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.
I was able to take these images of Venus and Mars Saturday night. The goal was Venus with Mars being way past its last opposition of about 9 months ago and now only about 3 arc seconds across.Venus on the other hand is close to one arc minute across and will only get larger for a few weeks before passing under the Sun and entering the morning sky. Venus was shot with a red filter and even so each frame was a quarter of a millisecond. It was the best 20% of 10,000 frames. Mars was best 20% of 5,000 frames with no filter and each frame was one millisecond. Last night Mars was about 6 magnitudes (or 225x) dimmer than Venus. Both were low in the west and seeing conditions were awful. Also seen but not imaged was Uranus, which was pretty close to Mars.
Rufous Hummingbird visiting Stout House in Little Rock for the third winter in a row. I arrived at 6:30, he was there before I could set up. I watched him for a couple hours.
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.
That sounds better than Penumbral Eclipse of the Moon which is more accurate. I was on my way out the door when I saw and remembered the eclipse so I ran back in for the camera. I was surprised about how easy it was to see this non-umbral eclipse. That said, look to the upper left near the Mare Frigorus area. That looks darker there. I don’t know how close the umbra came tonight but its easy to imagine it’s right there off the edge of the Moon.
Here’s a picture of the left to right from this evening. The Moon and Venus are, respectively, always the second and third brightest objects in the sky not counting occasional supernova. Mars on the other hand gets really bright for a couple months every two years and then fades to relative obscurity. So it is here. The Moon is about 500 times brighter than Venus and Venus is perhaps 100 times brighter than Mars right now. For this picture, I used a regular DSLR with a 300mm lens set to 6400 ISO and a 1/80th second exposure. That was the shortest exposure that showed Mars and while longer exposures showed the ruddy color of Mars they also blew out the Moon and Venus. So, I had to compromise.
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.