Dobsonian

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Contents

(n8vi's 12.5-inch f/5 dobsonian telescope)

14-inch dobsonian telescope tube (concrete pouring tube) plus the tailgate, the rear of the mirror cell, installed to check size. I have since painted the tailgate black (as you can see in the below image) and added locking ("push") screws.
The completed mirror cell with mirror installed.
Spider, secondary mirror, and focuser installed
August 27, 2012: power out at i3. What to do? Lets try shimming the primary mirror into place to see if this thing works at all. It sure does, and here's the picture taken with a cell phone through a cheap huygens eyepiece to prove it. This in turn proves that the tube is long enough (by at least, it turns out, a foot).
Painting the tube was a lesson in never assuming one has enough masking. Somehow the entire inside of the tube has a light misting of red paint now.
OTA (optical tube assembly) finished and ability to collimate and focus verified.
Altitude bearing complete, runs, as the dobsonian spec requires, "smooth like butter"
Azimuth bearing complete but a little wobbly. Need to sand down one or two of the teflon pads. Then it's just clearcoat and I'm done!


A friend of mine gave up on finding time to start his project to turn the pile of telescope parts he got off ebay into a telescope. The project has become mine. The parts I inherited were a 12.5-inch f/5 mirror, a metal cell for it to live in with a 15.5-inch outer diameter, a rack-and-pinion focuser with a bent pinion, a secondary mirror, and a spider vane assembly. Supposedly, he was supposed to receive a tube, but it never arrived.

The dobsonian is currently living near the middle of i3's west wall.

Tasks

OTA (optical tube assembly)

  • determine focal length of mirror by pointing it at a white board next to a light source and changing its distance until the reflection of the light source on the board is in focus. Divide this distance by two to get the focal length DONE: 65 inches, which makes this 12.5" diameter mirror f/5
  • purchase concrete pouring tube with enough inside diameter to allow cell for 12.5-inch mirror to move around a bit with a length of the focal length of the mirror or longer. DONE. 14 inch ID, 72 inch length
  • build new mirror cell back ("tailgate") for new tube inside diameter. Drill holes for ventilation, plus holes to match the asymmetrical tuning screws on the cell front. Add locking screws. DONE, although the locking screw heads and tuning screw heads collide, but perhaps I'll replace them both. I'd much prefer replacements with knurled circular heads. Maybe I'll make them ...
  • drill focuser hole, with notch for rack DONE
  • drill holes for and mount secondary mirror DONE. It even collimates!
  • Set primary in back of tube, lock it in place with shims, and move the mirror around until something several miles away comes into focus with the focuser at midrange. Do this for a number of observers and average the results. Try it with several eyepieces. A good site to try from is the parking structure at work, from which the Detroit Zoo watertower, five miles away, is visible. System test done, wedged primary mirror in with wood shims and viewed moon (see photo below). Mirror was almost a foot into the tube, however it was still not far enough in for the high power eyepiece (9mm with 2x barlow) to focus. DONE, 31 Aug. Found align point on my lunch break, marked with pencil, checked focus of detroit zoo water tower and a prominence on a roof about 1.5 miles away from the top of work's parking structure. Looks good. Later got a good view of Venus, primary still held in with shims.
  • about now would be a good time to make a cover for the end of the tube, so when the primary is installed, we can keep dust off it.
  • buy a telrad, they're cheap! DONE, arrived, looks pretty awesome.
  • install mirror in the average spot given all the marks from different observers DONE. Looks quite good with the cell held in place with three brass screws sitting in brass grommets. also, this averaging stuff isn't necessary, different eyeballs aren't that different, but different eyepieces are. I checked with a 25mm eyepiece and a 9.5mm eyepiece in a 2x barlow.
  • buy a second smaller tube and experiment with what's necessary to paint it and make it not look like crap. Actually experimenting now with a piece cut off the end of the tube when it was cut to length. May or may not use gap filler to remove the spiralling on the outside of the tube before primering
  • dismantle optical tube assembly and cut the tube to length. DONE
  • Sand the inside to rough up the light wax coating on the inside, and paint the inside matte black DONE. Not pleased with how it looks, I used far too course of sandpaper and tore up the inside of the tube, but functionally it should be okay. May sand and repaint. looked at this again today, it looks fine ... hell, the mild torn-upness of the tube probably baffles light better
  • fill the gaps created by the spiralling paper that makes up the tube with wood gap filler (got this idea from model rocket builders)DONE. Probably a bad idea, I don't have the patience to sand this properly, so it'll probably look worse than it would have without gap filler
  • sand that DONE
  • prime the outside of the tube had to stop five cans of primer in, still not totally uniform though
  • sand that DONE. Now looks much more even.
  • paint the outside of the tube DONE
  • Clearcoat the outside of the tube, depending on results with test tube DONE
  • repaint the inside of the tube because clearly there wasn't enough masking while painting the outside DONE. Used special low-reflectivity paint. Looks good.
  • figure out how to do tube rings, ie, metal rings at the top and bottom of the OTA to prevent denting and tearing Apparently i3 has a machine that magically turns strips of metal into rings, and I have a shop open on Saturday right next to my house that sells metal strips. Just need to decide width. DONE. 8.5 feet of aluminum from metal mart ($10) -> ring roller -> Sexton Enterprises for welding ($15) -> wire brush surface -> looks pretty awesome
  • reassemble tube DONE. Sitting on the west wall at i3 with a plywood sheet on top of it because I still haven't gotten around to making a dust cover.

Dobsonian-style ALT-AZ Mount

  • figure out bearing surfaces Well, the standard dob bearing is Teflon against ebony star laminate, right? Well, buying a sheet of laminate in normal person quantities is a little less than a hundred bucks. You can sample an 8 x 11" piece, but that's a little small for my purposes. [This] place has some smaller quantities though. DONE - teflon on smooth laminate works fine, despite what people say. Also, Dave A. had some sitting around. Thanks Dave!
  • perhaps acquire a small piece of Teflon and wander into countertop places looking for a good surface to run the Teflon on for the bearing surfaces of the mount how about find teflon in my living room? I have found a vinyl LP for the azimuth bearing. Contemplating brushed aluminum for the elevation bearing. DONE, see above
  • build a box around the tube with the semicircle alt bearing surfaces attached to the center of either side. Slide the box until the bearing surfaces are at the center of balance. Put a screw or two into the box from the inside of the tube. Box built, thanks to the help of Dave A with shopping for the right kind of plywood, rabbeting, and not killing myself with a tablesaw. DONE. Balanced, laminate applied to bearing edges. Thanks again Dave A.
  • Attach hefty brass handle to top of tube box. DONE
  • build the alt mounting, ensuring it is tall enough such that the telescope can point straight up. This means that the balance point will have to be sqrt((balance point)^2+(tube radius)^2) high, for the most forward-heavy configuration. DONE. I scaled it so that the eyepiece is at standing eye level when the scope is pointed straight up. You can't escape Dobson's hole though, as you'd have to lie down to view below 20 degrees. May be augmenting rocker box height prior to worrying about azimuth bearings. Rechecked, this time allowing for observing from a chair. It's good from 90 to 20 degrees, with a transition from standing to sitting at around 45 degrees. I call that damn near perfect.
  • build the az mounting, with some sort of screw or bolt protruding from the center of the bottom surface through a hole in the center of the top surface. Possibly we can mount a gear or something here later for motorization. The floor of the rocker box will be square and will rotate on a round ground board so that the furthest protruding part of the base is always a consistent distance from the operator's feet (no surprise trip hazards!). The ground board will have three feet. added a floor board to the rocker box last night after I realized how this should work. Ground board added with teflon pieces into a vinyl LP. I now have functional bearings on both axes. Three feet added, tough rubber, but they may be contributing to the wobble issue.
  • attach two handles to mount (Nope, there will be either one monkey handle or one brass handle up front).
  • stain and varnish the mount or whatever (I will be clearcoating. Nothing more, nothing less.)

Completion

  • Install telrad. Line it up, once again, by lying the tube flat and pointing at terrestrial objects. DONE. Lined up on the moon then slewed to Vega; Vega was then immediately visible in 9mm eyepiece with no correction. Easiest viewfinder I've ever used.
  • FIRST LIGHT (next clear night after this)

Possible Mechanization

The following steps may or may not happen if I decide I like poncet platforms, which is definitely a possibility.

  • write code that fits in an AVR or MSP430 that can pull date and time from a GPS and convert that time into an posix-style integer and back, and can pull lat and lon and convert them into radians and back. DONE
  • write code that converts RA and DEC to ALT and AZ for standalone earth rotation track mode MOSTLY DONE
  • find specs for a scope control protocol full-sized stellarium supports DONE: Meade's sucks, I'm using celestron's
  • make the above happen inside aforementioned AVR
  • talk to Steve about motorization
  • design a board for this uC to live in with motor control and GPS hardware
  • provide serial access to this board through a header that can accept [this] board.
  • produce a small similar board with an RJ45 connector on it to do serial in a cisco-style connector
  • learn to code for android STARTED, I need to put more time into this
  • design basic keypad controller for android that transmits celestron-style commands to the board over bluetooth
  • either buy [Sky Safari Plus] or talk to the [stellarium mobile] people about helping add a scope control module

Tube

I bought a 14-inch ID cardboard concrete pouring tube for this. I was unable to find anything with a 15.5-inch ID, so I needed to rebuild the back of the mirror cell (see below).

The next steps of building this behemoth will all be centered around the tube - sanding the light wax coating off the inside of the concrete pouring tube and painting it matte black, drilling the focuser hole, installing the spider vane assembly and installing the primary mirror. After all that's done, I'll be tearing it all apart and painting the outside. And all of those steps contain fiddly substeps that involve figuring out placement of things. And possibly using a few shims.

Cell/Tailgate (Primary mirror mount)

I rebuilt the back of the mirror cell out of birch plywood, which I painted matte black. This was a most educational process in which I learned that pine plywood is terrible, beam compasses are awesome (the tailgate measurements were done solely with a compass), oil paint is for people who know what they're doing (I just grabbed the first thing that said matte black), wood glue is pretty sweet and easy to use (I attached three pieces on the back of the tailgate to make hitting the tailgate with the screws that will attach it to the tube easier), and that despite how imprecise hole saws look, noone can tell at this scale.

Spider (Secondary mirror mount)

I fretted about drilling the focuser hole and spider mount holes in my pristine tube for quite a while. Turned out it was quite easy - first, I sanded the edge of the tube flat and checked it with a square and a flat piece of board. Then I drilled the eyepiece hole one radius' distance from the edge, with a urethane foam mold of the tube's inside behind the drill bit to prevent tearout. I then filed out a notch for the rack of the rack-and-pinion focuser to fit in. I laid the spider on the outside of the tube to figure out how far back from the focuser hole the spider mounting screws would go. Then I tied a piece of string around the tube, measured all around to ensure it was the same distance from the end everywhere, pulled it off, folded it into eighths, hit one side with a sharpie, and put it back on. Now I had sharpie marks every 90 degrees. I remeasured everywhere to ensure the string was again the same distance from the end everywhere, and drilled the holes. The springiness of my spider vanes eased installation. Successes that surprised me at this stage: I didn't ruin the thing by drilling anything in the wrong place, the secondary doesn't bounce all over the place, and the focuser draw tube fits perfectly in the hole I drilled.

Another thing I learned here is that collimation of the secondary mirror is likely much more easily done without the primary installed, and instead with a light at the end of the tube. (I made a quick collimation eyepiece out of the top of a soda bottle and some masking tape with a hole poked approximately in the middle)

A recent idea I had was to make four small notches in the cell end of the tube. I could then remove the primary and insert two pieces of string (which would cross in the center of the tube, right?) to align the secondary.

Notes