Trixie, a 13.1" f/4.5 Astrograph

May 2008 to July 2008

I had been avoiding "large" telescopes, because weight is critical to an astrophotographer,
 and I was worried about overtaxing my mount. Also, the price of mirrors goes way up once you
get past 10" or so.
Then I found an offer I couldn't refuse, a 13.1" f/4.5 primary and matching 3.1" secondary
 mirror from a Coulter Odyssey scope. Normally, these go for about $600 on Astromart, but
 this one was only $400, and I jumped on it.
When it arrived, I was stunned at the size of the mirror. I've had only 8" telescopes to 
this point, and the 13" was clearly of a different scale of mirror entirely. For its size,
the 13" primary was relatively light at 10.5#, because it's only 1" thick. That alone made me
start to think of this as a possible astrograph.
I wanted to keep the total weight of the OTA under 30# when fully assembled. The photo rig adds
another 10# or so, and I didn't want to approach the weight limit of my G-11 mount (which claims
60#).

I decided on a 3-tube style, for 3 reasons. First, I felt that it would be lightweight compared
to an 8-pole truss design (either of which is lighter than a solid tube). Second, because the 
design calls for "just rings and tubes", it would require minimal woodworking skill to execute.
Finally, I like the spare-but-functional look of the design.

In order to add a personal touch, I decided to cut the rings in a Reuleaux triangle shape, 
instead of making them circular. The shape can be constructed with compass and straightedge, and
also leaves a lot more "meat" of wood near the attachment points for the tubes, which seemed like
 a good idea.
The primary came with a mirror cell. The secondary came attached to an object that I refuse to
give the title "spider" to, so I needed to order (or build) a spider. After playing with some 
designs that would have reuqired me to build the spider, I decided to buy one, and that choice
informed the rest of the upper tube assembly (UTA). Using The Book (Kreige and Berry's excellent
_The_Dobsonian_Telescope_) as a guide, I calculated that the hole in the upper ring should be 14.25"
(which is enough to allow a 1degree FOV for the 13.1" scope). This effectively became the "inner
diameter" of the "tube", from which the rest of the scope was scaled to fit.
With the tube ID, I could order the spider, and also figure out how much wood to buy.
It turned out that 2 rings would fit on a quarter-sheet (2'x4') of plywood. So, with 3 rings to make,
I bought a quarter sheet of 1/2" Birch ply and a quarter sheet of 1/2" red oak ply. I bought the 
"cheap" stuff at my local home improvement store. I could have paid twice as much to get better 
ply from a specialty store, but given the abuse to which I put the wood, it would have been a 
waste of money, and I think that the ply I got looks fine, 'nuff said.

The parts list is pretty simple:
primary mirror
secondary mirror
mirror cell
spider and secondary holder
focuser
wood
tubes
screws, bolts, and brackets to hold it together

The scope took several weekends to build. If I'd been able to work on it straight through, it wouldn't have 
taken more than a week of work. Here are some photos of the construction in progress.

Now that the woodcutting is done, I moved on to varnish. Varnish turns out to be a bit tricky to work with. Bugs and bubbles are two enemies that I did not quite vanquish. However, I did get 4 coats of varnish on each side before I had to be out of town for a couple of weeks, so I left the varnish to set up, and when I got back, voila! Beauty.
The tubes are going to be aluminum, but because of my design, they have to be cut to exactly the correct length. So I decided to use some spare 2x2s I had lying around to get the length right, then I'd know how long to cut the aluminum. I'd had inklings as I got each new piece of the scope that this was going to be a massive object, but I really wasn't mentally prepared for *how* massive. Once I cut the temporary "tubes", I could knock the scope together pretty quickly, and there it was. The photo of the OTA standing in the hall doesn't really do it justice. When it's standing up like that, the upper ring is at armpit level. It's big.
To check the focal length, I set the scope up on a pair of sawhorses, and pointed it at a radio tower several miles away atop one of the local hills. These shots are at 70mm, 300mm, and 1500mm, respectively.
And with the aluminum tubes finally cut, here it is! Trixie weighs in at 26 pounds as she sits. If you look closely, you'll notice that there are no mounting plates and also no holes in the already-varnished wood to receive them. Once I cut holes for the plates, I'll be ready for first (star) light.
First light! I decided to press ahead and put on the mounting plate tonight (same day as the tubes got cut). Just in time for night to fall, I finished up and got Trixie on the mount for the first time. With no finderscope, the first night was a little rough until I got the G-11 synched up for the first time. After that, it was just a really great test drive. I got photos of Vega and M27 (each is a single 30sec unguided frame), and even spent a little time at the eyepiece. The difference between the 13" and the 8" scopes is amazing. There are a *ton* more stars, and the bright DSOs (M27, M57, and M17 in particular) are noticeably brighter and more detailed. WOW.