Vibrations in Telescopes - General Considerations
Telescopes are made up of two parts each of which must function exceedingly well in order to have a satisfying viewing and/or imaging experience. A series of articles on this site has discussed some general considerations and the flaws on the LX 200 design including what can be done about some of them. In some ways the optical part of the telescope is simpler to design and to manufacture because the standards are so well defined and the optical tests are so highly refined.
The mount for the instrument can take on many forms and specifications for the mechanical accuracy of the mounts have not been well established. The mount has a variety of defects ranging from its ability to track the object to vibrations which fuzz up the image. The sources of these defects are many. Some of the ways to improve the mount have been discussed in other articles on this site. This article will concentrate on what can be done to improve a mount that is deficient in typical ways.
The deficiencies can be divided into several regimes. One is the
defect of inaccurate tracking of the objects as the sky rotates.
This defect consists of two parts. Lack of linear tracking and
field rotation. These defects can be fixed by using a CCD chip to
drive the telescope tube locked to a star and aligning the telescope to
the polar axis of the earth. It is relatively easy to fix these defects.
Methods are well known and described elsewhere.
The second class of deficiencies, vibrations of various sorts, are much more difficult to fix. These deficiencies are due to basic weaknesses in the design of the mechanical structure of the telescope mount. All mounts are compromises from a mechanical viewpoint. They should be absolutely rigid and move with the smoothest precision. The requirements for excellence in these characteristics have to be balanced with size, weight and cost of the mount. Defects can be worked on in several ways. One method is to use the Active Optics approach. In some ways this is the ultimate solution that can be used to stabilize images that are wandering about, either due to mechanical wobbles of the telescope tube or due to wobbles in the atmosphere. This is the ultimate stabilization scheme since it is a direct and absolute scheme for placing the star on a pixel and holding it there.
This method should work well. There is every indication, from users, that the AO unit supplied by SBIG works to accomplish the assigned task. The only problem with the AO scheme is that it has a very limited frequency response range. The maximum correction rate is about 40 corrections per second. This seems to be fast enough to correct atmospheric fluctuations of many sorts and it is certainly fast enough to correct any tracking errors in the telescope drive. The makers suggest that it can correct frequencies of 0.1 of the correction rate or about 4 Hz. Unfortunately almost all of the vibrations, instabilities, wobbles or whatever name you wish in the telescope mount are at much higher frequencies. Measurements on the LX fork mounts place the wobble frequencies mainly at 8 to 16 Hz. There are some at frequencies of 16 to 50 Hz as well but they are generally small. These large, low frequency wobbles are as large as 10 to 20 arc seconds and are too fast to be corrected by the AO type unit. (at least the digital chip based type because it its slow readout time). These vibrations can be reduced and damped by other means. There are well known passive and active means to control mechanical vibrations. The area of vibration control is a long study all its own. I will describe only one method applicable to telescope stabilization.
The method is active vibration control, which consists of a vibration transducer and an inertial transducer. The scheme is simple in principle. A sensitive transducer picks up any tendency for the telescope to vibrate and a controller/amplifier drives the transducer to counteract the tendency to vibrate. Vibrations can easily be reduced by 10 to 100 times. The tricky part is to place the transducer at a location on the telescope, which is a complicated structure, so that it applies the correcting force at the center of gravity of the telescope. This point is generally not easily available so one must look for a point of application that damps the vibration without introducing other motions. This is known as controlling the modes of vibration. In many cases several transducers are needed to control the vibration of a complex structure. A vibration control system of the sort described, as it would be applied to the LX200 mount, is under development at this time.
In summary: There are vibrations, oscillations and wobbles of several kinds and several ranges of frequencies that telescopes exhibit some of which can be fixed. Each regime of vibration requires its own type of solution. There may not be a single solution to all of the problems. An attractive solution would be an AO type system which works up to 20 to 40 Hz. This would require a digital CCD chip readout of 200 to 400 samples per second. This quickness is currently out of reach. At some point it is appropriate to consider the efficacy of all the fixes. If it costs $1000 to fix a mount with defects, it might be better to spend those resources on a better mount in the first place.