Look Mom, No Forks! My Adventures Deforking a NexStar 11 GPS

About 18 months ago, I was graciously given a telescope by a member of my town’s local astronomy club: a Celestron NexStar 11 GPS. I could hardly contain my excitement! This scope would give me the ability to see things I couldn’t with my smaller refractors. It also presented me with a new challenge; getting it to work for long-exposure, deep sky objects. Here’s how things have turned out so far.

A Beast of a Telescope

First built in 2001 (mine’s a 2002 model), the NexStar 11 GPS was one of Celestron’s flagship telescopes and consisted of a C11 Optical Tube Assembly (OTA) and a forked ALT/AZ mount. It carried a $3500 price tag at the time of its release, but it came with literally all of the bells and whistles, some of which still don’t come standard on similarly-priced scopes today. Its coolest feature, in my opinion, was that it had built-in GPS and it could align itself automatically. You literally got it level, pushed one button, point at a star or two, and five minutes later, it was ready to go.

That said, you don’t get scopes this big without some serious heft. The forked portion of the scope weighed in at 65 lbs and the tripod tacked on another 26lbs. It was massive, and only the biggest of bear hugs could move it around. I’m not kidding.

Figure 1: My 6-year-old son standing under the C11, when I still had it on the fork mount.

Even so, the 91lb curb weight was possible to move from my porch to the yard all in one piece when done properly, and it was a lot of fun to use that way. I could get home from work and be looking at the moon and planets with my wife and kids in just a few minutes. The only problem was that I really wanted to get tighter images of star clusters and nebulae with this scope, and you just can’t do long exposure photography straight from a forked mount (because of field rotation). To do that, I had two options; either add a wedge between the tripod and the forks, or remove the telescope tube from the forks and put it on an equatorial mount. The first option would add another heavy chunk of equipment to the mix, and I’d probably have to disassemble the whole setup each time, thereby losing its appeal as a “grab and go” scope (not to mention the added $300 cost of the wedge). Since I already had a capable equatorial mount, I decided to go for option two.

The Swap

Before I could get to work removing the C11 from the forks, I needed to make sure I’d be able to mount the tube to my equatorial mount, an Orion Atlas Pro. Many equatorial mounts like my Atlas use what’s called a “dovetail bar saddle” to attach scope tubes to them. The saddle is trapezoidal and clamps down onto a dovetail bar attached to the OTA to keep the entire assembly from falling off. Because my C11 was attached semi-permanently to the fork mount with pins and screws, it didn’t come with its own dovetail bar, so I had to order one.

Figure 2: Left, a dovetail saddle, similar to the one on my Atlas Pro mount. Right, the 11″ Celestron dovetail bar I ordered.

Once the dovetail bar came in, I opened the box and quickly found that the screws that came with it weren’t going to be quite long enough. So I went to Lowe’s and bought slightly longer screws. Now you have to be careful about doing this because screws that are too long can end up pushing into either the primary mirror at the back or the corrector plate at the front of the OTA. I can’t remember what length I ended up going with, but by eyeballing the original screws (see figure 4 below) against the dovetail bar and the body of the OTA, I chose the next-shortest length available. I only needed about 1/4″ longer than what I had to begin with. New screws in-hand, I got to work removing the OTA from the forks.

Figure 3: Left, the new dovetail bar and the C11 OTA still attached to the forks. Right, getting one of the arms loosened up. The two screws at the bottom needed to come out. There were two other large bolts under the handle area (not pictured) that also needed to be removed to loosen the arm.

Once the fork arm was loosened up, it was pretty easy to pry it open just far enough to pop the pins out of the OTA tube. There were also two long arm-like structures running the length of the tube that had to be removed as well. Next step was to get the new dovetail bar onto the scope. There are two places you can mount a dovetail bar, and I’m not sure if it matters which one you choose. I ended up bolting mine to the side of the tube that shares the carry handle (seemed to me that it would more likely be the “stronger” side, if any side is strongest in the first place).

Figure 4: Left, the bottom of the OTA, near the handle. One end of the dovetail bar bolts in where the two right screws are. Right, the top of the OTA. The two bigger screws are where the other end of the dovetail bar bolts into.

 

Figure 5: The C11 Deforked!

Now that the C11 was free of its ALT/AZ fork mount, I could test it out on my Atlas Pro. I was admittedly a little nervous – even apart from the forks, the C11 weighs somewhere around 30 lbs. This would be the heaviest OTA I ever tried to use on my Atlas. The Atlas Pro is rated to carry 44lbs, so it’s still well below the max payload rating, but you have to lift the tube rather high and in a somewhat awkward position to get it attached, and I was worried I’d drop it. But it was too late to turn back now, so I gave it ago, and after a couple minutes of figuring out how to contort my body in ways I never thought I would, I got it mounted.

Figure 6: The C11 mounted on the Atlas Pro, looking at Jupiter through the trees in my neighbor’s back yard.

The mighty Atlas Pro doesn’t look so might with that massive C11 on it anymore, now does it?  Disproportionate appearances aside, it handles the weight of the OTA just fine, at least for planetary imaging. But I really wanted to see what this thing would do when given tougher targets.

Swap Complete, Now What?

In my excitement to do this mount swap, I had completely forgotten about guiding! To the uninitiated, guiding involves using a second camera and a computer to keep the scope on target during imaging. I have a separate guide scope that I use with my shorter scopes, but the C11 comes with nowhere to mount it, and I was concerned about getting too close to the maximum weight limit of the Atlas by adding that second scope. The focal length of the C11 is also quite long, so using my guide scope would probably not be practical. I needed to invest in an Off-Axis Guider.

I ended up choosing the Orion Thin Off-Axis Guider. It’s a pretty cool little gadget that uses a prism to deflect a small amount of light from the main OTA into the guide camera. It’s a lot trickier to use than a separate guide scope because the guide camera no longer sees a portion of the same field of view that the main camera sees. It also operates at the same focal length as the main scope, which can make finding a guide star pretty tough. There’s definitely a learning curve, but I gave it a few months of trial and error and finally got it down. But, because it is attached directly to and receives light from the main scope, it will never suffer from issues like differential flexure that can sometimes happen with a separate guide scope, and yields better guiding performance.

Another thing I needed was a Field Flattenter/Focal Reducer. Camera sensors are flat, but the C11 projects a curved field of sharp focus, so in order to take halfway decent long exposures with a DSLR on this scope, I needed to correct this field curvature. I ended up buying Celestron’s F/6.3 focal reducer. There are better reducers out there, but this is the best one that my budget would allow.

First Long Exposure

This past weekend, my family and I went to Navajo Dam, New Mexico, on our first camping trip of the summer. It’s a little late in the moon phase for nebulae, but I wanted to test my C11’s newfound long-exposure capabilities on something a little brighter anyways, so I chose M13, the Great Cluster in Hercules. Here’s how things turned out.

Figure 7: Messier 13, the Great Cluster in Hercules. Taken with my deforked C11 and a Canon EOS 70D. 33x120sec lights @ ISO400 and F/6.3, 60 bias, dark, and flat frames, calibrated and stacked using some custom software, Lightroom 6 for color stretching, and GIMP 2.8 for healing out some residual walking hot pixels.

This image is slightly cropped as the field of view was a little wide for my tastes and showed some coma in the corners. I’ve heard that the coma issue can be fixed by replacing the Celestron Flattener/Reducer with one from Starizona, so I may try that next when budget allows.

Final Thoughts

For now, I’m really happy with the results. It’s been a long, and sometimes frustrating road, but I’m glad that this setup is working. I’m looking forward to taking it out on more camping trips this summer, improving my skills, and of course, sharing the results with you!

Are there any projects you’re currently working on or have recently finished? Feel free to share in the comments below!

Note: for a more-recent update on what I’ve been able to do with this scope so far, read my latest blog about it here.