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Meade 7 inch MAK revised

Posted by Steve Hollenbach   12/19/2008 08:00AM

Meade 7 inch MAK revised
Meade 7” Maksutov and Internal Counterweight Removal (revised 12-18-08)
By Steve Hollenbach

I’ve written three articles on the Meade 7” MAK. Since then I’ve done a bit more work so it seemed an update was in order. This article is intended as a record of my efforts to remove the internal counterweight and make other improvements. It can be used as a guide to working on your telescope, but any responsibility for damage rests with you.

Some Background on the Meade “Big Mak”:

After observing through many scopes over the years, I still consider the Meade 7 inch Maksutov to be their best OTA. It is unfortunate these are no longer in production. All had an 8.25 inch oversized mirror and a meniscus made of BK7 glass giving a 7 inch aperture. This allows a wider field of view than many Maksutov designs of similar focal length and provides excellent light gathering.

There were three basic models based on the type of mount; the LX 50, LX 200 and OTA. The OTA models had no internal counterweight. The LX200 GPS capped the line near the end of production, and introduced the mirror lock. The optical components enjoyed progressive development, including the standard MgF2 coatings, the EMC coatings on the meniscus which became standard on the LX200 series, then near the end, the UHTC on all glass surfaces.

If you are looking for one on the used market, prices range from about $600 on up to $3,500 depending on condition, mounts, eyepieces and other accessories. Try to get one with EMC or UHTC coatings as having the optics recoated is a rather difficult process. The EMC coatings were an improvement. The UHTC coatings are considered the best. The standard coatings are not bad, but under good seeing conditions the difference can be noticeable. If you do a lot of imaging, look for one with the mirror lock. For visual observation I set it for slight friction.

Many of us are removing the OTA from the fork mount to fit the scope to a German Equatorial system. That makes the internal counterweight obsolete. The counterweight partly blocks the airflow for cooling, and itself must be cooled; to the tune of eight pounds of cast iron. That’s much more mass than the mirror. Removing it really does improve cooling times, but requires the disassembly of the OTA.

Most of us are reluctant to open a good scope, and with good reason. This operation involves an element of risk. However, while the scope is reduced to its component parts, other valuable modifications and maintenance can be done.

The counterweight and cooling:

In the earlier scopes, the weight blocks the airflow from the fans almost completely. In GPS scopes (with the lock) the airflow is a little better but the weight blocks the fan and vent by about 50% and is slightly thicker. Removing the counterweight will significantly improve the cooling, reduce overall weight by about 8 pounds, and allow you the opportunity to clean and grease the internal parts of the scope. It also provides better positioning on a German Equatorial mount when balanced. Opening the OTA lends itself to installing a handle on the back or upgrading to a fine focuser. A second fan to pull air out is unnecessary.

Removing the internal counterweight should only be done if the OTA will be permanently removed from the fork mount. If you are installing a Vixen or Losmandy rail, the screws holding the rail and OTA together can be long enough to touch the mirror. I caught this on my second scope after noticing some difference in collimation. The screw was touching the side of the mirror; not good.

This procedure will not solve the image shift problem, but having the scope apart will give you a chance to re-grease the baffle tube and check the run-out play between the focusing screw and the mirror mount. Image shift can be reduced.

Removing the internal weight from the LX50 was actually pretty easy compared to the later models with a focusing lock. No sawing or carving on the scope or the weight was necessary. However, on my LX50 the collimation was affected at some point in the process; most likely the rail mounting screw.

As mentioned before, exercise caution. The inner workings of this or any telescope are easy to damage. Here’s a brief list of what can go wrong:

1. Damaged mirror or meniscus---pretty much total the scope
2. Damaged baffle tube---very likely have to sell the optics to the guy who dropped his mirror in #1.
3. Tweak the baffle tube, the scope back or the tube and aperture ring—difficult collimation job.

Well, the article is about removing the counterweight, so let’s get ready.

You’ll need the following:
• a clean work area
• a pair of Safety glasses –How much is one of your eyes worth?
• hex wenches (Allen’s) 1/16” for the fan & vent, 5/64” meniscus ring & focuser, also 3/32” for a Vixen rail or 1/8” and 9/64” for a Losmandy rail.
• blue masking tape to mark parts; the blue seems to leave less residue
• two large screw drivers
• a clean, flexible paint scraper
• a pad or old rug
• a big flashlight for inspecting the tube
• a pack of disposable cotton gloves – lab quality
• a bag of cotton balls (for glass parts)
• a roll of paper towels (for mechanical parts)
• a clean plastic bag
• a 50/50 mix of isopropyl alcohol and distilled water in a squirt bottle

And for the models with the focus lock:
• a good hacksaw or if you like a Saws-all with a new blade—courage now
• a Dremell tool with a cutting wheel
• a small hammer
• a 2x4 about a foot long, yes that’s what I said

Ground Rules:
Safety first-- Your eyes and hands are the likely victims.

Protect the optics-- use only clean cotton gloves never your fingers. There is no need to remove the glass components from their rings or center mounts. Give these subassemblies a safe place to hide for a few hours.

The scope tube and back—equally important and they support the baffle tube These must not be damaged, and are the key to Meade’s no collimation design.
They might get dirty in the process, so be prepared to clean them.

The mirror might fall out if the clip ring is missing. Face the tube up or on its side.

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.

Here we go…
Step 1
Remove unnecessary rails, finder mounts, fan, vent and visual back from the scope. If you still have the cap for the back, install it. The hex key I used to remove the fan was a 1/16”, but it was chrome plated. Un-plated 1/16” inch and a 1.5mm were both too small and a 5/64” was too big. I couldn’t find a 1.59mm or a 9/128” hex key. So you might have to invent a tool if there’s the slightest wear on the heads. An alternative would be to use a fine center-punch to make either 3 or 6 tiny dimples around the hex hole. This would provide a tighter fit for the tool. Or paint the tool.

Step 2
There are between 3 and 6 screws holding the meniscus assembly to the front tube ring. The aperture ring holds the glass assembly, and the tube ring is Expoxied to the OTA. Using some blue tape, mark a spot on the inner and outer rings so you can properly index the assembly to the tube ring when it goes back together. Two marks are better than one, but not on the glass!

Step 3
With the OTA facing up, remove the screws. The meniscus should lift right out. Store it face-down on a flat clean towel and tape the screws right next to it.

Step 4
Lay the OTA on its side and if it has a locking ring turn the 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. Some posts are too long to allow the focuser to be removed with the mirror still inside the tube. If that’s the case loosen the set screws on the knob, and unscrew it. You will still have to take the flange/bearing assembly out to get it back in again. The inner brass threaded piece should slide right through the bearing flange.

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. Set the focuser aside on a paper towel (greasy parts) with its screws. There is a small flange on the locking knob gear 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.

Step 5
Face the OTA so the open end is up. The mirror should be resting comfortably at the rear. Now remove the locking ring from the baffle tube. It’s finger tight. Feel around for the gap in the ring. Get a thumbnail under an end and gently inch it up the baffles till it comes off in your hand. I did not cover the mirror with anything because the grease from the baffle tube would get on the towel in some 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.

Step 6
Put on a pair of clean cotton gloves and reach down the tube with one hand. The mirror is mounted on what appears to be a large dark gray cast iron bushing. Grip the cast iron 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 tube ring. The mirror must be tilted to clear these gaps. On locking models the locking ring might fall off. That assembly is easy to screw right back on again. Set the mirror assembly on a clean paper towel face up. You should now have a bare OTA with a big black weight at the bottom.

Step 7
On the locking types 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 may need to be deepened slightly. I made them wider, but not deeper. 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 and was loud as anything. So I recommend just making the gaps bigger. One gap has a necessary screw hole next to it. Be careful not to damage that.

When sawing, protect the baffle tube with a plastic bag! It took me hours to get the metal shavings off of it on the first one I removed. On the LX50 the weight fits through the gaps so there is no need for sawing or enlarging them.

Step 8
The weight is held in place with black polyurethane caulking. On my GPS scope it was very difficult to remove and required lots of prying and scraping. My LX50 was so easy the weight could have been shaken loose. So, a wide range of cohesion might be encountered. On both models you pry between the fan holes and the weight, being careful not to pry too hard.

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. Scrape 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 the gaps are big enough turn it sideways it should fit right through. On the LX50 it was very easy.

Step 9
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. Just cover the uplifted dent with some blue tape and hold the hammer to strike as flat as possible with strong taps rather than light bangs. Wish I could quantify that a little better.

Next run your hand down the walls of the tube to make sure you didn’t bend it. It’s very 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 may 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.

Some people asked about flocking the interior. This would be a good time for it, but I didn’t, and don’t know the longevity of the peel & stick stuff. The black paint Meade used does have a slight sheen to it. Mask the baffle tube carefully if you intend to use paint. If you want to paint the outer surface of the OTA, this is a good time for it. The best way to keep paint off the glass is for the glass to be no where near it.

Step 10. Reassembly
Once you have the tube as clean as it can be, and any painting or touch-up is done and really dry. Grease the baffle tube with a very thin layer of red grease. On the LX200 I used the clean grease from the mirror assembly to lubricate the baffle tube. On the LX50 I had some Peterson Engineering red grease and I like it. It doesn’t take much, but its thickness will have a lot to do with how well the image stays still. Meade’s red grease is petroleum based and it does emit gas inside the tube over time, but has very good self cohesive properties. You don’t want grease dripping and dropping around the insides of the OTA. There are other lubricants available. Some are listed below.

The locking 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. It works like a chuck on a drill, gripping the shaft when turned.

Everything goes back together pretty much 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. To find smudges I tilt the mirror at various angles with a bight light and dark background. Some you’ll see head-on others you’ll only see at low angles.
Wear the cotton gloves.

A note about cotton balls for cleaning. They are soft enough, but leave little hairs on everything. The baffle tube likes to snag the cotton. Use each cotton ball only once. If they pick up a microscopic piece of grit, and they will, you don’t want to rub that around on the mirror surface. I went through a bag. They’re cheap.

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. 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. If the post was too long to tilt it off, it will be too long to tilt it on, and the focusing screw will have to be installed before lowering the mirror into place. 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. Same as cleaning a mirror, the cotton gloves and cotton balls. Use all new ones and only once. 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 sub-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, and Meade did not allow for these. It’s all minor movements of 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 LX200 scope was out of collimation when I first got it. The fault was not inside at all, but the motor focuser. It was installed cross threaded and was off about two degrees. That’s more than enough to make a mess of things. Not all collimation problems are in the OTA.

Its 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 inside and outside focal extremes. The disk distorted evenly as the star drifted out of the field. I repeated the test not so far out of focus where three rings appear between the outer and inner. It looked perfect to me.

I repeated these tests with a 12mm and 8mm EP, and had good results in every test. Get used to doing this simple star test. It’s a quick check, but it works.

The LX50 OTA was slightly out of Collimation when rebuilt. I’m sure it was the screw coming into contact with the mirror. Its performance is OK, but not spot-on, so that will take a bit of work.

The counterweight removal procedure takes the scope apart to its component sub-assemblies. While you’re doing all this work to get better performance make sure you take the opportunity to clean and inspect everything. You might also consider the following improvements:

• Feathertouch focuser from Starlight Instruments – good for this scope
• Peterson Engineering EZ focus kit if you keep the factory focuser
• 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 uses blue “Glube” which they say is much better than the red petroleum based grease. It emits no gas. They don’t usually sell it over the counter, and it is surprisingly expensive. I haven’t tried it yet but might.
• Fomblin perfluorinated grease is another possibility. I found it to be an excellent lubricant, but it lacks the very thick viscosity needed for the mirror movement. I used it on the focusing screw assembly.
• Flocking---Scope Stuff sells a peel & stick material. I have not seen it yet, but will soon. I also did a bit of homework at fabric stores, and found an upholstery fabric which has a print on the front. The back is so black you can’t see any sheen at any angle. Most fabrics do reflect a little, but this stuff was like a black hole. I’ll try a sample to test for suitability in an OTA. Keep in mind this scope already has excellent contrast.

A very thick lubricant should help stabilize the mirror on the baffle tube which is the source of the image shift. Another source of image shift is a loose focusing knob assembly. This can be checked before you re-install the meniscus, by observing the movement of the focusing knob while pushing and pulling the mirror assembly back and forth. Peterson Engineering makes a kit to solve this. That kit is not needed if you buy a whole new focuser, but it is inexpensive and works well on the factory knob assembly.

Chasing the focus is another source of frustration. I’ve been told Saturn looks focused one second but not the next. That’s not image shift, it’s a sign of poor seeing conditions. Yes, the Earth has an atmosphere. Chasing it with the focus knob won’t improve conditions. Get a good focus and leave it there.

About my two Meade’s:
I happen to have two OTAs. I’ll call them LX50 and LX200GPS just because that’s where they came from. Both are off the fork and minus the counterweight.

The LX50 has the EMC coatings and was recently cleaned. This is the one with the collimation problem. It’s just noticeable in the airy disk unless you really rack out the focus then it goes away. Much of the problem was eliminated by moving the meniscus, but some error persist. I’m sure it’s mirror angle, I’ll have to spend more time checking it out.

My LX200GPS has the UHTC coatings and the mirror lock. I was very fortunate that this one had very little image shift and nearly pristine optics. Collimation is bang on, even after removing the counterweight. It has the Feather-touch focuser, dual rails, and the Virtual View visual back. I’m reluctant to do anything else to it, but might flock the interior.

Back to the LX50. I’m not reluctant to do something to it. I have considered flocking the interior of the tube. It’s surprising how reflective flat black paint can be. I am also considering using it as a test bed for a new mirror mount. The new mount will be an attempt to control image shift and give the scope some adjustments for collimation via holes in the back.

The EMC coatings are only on the meniscus, not the mirror. While I have the mirror out of the OTA, recoating it becomes an option. I am considering protected aluminum, but also a dielectric coating. I have not considered refiguring, as the mirror and meniscus have to be matched, and they aren’t far off now.

There is one other issue I had with the Maksutov OTA. The focal length needed for the very good planetary viewing I enjoy prevented me from using the scope for deep space. So, it was not as versatile as some of its competitors. For a wider field I cobbled together a focal reducer that fits like a Barlow. It’s an Anteres .70 in a 2 inch format on a spacer and extension barrel. At the visual back, and using a 27mm Panoptic, it gave more than a 1.6 degree TFOV. That exceeds the theoretical limit for a 7 inch aperture of 2670mm focal length. Told you the oversized mirror was a good thing. It was enough to get the Pleiades in view. For more on that please read my other articles.

Clear Skys and Calm Conditions---