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| Section 1 - C11 SCT Disassembly | |
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Ok, here we are at the beginning. Pictured here is my 2000 model C11 ready for disassembly for the first time. Make a conscious effort to keep in your mind that this thing must be reassembled *exactly* in every way to avoid optical misalignments or other performance issues that may not exist before hand. Don't worry about your room for error - although there may be a tiny bit, you will be better off assuming there is none. Having a clean work area is also important. You may want to first remove any accessory mountings or dovetail plates at this time. ** Supplemental: There seems to be an ongoing debate as to how precisely you need to keep the corrector orientation with its original position. Some claim that the manufacturers say it only needs to be within about 20% or so of the original orientation while others believe it should stay as close as possible. Regardless, I see no harm in assuming it needs to be precise and will treat it as such in this documentation. |
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Before you begin,
decide on a method that you will use to precisely mark both the rotational
and lateral position of the glass corrector plate. If you don't plan on
messing with the secondary (recommended), then still take the time to mark
it's orientation. You don't want to find that it has moved after you've
already removed the corrector plate (or anytime for that matter). The secondary
should never be able to move freely. While the scope is upright, remove the eight screws holding the retaining ring in place and place them in a ziploc bag labeled "corrector ring screws" or something. This ring basically keeps the corrector plate from falling outwards - hence the reason for setting it upright first. As you begin to remove these screws, take careful notice how tight they are initially; slightly tightening one or two slightly before you loosen them may give you a better feel. The point being, with mine anyway, that they are definitely not all that tight, but tight enough to hold them in securely. Just keep this in mind for reassembly. |
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Now we can see the outside edge of the corrector plate and a few cork and paper shims (I suspect there may be variances in the number of shims and their thicknesses from scope to scope). I chose to make a number marks by using small pieces of tape across the corrector edge onto the tube housing and then with a utility knife separating the tape between the seams so that both halves were exactly lined up. First I made 4 marks to record the rotational orientation of the corrector at 90 degrees each. Then I marked the beginning and ending edge of each shim stack (you'll want to put these shims back in the same spots as well). |
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Here is a close
up of my tape markers. On each of the shim marks, I wrote codes on each.
The one shown is marked "2-2" meaning shim #2 - 2 pieces of cork
(or paper in some areas). I had a total of four shims around the corrector;
two had two shim pieces stacked, one had three pieces and one had a single
piece. The trickiest part of the entire disassembly process for me was pulling the corrector out while keeping track of where the shims went. I tilted the scope at about 10 degrees before I pulled the corrector plate out (just carefully grab the secondary housing and pull straight out). One of my shims stayed in there while the other two fell apart as the corrector came out. Fortunately I had another person helping at this stage whose job was to keep an eye on which shim fell where while I pulled it out. Just do this part slowly and you should be OK Don't forget to put the shims aside where they won't be disturbed. I put mine on a piece of paper where I had written next to each the corresponding codes I previously made on the tape markers. ** Supplemental: After further discussions with others on cork shims, it is safe to say that here is yet another debate about just what the shims are for. Some claim that the main purpose for them is simply to pad the corrector plate so that it doesn't rest on the hard surface of the corrector housing, rather than provide any concentric alignment (which would be very slight indeed and a seemingly inadequate material to maintain such a precise spacing). So, with that in mind, taking the precautions above may be over kill and not nearly as critical. You still want to keep track of the shims for re-assembly. What their purpose is I'll leave to your consideration. |
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This is a shot of the tube after the corrector plate has been removed. Put the plate aside where it will be safe. Also, it's worth noting to make sure your little tape marks are nice and secure and not in danger or peeling off anytime soon. If they are, reinforce with more tape strips. |
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Here is a shot of the C11 corrector plate and secondary mirror assembly. It can be safely stored resting face down on the secondary housing which also protects the recessed secondary mirror. The C11 corrector glass, is about 1/4" thick and weighs about 3 lbs. or so. |
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First,
let me note that you do not want to bump, graze or touch the primary
mirror in any way - it WILL leave a spot
and cleaning the primary is much more difficult and risky than cleaning
the corrector plate. Not to overly alarm you, a spot or smudge
does far more damage in your mind than trying to remove it and you probably
won't even be able to notice performance degradation from a small flaw.
So, unless it's big, better to leave it alone, but always keep your
optics as clean as possible (we have enough visual obstacles to get
overcome as it is!). Now you want to remove the tube body from the rear casting. There are seven screws around the tube that are fastened with seven accompanying nuts on the inside of the scope. With the scope pointed upright, slowly and carefully reach inside the tube with one hand to hold the nut while unscrewing the screw on the outside with the other hand. There are no lock washers on them so just using your fingers should give you enough pressure to keep the nuts from rotating while you do this. Be careful because your fingers will be very close to the primary mirror. If a nut falls down to the bottom, it's no big deal; you can get it when the tube is off. If you have a removable dovetail plate on the scope, you don't need to remove it. There were two other screws in the casting body with no attaching nuts that also did not have to be removed in order to get the tube off. After the screws and nuts have been removed, you can lift the tube straight up and off the rear casting. It may take some light jarring side to side to get it moving. ** Supplemental: It should be mentioned here that if your intention is to simply remove the primary mirror, you do not have to remove the tube from the rear cell. I have been told, however, that the corrector ring at the tend of the tube may have to be removed first due to the fact that the diameter of the primary is larger than the inner diameter of the corrector ring. |
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At this point, there are two things that need to be done in order to remove the primary mirror. First, bring the primary mirror all the way to the back and remove the focus knob assembly. Loosen the set screw on the knob to remove the focus knob and remove the three screws on the black bracket. There is also a screw at the center of the focus shaft where the knob was. Now you should be able to just unscrew the focuser bearing assembly off of the focus shaft by hand. |
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The second task before removing the primary mirror is to remove the mirror shaft retaining ring from the middle of the primary baffle tube. It is a metallic ring sitting in a groove that is held on by it's own tension. Since it winds around the baffle tube only about 360 degrees, you can put your fingers on each end to pull it open enough to slide it off the tube. Be careful not to scratch the tube. After the ring is off, you can then slide the primary off the end of the baffle tube. Needless to say, exercise caution here as the primary is pretty heavy. I would estimate the C11 mirror weighs around 10 lbs. if memory serves. After I removed the primary, I placed it inside a fresh trash bag until I was ready to work with it, being careful not to let anything touch the surface. |
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This is a shot of the empty rear casting. If you are planning on doing any kind of drilling into the back, it's worth noting now the molded support ridges inside. These can be a pain to work around and you don't want to drill into one from the back or it will make your day a long one. It's about 1/8" thick at the back around the ridges so it's not too bad to work with and the fact that it's aluminum is a plus. |
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A shot of the primary after removal. You can make your own assessment, but the primary ring on my C11 was only finger tight and I wouldn't recommend bothering with it unless it is very loose. For those curious, it is threaded on the inside and rests on a cork pad (tightens clockwise). Over tightening this ring could introduce warping of the primary's spherical profile. I know very little about the ring / torque relationship, so if you need to adjust it then contacting Celestron first may be wise. I just know it's good practice not to mess with things that don't need messing with:) |
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Here is a side shot of the C11 primary mirror. Notice all of the orange RTV filled at the base. Unevenly filled RTV seems to be the norm, don't be alarmed. RTV is the firm rubber adhesive used to mold the primary glass body with the rest of the aluminum housing which includes the focus arm and accommodates the baffle tube. |
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Lastly, here is
a bottom shot which clearly shows the focus arm and it's attachment to the
baffle tube. Hopefully these three mirror shots will give some helpful information
to those trying to design and implement a mirror locking mechanism. This concludes my Celestron C11 disassembly section. The content below covers all of the modifications I have made to the C11 in order to achieve some small, but handy performance gains. |
| Section 2 - Cooling Fans | |
| Click
here for a listing of parts
used in this portion of the project from Mouser
Electronics. Please also read over the "Cooling Fans" notes in the conclusions section for important results of this mod. |
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The first step in adding cooling fans to the C11 is taking the time to decide where they (or it) will be placed. As noted in the disassembly section, the inside of the rear casting is lined with radial ridges that will affect where you choose to drill. The other factor is to consider the accessories you will be using and that there is no interference there. You may want to install one or two fans. I chose to install two, but in seeing how much air flow they produce you could do fine with one if you wanted. This illustration shows the first cut which is 2" to match my particular fans. I used a circular drill plug with starting bit, much like the ones used to drill out door knob holes but made for metal. Needless to say, this job killed the bit pretty quickly, but it worked. I recommend that you cover as much of the areas surrounding the holes as possible as the shavings get everywhere (the voice of experience:) |
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A drill press is recommended to keep everything perpendicular, but if you have another person to spot you it can be done by hand if your skilled and take your time to do it right. After the two holes were drilled, I also drilled and installed a toggle power switch and 12v power jack located under the focuser to allow me to power the fans from a standard auto cigarette jack; pretty much the standard for astronomy field battery power. Take care not to nick or damage the baffle tube while drilling in any way. The seal between the baffle tube and the primary tube must be tight and smooth with no surface flaws. |
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The
two fans have been installed here to blow into the tube and directly on
the back of the primary as opposed to one in and one out. The reason being
that having one fan reversed and blowing out created an issue of not being
able to keep dust out when not in use. The filter screen would have been
placed behind the fan making it inaccessible to cleaning and I didn't want
to leave it off. The power switch and 12v jack have also been wired. The wiring was neatly tucked away to prevent any possibility of interference with any moving scope parts. I used plastic epoxy welder to secure the wire in a couple of spots. One thing to mention is whenever I bolted anything on the inside of the casting during this project, I always added lock washers with lock-tight since you don't want to risk anything getting lose and falling off once it's all back together. It's probably understood, but always make sure any hardware you use is either stainless steel, brass, or nylon based to prevent corrosion. |
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Here
is a shot of the fans after installation. Notice I also ordered special
snap on grills that fasten to the back of each fan (see below). The grill
contains filtration media that screens out some of the dust that could be
brought into the tube during use. It can easily be replaced or cleaned when
needed. One of the screens can be seen next to the scope in this photo.
Another thing you may notice in this photo that wasn't previously mentioned is the addition of two brushed stainless steel handles on the left and right side of the casting. I happen to see these at Home Depot and had a hunch they might come in handy. They really look pretty nice- stock almost. They come in two sizes; one about the width of the original Celestron handle and a shorter one about 3/4 that width which is shown in this photo. |
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This
shot illustrates the extra fan accessory filtration grill available from
Mouser. I ordered extra filter media and added an extra layer to each fan
for more dust protection. The fans can still breathe easily. One nice thing
about these is they quickly pop out for cleaning when needed and if in the
future I find another type of filter media that works better it can easily
be cut to fit. ** Supplemental: I've been using the fans almost every evening I've set the scope up for the better part of a year now and have not seen any significant evidence of internal dust on the optics, nor in the filter media. So far so good. |
| Section 3 - Tube Flocking | |
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Most
mass produced SCTs have a few corners cut to improve profit margins. Spraying
the inside of the OTA flat black rather than lining it with flocking paper
is one of them. Fortunately it's not that hard to fix this yourself if you've
already got everything apart. I ordered a sheet of flocking paper from Protostar
that sells by the inch from a 30" wide roll. It is amazing how much
darker you can get the inner wall of the tube. It's much like velvet, but
with shorter fibers. The flocking paper is made so as not to shed the fibers,
but I would recommend that you go over the surface before final reassembly
with a tape roller because it does shed just a little bit. Something else
that is as important as flocking the tube, at least with most C11's, is
to be sure to paint the tips of all the shiny screw heads that poke inside
the tube. I didn't notice this problem when I had a C9.25, but it is very
noticeable on my C11. You'll have to do this step when you get ready to
replace the tube body back on the rear casting. ** Procedure Notes: This can be a tedious job by yourself, but even with two people working in the small confines of a 12" tube body it is a cumbersome task. One tip I can offer is place plenty of light shining inside the tube where you will be working. The flocking paper's effective ability to absorb light hinders you here. What I did before I applied the flocking paper is carefully and lightly cut the backing paper into about 4 or 5 segments so it would peel off in those segments. It is easier to deal with the adhesive backing in smaller chunks, peeling back each one as you get to it. Since most tubes are rolled from a single piece of material, they have a slight seam where they are molded together. You can use this seam as a tool to line up the first edge of your flocking paper to ensure that it starts straight. Using a dowel, or in my case a piece of scrap PVC pipe, can be handy for rolling on the paper evenly from the felt side to prevent trapping air bubbles. The next tricky stage of flocking the tube is cutting the length just right since it is not easy to gauge. The plus side is that flocking paper is so flat and black that any seams will not easily show. So, if you slightly under cut your length, you can cut another strip to patch in the gap with no loss. Flocking paper doesn't stick well to itself so you probably want to avoid overlap. This may also make it more prone to peel up over time. Initially you will want to go ahead and cover over the screw holes in the tube and then go back with a sharp blade and cut them out. Before you put the tube assembly back together completely, I would recommend winding something like wide packing tape (or use a pet roller) around your hand and dab around the inside to pick up any debris stuck on the flocking paper or lose fibers. Please see my post-flocking comments below in Section 6. |
| Section 4 - Mirror Locking Bolts | |
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My
mirror lock design isn't all that sophisticated; no fancy pistons or tension
springs. Because of limited resources and a fairly strict budget, I wanted
to approach the problem with a simple and functional solution. No matter
how much you spend on a locking bolt modification, the end result is simply
applying safe, consistent pressure on the primary to prevent it from shifting.
Just about everything I've used on this project was acquired from Home Depot;
you just have to be a little inventive. There isn't a lot of documentation
out on the web regarding primary locking systems other than a single bolt
design. I felt that a single bolt may help the situation, but isn't the
best solution as it only addresses one point of weight pressure on the primary. The design is simply three carriage bolts that thread into the rear casting at 120 degrees apart. I found some black plastic knobs that I put on the ends for a better grip and feel. After the bolt holes were drilled into the back, I put the primary on the scope and slid it back to mark where each bolt made contact. Once I knew where they hit, I cut three pieces of aluminum to use as the contact surface. To mount them, I generously used black Permatex silicon sealant (available at automotive stores) to provide a very cushy mounting base. When cured, it's like thick Jell-O, but also has a strong adhesive property to keep the aluminum contacts in place. Each contact was shimmed with a metallic pin under the outside edge (also sealed in the Permatex) in order to make the contacts level with the bolt ends since the back of the primary is curved (any type of spacer would suffice). ** Supplemental: An important factor to keep in mind when doing this mod is to take careful consideration where the location of the bolts will make contact with the back side of the mirror. Too far to the edge and it will be easy to warp the primary while under pressure; too close to the inside and it will require a lot of torque to lock it down. Without any experience in judging the optimal location, I just chose a spot that looked to me to be a balance between mirror thickness and distance away from the edge. The result ended up being about .75" to the inside starting from the midpoint. Click here for an illustration. This certainly may not be a reliable formula to use when figuring locations for different mirror sizes as they vary in thickness and, thus, vary in the amount of pressure they can safely sustain. The spot I chose for the C11 mirror has so far shown no distortion effects under enough pressure to lock it. Also, if your scope suffers from more than normal mirror flop before hand, it will require more torque to compensate, risking distortion (fully greasing the mirror tube / baffle tube surface would be a good idea here). |
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One issue that had to be solved was how to thread the holes in the rear casting. I felt that given the thickness of the casting, there wouldn't be adequate depth for tapping the holes. The best solution for this seemed to be finding a flange mounted threaded nut and use the holes in the casting to aid in securing the bolts. I did find such parts from a few online companies, but unfortunately not at a cost effective price (they varied from $30 each to $80 each). What I ended up doing was, once again at Home Depot, finding a threaded "tee" nut that was made for wood; it had spikes to dig in on the bottom of the collar. Anyway, I bent these back to make a flat surface and drilled four holes around the collar and into the appropriate locations on the back scope casting to bolt them in (use lock-tight and lock washers). It wasn't pretty, so I just epoxy welded a washer over each threaded nut to cover the screw heads. So what you see here in this photo is the final result with the threaded nuts popping up through the washers ready to accommodate the bolts. Short 1" bolts can be inserted in each hole as covers if I decide not to transport the scope with the long bolts sticking out. There is a little play when the bolts are threaded in before they make contact with the primary due to the fact that the threads of both the bolts and threaded nuts were only available with coarse threads. Once they tighten up, they are very secure. |
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These are the home built locking bolts (carriage bolts) with plastic knobs fastened tight by locking nuts underneath. After I get some experience using them, I may opt to shorten them once I find out what the comfortable mirror distance is. Currently they are 5" long. |
| Section 5 - The Finished Product | |
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Here is a final photo of the C11 modifications: Locking bolts, two cooling fans with switch and power jack, and two additional brushed aluminum side handles. The mods took two weeks to complete, working evenings and weekends. It took another couple of weeks before it actually started to research and plan what I was going to accomplish and acquire all the parts I could get before hand. I really wanted to get it all done as soon as possible to reduce the risk of tainting any exposed optics and lessen the stress factor. After all I was planning on cutting more holes in my pride and joy than Swiss cheese! |
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While everything was disassembled I wanted to go ahead and do anything I had even briefly thought I would eventually do. One of those things was to clean both sides of the corrector plate with a home brew cleaning solution made from ethyl alcohol, distilled water, and a drop of dish washing detergent using 100% cotton balls. While this was going on, I used a small air brush air compressor to quickly blow dry any residual solution before it could spot. Using air compressors on optics is a debatable topic, but knowing this is an oil-less rubber diaphragm compressor and doing previous tests I am confident that my particular compressor is safe for optics. You can't tell a lot from this photo, but the corrector now is cleaner than I've ever seen it. Cleaning the primary mirror is an entirely different game and unless you really have significant muck on it I'd opt to leave it be. I did run the compressor's air jets over it and get some debris off safely, but I don't think I'd ever get the courage to try and clean it unless it really needed it. Before this whole project came to a close with the replacement of the corrector plate, I used a bright flash light and scanned for the tiniest bits of dust anywhere I could find on the tube walls, primary, and corrector. I also thought it would be a good idea to run the fans for a while to be sure they didn't stir up any construction debris that I couldn't get to after the fact. Too bad it won't stay this clean, but you've got to let it go somewhere along the line! |
| Section 6 - Results & Conclusions | |
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To fully gauge the pros and cons of the modifications,
it will be necessary to keep an on going review as I gain more field time
with the scope, rather than a one time impression. I will update this
section as needed to reflect any new findings that occur, good or bad.
Please have patience with this section as living in severely light polluted
Atlanta forces me to drive a minimum of 1 hour every time I want to do
any in-depth work with my scope which prevents me from doing it as often
as I would like. |
