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Home > Articles > How To > Advanced > Flocking a Celestron 9.25” SCT

Flocking a Celestron 9.25” SCT
By Jay Emirzian - 6/18/2005

Introduction:

Before actually flocking your telescope I recommend investing time to understand the different schools of thought regarding the procedure. Read the online forums and reviews, spend time talking with people to get their perspective before making a decision. In my opinion, it was the cheapest upgrade I could have done to my telescope that gave great results.

The purpose of flocking the inside of any telescope is to reduce the amount of stray light entering the tube from bouncing up into the eyepiece. To get a better understanding of this in a different perspective, we can use sound waves as an example. Regardless of the type of wave, sound, light or other, our senses do a good job picking them up. They also don’t discern what the mind wants to see or hear. Our senses will be stimulated regardless of whether we wanted them to be or not. To give an example, think of a recording studio. The inside walls of a recording studio are dampened with a sound deadener to prevent an echo from entering the microphone. The sound deadener’s job is to absorb the wave, and prevent it from bouncing. This echo will be heard by the microphone and later transposed into the recording. If the amount of unwanted sound can be reduced, the music will be more defined. Now think of this in terms of a telescope. If the amount of stray light can be reduced from hitting the mirrors, the more detailed your image will be. Below I have listed a cookbook style list of materials and methods of flocking the inside of an SCT.

I have also listed two additional modifications that can be performed on the telescope while it’s disassembled. They can be found in the methods section of this article listed under ADDITIONAL MODIFICATIONS #1 and #2. Although it won’t help with the performance of internal reflections, they are great additions to performance. The telescope will already be disassembled and these upgrades only take minutes. It’s worth it.

Although this project can be done by one person, I highly recommend having another set of hands to help where necessary. I listed where that extra set of hands will be appropriate thoughout the article. It will make the installation proceed more smoothly.

Materials:

Protostar flocking paper*
Newspaper
Masking tape
Super 77 spray adhesive**
Clean cotton towels
Dow Corning High Vacuum grease
Flat black model paint and a paint brush
Zip-lock sandwich bags
Tray for holding small parts

Tools:

Yard stick
Ruler
Screw driver (Phillips)
Razor blades
Permanent marker (fine tip black)
Lint roller (sticky paper)
Pair of needle nose pliers
Straight forceps (preferably locking)

*I recommend using the Protostar paper instead of black felt because felt will shed small fibers on the mirror after the job is done and the OTA will be closed up. The Protostar paper won’t release fibers onto the mirror when installed right. This will save time and headache later.

**I am a firm believer in 3M products. They make a spray adhesive called Super 77 that works great. If you can’t find that, another product they make is High Strength 90. I believe the High Strength 90 is more expensive and is a stronger adhesive, but the Super 77 does the trick just fine. You can pick up a can at your local hardware store for about $8.00USD. High Strength 90, I believe, is a few dollars more.

Method: [READ COMPLETELY BEFORE STARTING]

If you don’t feel comfortable with the methods below, don’t start the project!!!!

Make sure you have a nice flat clean work bench to disassemble your telescope. At least 6 ft by 4 ft. The cleaner your area, the better.

Lay the cotton towels over your work area to prevent the mirror or other optics from being damaged.

Make sure you have a place on the table to place the pieces of your telescope before you start disassembling. The less time the parts stay in your hand the less of a chance you have of dropping them.

How to remove the corrector plate:

GETTING STARTED: The one thing to remember here is putting back the telescope EXACTLY the way it came apart. The philosophy about disassembling any telescope is based on returning it to its proper working order. The corrector plate is oriented a certain way with respect to the primary mirror and it is absolutely imperative that it be put back that way. Put a rolled towel under the front of the telescope so when the screws are removed there is no chance of the corrector falling out. There is a metal plate that is sandwiched between the corrector and the screws. Mark a line with the permanent marker on the face of the metal ring in a straight direction towards the front cell. Continue that line onto the front cell. This will allow you to align the screws and metal plate again when they are removed. This may sound childish, but, be sure not to touch the marker to the corrector plate.

REMOVING THE SCREWS: Once the line has been marked, loosen the first screw and be aware of how tight it was. You want to use this much torque (which isn’t much) when reinstalling the screw back into its hole when replacing the corrector plate. It might be a good idea to hold the screw driver as close to the tip as possible without touching the corrector. This will decrease the chance of hitting the corrector with the screw driver. TAKE YOUR TIME. There is no need to rush this. Now loosen and remove all the other screws and place them in a zip-lock sandwich bag and label it “corrector screws” so you know what’s what later.

REMOVING THE RETAINING RING: Now its time to remove the metal ring from the front cell. Check to make sure your black line is visible so you can reference it later. If its not, make it darker!! This metal ring usually has some type of butyl rubber on it to protect the corrector from being scratched. Sometimes this butyl can adhere to the corrector making it hard to get off. Take your time with removing the ring... it will come off.

MARKING THE SHIMS: Once the metal ring is removed look carefully around the corrector plate. Sometimes the factory will install shims to keep the corrector in a specific spot. If your telescope has these shims, mark their placement on the front cell. This may sound obsessive, but if there is more than one shim, number the marks near your lines so you can put them back in their corresponding places. Number the shim as well once the corrector is off.

MARKING THE CORRECTOR POSITION: You also need to place a small line on the corrector and the front cell to make sure the corrector goes back in the same way. Do this just like the metal ring. Make sure you don’t make the line so long that it sticks out past the metal ring. If you do, you’ll be looking at a nice black line on your corrector…Not good. Please be sure all your marker lines are dark enough to see later. Marking the corrector is a VERY IMPORTANT STEP. It allows you to match the alignment of the primary with the corrector when the scope is put back together.

REMOVING THE CORRECTOR PLATE AND SHIMS: Now you can remove the corrector. Usually there is a piece of cork around the front cell to again protect the glass from being scratched. The corrector may be stuck to the front cell like the metal ring was to the corrector. Again, take your time and ease it loose. Don’t touch the optical surfaces. You might be able to get a nice grip on the central obstruction where the secondary is housed to help lift the glass off. Remember those shims? Don’t loose them!!

STORING THE CORRECTOR PLATE: Once the corrector is off, your primary and secondary mirrors will be exposed to the outside air. I find it’s best to keep the mirrors facing downwards or sideways as much as possible to prevent things from dropping on them, dust included. Put the corrector in a safe place out of harms way. I like to face the secondary mirror downwards to prevent dust from falling on it. Good job, but there is still much work to be done.

REMOVING THE DOVETAIL: Removing the dovetail is rather easy. Two large phillips head screws may be found on either end of the underside of the CG-5 dovetail. Remove them and remove the plate…..simple. Place the screws in a bag and label them for reinstallation later.

REMOVING THE FRONT CELL: Now you can remove the front cell. There are usually four Phillips head screws spaced 90º from each other that hold this piece on the tube. Inside the tube the screws are attached to small nuts. Again, mark the front cell and the tube with a small line to make sure they go back the same way they came off. Now holding the tube on its side, hold the nut with the needle nose pliers and unscrew the screw. **Please be sure the mirror sits perpendicular to the table so that if you drop a nut it won’t hit the mirror.** Remove the other 3 screws and carefully detach the front cell from the tube.

REMOVING THE REAR CELL: Now that you have taken off the front cell, it’s time to remove the rear cell, where the mirror is housed. Another set of hands will help during this step. Before you start, rack the focuser all the way in so the mirror is as close to the back of the OTA as possible. Remember to place another marker line on the front cell and tube of the OTA so the tube goes back the same way it came off. I would recommend placing two lines next to each other, on the rear cell, so you can discern the front of the tube with the back of the tube. Now comes the tricky part. Looking down the tube you will see a small space between the mirror and the tube. Between this space you will see the nuts that hold the screws of the rear cell. With your forceps, lock onto one of the nuts inside the OTA down below the mirror. Have your partner remove the screw from the outside of the rear cell. When it is removed, carefully lift the forceps with the nut out of the tube. It might be a good idea to cup your hand under the forceps to make certain nothing falls on the mirror. TAKE YOUR TIME……This step and putting the screws back are the most critical, as you could slip and scratch the mirror. The locking forceps will make this job much easier. Remove the other three screws the exact same way as the first. Carefully detach the tube from the rear cell.

You have now successfully removed the piece that will be flocked. Make sure that, when you start the flocking procedure, you work in a different area then where your telescope pieces are sitting. Remember: You will be working with spray adhesives that will ruin any optical surfaces they contact.

FLOCKING: The Protostar paper comes with a set of directions which explain two ways of flocking the inside of a telescope. One way is to install the paper in one piece and the other is to cut the paper in strips and install them lengthwise. They also recommend one way over the other. I would spend the extra time and install the flocking paper the way they recommend, cutting the paper into strips. You have already painstakingly disassembled your telescope, why not go the extra mile and make it last.

Start by cutting the Protostar paper into 1.5 inch strips that are at least 6 inches longer then the tube itself. Cut at least 30 strips. This step was the most time consuming, but spend the time and your job will look like it came from the factory. It is very important to cut the strips in parallel sections. If the strips are not parallel you will have open seams along the inside of the OTA. Although it really won’t affect the performance too much, I’m a perfectionist and want the job to look nice and perform well. Ideally a very large paper cutter would be great. But not many people have something like this readily available. So you can use a nice yard stick, ruler and a sharp razor blade.

Now that the paper is cut into lengths, you'll want to protect the outside of the tube from overspray. Take the masking tape and wrap it around the ends of the tube, making the tube look extended. I would say extend the tape at least 2 inches so there is no chance of hitting the outside with spray adhesive.

Take the tube outside and lay newspaper on the ground so you have a working area that won’t scratch the paint on the outside of the OTA. I recommend doing this step on a sunny day as the adhesive will tack (dry) better. Give the inside of the tube a nice thin even coat of adhesive. The trick here is to make the adhesive a uniform color and thickness on all sections of the tube. This will give you an even coat. Remember, a little of this stuff goes a long way. Let the adhesive dry for about 5-10 minutes before you start installing the paper strips.

This next step is when you’ll need that extra set of hands. I recommend placing the tube on a padded chair and the two if you sit on the ground at opposite ends of the OTA. Strip by strip, hand the paper through the tube and line them up parallel. The first piece of paper is the most critical. You have to get this piece straight down the tube or the others will follow the same angle. Run your fingers over the flocking paper to adhere it to the glue. Spray adhesive usually works best when both sides of the pieces being glued together have a coat of adhesive on them. Because the Protostar paper is adhesive-backed, there’s no need to spray the strips of paper. You could do more harm then good spraying the back of the paper. When you place the flocking paper inside the tube, let a bit of the paper extend over the ends. This will allow you to cut a nice sharp finish line on the paper with your razor blade. I would recommend cutting each piece as you install it. Continue installing each strip into the tube and cutting them along the way. The last piece of paper will have to be measured and custom fit.

Once all the pieces have been installed, run your hand down the tube on the paper to make sure there is good adhesion.

Now, with your sticky lint-roller, roll the inside of the OTA. I would spend some time doing this step. Spare no time to remove those loose fibers that might fall onto your mirror later. Although the fibers won’t hurt your mirror, you wouldn’t want to remove the corrector again to clean it.

Cut the screw holes out with your razor blade for the front and rear cell screws.

Your tube is now flocked!! Now the only thing left is to put your telescope back together.

ADDITIONAL MODIFICATION #1: This next step is an additional upgrade you can do for your scope while it’s apart (I highly recommend this). Although it’s not necessary to do this step, it can be a nice little addition and doesn't take a lot of time. The grease that is on the mirror shaft is most likely cheap Chinese grease. Have you ever wondered why your image shifts when you turn the focus knob back and forth? It’s because the mirror is sticking along the shaft (that long tube that extends from the middle of the mirror). Get yourself some Dow Corning High Vacuum grease (You can easily find information by Googling the phrase, “Dow Corning High Vacuum grease"). The reason for using a vacuum grease is so the grease, over time, won’t do what they call “degassing” or “out-gas”. This is when the impurities in the grease evaporate inside the tube and form a thin coating on your mirror. Over time the heat from the day’s sun can out-gas the grease. The Dow Corning grease won’t do this, as it is specifically designed for laboratory applications. I have listed a web address above where you can purchase and learn more about the grease. Only use a sparse amount, just enough to make the surface of the shaft slippery. Don’t get any grease on the mirror!! Don’t use anything else but your finger to apply it. Once you have a nice thin film of grease on the mirror shaft, rack the focuser back a forth a few times to work it in. You’ll notice how much smoother your focus will be. I was amazed with the results on my scope!!

Before you start putting the scope back together, paint the nuts that hold the front and rear cell black so they won’t give off any internal reflections inside the OTA.

ADDITIONAL MODIFICATION #2: There is another “how to” step you can complete for the telescope, which a gentleman named PJ Violin has explained in detail. This article explains how to collimate an SCT with great precision. Again, it’s great to accomplish all these modifications to your telescope while it is apart. This way the scope is only dissembled once. Visit the Cloudy Nights review site for PJ Violin's article.

PUTTING THE SCOPE BACK TOGETHER: I will go through the process of putting the scope back together rather quickly as it should go back together the same way it came apart. The lines you marked on the pieces will allow a quick and easy reassembly.

The toughest part of getting your scope back together is getting the rear cell attached to the tube. Two people, again, can be an advantage over one. Put the rear cell and the tube back together and line up the two marks. Use the forceps to align the screw with the nut and turn the screw until it catches the threads. Continue this process with the other 3 screws. This is very tricky, but once it’s done your into the home stretch. REMEMBER…..TAKE YOUR TIME. This is a very tricky step and you don’t want to harm the primary mirror.

Place the front cell on the tube, line up the marker line, and install the screws with the nuts on them.

Place the shims back in there spots and install the corrector plate with the lines matched up. VERY IMPORTANT STEP.

Carefully place the metal ring back on the corrector with the lines matched and install the screws. Remember to tighten the screws using the same amount of torque you felt when removing them.

Your telescope is now back together.

The only thing left to do is collimate your telescope. I have constructed a lens for the Mini-Mag light with a precision hole to aid in collimation. If you would like to learn more about it or need help collimating your scope, please contact me via the Astromart message system.

Conclusions:

Now that your telescope is back together and collimated, take a look down the tube. The internal reflections should now been completely eliminated. Why the factory can’t install this flocking paper is beyond me. In my opinion it will really improve the image quality of commercially made, mass produced telescopes. I would love to hear your comments reports of the before and after results with your scope. How is your planetary detail now? Double star images? Remember, if you have any questions about the above procedure, please don’t hesitate to contact me. I would be more than happy to help in any way I can. Many people have helped me in this hobby; it is my turn to give something back!!!

Clear skies!!!!!

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