Mirror Lock-down Bolts for Celestron Telescopes

Introduction

One of the biggest problems with Schmidt-Cassegrain telescopes is mirror flop. Schmidt Cassegrain telescopes are designed to focus by moving the primary mirror. Since the mirror is meant to move, it can unpredictably shift position, especially when crossing the meridian. To correct this problem, the primary mirror can be locked in place by means of lock-down bolts and focusing accomplished with an external focuser. We are using the temperature controlled focuser from Optec.

This article describes how to install the mirror lock-down bolts on a C-11. The procedure requires you to completely disassemble the telescope. I am providing this information for your consideration only. I do not recommend you perform this procedure. Nor do I assume any liability if you mess up your telescope. Please do not attempt this installation without expert assistance. I did not do this work myself. Gary Burgess, a machinist with many years of experience did all of the work for me.

I also want to point out that, at least for the 14-inch Celestrons, you can order the optical tube assembly with the bolts already installed. This is obviously the best route to take.

Procedure

	Begin by removing the corrector plate. First, put a small piece of masking tape on the corrector plate frame (not on the glass itself) and another piece of tape on the telescope. This will mark the orientation of the corrector plate for reassembly. Remove the screws around the rim of the corrector and carefully remove the plate.
		Again using tape, mark the orientation of the rear cell relative to the telescope tube.
	Then remove the screws that attach the tube to the rear cell. The tube should be very tight in the rear cell. To remove it, use a hammer and a thin piece of wood.
		The picture to the left shows the rear cell after the tube has been pulled off. About half way up the central baffle is a retaining ring. Carefully remove this ring. Spread the ring just enough to slide over the central baffle. If you spread it too far it will permanently deform.
	Next remove the focuser knob from the back of the rear cell. The knob itself is attached with a set screw. The retainer ring is attached with three screws that must be removed.
		Finally, the nut on the focuser bolt must be removed. When this is done, the mirror should slide up and off the central baffle.
	Drill and tap two holes separated 120 degrees from the focuser hole . The size of these holes should correspond to the bolts you will be using.
		We used 1/4 20 threaded rods. On the mirror end of the rod, we epoxied a nylon cap. The rod was then threaded through the hole and an aluminum, knurled handle was threaded on the outside end.

Reassemble the telescope in reverse order, first installing the mirror, the focuser, and the retaining ring. Then reattach the tube to the rear cell. Finally, install the corrector plate.

Conclusion

To use the lock-down bolts, first unscrew them as far as possible. Then focus the telescope using the primary mirror. The external focuser should probably be near its midpoint. Or if you are focusing on a particularly warm or cold night, adjust accordingly. When the telescope is focused, screw in the lock-down bolts until they just make contact with the mirror. The bolts are only meant to keep the mirror from moving so a lot of pressure is not needed. Finally, it would probably be a good idea to check your collimation.

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