Walt's Dob

In late 1999 I had a sudden desire to build another telescope. I had built several back in the 70's and 80's with a moderate amount of success, but after children came along and a move to Florida, priorities had changed. Now, with older children and semi retired, I decided I needed another scope. After looking through several issues of Sky&Telescope and other publications, and visiting a few of the many websites, I decided on a 10" Dobsonian. My last scope was a 12.5" Dob than I sorely missed, but I didn't want to tackle another 12.5" after so long a time. I must say that the "sticker shock" of commercial scopes as well as parts to make one really got me! I finally decided to order a mirror from Coulter, as it was obvious that I could buy a ready made mirror as cheap or cheaper than doing it myself. Besides, I figured it would take me 5 or 6 months to finish a mirror, so buying might be faster as well. NOT!! I called Coulter in early January as an initial inquiry, and then called back around the end of the month to place my name on the list. I wanted a f/7, but was told they could only offer f/6.3, and some were due in soon from the coating company. OK with that, I reserved one and proceeded to gather up the materials and further info to build a mount. I must pass along thanks to Ray Cash for his web pages with all the info available and plans for the Dobsonian scopes. The resource lists really make getting all your ducks in a row an easy project. Because of that, and some extra time available, I had the mounting essentially complete by mid February, but still no mirror from Coulter. The last call to them informed me that a crate of supposed 10" mirrors arrived, but upon opening they turned out to be 8" instead. That's when I decided to call Willmann-Bell. Less than a week later I had the kit in hand. I literally started grinding that evening.
One website I visited was Bob May's on using a metal pipe end to generate the rough curve and then make a tool using ceramic tile and dental stone. Our club in Bowie, MD had made a tool using the hex-tile before, but we epoxied them to two laminated and fiber glassed plywood disks. It worked fine, but the epoxy and fiberglass was a sticky mess, and the dental stone approach had appeal. So, what the heck, I tried it!
First, I couldn't find a cast iron end cap the appropriate size (around 4 to 5 inches), so got a 4" closet flange instead. I screwed a short PVC nipple into it for a handle, and started grinding with the 60 grit. Amazingly, the center hole starting coming in like magic, and by 10 PM that evening I was down to .050" sag and almost out to the edge! Now this was FAST! A few more sessions around the barrel the next day and I was ready to try the dental stone technique. There are several good sites on the web (Russ Durkee, ATM Resource List) that talk about and show how-to-do pictures of the process. Needless to say it works. The thing that impressed me the most about working with the dental stone/hex-tile tool is the grit has the ability to move about freely increasing the rate of work. Also, there were absolutely no problems with sticking, even at the 5-micron stage! I used so little grit at each stage that I have enough left to make several more mirrors! Plus, I still had the 2nd blank that came with the kit. I ended up selling that blank over the internet to another ATM'ers in Michigan. His name is Mark Femminiro, who has moved to the Orlando area and, guess what?!! He is now a member of CFAS!! The polishing went a little less trouble-free, but still relatively painless. In past mirrors I had used a button-lap mold to make the polishing lap, but this time I couldn't find one readily, so I made a sawed lap. Labor intensive!! I must admit, though, that all the literature tells you to be patient and any lap can generally be whipped into shape. It proved to be true, as once I had good contact, the polishing proceeded as expected. The figuring also went smoothly, until I too late realized that I had overcorrected and had a hyperbola, about a 20% overshoot. Several sessions of cold pressing and short strokes got me back to a nice looking parabola. I ordered the secondary and was able to finish the tube unit and star test as well. The star test looked very nice, classic diffraction pattern, nice airy disc, close to the same inside and outside focus. The confirmation was the ronchi grating test. With only 2 or 3 bars showing, they were as straight as a board, with no sign of a turned edge. I was very pleased!!
As far as the mount and tube unit are concerned, two points are worth noting. First, the little rollers I used to allow the tube to rotate. The eyepiece stands at 70" at the zenith, so it's nice to be able to rotate the tube at lower altitudes for better eyepiece presentation. The other item is the two aluminum rings I placed inside the tube at the mirror cell mounting point and the spider mounting point. This strengthens the sonotube so the bolts/screws can be tightened securely.
When I built the scope, I had a small station wagon that would allow me to transport the scope to dark sites for viewing. However, now that I only have a 4 door sedan, I have decided to shorten the focal length of the mirror to 60” so I can fit the scope in the car. So, I have started grinding again on the mirror and have it pretty close to my aim point as of this writing. In the meantime I am enjoying my recently completed 8” f/6 Dob using one of Dan Cassaro’s thin plate glass blanks. It really came out nice, and I can use it either as a Dob, or with my recently acquired German equatorial mount.