Quote:
Originally Posted by glend
Castors are a bad idea in my opinion because they will transfer shock and vibration directly to the structure, with possible effects on collimation. An inflatable tyre trolley would be better. Secondly, if you are taking the dob to a dark site the castors will be useless on the ground and sink in or lead to instability.
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I agree 100%. Predominantly because castors will only work on a hard flat surface like a concrete, or bitumen pad or path and that is the last surface you should be setting your telescope up on to observe, particularly in the Australian Summer. While concrete or bitumen is convenient and easy from a transport and setup point of view, thermally and optically it is a poor choice. Get a pump up tyre trolley and wheel the scope off the concrete and 20 feet or so onto the grass, if possible. Then try to observe in the direction away from the concrete if possible. A good test after it has been a hot day is to observe a star not over any concrete or structures that hold heat like houses and defocus on a star outside focus and observe the thermals. Then do the same thing with the scope aimed over a concrete path or a house emitting thermals and observe the difference in the stability of the defocused fresnel rings.
Concrete and bitumen holds the heat from the day and emits warm thermal currents continually through the night as it cools. The best surface to set up any telescope on, regardless of mount type, or design, is on short mown grass, or dirt with a tarp over the dirt to keep things clean. The grass doesn't hold the heat and stops the dirt underneath absorbing any heat. Irrespective of the grass, dirt holds a lot loss heat than concrete or bitumen.
A lot of people claim they are experiencing "poor seeing" when they have their scope set up on concrete or bitumen. In fact in many cases the general seeing is excellent, but is bad in their immediate vicinity because of the thermals emanating from the concrete or bitumen they are set up on.
Cheers,
John B
