Removing the Internal Counterweight - Meade 7” Maksutov
I can’t take any responsibility for any damage you might do to your telescope, but in all fairness, I should tell you to use caution. Read the whole procedure before you make a move!
If you intend to keep your MAK on its factory fork mount, don’t remove the counterweight. It’s a necessary part of the instrument. If you wish to mount the OTA on a German equatorial mount, then removal is possible.
A few were made as OTA only scopes and have no counterweight. If the back of the tube is much heavier than the front, it probably has the weight inside. To make sure, just remove the fan or the vent. The big ugly chunk of cast iron is it.
The Meade 7” MAK is a very good instrument, but has a few drawbacks:
• Heavy weight OTA
• Out of balance for a G-E mount
• Slow cooling time
• Mirror shift
Removing the counterweight will solve the balance problem and lighten the OTA. It helps cooling by removal of mass and by opening up the air flow. It will not solve the image shift problem, but having the scope apart will give you a chance to improve upon that as well. More about that later. So, first two problems solved, and the others reduced.
Meade included an adjustable locking brake for the mirror assembly on the newer models. By leaving a little drag on the brake and with some practice that image shift is pretty much solved. These scopes were built for the GPS mount and can be found on the used market, but usually still mounted on a fork mount. If you intend to remove the internal counterweight from the non-locking type it’s about the same procedure.
The non-locking scopes are much easier to find used, but because they’re older the condition of individual instruments varies greatly. My second 7” MAK was bought used with electronic damage, but good optics, so it could benefit from the weight removal and a German Equatorial mount. If the optics of a scope are in poor condition it might not be worth the effort to open it up in the first place.
If you’re unaccustomed to doing ham-handed mechanical work on a delicate expensive mechanism, just stop and pay somebody. I would consider doing it for other people, but I’d have to charge money to pay for the scopes I break. Yes, that’s a distinct possibility.
Before you proceed, please keep in mind any changes you make to the OTA will void any warranties. In fact some shops might not take on the repairs from the damage you do. The task demands some skill and dexterity. You stand an excellent change of turning a fine instrument into a scrap heap. So, proceed only with a healthy respect for the value and rarity of the thing you’ll be wrestling with and prying on!
OK, your mind is made up that this evil weight has got to go. The German Equatorial mount awaits its new partner. So, let’s get ready.
You’ll need the following:
a pair of Safety glasses –about $3 US. How much is one of your eyes worth?
a variety of small hex wenches (Allen’s)
two large screw drivers
a good paint scraper
a small hammer
a 2x4 about a foot long, yes that’s what I said
a pad or old rug
a good hacksaw or if you like a Saws-all with a new blade—courage now
a Dremell tool with a cutting wheel
a big flashlight
a pack of disposable cotton gloves – lab quality
a bag of cotton balls and a roll of paper towels
a clean plastic bag
a 50/50 mix of isopropyl alcohol and distilled water in a squirt bottle
a clean area to work
a small amount of blue tape to mark parts
Safety first-- Your eyes and hands are the likely victims.
Protect the optics-- use only clean cotton gloves never your fingers and give the assemblies a safe place to wait out the war.
The scope tube and back—equally important and it supports the baffle tube—these must not be damaged, and will get filthy dirty in the process, prepare to clean them. These are the key to Meade’s no collimation design. Bang them up and never see well again. Remember this as you wrestle the tube and saw into the corrector mounting ring. There’s still time to bail out!
Face the tube up or on its side as long as the optics are in it. Some mirrors fall out and crash badly.
Keep all the hardware with the part it goes to, that way they won’t get lost. I leave several strips of blue masking tape out and just stick the screws to it.
Remove all unnecessary rails, finder mounts, fan, vent and visual back from the scope. If you still have the aluminum Meade cap for the back, screw it on.
There are three screws holding the meniscus assembly to the front tube ring. Mark a spot on the inner and outer ring so you can properly index the assembly to the outer ring when it goes back together. I used blue tape, but not on the glass.
With the OTA facing up, remove the three screws. The meniscus should lift right out. Store it face-down on a clean towel and tape the screws right next to it.
Lay the OTA on its side and turn the brake knob to full loose. Turn the focusing knob fully counter clockwise to move the mirror forward inside the tube. At this point both sub-assemblies can be unscrewed and removed. Take out the focuser first. Once the screws are out pull the mirror about 1/3 of the way back and wiggle the end of the draw screw off the mirror positioning post. That post is solidly mounted in the mirror assembly. Once the focuser is removed, the mirror will slide along the baffle tube at random. If the scope has been apart before and the lock ring was left out, it can slide right off the end of the baffle tube. OK set the focuser aside on a paper towel (greasy parts) with its screws. There is a small flange on the locking knob assembly. It must be wiggled off the gear by tilting it similar to removing the focuser, but it has no direct connection. Set it on the paper towel with its screws. At this point the back of the scope should be just a bunch of holes. Slide the mirror right back to the rear.
Face the OTA so the open end is up. If you face it down the mirror may very well slide right off to its doom. The mirror should be resting comfortably at the rear. Now remove the locking ring from the baffle tube. It’s finger tight. Get a thumbnail under an end and gently inch it up the baffles till it comes off in you hand. I did not cover the mirror with anything because the grease from the baffle tube would get on the towel in an uncontrolled way and probably get on the mirror’s surface. There’s no need for a death grip on the ring just a gentle grip and pull it off. Set it on the paper towel with the focuser. Now we’re ready for the mirror. It’s fragile by the way.
Put on a pair of clean cotton gloves and reach down the tube with one hand. The mirror is mounted on a dark gray metal casting. Grip the metal casting and remove the main mirror by sliding up the baffle tube till it’s free. If you’re not sure of your grip, skip the gloves and use extra care not to brush against the mirror surface with your skin or worse yet with red grease. The mirror is wider than the exit. There are two gaps in the front ring. The mirror must be tilted to clear these gaps. Have you ever scraped the coating off the edge of a hard to find mirror? That can happen when removing or installing it. Set the mirror assembly on a clean paper towel face up. The locking gear and compressor ring can fall off.
You should now have a bare OTA with a big cast iron weight at the bottom. The gaps that enabled you to remove the mirror are not big enough to remove the weight. Those come first. They must be enlarged about 3/4 inch side to side and away from the meniscus mounting holes, and deepened about 1/8 inch each. I did not deepen them. Instead I sawed a gap in the weight by rotating it 90* and supporting it with a 2x4. I cut through it with a Saws-all. It was a risky move, placed my left hand right next to a reciprocating blade and was loud as anything. So I recommend just making the gaps bigger. One has a necessary screw hole next to it. Careful not to damage that.
When sawing, I failed to protect the baffle tube with a plastic bag, but that would be a good idea. It took hours to get the metal shavings off of it, and I’m not sure I got them all.
The counterweight is held in place by what appears to be black polyurethane adhesive caulk. The cohesion must be broken. This is time consuming and a bit scary. First, you must use a screwdriver and paint scraper to get a foothold in one of the fan holes. Once you get started it’s not too bad. The photo shows my progress after about an hour. No, I didn’t come close to the threads on the back, but wish I still had the aluminum cap.
This looks worse than it is. Using screwdrivers you must pry down on the weight. The problem is the edges of the casting are your fulcrum. By prying down the weight you’re also prying up the edges of the holes. The adhesive will give way to time much better than force. Scrap a little more by forcing the paint scraper between the casting and the weight. The less surface you have glued together the easier this will be. That means get as much out as possible before prying. I cleaned about four radial inches. You’ll feel it give about 1/16th of an inch at first. Listen for the sound of rubber ripping; like tape being pulled from a smooth surface. Just keep flexing the adhesive and watch how much pressure you apply. If you dent a hole, it’s not the end of the world, but it will be no fun to repair. Once you get about 1/4 inch of flex the glue will rip apart nicely.
It’s a process of flexing the rubber adhesive just past its limit over and over again. It took about half an hour to get the weight to rip free. Others have used a hammer and chisel to break it free. Please don’t. I did not want to hammer away at it or set the tube in a press because that would alter the relationship between the tube back and the meniscus mounting ring. Those must stay in perfect alignment with the baffle tube to maintain collimation later. When I had the weight torn out to where that side was about 1 inch away from the scope back, I reached up inside and ripped it out by hand.
If you’ve made the gaps big enough with the Dremell tool the weight should come right out. To saw the weight itself I had to support the weight in the tube sideways with part of its curvature outside the tube. I supported it with a 2x4 and sawed through it as described above. It was a disturbing experience. Once the weight is removed you must hang it on a gold rope and use it as a dinner bell for guests you invited over for a star party. Well…that’s optional.
Clean and inspect the OTA. Inspect the back first checking the holes to make sure you didn’t pry one up. If so, that’s what the hammer is for. It’s easy to crack, but a little tapping should do. Don’t set the tube on its open end and bang away. That’s the shape that’s most critical to protect. Next run your hand down the walls of the tube to make sure you didn’t bend it. It’s unlikely. If the OTA is in good shape and you did not touch the baffle tube, it’s time to clean everything as well as humanly possible. The baffle tube may be covered with dust and red grease. The grease will be contaminated with metal filings and must be wiped off. Remember the plastic bag I forgot. There will be scraps of rubber. Get as much of it out as you can. It will be floating around in there for years if you don’t.
A note about cotton balls for cleaning. They are soft enough, but can leave little hairs on everything. The baffle tube likes to snag the cotton.
Step 12. Reassembly
Once you have the tube as clean as it can be, start by greasing the baffle tube with a very thin layer of red grease. I used the clean grease from the mirror assembly to lubricate the baffle tube. It won’t take much, but its thickness will have a lot to do with how well the image stays still. The red grease is petroleum based and it does emit gas inside the tube over time. There are other lubricants available. One is listed below. The brake assembly needs to be screwed about half way onto the mirror assembly. I didn’t get it right, but it was fixed easily by removing the locking knob and rotating the big gear with my finger till it felt snug. There’s a plastic collet or compression ring that must not be forgotten, otherwise you’ll be opening up the OTA again.
Everything goes back together in the reverse order it was taken apart. I dusted off the mirror with an air puffer (not my breath!) and found it to be quite clean. Some cleaning solution might be needed if your mirror looks dirty or has a smudge. Wear the cotton gloves. Tilt the mirror to get it past the gaps, then slide it down the baffle tube. Slip the locking ring into its position and be sure it’s seated. The locking knob comes next and the gear teeth must be engaged for it to fit back into its hole.
Next is the focuser. I happen to have a Feather-touch focuser and it’s well worth the investment. This is an opportune time to install one. Slipping the end of the draw screw over the post takes a few tries, but it can be done. Once these components are on, the fan and filter vent should be installed along with all the screws, and you’re done with the back of the scope.
We finish with the meniscus assembly. Remember those gloves. Mine was dirty from years of who knows what entering the OTA by way of the visual back. The secondary mirror and inside of the lens took a bit of cleaning. Be sure the mating surfaces of the rings are clean, line up your marks and gently lower it into place. Use the original screws, and the whole thing should fit the way it was built.
By keeping the mirror and meniscus assemblies intact you better the odds of everything going back together in collimation. A star test is next. For collimation there are three basic adjustments; all in the meniscus assembly. Misalignments include off center or tilted off angle, and out of phase or index. If it’s out of index you didn’t keep the marks lined up and you’ve got two other chances to get it right. It’s unlikely this will cause much astigmatism. It depends on how well matched the two main elements were. If it’s off center you just loosen the three screws and shift it around the .25mm possible. If it’s tilted you must make a set of shims. Yes this would mean shims and tightening and loosening screws all day. If it’s way off, you may have moved the baffle tube, and that is another story!
My star test was a success. I used a 27mm Panoptic EP and defocused Arcturus to about 50% of the TFOV. The inner and outer main rings were about equal in thickness, and hundreds of very fine diffraction lines showed just below my visual acuity, making the field look gray. The airy disk was symmetrical at both focal extremes, and distorted evenly as the star drifted out of the field. I repeated the test with a 12mm and 8mm EP, and had good results in every test. It’s only a quick check, but it works.
This procedure takes the scope apart to its component sub assemblies. While you’re doing all this work to get better performance you might consider:
• Feathertouch focuser from Starlight Instruments – good for this scope
• Virtual View rotating visual back by Starizona –comes in handy on a GE
• 2” diagonal dielectric mirror—mine is a TeleVue, but others work very well
• Extra Losmandy mounting rail for piggybacking yes it does add weight
• Starizona has “Glube” which they say is much better than the red petroleum based grease. I didn’t try it but might.
A thicker lubricant should help stabilize the mirror on the baffle tube which is the source of the image shift. Another source of image shift is chasing the focus. I’ve been told Saturn looks focused one second but not the next. Yes, the Earth has an atmosphere chasing it with the focus knob won’t improve conditions. Get a good focus and leave it there. Kendrick makes a great focusing aid, and so can you!
|All times are in (GMT-8:00) Pacific Standard Time Zone|