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Vixen R150S (SP-C6) 6" f/5 pyrex mirror
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Poster: EverlastingSky (see all of this user's photos)
For an excellent old Super Polaris instruction manual, download this:
For the original Celestron SP-C6 manual download here:

Celestron SP-C6 by Vixen. 6" f/5 Newtonian reflector. Sold by and still in production by Vixen as the R150S to the present day. Seems to have been first available under the Celestron label starting around 1984 and up to 1992. I read a number of quite positive anecdotal comments regarding this model. As a result of these comments (and additional short reviews I uncovered), as well as the curiously low price they sell for, I got one to try out.


Important tips to greatly increase the Vixen R150S (Celestron SP-C6) performance and ease of use:

Take the time and extra effort to really find the center of the mirror to within 0.3mm and place a mark there with a 0.5mm fine tip black marker. A fleck of titanium white acrylic paint can then be placed in the center of this black marker dot with a toothpick. It will reflect light at a certain angle and appear as a grey speck (instead of invisible black) as you look through the Cheshire eyepiece.

Get a good solid machined aluminum Cheshire eyepiece if you don't have one. And make a peephole sight to align the secondary under the eyepiece holder tube. It's easier than doing it with the Cheshire alone. Make one from a plastic plug or 35mm film can. Use a small strip of clear tape inside the eyepiece holder opposite the single set screw. To cancel out most of the error or offset the setscrew induces by pushing the Cheshire collimation eyepiece off center! Guaranteed substantially better collimation assuming a very accurate center spot (more like a speck).

Due to the sliding focuser moving the secondary mirror itself one can use a parfocal eyepiece set to eliminate focus induced mis-collimation. The idea is to remove the need to focus as much as possible. And collimate around the focal point of the most used eyepiece.

Replace the three 6mm machine screws that "pull" the mirror cell with new longer ones of 25mm plus. Add extra nuts to the head end to act as knobs to hold onto. Load these screws with springs. Tighten down the three remaining "push" screws and forget they exist. You won't be using them anymore.

Remove the disc of thin black rubber material behind the mirror. Bond the mirror to the aluminum cell with clear GE silicone adhesive in three large (25mm diameter) support pads. Use 2mm popsicle sticks as spacers under the mirror. Remove them when the silicone cures. You now have an airspace and better thermal isolation between the mirror and its cell. Air can now be forced around the mirror with a fan positioned over the large hole in the mirror cell - see below.

Relax the three mirror clips so that they only act as safety "catchers". Get that astigmatism inducing stress off of the mirror.

Line the rolled steel tube with cork and flocking paper. Blacken the metal edges and surfaces where the mirror cell attaches.

Adjust the brass focus sled gear to remove excess slack. Do this by loosening the two small Philips head screws on the outside of the focus knob assembly. With a finger on the small brass gear and the thumb of the same hand under the focus knob, pull/push upwards while re-tightening the two Philips head screws with the other hand. This can remove as much a one full millimetre of clearance between the brass gear and track. The focus knob assembly has a tendency to "fall" down and open up this clearance. This can add a lot of focus slack. Do not add any grease to the brass gear and track. If there is sticky grease already there then leave it there.

Tighten down the other two very small slot head screws that govern sled tension. They are on the outside of the focus sled opposite the focus knob.

The plastic focus knob is easy to crack if you over-tighten its embedded set screw. Carefully adjusting just how far down the brass threaded shaft the knob is positioned can tighten up other annoying wobble. Once adjusted perfectly I set mine permanently with epoxy to stop it loosening and moving around again. You might also look into finding a solid machined aluminum knob to replace the plastic one altogether.

Flock the secondary mirror holder, primary mirror facing side of the single vane, underside of the sled and inside of the eyepiece holder.

While you have the mirror out add a handle to the rear edge. You will have to drill holes. Find a simple stainless steel drawer "pull" type handle. Bend it through trial and error so that it conforms to the curve of the tube. Otherwise, to avoid this effort, get a single large knob type handle with single attachment point. Place it on the same side as the focuser. Mount it on the thin rolled steel and not the cast aluminum end ring because the screw head(s) that need to be inside will interfere with the mirror cell placement. So on the inside thin steel wall use washer(s) on the screw(s) heads to prevent flexing. Now you have something to grip as you guide the telescope on those wide field sky sweeps.

Look at the aluminum tube rings. Notice that there are two more machined holes opposite where they are mounted. A place to mount yet another telescope piggyback perhaps. Or another handle. Bridge these two points on the tube rings with a strip of aluminum to form another extremely useful handle - the telescope can be carried like a suitcase now (even with the Unistar Heavy still attached).

I added a 92mm computer fan over the cast aluminum mirror cell opening to help expedite mirror cooling. This fan isn't for dispelling the boundary layer on the surface of the mirror. For that purpose fans would have to be mounted on the side of the tube directing air over the mirror surface to be fully effective. The fan is a temporary experiment and is only attached with some clear GE silicone. Its purpose is flushing cold air over the back and sides of the mirror to maintain ambient balance. It also maintains a flow of air up and out of the telescope tube. A wet finger placed near the aperture or open eyepiece tube can detect the airflow. The fan is already a special "silent" variety and has an additional 25 ohm rheostat speed control which reduces the rpm to a near inaudible vibration free rate of around 700. It runs on a battery pack of 9AA cells. Current consumption is so low (about 80ma on lowest, 200ma on highest) that alkaline cells can handle the current draw and still yield close to their full 3000ma capacity assuming temperatures are not too cold. A fan filter was made by sandwiching a piece of vacuum filter material between two pieces of black plastic plant tray. Hot melt glue was used to seal the package together.

Next time I'll just build a Newtonian from scratch and not try to make a junk design work better... The inside diameter of the rolled steel tube is about 177mm. The mirror is 152mm. With only 25mm clearance, 12.5mm on each side of the mirror, between it and the steel tube... well. The tube is much too small and made of the wrong material. As it comes the mirror cell is awful. The focuser is awful. The moving secondary is absurd etc., and on it goes. Still, optimistic as always, I believe these can all be overcome and the result is a neat little "sleeper" scope that is as cheap as dirt and that usually has decent 1/6 wave Japanese made Pyrex mirrors. I always wanted to get inside one of these scopes' and figure out what made them tick and what they could do.

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