Horsehead
Simple Inexpensive C14 Mirror Lock
 Horsehead

Anyone who has imaged with a SCT has probably experienced "Mirror Flop" when near the meridian, and possibly at other orientations. This is caused in part by the basic design of the mirror mount itself. In order for the SCT to be focused, the primary mirror must move closer or further from the secondary mirror by sliding on the tube that directs the light to the back of the scope. The mirror is only held in place by one connection at the focuser. This single support point can not keep the weight of the mirror from shifting as it moves across the sky. The solution is to lock the mirror and use an auxillary focuser. The problem is there are only a few C14 mirror locks out there. I came up with my own version that took only one trip to Lowes (or Home Depot) and about 1 hour of assembly time.



Click on any thumbnail picture for the larger version
The C14’s primary mirror is mounted to a triangular 3 point support. The transport screws hold the mirror support in two places and the focuser makes up the third support point.
This is one the original shipping mirror lock down bolts. It is too short to reach the mirror after focusing the SCT with all the gear hung on the back. I started with a 3" 1/4 x 20 stainless steel carriage bolt.
My preference was to make a workable (but also nice looking) lock, so I cut off the carriage head and ground down about 1/4" from that end and rethreaded to 10-32. Next, I used a #10-32 x 3/4 Clamping Knob w/Female Insert to serve as the new head.
Screw the knob onto the modified carriage bolt, and you now have a longer mirror lock. Once the bolt made contact with the mirror support, it became obvious that I needed a way to secure the bolt so it wouldn't move , and to prevent it from coming loose. I added a 1/4 x 20 Well Nut.
The Well nut is rubber with a 1/4 x 20 brass insert, so it just screws on the bolt. When installed, the bolt is screwed into the mirror support in the same place as the original lock down bolts. I then screw the Well nut back down against the scope and the mirror is fixed. Here, the new locks have been installed and the mirror is now held by three (3) points instead of just one. Results, the mirror movement was dramatically reduced, but not totally eliminated.(sort of like a three legged stool, have you ever seen one wobble !!)


After I installed the locks, I pointed the C14 at the Zenith and locked the mirror. I then centered a star on the C14s chip and adjusted the Vixen so the same star was centered on the ST-237A chip. I then took a series of images that compared the location of the star on the Vixen/ST-237A after centering the star on the C14. The results were dramatically improved over no locks at all. The comparisons are located here: Pointing Comparison



For those interested, here is the parts list:

    1 package of Hillman 1/4 x 20 Carriage Bolts Stainless Steel Qty 4 ------ $3.64
    Pick-A-Nut #10-32 x 3/4 Clamping Knobs w/Female Insert Qty 2 ------------ $4.12
    Pick-A-Nut 1/4 x 20 Well Nuts Qty 2 ------------------------------------- $1.84
    Total out of pocket expense --------------------------------------------- $9.60 + Tax