Giro III & the C8

Last week I found the sky had cleared about 12:30 am and went out for an hour with this setup. The tripod is my own, homemade, walnut, not yet finished but very sturdy. That's the 100mm f/6 on one side, and the C8 on the other. In between is the Giro III that was my Christmas present (I have a very understanding bride who this year let me pick my own present). Just under the Giro III and above the tripod hub is a 60mm-wide rubber washer I obtained from Lowe's.

The Giro III is a fine piece of work. The C8 with the WO focuser and the 8mm Baader eyepiece is pretty heavy, but the Giro III handles it quite well. The 8mm provides a magnification of 254x. I thought that might be too much for the mount, but it was not so. The field of view is very small, just a quarter of a degree. To illustrate, M13 pretty much filled the view. I was just able to put Jupiter and three moons in the view; the moon furthest out would not fit or just barely fit with the planet quickly moving outside the view (Jupiter was awesome to behold that night at that magnification). Most things I see in that view disappear in about a minute tops, and they filled the middle 50% of the view for only 20 or 30 seconds. That means that I have to move the scope often. Moving the scope is not a problem, really, for me: I'm a dob fan. But dobs are so large. With the Giro III, I was hoping to get the benefits of a dob mount for my refractors and SCT. And that's about what I have. The primary issues for a mount of this type are, for me, backlash and settling time. Settling time is less than a second with this setup (with my rubber washer in place), and that's if I actually move the tripod. Just rapping on the tripod does nothing. Bumping the scope itself requires the less-than-a-second settle. If the scopes are balanced well, there is virtually no backlash. On this night, I was balanced a little too far backward for work at the zenith and tightened down the altitude axis to compensate, and even then the backlash was only 0.03-0.04 degrees, well within workable limits, even at 254x. I will definitely not hesitate to put the 8mm in again.

There are some drawbacks. My eyepieces do not all weigh the same, and sometimes I have to re-balance when switching eyepieces. That's a pain if I have to find objects twice. But the virtue of the dual arm setup is that I do not need to switch eyepieces. I use my 4-inch as a giant finder (35mm ultrascopic eyepiece, 17x) and the C8 for a closer view.

[Update: I later removed the rubber washer. The mount handles vibrations almost as well without it, and I was continually (once every four or five uses) having to tighten the bolt holding the mount when the washer was in place. On balance, I decided it was not worth the effort. The mount performs beautifully without it, too.]