Backyard Voyager
Flocking the C11 Optical Tube Assembly by Gregory Wuestenhagen
What is flocking, and why do it? Flocking is a material that is very very non-reflective, more so than any paint you can buy. The reason for flocking an optical tube is to cut down on internal reflections that reduce contrast. Compount optical systems like Schmidt Cassegrains (SCTs) and Maksutov Cassegrains (MCTs) suffer most from light bouncing around inside the optical tube reducing contrast. It is my opinion also that 70 percent of the problem causing SCTs to produce "fat" star images and light scatter around bright objects like Jupiter or Sirius is internal reflection. It is also my opinion, based on experience, that SCTs will, or example, split double stars more cleanly and resolve globular clusters better, show more extent on nebulae and greater differentiation on contrast features on Jupiter and Saturn with flocking.
Look at a typical unflocked C11 optical tube and a flocked tube:
The light you see bouncing around in the tube is only sunlight being reflected off of the walls of a room - no artificial light has been introduced into the tube. Now lets look at the tube after it has been flocked (same tube, 2 hrs later, same room and brighter natural lighting):
see the difference? No light bouncing around inside the tube. Now let me tell you this - I've flocked a C5 and put it up against an unflocked C8 on deep sky objects, and the flocked C5 shows more contrast and nebulosity extent, cleaner images on bright objects, and more resolution on globulars. I think flocking has the effect of improving an SCT by two magnitudes of light grasp. How can this be? Optical authorities will tell you that an SCT system looses
about 30% of its light grasp in internal reflections and loss of contrast. Flocking is the easiest way to solve this problem. Interested in learning about how to flock etc.? Read on.....
Flocking material is available from several suppliers and in two types. I have exclusively used flocking paper made by 3m and sold by Edmund Scientific, and I use the self adhesive type. It is more difficult to work with, but stays in place better over the long haul.
The best way to hold the ota tube and maneuver it to meet your needs during the process is to keep it on your mount. This will work even if you are going to completely remove the tube from the rear cell as long as you remove the front
bolt(s) from your dovetail plate. Make sure you have excellent lighting available. When the flocking starts going in it gets very very dark and hard to see inside your tube.
Before disassembling your SCT for flocking, you need to make some reference markings so that you can reassemble all of the components in the same orientation. If you do not do this, it may not be possible to achieve collimation again with your instrument.
I use masking tape. Place across junctures of the front and rear cells and the tube, then cut carefully with a sharp knife. lets you realign these components as they were before you started. Marking the front and rear cells is only needed if you are going to fully disassemble the 'scope in order to flock it. I usually don't fully disassemble with a larger SCT because it's easy enough to gain access to the innards without doing so.
These marks are critical - you need to be able to realign your corrector plate in two axes and also to re-center it after disassembly. I use masking tape and a straight edge. The cross on the secondary cell aligns with marks on the tape to the top and to the right of the cell.
Tip the ota UP a little to prevent the corrector plate assembly from falling out when you remove the retaining ring. Watch carefully for any loose shims - you will need to reinstall them in their correct positions. Actually, I prefer to shim the corrector plate with the the fuzzy side of self stick black velcro. I place it all around the corrector plate recess in at least 6 equally spaced locations. This is more durable and centers the corrector plate quite well. If you find that your corrector plate is greatly off- center when you first remove the retaining ring, and if your scope was attaining good collimation that way, try to reproduce the same alignment on re-assembly.
See the difference between the flocked surface and the original tube? Look at that reflection. Just natural light bouncing off of the mirror.
You can let successive piece(s) of flocking overlap by 1/8" or so. If you overlap much more the piece will tend to curl up a little.
Here's what it looks like when you're done. Its so dark it will fool your camera's exposure system!
While you have the corrector plate and secondary off let's give it some attention. You will find that the "black" material inside the secondary cell is also very reflective:
This side view shows the reflection on the inside of the secondary holder. It is very important to control reflections around the secondary mirror. Even a baffled SCT or MCT needs this attended to. Don't try to line the inside of the corrector in one piece, do it in two or three. Be aware that it is not a cylinder inside but is conical. The sides slope somewhat outward. After cutting and fitting flocking, let's look at the result:
There is still some reflection in the very base of the secondary system, but for the most part it is much much darker in there!
Flocking the exterior of the rear baffle tube is optional. I think it helps, but it has to be done carefully because of its proximity to the main mirror and because the baffle tube on some SCTs is multi part and the parts slide or "telescope" in and out with mirror movement. If you elect to flock the outside of your baffle tube, turn your focuser to its limit in each direction and observe the action of the baffle tube before you apply any flocking material. I am not entirely happy with the inside of the baffle tubes either.
Some are completely smooth and shiny (low end SCTs) and others have milled ridges like mini-baffles, but are still obviously too shiny. I have not at this point addressed flocking the inside of the baffle tube, since it would be pretty much impossible to remove self adhesive flocking material once applied. I may experiment with flocking paper that is not self adhesive and is attached from an accessible place outside the baffle tube. In the mean time, I think that removing all extraneous reflections inside your ota mitigates if not eliminates the need to flock the inside of the baffle tube.
When you replace the corrector assy. into the tube, line up your marks rotationally and make sure you replace shims as needed. Once you have both sets of marks lined up, replace the retaining ring. The screws in the retaining ring should only be tightened
as much as you can with two fingers. If you over tighten, you can distort the optical figure of the corrector plate. Screws should be tightened in an order so that you are replacing opposite screws as much as possible to even out the tension on the retaining ring.
When you're done it looks like the image at right:
One last thought - you might want to flock the inside of your flexible dew shield, too - unless it's an Astrozap unit, it's too reflective inside.
Enjoy!
Gregory Wuestenhagen is an amateur astronomer and member of the Northern Virginia Astronomy Club. You can visit him at his website: http://www.geocities.com/g_wuestenhagen/