It's been nearly a year since I last pulled out my 11-inch Schmidt-Cassegrain and its Celestron CGE Pro mount. I describe some of the issue in my series of posts last year about the Texas Star Party (starting with night 3's post), but I'll give you the rundown.
It all started at the 2017 Hidden Hollow Star Party, in late September of that year. Just a few months prior, I had roaring success with the CGE Pro at the Green Bank Star Party in mid-July, accomplishing 8 minutes of guiding at least, which for some isn't much, but for me is a lot! (And by that I mean, I could take an 8-minute image and not get any star streaks from tracking/guiding errors). At Hidden Hollow, however, I started having issues with guiding -- my guide plots were all over the place, and I couldn't take images longer than like 30 seconds of so without streaking. This was terrible! I finally figured out that the declination axis had some kind of jitter -- when I slewed slowly, I could literally see the stars bouncing back and forth. The right ascension axis was still buttery smooth. Fellow club member Jim and I popped open the casing and took a look, but nothing looked out of place. I could, however, rock the dec axis back and forth by hand, which was not a good sign. But it started getting cold as soon as I got home, so the mount stayed indoors all winter.
It didn't warm up enough that spring before the Texas Star Party for me to pull it out and tinker, so I brought it as it was down to TSP. I figured out how to remove the cover while keeping the telescope loaded, and observed that the worm gear had some somewhat separated from the main dec axis cog wheel, which was causing all of the slop. I tightened up the bolt that pushes the worm into the cog, but finding the exact right position for it was tough -- I didn't want to cause binding as well. Some folks around me offered ideas, and I even talked to the Celestron people who were there, but unfortunately they didn't have any of their engineers in attendance. They were flabbergasted to hear that the CGE Pro was having this problem, especially after it had been Hypertuned!
And finally here we are, a year later. Time to see where things were at.
I got home somewhat late from work, and was halfway through the first episode of the last season of Game of Thrones when I looked at my watch and saw that it was already 8:20 PM! So I paused the show and started pulling my gear together to take downstairs to the front yard to set up. I just needed to see some stars so I could guide. It was pretty clear that night, although chilly in the mid-40s. But looking at the forecast, it would be my only opportunity before TSP most likely.
Once I put the cats away in the den so I could leave the door open, I picked up my CGE Pro mount head in its box -- I forgot just how much that thing weighed! It's about 60 lbs! And I have been slacking on the gym-going over the winter. As I started to descend the stairs inside my apartment, my legs began to wobble, so I had to change tactics. I put the box on the stair a few stairs above me, and then braced it on my legs to move it down one step at a time. It was slow going, but much safer and easier. (I almost fell down the stairs with it once). All of my the rest of my gear felt light by comparison! All told, I moved about 250 lbs of gear or so down the stairs.
"The Beast" loadout: box with the equatorial section, pier, cables, nuts, and bolts; C11 telescope; 80mm guidescope; mount head box; three counterweights; tripod; computer table; tackle box; cameras; battery; filter wheel; and some warm clothes.
It took me about an hour to set it up, which went pretty smoothly for not having done it in a while. They only thing I couldn't find was the SCT-2-inch connector, which I ended up finding in my ZWO camera bag, where I had strategically left it previously to use with my C8 for planetary imaging. I went ahead and moved it to the tackle box, which is where I expected it to be. The tackle box is like my favorite thing; it's enormous and probably weighs about 30 lbs fully loaded at the moment, but it was everything I need: batteries, cables, adapters, extension tubes, parfocal rings, nuts, bolts, hex wrenches, screwdrivers, scissors, velcro, vice grip, pliers, cable ties, and more.
After attaching the camera and filter wheel, I got it balanced, and realized I needed that third counterweight! I could hear the tripod creaking as I moved things around. I tightened the leg leveling bolts and the stabilizer a bit more.
I haven't turned it on since last May :(
I was going to look visually through it first to see if the jitter was still there, but since the camera was already attached, I decided to go ahead and polar align it, align it, and see how the guiding would do in its current state. I firmed up the worm gear pretty decently last year, but the data I took after that were relatively low in altitude, so the guiding may have been affected by the atmosphere.
One annoying thing about this mount is that you have to re-position the altitude adjustment screw for latitudes above 40 degrees and latitudes below 40 degrees. When you get close to 40, however, you're really running out of screw at least a few degrees before that. Plus, one of the bolts you need to remove to do the switch is stripped! So I can't actually do it. Fortunately, one of my friends at the Green Bank Star Quest in 2017 came up with a simple and ingenious fix -- give the screw a little extra length with a giant nut!
So simple! And works so well!
I set the altitude marker near my latitude, checked that it was pointing mostly north, and then fired up SharpCap to do a proper polar alignment. The Pro version of SharpCap has an excellent polar alignment tool, in addition to others, and it's only $15/year. I got everything connected, and after finding focus, I started the procedure. I could see numerous stars easily, but it was unable to plate solve the image! I messed with gain, noise, and exposure settings, but was getting nothing. Then I wondered if I was just so far off the pole that it was outside the plate-solving database area it had. I looked through the Telrad - and yeah I was way off in altitude! But I had set the altitude using the marker, hmm...oh wait, the nut adds like five extra degrees of height *smacks face*. So I adjusted both axes until Polaris was near the center of the Telrad, and then tried SharpCap again. Plate solve no problem!
Now, if you've been following my blog or have spoken to me before, you have heard me say how terrible the altitude adjustment system is on the CGE Pro (Celestron have a much better one on the CGX-L, I will note). Basically, the gist of the problem is that when you turn the altitude knob, you are pushing against the entire weight of both the mount head and whatever telescope, counterweights, and other gear you have loaded. For me, this means I'm trying to turn a knob against over 160 lbs! Turning by hand is impossible, and I borrowed a belt wrench at last year's TSP, but that was also pretty difficult. But, last year, I noticed that the hand knob had holes in the side all around it. So after I got back, I went to Lowe's and bought two long rods that fit the holes perfectly. Now I can get all kinds of torque, and it turns no problem! I do have to take them out every quarter turn to shift to the next set of holes, but it's not too bad.
While I was polar aligning, I noticed that the stars seemed less in focus near the edges than the center. This suggested I needed to collimate. I decided to try what I've been meaning to try -- collimating with my camera! I slewed over to Capella, defocused the star, messed with the exposure time to get a nice image, and then saw that my donut was, indeed, a little asymmetrical.
One of the cool things about collimating a Schmidt-Cassegrain is that you can literally point to one of the three adjustment screws and see which one you are pointing at in the image. This makes it very easy to know which one to turn! So I made the adjustments, and got it probably fairly close.
After that was all done, I aligned, which went pretty smoothly -- after polar aligning, now each star landed in the field-of-view of my camera, and then I just had to center it in the camera view. I did two western stars and two eastern. I did notice, however, that the alignment didn't seem to get better - even after the fourth star, they weren't landing much closer to the middle of the image. Weird.
Next, I needed a quick imaging target -- I could have just dropped it anywhere to check on guiding, but I figured I may as well grab a few images while I was at it. So I popped over to M13, but I stopped by nearby Alphecca first so I could focus the guide scope. I had to add an extension tube to get my QHY5L-II to focus into my Celestron 80mm, but once that was added, Alphecca first appeared as an enormous circle on the bottom of the image, so I just had to jog the scope up a bit so that it would be centered in the guide scope for focusing.
Finally, with all that done, it was time to calibrate PHD for autoguiding. I got it connected, and was just about to hit go when...my tablet shut down! Running both my ZWO ASI1600MM Pro and my QHY5L-II off of it were just too much for the battery while plugged into the 1.5A USB power on my Celestron Power Tank. I really need to be on 2.4A to run cameras, but an even better option is to just power the USB hub. (The ZWO's cooler runs on 12V power, but the camera itself can run on 5V USB power alone, which is handy for planetary imaging when sensor temperature matters less). So I ran upstairs and grabbed my 2.4A smartphone battery and a micro USB cable to power the USB hub that everything was plugged into, got everything hooked up, rebooted the tablet, and was rolling again not too much later.
I calibrated PHD...and got a non-orthogonality warning! It did look a little non-orthogonal, and there were a few bad points.
I accepted it anyway so I could see what it really did to the images. I flipped over to Sequence Generator Pro and finally took my first image of the night, around 11:15 PM, of M13. I took a 3-minute image to start, but I could already see some pretty bad streaks.
Cropped view of a streaky M13.
I tried re-calibrating, but it looked even worse.
And guiding looked bad.
So I finally took off the camera and filter wheel and threw an eyepiece on, which I should've just done in the first place to save me a bunch of time. Sure enough, the problem I started seeing in 2017 was back, with a vengeance! Slewing in dec at slow-ish speeds made the stars jiggle back and forth as they slewed, making little streaking lines to my eye. I could hear it happening too -- it almost sounds like something is sticking or dragging. It was worse pointing west than east, but it was still present looking east. It's bad enough that I can probably work on it during the day and not need to see how the stars are affected because the sound is so obvious. Maybe I'd be able to see the jitter looking at trees or something too from inside my living room. I'm hoping to get a chance to take a look inside and tighten things up during the daytime before I have to leave.
I tore down shortly after that, around midnight. My fingers started to freeze because so many tasks require having ungloved fingers, like attaching cable ties, loosening thumbscrews, etc. I hauled everything up the stairs as quietly as I could so as not to disturb my downstairs neighbor, but I'm pretty sure she hates me, haha. For the mount head, I did the same thing going up as coming down: I used my legs to brace it and push it up one step at a time. My knee had started to hurt from accidentally turning it out a bit when I lifted, so I ended up having to do bad-form lifts using more back than legs. My back was sore for three days! Whoops...
Can't wait for the Texas Star Party! Hopefully I can get things to work! I think I've got a solution for the dec axis sticking/freezing on my Celestron AVX as well, but haven't had a chance to finish implementing it yet. I'll write a post on that whole process when it's done.