Tripods and piers 381 Steadiness

Really sturdy telescope mounts are not portable, and vice versa. At an observatory, you can tap the side of the telescope tube, and the image won't move; not so in the field. A good portable mount will shake for about one second after you lightly touch the side of the eyepiece; a light-duty mount will shake longer.

For visual astronomy, you can use a relatively light-duty mount as long as it doesn't shake while you're observing. Basic astrophotography requires a medium-duty mount, and serious astrophotography, a heavy-duty one. Based on personal experience, I classify the Meade ETX-90 mount as light-duty; the Celestron NexStar 5 as medium-duty; and the 8-inch (20-cm) LX90 at the low end of the heavy-duty range. The LX200 is sturdier than the LX90. Really heavy-duty mounts, some of them barely portable, are made by Losmandy, Astro-Physics, and Software Bisque (Paramount mounts).

There are two ways to control vibration: prevent it by using strong materials, or absorb it quickly after it starts. Unfortunately, these two strategies work against each other. The strongest material, steel, tends to "ring" for a long time when tapped. The best vibration-absorbers, wood and rubber, are subject to flexure. For permanent piers, steel embedded in concrete is much better than steel alone because concrete absorbs vibrations.

Figure 3.6. Vibration-reducing pads consist of a layer of soft rubber sandwiched between two layers of hard rubber.

Vibration in portable tripods can be reduced by putting rubber pads under the tripod legs; Celestron and Meade sell pads for this purpose (Figure 3.6). I find that they work miracles with Meade tripods (which are very hard and stiff) but don't help as much with Celestron tripods, which already have soft rubber feet for the same purpose.

A low tripod is a steady tripod, which is one reason many observers prefer to sit on a stool when observing. On the other hand, a tall tripod gets the telescope farther away from the ground and its associated air currents.

Occasionally, vibration is caused by the telescope's stepper motors. Such vibration is usually evident only at high powers and is very sensitive to the resonant frequency of the telescope and mount. This in turn makes it depend on the length of the tripod legs, the presence or absence of rubber pads, the weight of the eyepiece and dewcap, and the position of the telescope. With very lightweight mounts, the best tactic might be to turn off the drive when you want to use high power. On Meade Autostar telescopes, you can do this by selecting "Sleep telescope". If problems are persistent, relubrication of the worm gears may help.

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