Celestron C11 Secondary Modification

Celestron C11 Secondary Modification:
Springs on the Secondary Mirror for Collimation

The purpose of this modification it to apply compression springs to the secondary of my Celestron C8 and C11. I wanted to show that, with springs installed, it was as easy to collimate the SCT as easily as a Newtonian! It proved to be the case. I chose to use the C8 at first because it is Faster compatible and the secondary was readily removed. The C11 required removing the corrector. Be sure to follow the correct procedures when doing this, particularly the need to return the corrector to the same "rotational" position as when removed. Simply mark the corrector and OTA with an index mark after removing the retaining ring. At the same time, you'll have an opportunity to clean the inside of the corrector, install black "flock" paper or clean the inside of the OTA itself.

I wanted to find supplies easily and readily available. I found that Orchard Supply and Hardware has a nice inventory of springs. The cost of these was $2.49!!

Here's an overview of the procedure:

Fig 1. Springs from OSH

The specification on the spring package reads:
5/16x5/8x.026"
7.96x15.876x.660mm

Century Spring Corp
800-237-5225
www.centuryspring.com

Fig 2. The compression springs
are about one-inch in length

Fig. 3. I simply stood the springs on the backing plate of the mirror. You could use clear finger-nail polish as an adhesive if necessary

Fig. 4. I found that 6/32x1" bolts worked nicely. Note that the spring is compressed about 1/2 its length.

Be careful not to screw the bolt too far into the backing plate. It can go all the way through and touch the secondary. Screw in too far, and the secondary will come away from the backing plate. It is attached with 2-sided tape. If you do manage to dislodge the secondary, loosen the offending bolt and press the secondary firmly against the backing plate. It will re-affix itself. Turn carefully until you can feel that the bolt is fully into the backing plate and is near the back of the secondary. Using 1" bolts or thumbscrews and 1" springs will compress the springs about 1/2 its length. As you collimate the secondary, you will (at first) ""loosen"" the screws. You'll find plenty of play. The "center" pivot post is simply left alone. You can see in the picture (fig. 4) that it no longer touches the secondary backing plate.

After reinstalling the secondary, I used the visual method to reach a quick collimation. I placed the OTA on a flat surface and stood back looking directly into the OTA. I closed one eye and centered the secondary on itself. You can move back until you start seeing concentric rings around the secondary -- "resembling" a star-test. Turning one of the three collimation screws at a time, I was able to bring the rings concentric in about a minute or so. The final collimation was using a star (or artificial star).

I also found that Bob's Knobs (www.bobsknobs.com) can provide you with 6/32x1" long thumbscrews. He will also provide you with a small assortment of spacers (for underneath the knob) if you mention the application. Also, Scope Stuff (http://scopestuff.com) also has an assortment of thumb screws varying in size from 1/2" to 1" in length (http://scopestuff.com/ss_scr1.htm)

Any questions or comments? Send me an email:

-----Dennis Steele [email protected]