11” Celestron SCT and the Losmandy G11 Mount
Schmidt Cassegrain Telescope
f/10 - First Light about 2002
This is the C11 Schmidt Cassegrain Telescope (SCT) from the early 2000s. When I purchased it it was sold as the CG11. The C11 part was for Celestron's 11" SCT and the G11 was for Losmandy's G11 German Equatorial Mount (GEM). This is a solid combo and tracked visually nicely. I used it on and off until about 2005 when I started building larger Newtonian telescopes.
The CG11 was barely used for 17 years. In 2022 I decided to take a shot at digital astrophotography. This caused me to dust off the G11 mount and upgrade it with new motors and go-to electronics. In mid 2023 I did the Level 6 firmware upgrade to the mount. This was unusual because the motor controller firmware in the two PIC microcontrollers needed to be updated in addition to the SD card in the controller and the hand controller. Fortunately that went well.
I'm slowly rigging up the C11 for astrophotography but at f/10 the C11 is a slow gatherer of photons. I decided to try the G11 mount with the little Megrez 80 first. After some bumps and a reasonable amount of success I swapped out the Megrez 80 for the C11. Due to the much higher magnification and the potential for mirror shift I employed an off axis guider for tracking.
Imaging Train
Here's the C11 with the Celestron focal reducer (F/10 becomes f/6.3) that I wanted to try with the camera. The Celestron reducer requires 105mm back focus. This is accomplished by adding the Focal Reducer, the Celestron T-Mount adapter, the ZWO Off Axis Guider with a ASI174MM guide camera, ZWO v2 Filter Tray and, finally the Camera. This option will use the off axis guider instead of a guide scope to align tracking with any mirror movement. While HyperStar is an option I really wanted to try it with the longer focal length. The camera is the ASI 2600MC. I have been a bit disappointed with the vignetting where the outer part of the image is much darker than the middle. This may need a wider light cone. I'll get back to this.
Electronic Automatic Focuser Bracket
Like the Megrez this old telescope didn't have a bracket option that would connect the electronic focuser to the C11.
The 3D printed EAF bracket. While ABS plastic is a pain to use (glass mat with lots of hair spray to get it to stick) the end result is much more heat resistant than PLA plastic.
Here's the EAF bracket mounted using the 3D printed motor to focuser shaft coupler. I tapped for 4mm cap end screws in the ABS plastic to bind it to the two different sized shafts. The bracket and motor coupler have held up well.
SCT Collimation
One really nice side effect of having a astro camera connected to your SCT is that you can look at the computer screen's image of in and out of focus stars, zoom in and tweak the collimation perfectly. I now have the best collimation that I have ever had on this telescope. I should add that having Bob's Knobs on my secondary mirror made the process quicker.
