In December I bought a used Orion XT10g GoTo Dobsonian from a local guy through Facebook Marketplace. It was a really good price, but that was for a reason. It had problems, some of which I just didn't know to look for when I bought. Here is a list of challenges and how I fixed them. I'll try to include pictures:
1) Collimation was way out of whack. So was the laser collimater.
2) Secondary mirror was hard to adjust.
3) Primary mirror was very dusty. Cleaned it.
4) Buttons on SynScan hand controller were not responding.
5) Goto alignment way off. Elevation readout completely bonkers. Broken encoder?
6) My ignorance about 2" eyepieces.
7) The azimuth motor is creaking/knocking/lurching while tracking.
8) Still to do: Missing dust guard and lurching/knocking azimuth drive.
1) COLLIMATION:
I met the seller in a Target parking lot. I could tell the mirrors were way off, but I was familiar with collimation. I bought a laser collimator new, but the collimator itself ended up being way off. I built a jig to collimate the collimater. I used a hole saw to drill a 2" hole in a piece of wood and clamped it to my workbench (hole pointing down), then put the 2" eyepiece adapter in it. The laser collimater was slightly loose, so I wrapped it with one layer of blue painters tape. Nice tight fit! (see photo). I rotated it in 90 degree increments and marked a spot on a piece of paper on the floor (again, see photo). I pulled the silicone glue out of the collimator set screw holes and adjusted them until rotating the collimator 90 degrees did not produce a wandering laser dot. This was much harder than collimating the telescope because the set screws are incredibly sensitive. Later on, I increased accuracy by tilting the wood board vertical (laser pointing horizontal across the garage) and projecting to 10 feet away and then re-collimating the laser. When the collimater is rotated, it transcribes a small circle with a diameter of about 1/8 of an inch.
2) SECONDARY MIRROR HOLDER WAS HARD TO ADJUST:
I had to adjust the secondary mirror in almost every direction, including shifting the spider (loosening one nut and tightening the opposite nut). The secondary mirror stalk had divots in its base from overtightened set-screw settings, making adjustments very tricky. I placed a washer made of milk-jug plastic between the collimation screws and the secondary stalk. I don't know why Orion uses set screws on their pricier scopes like the XT10g, but they did. I replaced those with Bob's Knobs ($21). Very pricey, but worth every penny. I can collimate the secondary without fussing with any allen wrenches.
3) PRIMARY MIRROR WAS EXTREMELY DUSTY:
The primary mirror was very dusty. I followed the Orion instructions. Filled sink with lukewarm water, some dish soap and cap of isopropyl alcohol. After soaking the mirror for a while, I used cotton balls to wipe straight lines. One cotton ball per wipe. Then I washed it all off with distilled water. It was impressive how all the water just slid off, leaving no drops. I re-installed the mirror leaving a business card thickness gap between the surface and the four hold-down clips. See photo.
4) BUTTONS ON SYNSCAN HAND CONTROLLER WERE NOT RESPONDING:
The ESC button wouldn't work. The instruction manual said to push ESC when you had a problem, but it did nothing. I opened up the SynScan hand controller and found that the gold contacts had dirt buildup (see photo). So I used a pencil eraser to rub the contacts clean. Problem solved!
5) TRACKING/GOTO WAS WAY OFF. ALIGNMENTS PASSED, BUT GAVE JUNK BEHAVIOR:
I first noticed something was wrong because tracking was way off. That's the simplest automated feature. I did the star alignments for GoTo and it was laughably off. I'd select the Orion Nebula and it would slew to some point in a galaxy far far away. So I took it in the garage and started testing azimuth and elevation separately. The SynScan controller has a Show Position feature that displays the azimuth and elevation angles. I leveled the tube and pointed it north, then turned the power on. The readout for azimuth was 0 degrees. I then rotated azimuth physically in 90 degree increments and the readouts were spot on. Then I started elevating the tube and WOW, the readouts were all over the place. I'd move to 45 degrees and it would say 15 degrees. I'd go back to horizontal and it would say -15 degrees. It changed every time. Bad encoder? Big money? So I removed the cover to investigate and found that BOTH set screws on the encoder shaft had worked their way out (see photo with red circle). I put some Lock Tite on the screws and put them back in. While it was all open, I re-greased the gears too. Elevation was spot on after that.
6) MY IGNORANCE ABOUT TWO INCH EYEPIECES:
When purchased new, the Orion XT10g comes with two eyepieces: a 1.25" 12.5mm illuminated reticle eyepiece and a 2" 28mm Orion DeepView eyepiece. When I bought this scope, the batteries were dead in the reticle eyepiece and the 2" 28mm eyepiece was missing. The batteries were also dead in the red-dot finder. Replacing the batteries was easy enough. The reticle and red-dot finder work great now. I bought a used Orion DeepView 28mm and, for good measure, I bought the 35mm big brother to it. The 28mm produced okay images but with blurry edges and the 35mm was unusable with the pincushion and blurriness (I've heard some say the stars are "seagulled"). I learned that my telescope was "fast" with a focal ratio of f/4.7 and long 2" focal length eyepieces have problems with "coma" when used on fast scopes. Put the same eyepieces in an 8" diameter f/6 telescope and they probably look great. So if you plan to use big 2" eyepieces in a fast scope, you need to buy a coma corrector or spend big on eyepieces that work their magic to flatten out that coma effect. Note: My 1.25" eyepieces all produce terrific views, even the 32mm.
7) AZIMUTH MOTOR IS CREAKING/KNOCKING WHILE TRACKING:
The azimuth motor makes a creaking/knocking sound while tracking. I haven't figured out a solution yet. Suggestions welcome. Here is the sound:
https://youtu.be/WOy1qiSktVk
8) Things left to do:
A) The telescope was missing the white dust belt wrapped around the azimuth motors. I'm still looking for a good solution and I think I've found it with 1"x1" air conditioner insulation foam strips.
B) I plan to glue a permanent circular bubble level on top of the azimuth housing for ease of leveling the mount.
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Edited by JJDreese, 21 January 2021 - 09:07 AM.
