Last night, Molly the 16-inch f/4.5 Dobsonian telescope had her public debut at the River Ridge Observatory. It was mostly a successful debut. The images were bright and clear. We looked at the Orion Nebula (M42), Bode’s Nebula (M81), the Cigar Galaxy (M82), M79 in Lepus (one of the few globular clusters in the winter sky). I even chased the International Space Station for a bit until I lost it. I could see that it was oblong instead of a point or circle but I was so busy trying to keep it in view I couldn’t really observe it.
The elastic variable counterweight system worked at first at all angles but later had trouble. I’m not sure what to make of that, maybe temperature. I need to see what weight I can reduce from the top or transfer to the bottom. Also, I brought a light shield for the upper assembly but forgot a way to attach it. That said, I think we just need tweaks now.
I put the 16 inch scope together for the first time tonight. The six truss rods attach with bicycle clamps, collimation knobs and a power button for the fan at the top of the mirror box. I’m going to shorten the ground board legs and the truss rods a bit before I’m done. The six truss rods are connected to each other and as identical as I can make them. I need to add a bungee or something to keep them together while attaching. Plenty of fine tuning to do.
I’ve been doing mostly small stuff on the scope since the last post but today I made a couple of major steps forward.
- I cut the Ebony Star Formica for the altitude bearings and glued them to same. I’m waiting for the contact cement to cure.
- Took the relevant parts back to the River Ridge Observatory for second measuring session. This time I also took my truss rods so that I could mark them. Then I cut them. I cut them long (hopefully) and will remove a little at a time until they are just right.
I’m very close to first complete assembly but I didn’t want to mess up the contact cement by rushing it. After that will come the trimming of excess truss rod mentioned above, plus excess board for focuser and telrad, probably balancing. Truth be told the design doesn’t allow for proper balancing in advance. Hopefully it will be bottom heavy as that will be easier to address.
To follow up on yesterday’s post, today I took the mirror box and the upper telescope assembly to the River Ridge Observatory so that I determine the proper separation. Besides the sky, the site has a view of another mountain across the valley about two miles away. Since yesterday I attached the focuser and telrad boards to the UTA and added the focuser. You can see my hi-tech boards and clamps.
Note: when walking around with a 16 inch mirror in the sunlight be careful not to burn your retinas out.
Things are coming together. I’ve recently painted everything that needed painting. If I had to do it over, I think I would have gone with stain but it’s too late to fret over that and I can’t rebuild something until I build it first. The red is a little darker than some of these pictures. More like the last picture than the second.
The first picture shows the mirror cell resting on the threaded rods that will be used for collimation. In the center of the cell is a fan and at the edge is a push button toggle to turn it on/off. The fan will be used to help the mirror reach equilibrium. I decided early on to go with a simple cell with 18 felt pads in the best locations for support.
Next, we see the 16″ (or 0.4 meter) mirror safely in the box resting on the cell. You can see the three collimation knobs that will allow me to collimate from the top rather than the bottom. Hidden behind the knob on the right is the push button from the other picture that will let me turn the fan on or off.
Finally, for this post, is the upper tube assembly with the secondary mirror and spider. Next I need to attach the board for the focuser and focuser itself. After that I can place the mirror box above and the UTA below on jig to determine the optimal separation and from that the length of the truss rods.
I’m hoping for first light in about a month.
The custom made secondary holder and spider should be here by month end.
I have 7 1.25” aluminum poles, one as a spare. May get one more if I have the opportunity to avoid cost of shipping.
Here is a prototype truss connector I made yesterday plus another picture showing it attached to a simulated upper ring with a cam latch. I’m hoping that will work, one of the scopes I’ve studied uses them and they will be very convenient. Finally a shot of all six. You can tell they are handmade and each is unique. Later I may revisit this but for now I’ve had enough of working with aluminum. Fortunately I have enough aluminum channel (barely) to redo them.
Here we have a simple plywood disk mirror cell in the mirror box. there are 18 felt pads at the calculated points for support of the mirror.
Here I’ve cut out two 30″ diameter altitude bearings and matching rocker box sides from a single 2’x4’by3/4″ sheet of plywood.
Here we see the altitude bearings attached to the mirror box, resting on the rocker box with the mirror resting on its cell.