I think that I mentioned the long focal length 8" newtonian that I have been building in hopes of getting it ready in time for the TSP. Well, I made it...sort of anyway.
I finished the 7' long tube and finished modifying the mount of my old 8" f/6 on Thursday night prior to departure for Texas on Friday. The scope was actually completed on the observing field at TSP on Sunday. I had to wait on delivery of a 1" secondary mirror for the instrument.
The mirror used is a 1/16th wave mirror (wavefront) that was given to me several years ago. I mounted it in a wooden cell on the Berry design except I used clips rather than the silicone sealer that Berry described. The mount is also modified with a hole that provided ventilation via a muffin fan powered by two 6-volt lantern batteries. Cool down takes about 30 minutes in the hot (100 degree) Texas environment.
The extended sideboards (32" tall) on the Dob mount were not adequate. They allowed the tube to sway in the slightest breeze. Fortunately for me, the wind subsided at TSP after midnight, so the views were steady.
Folks, let me brag a bit about the views afforded by this long focal length (75.5" or f/9.46). They were simply fabulous!
As anticipated, Jupiter was like nothing I've seen in any other 8" scope and inferior to very few much larger instruments including refractors! Following Io during transit was no problem. But what really surprised me was the views of deep space objects! I used a variety of Orion "Sirius" Plossls with NO filters. The detail in the Trifid nebula was unbelievable. The lanes were very well defined. The dark nebulae in the Lagoon was inky black with a very few pinpoints of light. Structure was easily observed in the Whirlpool and somewhat less pronounced but still detectable in M101.
Since this is an ATM forum, I'll not go into other details observed other than to say that the new scope compares very favorable to the APOs costing much more (to say the least!). Of course, it dosen't have the light gathering capacity of the larger instruments on deep space objects, but this is to be expected.
Improvements will consists of a new mount for the scope that will provide more stability. I will also be adding a four-vaned spider to replace the single stalk spider that expediency demanded. Other than that, I doubt that I'll do much of anything to it.
The flange joint that holds the two 3.5' sections of tubing together worked flawlessly. Nothing more needs to be done in this area that I feared would be a real problem. Collimation was maintained all through the night after adjustment before each observing session.
The mirror cell consists of the design Berry describes except that instead of the silicone pads, I used three plastic buttons for support on the 70% zone of the mirror. Three 1/8" conventional type clips hold the mirror loosely in the cell.
The tube contains 7 baffles that double as bulkheads. These provided all the rigidity necessary for the long tube (square 2mm plywood). I am VERY pleased with the scope. It is a conversation piece when looked at during the day, and yet again a topic of conversation after dark when put into use! How many 8" telescopes have you used that require a ladder to view objects at zenith? <g>.
Mike * RM 1.3 * Compatible: Blows up a little later than Incompatible