoiding the thick grease Celestron uses on the gears to minimize slop. It sticks to everything and the grease needs to remain clean.
Now for a look at the inside of the housing:
Figure 3: Focuser Housing is Cast Aluminum with 2 Nylon Shims- Only One Visible Here
Bottoms up! Two nylon shims are employed to keep the draw tube centered in the focuser housing. The other one is opposite the shim you can see. A closer look reveals the extent of the problem:
The nylon shims have raised ridges that are nearly worn flat and the bottom of the housing is showing damage where the draw tube is moving laterally in the assembly. This is NOT COOL but it does explain the problem. The solution, however, still eluded me. Understanding WHY Celestron might choose two cheap nylon shims to center the draw tube helped me craft a fix. Ease of assembly and low material costs are only part of the equation. Friction is the enemy Celestron is up against. The draw tube is currently too loose in the housing so I want to tighten the works by reducing the gap between housing and draw tube. But…if there is no tolerance between the draw tube and housing the focuser will be impossibly tight and any material expansion or contraction (heating/cooling) could seize the mechanics completely. These nylon shims have ridges to minimize contact with the draw tube wall, thereby limiting friction, but they are sized thick enough to keep the draw tube reasonably centered in the housing. The problem is, I’ve worn down those ridges and begun to tear into the housing wall.
The solution has to balance smooth operation while minimizing play. Ordering replacement shims was a non-starter; they have already failed once. Why set myself up for failure again with a half-solution?
To find a permanent solution using the proper materials required a call to an expert. I have a buddy, Chris Larsen, who designs spas for these guys. Chris took Mr. McGuire’s exhortations in “The Graduate” seriously when it comes to plastics; the man breathes polymers and can wax poetic about PETE. It took him 5 seconds to solve this one-
Chris: I have one word for you. (ha!) Teflon.
Me: Huh? I’m not making eggs here, we’re rebuilding a telescope focuser.
Chris: Consider making eggs on your scope a side benefit. Seriously, get some Teflon/PTFE and then show me some stars. It’s an incredibly durable substance, has a very low coefficient of friction (nothing sticks to it, right?) and has nearly zero deformation with wild temperature swings. All that, and you can buy it in sheets at almost any thickness you need with an adhesive-ready side.
Figure 4: Teflon/PTFE Sheet, Front
Figure 5: Teflon/PTFE Sheet, Adhesive-Ready Back
I ordered a 12″*12″ sheet of 0.020″ thick Teflon. A quick check with the calipers reveals the material is spot on and consistent. After tax and shipping I was all in for under 20 bucks. So far, this is looking better than spending $300 on a new focuser for a telescope that runs about $150. Let’s get fixin’!
First, I need to remove those nylon shims and rough up the inside of the housing, particularly where the shims once lived. This prepares the surface for glue and insures no bumps on the surface. I did not want to take any material from the housing as the wall thickness needs to remain consistent; I needed to be careful here. My Dremel wire brush on low speed and some hand sanding with steel wool did the trick nicely. Here’s the result:
Figure 6: Focuser Housing, Prepped for Repair
Before I started sanding I masked off all of the channels and grooves with 3M Long-Mask tape. This is important as dust must NOT get embedded in the grease used on the gearing. Long-mask tape is designed to pull off without leaving any adhesive behind, even after sitting for a few months. I finished the roughing, cleaned out the dust and removed the tape. Now the fun begins. I cut a piece of the Teflon roughly to size for a test fit and to practice negotiating it into place. Once glue is in the works your fingers may not be nimble enough to position the material on the fly. Practice was definitely helpful here.
Figure 7: Teflon Inside Housing for Test-Fit
Notice the sizing of the Teflon sheet is not perfect. It is a little big, so it covers the interior completely but it is still small enough to avoid overlapping ends. Pi times diameter is your friend. I made a mistake in the first cut but I had enough material in the 12″*12″ sheet to cover this housing 5 times over. If you make a mistake, just cut another piece. Since the Teflon is roughly the right size, I pulled it out and applied spray glue to the adhesive-ready side of the Teflon and negotiated it into position. This was easy, really. I just rolled up the Teflon so the ends overlapped a little and was able to slide it in without contacting the housing walls. Once fully inserted, I popped the Teflon open with my fingers and pressed it against the housing wall. Don’t worry about glue on the Teflon side, we’ll clean that later.
Now we need to work fast for about 10 minutes.
The draw tube will not fit at this stage because we still need to trim the Teflon to the contours of the housing. Note there are two major grooves in the housing. The one on the left in these pictures is the slot for the rack and pinion gears. The dado on the right is for our focuser lock. The first cut needs to be on the left, to accommodate the rack. I whipped out the Exacto and trimmed just enough Teflon to allow the rack to pass through. For safety, I then inserted the draw tube to both test the fit and press the Teflon evenly around the housing while the glue firmed up. I let it set up for half an hour while I brewed and enjoyed some coffee (Colombian Supremo, fine grind), then came back to finish the trimming.
Figure 8: Teflon Glued In, Trimming 75% Complete
Figure 8 shows the Teflon about 75% trimmed in the housing. The loose bits on the left are all that remain. Negotiating the razor knife inside the housing was no trouble at all. I have special blades for wacky angles but I did not need them. Remember, Celestron used two half-inch wide nylon shims here so cutting away more material on accident is not a problem at all; you will still have 15 times more shim surface than your focuser started with. Also notice that my Teflon is now in two pieces. Finished, it looks like Celestron designed it that way. If only.
One last test fit with the draw tube tells the whole story. Without the pinion gear and spring to press on the rack and tube wall, the draw tube gently slides through the housing and will slide out (slowly) if I hold it vertically. This is perfect. I have no unneeded friction. How about the rock/play? Fully inserted, there is no rock at all. Fully extended, the draw tube moves less than 1mm laterally at the top. So far, this is success!
After carefully reassembling everything this solution works like a charm. I no longer have 2 nylon shims in my focuser, I have a Teflon bearing.
Fast forward 3 months:
The action is smooth night after night, the draw tube does not stick in the housing and I seem to have inherited some unanticipated benefits:
- The image no longer shifts when I lock the focuser in place
- I can adjust the tension on the rack and pinion more finely with those four screws now that the draw tube has more surface contact with the housing (well, with my Teflon, anyway). That means I can adjust the focus more finely without risking a runaway eyepiece.
- If the focuser expands and the action gets a little stiff, no worries. The Teflon simply will not stick. That, and the Teflon does compress by .001″ or so if there is sufficient pressure. My focuser is rock solid.
- The leftover Teflon makes for some awesome furniture foot pads!
In all, this was a major upgrade for me, not just a repair.
Some follow-up and opinion:
Celestron, I’m looking at you. I will reiterate that it was me, the owner, who abused the focuser by overloading it with gear. I am also well aware a rack and pinion focuser is pretty much the bottom of the barrel for the purpose I employ the scope but seriously…two nylon shims? What the heck? Yes, they’re cheap. Yes, they can be assembled by untrained workers in an anonymous factory somewhere. But you can do better. You make some ridiculously awesome gear. With the right tooling (which I already have in mind), this fix would cost no more than $0.50 per focuser to implement and it has already improved my satisfaction with the C6-N by leaps and bounds. Think about it. I hereby give you permission, nay, encourage you to use this modification in your future designs. All I ask in return is a nod, a shout-out, or maybe a 10″ Newt for my CG-5.
One has to dream, right?
First off, doing any of this is likely to void any warranty you have and may or may not solve your specific problem. While rack and pinion focusers are generally similar in design you might have a hangy bit here or pokey bit there that precludes this approach. Use your head and remember: this is a one-way street. Once you remove the factory shims they will probably be destroyed and your options will be limited. Second, while the rack and pinion is simple, you should already have some ability to turn a screwdriver and re-assemble the parts later. I therefore take no responsibility for you destroying your focuser. This guide assumes you are already past the point of living with the problem of rack and play.
- Of all the things you need to succeed a set of calipers is the most crucial. Buy or borrow some but do not even begin unless you can get hold of this measurement tool. They’re under $20 at Harbor Freight.
- The grease is annoying but don’t lose or contaminate any of it, it’s important.
- If you are aggressive and really want a perfect fit, buy the Teflon a couple of thousandths of an inch thicker than you need. Teflon can be sanded down with ordinary sandpaper. Sure, it takes a while and you need to constantly measure with the calipers but perfection can be had with some elbow grease.
- If you aren’t aggressive and just want to go from horrible to passable, buy the Teflon as close to your required thickness as possible without going over. This is the simpler route.
- Did you screw it up? Start over. You’ll have plenty of material to try 3 or 4 times.
What you need:
- Calipers (dial calipers capable of measuring .001 inch or less)
- Teflon sheet at the required thickness, 2-3 times what you will actually use
- Small razor knife, thin handled
- Screwdrivers and a wrench for removing the focuser
- Long-mask tape (go with 3M, you will thank me)
- Spray glue
- Dust free containers for your parts
- 200+ grit sandpaper or 000 steel wool
- Carefully remove the focuser from the scope. Don’t drop anything on the primary mirror!
- Disassemble the focuser starting with the 4 screws holding the pinion gear to the rack and immediately separate each piece in a dust free container. Don’t touch the grease or allow it to become contaminated in any way.
- Remove the draw tube from the housing.
- Use your calipers to measure the inside diameter (ID) of the housing and the outside diameter (OD) of the draw tube. Take many measurements (at least 4) and average them out. Measure the inside of the housing BELOW the entrance bead or lip to get an accurate reading. If you use the scope under the same temperature conditions night after night, warm or cool the tube and housing to that temperature before measuring for the best accuracy.
- Subtract the OD of the draw tube from the ID of the housing. Now divide by 2. E.g., if your housing ID is 1.600″ and your draw tube OD is 1.550″ the difference is 0.050″ (five hundredths). Dividing by 2 gets you to 0.025″ (2.5 hundredths). This (0.025″) is the maximum thickness of the Teflon shim you need.
Order some Teflon. A 12″*12″ sheet should work, but make sure it is-
- Adhesive-ready on the back side and…
- At least 3 times as much material as you need to cover the housing (Pi * ID” * Length” = Square Inches to Cover) and…
- No thicker than your calculation from step 5. Less is better than more.
- Using your long-mask tape, cover and seal off any grooves in the housing where grease is present or through which gears may pass.
- Carefully remove the original nylon shims and smooth out the area beneath them, removing burrs and glue without removing any of the housing material (aluminum or plastic). Fine grit sandpaper or 000 steel wool is perfect.
- Thoroughly clean out all of the dust from the housing. Compressed air works great. DO NOT use a painter’s tack cloth. You don’t want residue left behind.
- Remove your long-mask tape.
- Cut a piece of Teflon slightly longer than your housing barrel and a hair shorter than your housing’s inside circumference (Pi * Diameter is your friend).
Test fit the Teflon, sliding it in from the bottom and stopping flush with the bottom of your housing’s top bead or lip. This will mean the Teflon stops short of the top of the housing against the bead and will stick out of the bottom before trimming.
- The ends of your Teflon strip should line up with your housing’s rack groove or dado. This will make fitting and trimming easier later.
- If you are careful and trim the Teflon closer to its final length you can test fit the draw tube now, too. This is a good idea.
- Trim the Teflon as needed so it can be rolled up fitted in place with the least amount of dexterity. Re-test the fit.
- Apply spray glue to the adhesive-ready side of the Teflon, edge to edge.
- Insert the Teflon to the right depth (flush with the bottom edge of the housing “bead”) and press to the housing sidewalls with your fingers. Don’t worry about glue on the Teflon face. It will come off easily later.
With your razor knife, trim the excess Teflon away from the groove where the focuser rack will slide and insert the draw tube.
- Check the action. It should be smooth. The draw tube should want to slide out slowly if you hold it upright. If the draw tube sticks look for bubbles, overlaps (etc) and eliminate them.
- Leave the draw tube in place for 30 minutes to let the glue set.
- Remove the draw tube and carefully trim around all the inside and outside housing edges with your razor knife.
- Gently clean any glue or grease from the Teflon. It should rub off easily.
- Re-insert the draw tube, reassemble the focuser and attach to your scope.
- Collimate your scope like it got dropped from a moving truck. It just did.